DISCUSSION ASSIGMENT

*One of the entries MUST ask a question about a concept/idea presented in the required readings in the textbook from the chapters covered this week. This should be a question pertaining to material that you personally do not understand or need clarification on and should be at least 40-50 words in length. Question topics cannot be claimed, and it is one question topic per student. This will aid in diversifying the discussion. Broad categories are posted already in the discussion board. Post your question under the category to which it best applies. State your question in the subject line of the post. (Do not use generic titles such as week 1, post 1, etc., and try to avoid duplicating the category name.) This will create a list of questions and that everyone will be able to see.

Student submitted an appropriate post about the material. This includes asking/presenting an original question in a grammatically correct and logical manner.
Assignment submitted on time and on a different day than other posts.
Assignment met word count AND Word Count (WC) is stated at the end of the post.

The purpose of this Biology Discussion is to help each other understand the main concepts presented in the chapters covered this week. In Week of General Biology, we will be covering Chapters 8 – 10. *One of the entries MUST ask a question about a concept/idea presented in the required readings in the textbook from the chapters covered this week. This should be a question pertaining to material that you personally do not understand or need clarification on and should be at least 40-50 words in length. Question topics cannot be claimed, and it is one question topic per student. This will aid in diversifying the discussion. Broad categories are posted already in the discussion board. Post your question under the category to which it best applies. State your question in the subject line of the post. (Do not use generic titles such as week 1, post 1, etc., and try to avoid duplicating the category name.) This will create a list of questions and that everyone will be able to see.

· Student submitted an appropriate post about the material. This includes asking/presenting an original question in a grammatically correct and logical manner.

· Assignment submitted on time and on a different day than other posts.

· Assignment met word count AND Word Count (WC) is stated at the end of the post.

Chapter 10

Patterns of Inheritance

Essentials of Biology

SEVENTH EDITION

Sylvia S. Mader Michael Windelspecht

Because learning changes everything.®

10.1 Mendel’s Laws

Gregor Mendel

Austrian monk

Worked with garden pea plants in 1860s

When he began his work, most acknowledged that both sexes contributed equally to a new individual.

Unable to account for presence of variations among members of a family over generations

Mendel’s model compatible with evolution

Various combinations of traits are tested by the environment.

Combinations that lead to reproductive success are the ones that are passed on.

Mendel’s Experimental Procedure

Mendel’s experimental procedure:

Used garden pea, Pisum sativa

Easy to cultivate, short generation time

Normally self-pollinates but can be cross-pollinated by hand

Chose true-breeding varieties—offspring were like the parent plants and each other

Kept careful records of large number of experiments

His understanding of mathematical laws of probability helped interpret results.

Particulate theory of inheritance—based on the existence of minute particles (genes)

Figure 10.1 Mendel Working in His Garden

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(a): Pixtal/age fotostock

Figure 10.2a Garden Pea Anatomy and Traits

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Figure 10.2b Garden Pea Anatomy and Traits

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One-Trait Inheritance

One-trait inheritance:

Original parents called P generation

First-generation offspring F₁ generation

Second-generation offspring F₂ generation

Crossed green pod plants with yellow pod plants

All F₁ are green pods.

Had yellow pods disappeared?

Figure 10.3 One-Trait Cross

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Punnett Square

Punnett square:

Shows all possible combinations of egg and sperm offspring may inherit

When F₁ allowed to self-pollinate, F₂ were 3/4 green and 1/4 yellow.

F₁ had passed on yellow pods.

Mendel’s Interpretation

Mendel reasoned

ratio only possible if:

F₁ parents contained two separate copies of each heritable factor

(one dominant and one recessive)

Factors separated when gametes were formed and each gamete carried only one copy of each factor.

Random fusion of all possible gametes occurred at fertilization.

Mendel’s First Law

Mendel’s first law of inheritance—law of segregation

Cornerstone of his particulate theory of inheritance

The law of segregation states the following:

Each individual has two factors for each trait.

The factors segregate (separate) during the formation of the gametes.

Each gamete contains only one factor from each pair of factors.

Fertilization gives each new individual two factors for each trait.

One-Trait Testcross 1

One-trait testcross:

To see if the F₁ carries a recessive factor, Mendel crossed his F₁ generation green pod plants with true-breeding, yellow pod plants.

He reasoned that half the offspring would be green and half would be yellow.

His hypothesis that factors segregate when gametes are formed was supported.

Testcross

Used to determine whether or not an individual with the dominant trait has two dominant factors for a particular trait

One-Trait Testcross 2

One-trait testcross, continued

If a parent with the dominant phenotype has only one dominant factor, the results among the offspring are

If a parent with the dominant phenotype has two dominant factors, all offspring have the dominant phenotype.

Figure 10.4 One-Trait Testcross

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The Modern Interpretation of Mendel’s Work

Modern Interpretation of Mendel’s Work

Scientists note parallel between Mendel’s particulate factors and chromosomes

Chromosomal theory of inheritance

Chromosomes are carriers of genetic information.

Traits are controlled by discrete genes that occur on homologous pairs of chromosomes at a gene locus.

Each homologue holds one copy of each gene pair.

Meiosis explains Mendel’s law of segregation and why only one gene for each trait is in a gamete.

When fertilization occurs, the resulting offspring again have two genes for each trait, one from each parent.

Alleles

Alleles—alternative forms of a gene

Dominant allele masks the expression of the recessive allele.

For the most part, an individual’s traits are determined by the alleles inherited.

Alleles occur on homologous chromosomes at a particular location called the gene locus.

Figure 10.5 Alleles on Homologous Chromosomes

a. Various alleles are located at specific loci.

b. Duplicated chromosomes show that sister chromatids have identical alleles.

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Genotype Versus Phenotype

Genotype versus phenotype:

Genotype—alleles the individual receives at fertilization

Homozygous—two identical alleles

Homozygous dominant

Homozygous recessive

Heterozygous—two different alleles

Phenotype—physical appearance of the individual

Mostly determined by genotype

Table 10.1 Genotype Versus Phenotype

Allele Combination	Genotype	Phenotype
A A	Homozygous dominant	Normal pigmentation
A a	Heterozygous	Normal pigmentation
a a	Homozygous recessive	Albinism

Two-Trait Inheritance

Two-trait inheritance:

Mendel crossed tall plants with green pods (TTGG) with short plants with yellow pods (ttgg).

F₁ plants showed both dominant characteristics—tall and green pods.

Two possible results for F₂

If the dominant factors always go into gametes together, F₂ will have only two phenotypes.

Tall plants with green pods

Short plants with yellow pods

If four factors segregate into gametes independently, four phenotypes would result.

Figure 10.6 Two-Trait Cross by Mendel 1

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Figure 10.6 Two-Trait Cross by Mendel 2

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Mendel’s Second Law—Independent Assortment

Based on the results, Mendel formulated his second law of heredity.

Law of independent assortment

Each pair of factors segregates (assorts) independently of the other pairs.

All possible combinations of factors can occur in the gametes.

When all possible sperm have an opportunity to fertilize all possible eggs, the expected phenotypic results of a two-trait cross are always

Two-Trait Testcross

Two-trait testcross in fruit fly

Fruit fly Drosophila melanogaster

Used in genetics research

Wild-type fly has long wings and gray body

Some mutants have vestigial wings and ebony bodies.

L= long, l = short, G = gray, g = black

Can’t determine genotype of long-winged gray-bodied fly (L Blank G Blank)

Cross with short-winged black-bodied fly (lowercase llgg)

Figure 10.7 Two-Trait Testcross

In this example,

ratio

of offspring indicates L blank G Blank fly was L lowercase l G lowercase g (dihybrid).

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Mendel’s Laws and Probability

Mendel’s laws and probability:

Punnett square assumes:

Each gamete contains one allele for each trait

Law of segregation

Collectively the gametes have all possible combinations of alleles

Law of independent assortment

Male and female gametes combine at random.

Use rules of probability to calculate expected phenotype ratios

Rule of multiplication—chance of two (or more) independent events occurring together is the product of their chances of occurring separately

Coin flips—odd of getting tails is ½, odds of getting tails when you flip 2 coins ½ × ½ = ¼

Figure 10.8 Mendel's Laws and Meiosis

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10.2 Mendel’s Laws Apply to Humans

Pedigree

Chart of a family’s history in regard to a particular genetic trait

Males are squares.

Females are circles.

Shading represents individuals expressing disorder.

Horizontal line between circle and square is a union.

Vertical line down represents children of that union.

Counselor may already know pattern of inheritance and then can predict chance that a child born to a couple would have the abnormal phenotype.

Pedigrees for Autosomal Disorders

Pedigrees for autosomal disorders

Autosomal recessive disorder

Child can be affected when neither parent is affected.

Heterozygous parents are carriers.

Parents can be tested before having children.

Figure 10.9 Autosomal Recessive Pedigree

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Autosomal Dominant Disorder

Autosomal dominant disorder:

Child can be unaffected even when parents are heterozygous and therefore affected.

When both parents are unaffected, none of their children will have the condition.

No dominant gene to pass on

Figure 10.10 Autosomal Dominant Pedigree

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Figure 10.11 Methemoglobinemia

Genetic disorders of interest

Autosomal disorders

Methemoglobinemia—lack enzyme to convert methemoglobin back to hemoglobin

Relatively harmless, bluish-purplish skin

Division of Medical Toxicology, University of Virginia

Figure 10.12 Cystic Fibrosis

Cystic fibrosis—autosomal recessive disorder

Most common lethal genetic disorder among Caucasians in the United States

Chloride ion channel defect causes abnormally thick mucus.

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Figure 10.13 Alkaptonuria

Alkaptonuria—autosomal recessive disorder

Lack functional homogentisate oxygenase gene

Accumulation of homogentisic acid turns urine black when exposed to air

Biophoto Associates/Science Source

Figure 10.14 Sickle-Cell Disease

Sickle-cell disease—autosomal recessive disorder

Single base change in globin gene changes one amino acid in hemoglobin

Makes red blood cells sickle-shaped

Leads to poor circulation, anemia, low resistance to infection

Eye of Science/Science Source

Figure 10.15 Huntington Disease

Huntington disease—autosomal dominant disorder

Progressive degeneration of neurons in brain

Mutation for huntingtin protein

Patients appear normal until middle-aged—usually after having children.

Test for presence of gene

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(both): ©P. Hemachandra Reddy, Ph.D

10.3 Beyond Mendel’s Laws

Incomplete dominance

Heterozygote has intermediate phenotype.

The best examples are in plants. In a cross between a true-breeding, red-flowered plant strain and a white-flowered strain, the offspring have pink flowers. Crossing the pink plants, the offspring’s phenotypic ratio is 1 red-flowered : 2 pink-flowered : 1 white-flowered.

Familial hypercholesterolemia is an example in humans. Persons with one mutated allele have an abnormally high level of cholesterol in the blood, and those with two mutated alleles have a higher level still.

Figure 10.16 Incomplete Dominance in Plants

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Figure 10.17 Incomplete Dominance in Humans

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Medical-On-Line/Alamy Stock Photo

Multiple-Allele Traits

Multiple-allele traits:

ABO blood group inheritance has three alleles

antigen on red blood cells

i = neither A nor B antigen on red blood cells

Each individual has only two of the three alleles

Both

are dominant to i

are codominant—both will

be expressed equally in the heterozygote.

Figure 10.18 Inheritance of ABO Blood Type

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Polygenic Inheritance

Polygenic inheritance:

Trait is governed by two or more sets of alleles

Each dominant allele has a quantitative effect on phenotype and effects are additive.

Result in continuous variation—bell-shaped curve

Multifactorial traits—polygenic traits subject to environmental effects

Cleft lip, diabetes, schizophrenia, allergies, and cancer

Due to combined action of many genes plus environmental influences

Figure 10.19 Height in Humans, a Polygenic Trait

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David Hyde/Wayne Falda/McGraw Hill

Environmental Influences

Environmental influences:

In response to UV radiation, melanin is produced.

Human production of melanin in skin increases closer to the equator to protect skin from radiation.

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Figure 10.21 Gene Interactions and Eye Color

Multiple pigments are involved in determining eye color.

(red eye): Mediscan/Alamy Stock Photo; (brown eye): stylephotographs/123RF; (blue eye): lightpoet/123RF

Pleiotropy

Pleiotropy:

Single genes have more than one effect.

Marfan syndrome is due to production of abnormal connective tissue.

Other examples include sickle-cell anemia and porphyria.

Figure 10.22 Marfan Syndrome, Multiple Effects of a Single Human Gene

* Life-threatening condition

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Linkage

Two traits on same chromosome—gene linkage

Two traits on same chromosome do NOT segregate independently

Recombination between linked genes

Linked alleles stay together—heterozygote forms only two types of gametes, produces offspring only with two phenotypes.

Figure 10.23 A Selection of Traits Located on Human Chromosome 19

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10.4 Sex-Linked Inheritance

Females are XX

All eggs contain an X

Males are XY

Sperm contain either an X or a Y

Y carries SRY gene—determines maleness

X is much larger and carries more genes.

X-linked—gene on X chromosome

Sex-Linked Alleles

Sex-linked alleles:

Fruit flies have same sex chromosome pattern as humans.

When red-eyed female mated with mutant white-eyed male, all offspring were red-eyed.

In the F₂, the

ratio was found but underline all of the white-eyed

flies were males

Y chromosome does not carry alleles for X-linked traits.

Males always receive X from female parent, Y from male parent.

Carrier—female who carries an X-linked trait but does not express it.

Figure 10.24 X-Linked Inheritance

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Pedigree for Sex-Linked Disorder

Pedigree for sex-linked disorder

X-linked recessive disorder

Male offspring inherit trait from the female parent—son’s X comes from the female parent

More males than females have disorder—allele on X is always expressed in males

Females who have the condition inherited the mutant allele from both their female parent and their male parent

Conditions appear to pass from male grandparents to male grandsons

Figure 10.25 X-Linked Recessive Pedigree

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X- and Y-Linked Disorders

X-linked dominant

Only a few traits

Daughters of affected males have the condition.

Affected females can pass condition to female offspring and male offspring

Depends on which X inherited from a carrier female parent if male parent is normal

Y chromosome

Only a few disorders

Present only in males and are passed to all male offspring but not female offspring

X-Linked Recessive Disorders

X-linked recessive disorders:

Color blindness

About 8% of white males have red-green color blindness.

Duchenne muscular dystrophy

Absence of protein dystrophin causes wasting away of muscles.

Therapy—immature muscle cells injected into muscles

Figure 10.26 Muscular Dystrophy

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(left, right): Courtesy of Dr. Rabi Tawil, Director, Neuromuscular Pathology Laboratory, University of Rochester Medical Center; (center): ©Muscular Dystrophy Association

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Because learning changes everything.®

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Figure 10.1 Mendel Working in His Garden – Text Alternative

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The chart shows the data as follows:

Trait 1. Stem length. Recessive: short; Dominant: tall. Trait 2. Pod shape. Recessive: Constricted. Dominant: Inflated. Trait 3. Seed shape. Recessive: Wrinkled. Dominant: Round. Trait 4. Seed color. Recessive: Green. Dominant: Yellow. Trait 5. Flower position. Recessive: Terminal. Dominant: Axial. Trait 6. Flower color. Recessive: White. Dominant: Purple. Trait 7. Pod Color. Recessive: Yellow. Dominant: Green.

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Figure 10.2a Garden Pea Anatomy and Traits – Text Alternative

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A. Flower structure. Pollen grains containing sperm are produced in the anther. When pollen grains are brushed onto the stigma, sperm fertilizes eggs in the ovary. Fertilized eggs are located in ovules, which develop into seeds.

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Figure 10.2b Garden Pea Anatomy and Traits – Text Alternative

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B. Cross-pollination. 1. Cut away anthers. 2. Brush on pollen from another plant. 3. The result of a cross from a parent that produces round, yellow seeds and a parent that produces wrinkled yellow seeds.

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Figure 10.3 One-Trait Cross – Text Alternative

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In the P generation, the green pod and yellow pod with gametes (uppercase g and lowercase g) are crossed. In F1 generation, all plants are green (uppercase g lowercase g). In F2 generation, the uppercase g lowercase g eggs and uppercase g lowercase g sperms are crossed. The offspring produced are uppercase g lowercase g, lowercase g lowercase g, uppercase g uppercase g, and uppercase g lowercase g. Except lowercase g lowercase g which is yellow the rest are green. F2 generation phenotypic ratio is 3 green: 1 yellow. The key is uppercase G equals green pod and lowercase g equals yellow pod.

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Figure 10.4 One-Trait Testcross – Text Alternative

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First the uppercase g lowercase g green pod and lowercase g lowercase g yellow pod are crossed. The possible genotypes produced are uppercase g lowercase g green pod and lowercase g lowercase g yellow pod. Phenotypic ratio: 1 green: 1 yellow. Next, the uppercase g uppercase g green pod and lowercase g lowercase g yellow pod are crossed. The possible genotypes produced are uppercase g lowercase g green pod. Phenotype: all green pods.

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Figure 10.6 Two-Trait Cross by Mendel 1 – Text Alternative

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P generation has green pod T T G G (all uppercase), and yellow pod t t g g (all lowercase). P gametes are T G (both uppercase) and t g (both lowercase). In F1 generation, all plants are tall with green pods uppercase T lowercase t uppercase G lowercase g.

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Figure 10.6 Two-Trait Cross by Mendel 2 – Text Alternative

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In Punnett square, the F1 gametes are TG, Tg, tG and tg. The vertical axis is marked sperm, the horizontal top axis is marked as eggs and the horizontal bottom axis as offspring. The F2 generation has gametes TTGG, TTGg, TtGG, TtGg, TTGg, TTgg, TtGg, Ttgg, TtGG, TtGg, ttGG, ttGg, TtGg, Ttgg, ttGg, ttgg. F2 phenotypic ratio is given as 9 tall plant, green pod (green highlight) : 3 tall plant, yellow pod (yellow) : 3 short plant, green pod (orange) : 1 short plant, yellow pod (blue). Key: T equals tall plant, t equals short plant, G equals green pod, g equals yellow pod.

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Figure 10.5 Alleles on Homologous Chromosomes – Text Alternative

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A homologous pair has the alleles (uppercase g lowercase g, uppercase r lowercase r, uppercase s lowercase s and lowercase t uppercase t) placed at a gene locus. The duplicated chromosomes show that sister chromatids have identical alleles.

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Figure 10.7 Two-Trait Testcross – Text Alternative

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Two-trait test cross has P generation genotypes LlGg (long wings and gray body) crosses with llgg (short wings and black body). In Punnett square, the F1 generation has gametes LG, Lg, IG and ig on the vertical axis marked sperms, ig eggs on horizontal top axis and offspring is marked on horizontal bottom axis. The offsprings formed are LiGg, Llgg, llGg and llgg. The F1 phenotypic ratio are 1 long wings, gray body (green highlighted) : 1 long wings, black body (yellow) : 1 short wings, gray body (orange) : 1 short wings, black body (blue). Key: L equals long wings, l equals short wings, G equals gray body, g equals black body. The F1 generation is 1:1:1:1.

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Figure 10.8 Mendel's Laws and Meiosis – Text Alternative

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The parent cell has two pairs of homologous chromosomes (uppercase a lowercase a, uppercase b lowercase b). In meiosis 1: Homologues can align either way during metaphase. At the end of meiosis 2, all possible combinations of chromosomes and alleles result: Uppercase a uppercase b, lowercase a lowercase b, uppercase a lowercase b, lowercase a uppercase b.

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Figure 10.9 Autosomal Recessive Pedigree – Text Alternative

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The pedigree shows that the parents are carriers for an autosomal recessive disorder. An affected parent (aa) on cross with an unaffected A_ (one allele unknown) on step 1 to form A_, Aa, Aa and A_ in step 2. The alleles in step 3 are Aa, Aa, A_, and A_. Alleles in the final step 4 are aa, aa and A_. Key: aa equals affected, Aa equals carrier (unaffected), AA equals unaffected and A_ equals unaffected (one allele unknown).

The text underneath reads:

Unaffected parents can produce children who are affected.

Heterozygotes (Aa) are unaffected.

Both males and females may be affected with equal frequency.

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Figure 10.10 Autosomal Dominant Pedigree – Text Alternative

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The pedigree shows that the parents are carriers for an autosomal dominant disorder. Two affected parents (Aa) on crossing in step 1 forms aa, Aa, A_, aa, aa and aa alleles in step 2. The alleles in step 3 are Aa, Aa, aa, aa, aa and aa. Key: AA equals affected, Aa equals affected, A_ equals affected and aa equals unaffected.

The text underneath reads:

Children who are affected will have at least one parent who is affected.

Heterozygotes (Aa) are affected.

Both males and females may be affected with equal frequency.

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Figure 10.12 Cystic Fibrosis – Text Alternative

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The cytoplasm contains the chloride ions (Cl minus in green) and water (H2O in blue), are trapped inside the cell. The defective chloride ion channel does not allow chloride ions to pass through them. The lumen of respiratory tract are filled with thick and sticky mucus.

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Figure 10.15 Huntington Disease – Text Alternative

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The micrograph at the left has, many neurons in normal brain and the micrograph at the right shows the loss of neurons in a brain with Huntington’s disease which is a neurological disorder.

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Figure 10.16 Incomplete Dominance in Plants – Text Alternative

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The crossing of two pink flowers as parents (R1R2 cross R1R2), gives the offspring which are given in Punnett square. The vertical axis is marked sperm, the horizontal top axis is marked eggs and the horizontal bottom axis is marked as offspring. The offspring are as follows: R1R1, a red flower; R1R2, a pink flower; R1R2, a pink flower; R2R2, a white flower. Key: 1 R1R1 equals red flower: 2 R1R2 equals pink flower: 1 R2R2 equals white flower. Phenotypic ratio is 1:2:1. The reappearance of the three phenotypes in this generation shows that they are still dealing with a single pair of alleles.

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Figure 10.17 Incomplete Dominance in Humans – Text Alternative

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The vertical axis ranges from 0 through 1000 in the intervals of 100. The approximate data are as follows: Normal: 160 through 260, heterozygote: 290 through 550, and homozygote: 600 through 1,020. The photo of a human hand on the side has bulges, marked as cholesterol deposits.

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Figure 10.18 Inheritance of ABO Blood Type – Text Alternative

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The parent alleles taken for mating are uppercase I superscript B lowercase i and uppercase I superscript A lowercase i in which allele is recessive and the uppercase I superscript A and uppercase I superscript B are dominant parent genotypes. On mating, the resulting offspring are given in Punnett squares. The vertical axis is marked sperm, the horizontal top axis is marked eggs and the horizontal bottom axis is marked offspring. The offspring obtained are uppercase I superscript A uppercase I superscript B (purple highlighted), uppercase I superscript B lowercase i (red highlighted), uppercase I superscript A lowercase I (blue highlighted), and lowercase i lowercase I (colorless). Phenotype ratio as 1:1:1:1. Key: Blood type A (blue highlighted), Blood type B (red highlighted), Blood type AB (violet color) and Blood type O (colorless).

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Figure 10.19 Height in Humans, a Polygenic Trait – Text Alternative

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In the graph at the bottom, the horizontal axis from short to tall, ranges from 62 through 74 in increments of 2. A bell curve starts at the origin, reaches a peak at height equals 68 inches and slopes down to the horizontal axis. The region under the curve between height equals 65 inches and 71 inches is shaded and labeled most are this height, and the areas on either side are labeled few. The graph at the top with the horizontal axis labeled height in inches, ranging from 60 through 75 in increments of 5, shows a photo of people standing in the shape of the bell-shaped curve. Please note the data are approximate.

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Environmental Influences – Text Alternative

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The vertical axis shows the frequency. The horizontal axis shows the number of dominant alleles ranging from 0 through 6 in the intervals of 1 units. The graph shows a bell-shaped curve indicating many phenotypes which are categorized from 0 through 6. The majority have dark skin in middle ranges (1.5 through 4.5) and only few have extreme range of skin. The data is as follows:

0: a a b b c c (all lowercase.

1: uppercase A a b b c c, a a uppercase B lowercase b c c, a a b b uppercase C c.

2: uppercase A a uppercase B b c c, uppercase A a b b uppercase C c, a a uppercase B b uppercase C c, uppercase A uppercase A b b c c, a a uppercase B uppercase B c c , a a b b uppercase C uppercase C.

3: uppercase A a uppercase B b uppercase C c, a a uppercase B b uppercase C uppercase C, uppercase A uppercase A b b uppercase C c, uppercase A a b b uppercase C uppercase C, uppercase A uppercase A uppercase B b c c, a a uppercase B uppercase B uppercase C c, uppercase A a uppercase B uppercase B c c.

4: a a uppercase B uppercase B uppercase C uppercase C, uppercase A uppercase A b b uppercase C uppercase

Chapter 8

Cellular Reproduction

Essentials of Biology

SEVENTH EDITION

Sylvia S. Mader Michael Windelspecht

Because learning changes everything.®

8.1 An Overview of Cellular Reproduction

Multicellular organisms begin life as a single cell.

Humans become trillions of cells because of cellular reproduction.

Reproduction continues as we grow to replace worn-out or damaged tissues.

Figure 8.1 Cellular Reproduction

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(a): Andersen Ross/Getty Images; (b): Ted Kinsman/Science Source; (c): Biophoto Associates/Science Source; (d): (zygote): Anatomical Travelogue/Science Source; (fetus): Steve Allen/Brand X Pictures/Getty Images

Cellular Reproduction 1

Asexual reproduction (example Binary fission)

Doesn’t require sperm or egg (required in sexual reproduction)

All cells come from cells.

Cellular reproduction is necessary for the production of both new cells and new organisms.

Two important processes

Growth—cell duplicates its contents (including DNA and organelles)

Cell division—parent cell contents divides into two daughter cells

Both processes heavily regulated

Cellular Reproduction 2

Chromosomes

DNA replication is the copying of DNA.

Full set is passed to each daughter cell

DNA is packaged into chromosomes.

Thickened complex of DNA and proteins

Allows easier distribution to daughter cells

Chromatin

DNA and associated proteins have the appearance of thin threads.

Figure 8.2 Chromosome Compaction

DNA is periodically wound around histones to form nucleosomes.

Just before cell division, chromatin condenses into chromosomes.

Humans have 46 chromosomes.

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(nucleosomes): Don W. Fawcett/Science Source

8.2 The Cell Cycle: Interphase, Mitosis, and Cytokinesis

Orderly sequence of stages that takes place between the time a new cell has arisen to the point where it gives rise to two daughter cells

Interphase

M (Mitotic) phase

Figure 8.3 The Cell Cycle

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Figure 8.4 Overview of Mitosis

Duplicated chromosomes are composed of sister chromatids joined at the centromere.

Each sister chromatid has identical DNA.

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Interphase 1

Majority of the cell cycle

Time when a cell performs its usual functions

Amount of time varies widely depending on cell

Three stages:

G₁

S—DNA synthesis

G₂

Interphase 2

Interphase is divided into three stages.

G₁—stage before DNA replication

Cell doubles organelles

Accumulates materials for DNA synthesis

Makes decision whether to divide or not

G₀—arrested—does not go on to divide

S—DNA synthesis

Results in each chromosome being composed of two sister chromatids

G₂—stage following DNA synthesis

Extends to onset of mitosis

Synthesizes proteins needed for cell division

Cell Division Occurs in the M (Mitotic) Phase

Cell division occurs.

Encompasses

Division of nucleus (mitosis)

Creates two identical daughter nuclei

Division of cytoplasm (cytokinesis)

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M (Mitotic) Phase

Distributes duplicated nuclear contents of parent cell equally to daughter cells

Each sister chromatid has the same genetic information.

Daughter chromosomes—separated sister chromatids

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Daughter Nuclei

Daughter nuclei produced by mitosis are genetically identical to each other and to the parent nucleus.

Every animal has an even number of chromosomes—each parent contributes half of the chromosomes to the new individual.

In drawings, colors may be used to indicate chromosomes contributed by the male or female parent.

Moving the Chromosomes

Spindle

Most eukaryotic cells rely on this structure to pull chromatids apart.

Part of the cytoskeleton

Spindle fibers are made of microtubules

Centrosome—primary microtubule organizing center

Spindle fibers may overlap at the spindle equator or attach to duplicated chromosomes.

Phases of Mitosis in Animal and Plant Cells

Mitosis is a continual process traditionally divided into four phases:

Prophase—chromosomes are visible under microscope in condensed pairs

Metaphase—chromosomes line up along equatorial plate (middle)

Anaphase—chromosomes are pulled to opposite poles of cell (apart)

Telophase and Cytokinesis—two distinct cells are visible under the microscopes.

Figure 8.5 Phases of Mitosis in Animal Cells 1

Interphase

During interphase, the eukaryotic cell duplicates the contents of the cytoplasm, and DNA replicates in the nucleus. The duplicated chromosomes are not yet visible. A pair of centrosomes is outside the nucleus.

Prophase

During prophase, the chromosomes are condensing. Each consists of two sister chromatids held together at a centromere. Outside the nucleus, the spindle begins to assemble between the separating centrosomes.

Prophase continues with the disappearance of the nucleolus and the breakdown of the nuclear envelope. Spindle fibers from each pole attach to the chromosomes at specialized protein complexes on either side of each centromere. During attachment, a chromosome first moves toward one pole and then toward the other pole.

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(all): ©Ed Reschke

Figure 8.5 Phases of Mitosis in Animal Cells 2

Metaphase

During metaphase, the chromosomes are aligned at the spindle equator midway between the spindle poles. The spindle fibers on either side of a chromosome extend to opposite poles of the spindle. Unattached spindle fibers reach beyond the equator and overlap.

Anaphase

During anaphase, the sister chromatids separate and become daughter chromosomes. As the spindle fibers attached to the chromosomes disassemble, each pole receives a set of daughter chromosomes. The spindle poles move apart as the unattached spindle fibers slide past one another. This contributes to chromosome separation.

Telophase and Cytokinesis

During telophase, the spindle disappears as new nuclear envelopes form around the daughter chromosomes. Each nucleus contains the same number and kinds of chromosomes as the original parent cell. Remnants of spindle fibers are still visible between the two nuclei. Division of the cytoplasm begins.

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(all): ©Ed Reschke

Phases of Mitosis in Animal Cells

Although mitosis is divided into phases, it is a continuous process.

DNA is replicated before mitosis begins.

Each chromosome consists of two sister chromatids attached at a centromere.

Red chromosomes are from one parent, and blue are from the other parent.

Mitosis is usually followed by cytokinesis.

Division of the cytoplasm

Begins during telophase and continues after the daughter nuclei have formed

Mitosis Differs in Plants and Animals

Plant and animal cells differ.

Plant—have centrosomes but lack centrioles

Animal—each centrosome has two centrioles and an aster (array of microtubules)

Cell membrane in plants form from center outward along a cell plate

Cell membrane in animals forms through a cleavage furrow (outer membrane toward the center).

Figure 8.6 Comparing Mitosis in a Plant and Animal Cell 1

Nucleolus has disappeared, and duplicated chromosomes are visible. Centrosomes begin moving apart, and spindle is in process of forming.

The kinetochore of each chromatid is attached to a kinetochore spindle fiber. Polar spindle fibers stretch from each spindle pole and overlap.

Metaphase

Centromeres of duplicated chromosomes are aligned at the metaphase plate (center of fully formed spindle). Kinetochore spindle fibers attached to the sister chromatids come from opposite spindle poles.

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(prophase, metaphase, anaphase, telophase): Kent Wood/Science Source; (prometaphase): ©Ed Reschk

Figure 8.6 Comparing Mitosis in a Plant and Animal Cell 2

Anaphase

Sister chromatids part and become daughter chromosomes that move toward the spindle poles. In this way, each pole receives the same number and kinds of chromosomes as the parent cell.

Telophase

Daughter cells are forming as nuclear envelopes and nucleoli reappear. Chromosomes will become indistinct chromatin.

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(prophase, metaphase, anaphase, telophase): Kent Wood/Science Source; (prometaphase): ©Ed Reschk

Cytokinesis in Animal and Plant Cells

Cytokinesis

Accompanies mitosis in most but not all cells

Mitosis with cytokinesis results in a multinucleated cell

Muscle cells in vertebrate animals

Embryo sac in flowering plants

Cytokinesis in Animal Cells

Cleavage furrow forms as anaphase ends.

Contractile ring, a band of actin filaments, forms a constriction.

Like pulling a drawstring

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(top): National Institutes of Health (NIH)/USHHS; (bottom): Steve Gschmeissner/Brand X Pictures/Science Photo Library/Getty Images

Cytokinesis in Plant Cells

Plant cells

Rigid cell wall prevents furrowing

Involves building of new plasma membrane and cell walls between daughter cells

Golgi apparatus produces vesicles.

Cell plate—newly formed plasma membrane

New membrane releases molecules that form new plant cell walls

Biophoto Associates/Science Source

8.3 The Cell Cycle Control System

Cell cycle must be controlled

Ensures that the stages occur in order and that the cycle continues only when the previous stage is successfully completed

Cell cycle checkpoints

Three of the many

G₁ checkpoint

G₂ checkpoint

Mitotic stage checkpoint

Cell Cycle Checkpoints

G₁ checkpoint

Cell committed to divide after this point

Can enter G₀ if checkpoint not passed

Proper growth signals must be present to pass

DNA integrity checked—if repair is not possible, apoptosis occurs

G₂ checkpoint

Verifies that DNA replicated

DNA damage repaired

Mitotic stage checkpoint

Between metaphase and anaphase

All chromosomes must be attached to spindle to pass.

Figure 8.9 Cell Cycle Checkpoints

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Internal and External Signals

Signal—a molecule that stimulates or inhibits an event

External signals come from outside the cell.

Internal signals come from inside the cell.

Kinases remove a phosphate from ATP and add it to other molecules.

Cyclins are internal signals present only during certain stages of the cell cycle.

Destruction of cyclin at the appropriate time is necessary for normal cell cycle progression.

Cell Cycle Signals

External signals

Epidermal growth factor (EGF) stimulates skin near an injury to finish cell cycle and repair injury.

Hormone estrogen stimulates lining of the uterus to divide and prepare for egg implantation.

Contact inhibition—cells stop dividing when they touch

Cells divide about 70 times in culture and then die.

Due to shortening of telomeres

Telomere—repeating DNA sequence at end of chromosome

Apoptosis

Programmed cell death

Remaining cell fragments engulfed by white blood cells

Unleashed by internal or external signals

Helps keep number of cells at appropriate level

Normal part of growth and development

Tadpole tail

Webbing between human digits

Figure 8.10 Apoptosis 1

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Figure 8.10 Apoptosis 2

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(photo): Steve Gschmeissner/Science Source

8.4 Cell Cycle and Cancer

Cell cycle is regulated by signals that inhibit or promote cell cycle.

Cancer may result from imbalance.

Cancer is a disease of the cell cycle in which cellular reproduction occurs repeatedly without end.

Figure 8.11 Development of Cancer

Cell (red) acquires a mutation for repeated cell division.

New mutations arise, and one cell (teal) has the ability to start a tumor.

Cancer in situ. The tumor is at its place of origin. One cell (purple) mutates further.

Cells have gained the ability to invade underlying tissues by producing a proteinase enzyme.

Cancer cells now have the ability to invade lymphatic and blood vessels.

New metastatic tumors are found some distance from the original tumor.

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Genetic Control of the Cell Cycle

Proto-oncogenes code for proteins that promote the cell cycle and inhibit apoptosis. They are often likened to the gas pedal of a car because they accelerate the cell cycle.

Tumor suppressor genes code for proteins that inhibit the cell cycle and promote apoptosis.

When proto-oncogenes mutate, they become cancer-causing genes called oncogenes.

When tumor suppressor genes mutate, their products no longer inhibit the cell cycle or promote apoptosis.

Figure 8.12 Role of Proto-oncogenes and Tumor Suppressor Genes in the Cell Cycle

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Other Genetic Changes and Cancer

Absence of telomere shortening

Chromosomal rearrangements

When the chromosomes of cancer cells become unstable, portions of the DNA double helix may be lost, duplicated, or scrambled. For example, a portion of a chromosome may break off and reattach to another chromosome. These events are called translocations.

Other cell cycle genes associated with cancer:

BRCA1 and BRCA2

RB gene

RET gene

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8.5 Characteristics of Cancer

Characteristics of cancer cells

Carcinogenesis—development of cancer

Cancer cells lack differentiation—do not contribute to body function

May be immortal—divide repeatedly

Have abnormal nuclei with abnormal number of chromosomes

Figure 8.14 Cancer Cells

Cancer cells

Do not undergo apoptosis

Form tumors—do not respond to inhibitory signals

Undergo metastasis (cells travel to start new tumors) and angiogenesis (form new blood vessels to nourish themselves)

Benign—contained within a capsule

Malignant—invasive and may spread

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N. Kedersha/Science Source

Figure 8.15 Development of Breast Cancer

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(b): King’s College Hospital/Science Source

Cancer Treatment

Either remove tumor or interfere with the ability of cancer cells to reproduce

As rapidly dividing cells, they are susceptible to radiation therapy and chemotherapy.

Damages DNA or some aspect of mitosis

Leads to side effects

Hormone therapy is designed to prevent cells from receiving signals for continued growth and division.

Prevention of Cancer

Protective behaviors

Avoid smoking—accounts for about 30% of all cancer deaths

Avoid sun exposure—major factor in development of most dangerous type of skin cancer, melanomas

Heavy drinkers are prone to particular cancers.

Protective diet

Weight loss can reduce cancer risk.

Increase consumption of foods rich in vitamins A and C.

Avoid salt-cured or pickled foods.

Include cabbage family members in the diet.

Figure 8.16 The Right Diet Helps Prevent Cancer

(leafy greens): Ingram Publishing/SuperStock; (blueberries): Purestock/SuperStock; (oranges): sanmai/Getty Images; (broccoli): Mark Steinmetz/McGraw Hill

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Accessibility Content: Text Alternatives for Images

Figure 8.1 Cellular Reproduction – Text Alternative

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The details of the panels are as follows:

a. Children grow: A woman hugs a baby girl.

b. Tissue repair: A set of nine photos shows the sequential healing of a wound.

c. Amoebas reproduce: The cell of Amoeba ruptures into two.

d. Zygotes develop: Human zygote with a fetus in the womb.

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Figure 8.2 Chromosome Compaction – Text Alternative

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a. At the top, a duplicated and condensed chromosome shows two labels including sister chromatids and centromere. Further, looped chromatin and zigzag chromatins are shown. Next, chromatin is coiled around specialized proteins called histones to form nucleosomes.

b. The micrograph of a DNA strand shows nucleosomes. They appear as beads on a string.

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Figure 8.3 The Cell Cycle – Text Alternative

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Interphase includes stages G1, S, and G2, leading to stage M (Mitosis). G0 stage is also mentioned in the interphase. In the G1 phase, growth occurs as organelles double. In the S phase, DNA replication occurs as chromosomes duplicate. In the G2 phase, growth occurs as cell prepares to divide.

In stage M, mitosis and cytokinesis occur including steps prophase, metaphase, anaphase, and telophase, along with cytokinesis. After completion of the M stage, two cells are formed.

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Figure 8.4 Overview of Mitosis – Text Alternative

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A chromosome consisting of one chromatid converts into duplicated chromosome with two sister chromatids, connected with a centromere, during DNA replication. The duplicated chromosome with two sister chromatids converts into two daughter chromosomes consisting of one chromatid, during mitosis.

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M (Mitotic) Phase – Text Alternative

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Cell Division Occurs in the M (Mitotic) Phase – Text Alternative

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In stage M, mitosis and cytokinesis occur including steps prophase, metaphase, anaphase, and telophase, along with cytokinesis. After completion of the M stage, two cells are formed.

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Figure 8.5 Phases of Mitosis in Animal Cells 1 – Text Alternative

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The stages are discussed as follows:

G 1, S, and G 2 stages are associated with the interphase of the cell cycle. During interphase, the eukaryotic cell duplicates the contents of the cytoplasm, and DNA replicates in the nucleus. The cell in this stage shows labels including centrosome, centrioles, nucleolus, chromatin, nuclear envelope, and plasma membrane.

The M phase of the cell cycle is divided into four phases along with cytokinesis.

Prophase: During prophase, the chromosomes are condensing. Each consists of two sister chromatids held together at a centromere. At this stage, the cell shows the parts including early mitotic spindle, centrosome, centromere, and chromosome with two sister chromatids. Next, prophase continues with the disappearance of the nucleolus and the breakdown of the nuclear envelope. At this stage, the cell shows parts including spindle fibers and nuclear envelope fragments. The chromosome area is also easily distinguished in two micrographs magnified 250 times.

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Figure 8.5 Phases of Mitosis in Animal Cells 2 – Text Alternative

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The stages are discussed as follows:

Metaphase: During metaphase, the chromosomes are aligned at the spindle equator midway between the spindle poles. Aster and chromosomes at spindle equator are easily seen in the micrograph magnified at 250 times.

Anaphase: During anaphase, the sister chromatids separate and become daughter chromosomes that are easily seen in the dividing cell and the micrograph magnified at 250 times.

Telophase and cytokinesis. During telophase, the spindle disappears as new nuclear envelopes form around the daughter chromosomes. Each nucleus contains the same number and kinds of chromosomes as the original parent cell. Cleavage furrow and nuclear envelope formation are seen in the cell and the micrograph magnified at 250 times.

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Figure 8.6 Comparing Mitosis in a Plant and Animal Cell 1 – Text Alternative

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Prophase: During prophase, the chromosomes are condensing. Each consists of two sister chromatids held together at a centromere. The plant cell at 900 times magnification shows cell wall and chromosomes and the animal cell shows duplicated chromosome, centromere, and spindle fibers forming.

Prophase continues with the disappearance of the nucleolus and the breakdown of the nuclear envelope. The plant cell at 500 times magnification and the animal cell show distinct spindle pole.

Metaphase: During metaphase, the chromosomes are aligned at the spindle equator midway between the spindle poles. The plant cell at 900 times magnification shows spindle fibers and the animal cell shows chromosomes at metaphase plate.

Anaphase: During anaphase, the sister chromatids separate and become daughter chromosomes. The plant cell at 900 times magnification and the animal cell show daughter chromosomes.

Telophase and cytokinesis: During telophase, the spindle disappears as new nuclear envelopes form around the daughter chromosomes. Each nucleus contains the same number and kinds of chromosomes as the original parent cell. The plant cell at 900 times magnification shows a cell plate while the animal cell shows cleavage furrow and nucleolus.

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Figure 8.6 Comparing Mitosis in a Plant and Animal Cell 2 – Text Alternative

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Prophase continues with the disappearance of the nucleolus and the breakdown of the nuclear envelope. The plant cell at 500 times magnification and the animal cell show distinct spindle pole.

Anaphase: During anaphase, the sister chromatids separate and become daughter chromosomes. The plant cell at 900 times magnification and the animal cell show daughter chromosomes.

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Cytokinesis in Animal Cells – Text Alternative

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The top micrograph at 4000 times magnification and the diagram show a distinct cleavage furrow. The diagram also shows the contractile ring that is visible in the second micrograph. The last diagram shows the deepening of the cleavage and eventually tends to separate the membrane in two by the movement of the cell cytoplasm towards the centripetal direction.

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Figure 8.9 Cell Cycle Checkpoints – Text Alternative

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The checkpoints of the cell cycle are as follows:

G1 checkpoint: The G1 checkpoint is present at the end of the G1 phase, before the transition to the S phase. Here, apoptosis can occur if DNA is damaged beyond repair.

G2 checkpoint: It ensures that mitosis will not occur until DNA is replicated.

M checkpoint: At this checkpoint, mitosis stops until the chromosomes are properly aligned.

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Figure 8.10 Apoptosis 1 – Text Alternative

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First is a group of normal cells. Next, a normal cell rounds up, and the nucleus collapses.

Then, the chromatin condenses and the DNA breaks into fragments.

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Figure 8.10 Apoptosis 2 – Text Alternative

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The plasma membrane blisters and blebs form. Diagram shows the blebs. Finally, the cell breaks into fragments. The micrograph of apoptotic cell magnified at 2500 times shows cell and DNA fragments.

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Figure 8.11 Development of Cancer – Text Alternative

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The steps are marked from top to bottom as:

Only one mutation in red appears across the yellow epithelial cells.

Two mutations appear across the epithelial cells with one teal cell at the center of a few red cells.

Three mutations appear just above the lymphatic and blood vessels, forming a tumor. The illustration shows a pile of teal cells with one purple cell at the bottom.

An invasive tumor, that is, a pile of purple cells appears.

The malignant tumor, which is a pile of purple cells appears approaching the lymphatic and blood vessel appears.

A purple cell appears in the lymphatic vessel. The distant tumor, which is a pile of purple cells appears invading the lymphatic vessel.

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Figure 8.12 Role of Proto-oncogenes and Tumor Suppressor Genes in the Cell Cycle – Text Alternative

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a. It demonstrates how a growth factor binds to a cell membrane-attached receptor. The phosphorylation of signaling protein, which leads to activated signaling protein, supports the activity of receptors. Labels include signaling protein, activated signaling protein, and phosphate.

b. Growth factor binds to receptor and activates proto-oncogenes through cell-signaling pathway. Proto-oncogene codes for a protein that promotes the cell cycle. If a proto-oncogene mutates, the resulting oncogenes may lead to uncontrolled cell division. Tumor suppressor gene codes for a protein that inhibits the cell cycle. Mutant tumor suppressor genes can lose this function. Labels are activation of proto-oncogene, promotion of cell-cycle, expression of tumor suppressor, and inhibition of cell cycle.

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Other Genetic Changes and Cancer – Text Alternative

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The ABL and BCR genes are located on the long arms of chromosomes 9 and 22 respectively.

The translocation of ABL gene from chromosome 9 to chromosome 22 creates the Philadelphia chromosome that contains the BCR-ABL oncogene.

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Figure 8.14 Cancer Cells – Text Alternative

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Yellow and red cells are labeled cancer cells and the green cells on either side are labeled normal cells.

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Figure 8.15 Development of Breast Cancer – Text Alternative

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A benign tumor in the breast is developed from a single mutated cell. The tumor becomes malignant and invades nearby tissue. The cancer cells travel through lymphatic and blood vessels, and metastatic tumors form.

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