Section 1

Reading Preview

Essential QuestionsI How does the reduction in

chromosome number occur duringmeiosis?

I What are the stages of meiosis?I What is theimportance of meiosis in

providing genetic variation?

Review Vocabularychromosome: cellular structurethat contains DNA

New Vocabularygenehomologous chromosomegametehaploidfertilizationdiploidmeiosiscrossing over

Multilin eGlossa

Figure 1 Homologous chromosomes carrygenes for any given trait at the same location. Thegenes that code for earlobe type might not code forthe exact same type of earlobe.

A pair of hbmologouschromosomes

eiosis

MAIN ~ Melosis produces haploid gametes.

Real-World Reading Link Look around your biology class. You might nou~that the students in your class do not look the same. They might be differentheights and have different eye color, hair color, and other features. This varietycharacteristics is a result of two sex cells combining during sexual re du

Chromosomes andChromosome NumberEach student in your biology class has characteristics passed on totby their parents. Each characteristic, such as hair color, height, or eyecolor, is called a trait. The instructions for each trait are located onchromosomes, which are found in the nucleus of cells. The DNA onchromosomes is arranged in segments called genes that control theproduction of proteins. Each chromosome consists of hundreds ofgenes, each gene playing an important role in determining the characteristics and functions of the cell.

Homologous chromosomes Human body cells have 6 chromsomes. Each parent contributes 23 chromosomes, resulting in 23 palof chromosomes. The chromosomes that make up a pair, one chromo.some from each parent, are called homologous chromosomes. Asshown in Figure 1, homologous chromosomes in body cells have thesame length and the same centromere position, and they carry genesthat control the same inherited traits. For instance, the gene for earlotype will be located at the same position on both homologous chromo~somes. Although these genes each code for earlobe type, they mightcode for the exact same type of earlobe.

270 Chapter 10 • Sexual Reproduction and Genetics

HaPl0~ and diploid cells In order to maintain the same chromo50me number from generation to generation, an organism produces

metes, which are sex cells that have half the number of chromo~mes. ~Jthough the number of chromosomes varies from one species0anot~~&~ in humans each gamete contains 23 chromosomes. The

symbOI ,~ can be used to represent the number of chromosomes in aamete A cell with n number of chromosomes is called a haploid cell.

~iaploid comes from the Greek word haploos, meaning single.The process by which one haploid gamete combines with another

haplo”1 gamete is called fertilization. As a result of fertilization, thecell now will contain a total of 2n chromosomes—n chromosomes fromthe female parent plus n chromosomes from the male parent. A cell thatcontains 2n number of chromosomes is called a diploid cell.

Notice that n also describes the number of pairs of chromosomes inan organism. When two human gametes combine, 23 pairs of homologous chromosomes are formed.

MeiOSiS I FoI~DABIncorporate in ormationGametes are formed during a process called meiosis, which is a type of from this section intocell division that reduces the number of chromosomes; therefore, it is your Foldable.referred to as a reduction division. Meiosis occurs in the reproductivestructures of organisms that reproduce sexually. While mitosis maintainsthe chromosome number, meiosis reduces the chromosome number byhalf through the separation of homologous chromosomes. A cell with 2nnumber of chromosomes will have gametes with n number of chromo

o. somes after meiosis, as illustrated in Figure 2. Meiosis involves two conrs secutive cell divisions called meiosis I and meiosis II.

MaleFemale (diploid)

(diploid) 2n2n

Grows intoadult male oradult female

Zygote(diploid

( IMeiosis

\ Male gamete 2n)

Meiosis

Fertilization(haploid) Figure 2 The sexual life cycle in animalsinvolves meiosis, which produces gametes.

Female gamete When gametes combine in fertilization, the(haploid) number of chromosomes is restored.

n Describe what happens to the number of chromosomes during melosis.

Section 1 • Meiosis 271

_______— Centromere

A pair of homologous chromosomes

Sisterchromatids

Figure 3 The homologous chromosomesare physically bound together during synapsisin prophase I.

Figure 4 The results of crossing over arenew combinations of genes.Determine which chromatidsexchanged genetic material.

Interphase Recall that .the cell cjrcle includes interphase prior to mitosis. Cells that undergo meiosis also go through interphase as part of thecell cycle. Cells in interphase carry out various metabolic processesincluding the replication of DNA and the synthesis of protçins.

ProphaSe I As a cell enters prophase I, the replicated chrôñ-iosoniesbecome visible. As in mitosis, the replicated chromosomes c~ñsist ofsister chromatids. As the homologous chromosomes condei~e, theybegin tolorm pairs in a process called synapsis. The homologous chro.mosomes are held tightly together along their lengths, as illustrated ~Figure 3. Notice that in Figure 4 the pink and green chiokb’somes ha~exchanged segments. This exchange occurs during synaps~s. Crossingover is a process during which chromosomal segments ar& &ichangedbetween a pair of homologous chromosomes.

As prophase I continues, centrioles move to the cell’s i~posite p0Spindle fiber~’form and bind to the sister chromatids at the ~entrome

MetaphaSe I In the next phase of meiosis, the pairs of homologousmosomes line up at the equator of the cell, as illustrated in Figure 5.In meiosis, the spindle fibers attach to the centromere of each homologochromosome. Recall that during metaphase in mitosis, the if~dividualchromosomes, which consist of two sister chromatids, line up at~ the cel1~equator. During metaphase I of meiosis, the homologous chiomosomesline up as pairs at the cell’s equator. This is an important disiinctionbetween mitosis and meiosis. .

AnaPhaSe i During anaphase I, the homologous chromo~.omeS separate, which is also illustrated in Figure 5. Each member.o~She pair isguided by spindle fibers and moves toward opposite poles of the cell.The chromosome number is reduced from 2n to n when the homolo~gous chromosomes separate. Recall that in mitosis, the sister chromatids split during anaphase. During anaphase I of meiosis, however, eshomologous chromosome still consists of two sister chroinatids.

TelophaSe I The homologous chromosomes, consisting of two sisterchromatids, reach the cell’s opposite poles. Each pole contaiiIs only onmember of the original pair of homologous chromosomes. Notice inFigure 5 that each chromosome still consists of two sister chrornatidsjoined at the centromere. The sister chromatids might not be identicalbecause crossing over might have occurred during synapsis. in~prQPh

-t

272 Chapter 10 • Sexual Reproduction and Genetics

visu lizing Meiosis

5Follow along the stages of meiosis I and meiosis II, beginning with nterphase at the left.

0 Metaphase I o Anaphase I• Chromosome centromeres attach to

0 prophase spindle fibers. • Homologous chromosomes• pairing of homologous chromosomes • Homologous chromosomes line up at separate and move to

occurs~ each chromosome consists of the equator. opposite poles of the cell. o Telophase Itwo chromatids.

• crossing over produces exchange • The spindles break down.of genetic information. • Chromosomes uncoil and

• The nuclear envelope breaks down. .~ ) form two nuclei.•spindles form. _______

~~_> ,c’ • The cell divides.

t \Interphase• Chromosomes Equator

replicate.• Chromatin condenses.

“Centrioles MEIOSIS I

3

0 Prophase II• Chromosomes

condense.• Spindles form in

each new cell.

Products • Spindle fibersattach to• Four cells have chromosomes.

formed.• Each nucleus

contains a MEIOSIS II ~ Ihaploid number Equatoof chromosomes.

•Metaphasell• Centromeres of chromosomes

line up randomly at the

0 Telophase II equator of each cell.• Four nuclei form

around chromosomes.• Spindles break down.• Cells divide. ° Anaphase II

• Centromeres split.• Sister chromatids separate and

move to opposite poles.

Conce in Motion Animation

Section 1 • Meiosis 273

During telophase I, cytokinesis usually occurs, forming a furrowpinching in animal cells and by forming a cell plate in plant cells. Polling cytokinesis, the cells may go into interphase again before the Sec0set of divisions. However, the DNA is not replicated again during thisinterphase. In some species, the chromosomes uncoil, the nuclear mernbrane reappears, and nuclei re-form during telophase I.

elosis IIMeiosis is only halfway completed at the end of meiosis I. During pro.phase II, a second set of phases begins as the spindle apparatus forms athe chromosomes condense. During metaphase II, the chromosomespositioned at the equator by the spindle fibers, as shown in Figure 5.During metaphase of mitosis, a diploid number of chromosomes linenat the equator. During metaphase II of meiosis, however, a haploid flum.ber of chromosomes line up at the equator. During anaphase II, the sistchromatids are pulled apart at the centromere by the spindle fibers, andthe sister chromatids move toward the opposite poles of the cell. Thechromosomes reach the poles during telophase II, and the nuclear mebrane and nuclei reform. At the end of meiosis II, cytokinesis occurs,resulting in four haploid cells, each with n number of chromosomes,asillustrated in Figure 5.

Reading Check Infer Why are the two phases of meiosis importantfor gamete formation?

a

Data and Observations

0 12 24 36Hours in meiosis medium

*Data obtained from: Shanks, et al. 2001.The Kar3-lnteracting protein cilcIp plapa critical role in passage through meiosis I in Saccharomyces ce,evisiae. GenebcS159:939-951.

(‘AREFILS IN BIOlOGY

Medical Geneticist A medicalgeneticist researches how diseasesare inherited, how to diagnosegenetic conditions, and treatmentsfor genetic diseases.

Inquiry Launch Lab

Review Based on what you have readabout meiosis, how would you now answerthe analysis questions?

.:IIATA IBased on Real Data*

raw Co clu&onsI ~

cjj

How do motor proteins affect cell division? Many scientists think that motor proteinsplay an important role in the movement of chromosomes in both mitosis and meiosis. To testthis hypothesis, researchers have producedyeast that cannot make the motor proteincalled Kar3p. They also have produced yeastthat cannot make the motor protein calledCikip, which many think moderates the function of Kar3p. The results of their experimentare shown in the graph to the right.

Motor Protein Effect on cell Division

Yeast that cannotmake Kar3p

— — Normal yeast— Yeast that cannot

make Cikip

I-,

0a‘5a0

CwS

‘I

U-

1 102

1 x i0~

1 x106

— —

Think Critically1. Evaluate whether Cikip seems to be impor

tant for yeast meiosis. Explain.2. Assess whether Kar3p seems to be necessary

for yeast meiosis. Explain.3. Conclude whether all motor proteins seem

to play a vital role in meiosis. Explain.

274 chapter io • Sexual Reproduction and Genetics

by ~ importance of Meiosisfable i shows a comparison of mitosis and meiosis. Recall that mitosis

of only one set of division phases and produces two identical

diploid daughter cells. Meiosis, however, consists of two sets of divisions0d produces four haploid daughter cells that are not identical. Meiosis

is 1~p0rta~1t because it results in genetic variation.

Review Personal Tutor

Table I Mitosis and Meiosis Concepts in Motion Interactive Table

AnaphaseTelophas

Chromosomereplication

Homologouschromosomesseparate duringanaphase I;sister chromatidsremain together

Crossingover

IChromosome

replication

4)4%

Prophase I

Synapsis and crossingover of homologouschromosomes

one division occua during mitosis. Two sets of divisions occur during meiosis: meiosjs I and meiosis II.ne up DNA replication occurs during intemhase. DNA rè’plication occurs once before meiosjs I.EMun.

Sister synapsis of homologous chromosomes does not occur. Synapsis of homologous chromosomes occurs during prophase I.. andte Two identical cells are formed per cell qicle. Four haploid cells (n) are formed per cell cycle.

The daughter cells are genetically identical. The daughter cells are not genetically identical because ofcrossing over.

Mitosis occurs only in body cells. Meiosis occurs only in reproductive cells.

Meiosis is involved in the production of gametes and providingMitosis is involved in growth and repair.

genetic variation in organisms.

MITOSIS Parent cell MEIOSIS(before chromosome replication)

Meiosis I

Prophase

Duplicatedchromosometwo sister

chromatids)

Metaphase

4, 2n 4

Homologouspairs line up aat the equator

/er chroma\separate during

anaphase

Chromosomesline up at theequator

2n

Metaphase I

Anaphase I

itt. ~Telo~hase IDaughter cells

of meiosis I

Haploidn=2

2n

Daughter cells of mitosis c c / Melosis IIccDaughter cells of meiosis II

Chromosomes do not replicate again;sister chromatids separate during anaphase II

Section 1 • Meiosis 275

Meiosis provides variation Recall that pairs of homologous chromosomes lineup at the equator during prophase I. How the chromo~ -

line up at the equator is a random process that results in gametes withdifferent combinations of chromosomes, such as the ones in Figure 6.Depending on how the chromosomes line up at the equator, four game~~with four different combinations of chromosomes can result.

Notice that the first possibility shows which chromosomes were o~1the same side of the equator and therefore traveled together. Differentcombinations of chromosomes were lined up on the same side of theequator to produce the gametes in the second possibility. Genetic Varia.tion also is produced during crossing over and during fertilization,when gametes randomly combine.

Sexual eproduction v.Asexua ReproductionSome organisms reproduce by asexual reproduction, while othersreproduce by sexual reproduction. The life cycles of still other organ.isms might involve both asexual and sexual reproduction. Duringasexual reproduction, the organism inherits all of its chromodo~esfrom a single parent. Therefore, the new individual is genetically iden.tical to its parent. Bacteria reproduce asexually, whereas most]Srotists

4 reproduce both asexually and sexually, depending on environmentalconditions. Most plants and many of the more simple animali~anreproduce both asexually and sexually, compared to more advancedanimals that reproduce only sexually.

Why do some species reproduce sexually while others reproduceasexually? Recent studies with fruit flies have shown that the rate ofaccumulation of beneficial mutations is faster when species reproducesexually than when they reproduce asexually. In other words, ‘~thenreproduction occurs sexually, the beneficial genes multiply fastcr overtime than they do when reproduction is asexual.

S

AssessmentUnderstand Main Ideas1. 4~ Analyze how meiosis produces haploid gametes.2. Indicate how metaphase us different from metaphase in mitosis.

3. Describe how synapsis occurs.4. Diagram a cell with four chromosomes going through meiosis.

5. Assess how meiosis contributes to genetic variation, while mitosis does nO~

6. Compare and contrast mitosis and meiosis, using Figure 5 andTable 1,by creating aVenn diagram.

wnrrng ta. Biology

7. lmagineyou are a chromosome going through meiosis. Describe whathappens to you and theother chromosomes.

Assessment OiiI

Y y57~

yYDiploid parent

SsYy

5

Y y

Meiosiscontinues

Possibility I Possibility 3~ () (p’) ~

Potential types of haploid gametesFigure 6 The order in which the

homologous pairs line up explains how avariety of sex cells can be produced.

SQctLQn 1•Section SummaryI DNA replication takes place only once

during meiosis, and it results in fourhaploid gametes.

I Meiosis consists of two sets of divisions.

I Meiosis produces genetic variation ingametes.

Think Critically

276 chapter 10 • Sexual Reproduction and Genetics