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8/13/2019 13_DetailLectOut

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Chapter 13

Meiosis and Sexual Life Cycles

Lecture Outline

Overview: Variations on a Theme

Living organisms are distinguished by their ability to reproduce their own kind.

Offspring resemble their parents more than they resemble less closely relatedindividuals of the same species.

The transmission of traits from one generation to the next is called heredityorinheritance.

Offspring differ somewhat from their parents and siblings, which demonstratesvariation.

Farmers have bred plants and animals for desired traits for thousands of years, butthe mechanisms of heredity and variation eluded biologists until the development ofgenetics in the 2th century.

eneticsis the scientific study of heredity and hereditary variation.

! "enetics has revolutioni#ed medicine and agriculture.

! Our ability to manipulate the genetic material, $%&, has raised some thornysocial and ethical 'uestions.

Concept 13!1 Offsprin" ac#uire "enes from their parents $y inheritin"chromosomes!

(arents endow their offspring with coded information in the form of "enes.

! )our genome is made up of the genes that you inherited from your mother andyour father.

"enes program specific traits that emerge as we develop from fertili#ed eggs intoadults.

"enes are segments of $%&. "enetic information is transmitted as specificse'uences of the four deoxyribonucleotides in $%&.

! This process is analogous to the symbolic information of language, in which

words and sentences are translated into mental images.! *ells translate genetic +sentences into freckles and other features that have no

resemblance to genes.

-ost genes program cells to synthesi#e specific en#ymes and other proteinswhose cumulative action produces an organisms inherited traits.

The transmission of hereditary traits has its molecular basis in the precisereplication of $%&.

Lecture Outline for *ampbell/0eeceBiology, 1thdition, 3 (earson ducation, 4nc. 13-1


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! The replication of $%& produces copies of genes that can be passed from parentsto offspring.

4n plants and animals, reproductive cells called"ametestransmit genes from onegeneration to the next.

&fter fertili#ation 5fusion of a sperm cell and an ovum6, genes from both parents

are present in the nucleus of the fertili#ed egg, or #ygote. &lmost all the $%& in a eukaryotic cell is subdivided into chromosomes in thenucleus.

! Tiny amounts of $%& are also found in the mitochondria and chloroplasts.

very living species has a characteristic number of chromosomes.

! 7umans have 89 chromosomes in almost all of their cells.

ach chromosome consists of a single $%& molecule associated with variousproteins.

ach chromosome has hundreds or thousands of genes, each at a specific location,or locus% along the length of the chromosome.

Like begets like, more or less: a comparison of asexual and sexual reproduction.

Only organisms that reproduce asexually can produce offspring that are exactcopies of themselves.

4n asexual reproduction%a single individual is the sole parent to donate genes toits offspring.

! :ingle;celled eukaryotes can reproduce asexually by mitotic cell division toproduce two genetically identical daughter cells.

! &n individual that reproduces asexually gives rise to a clone%a group ofgenetically identical individuals.

! -embers of a clone may be genetically different as a result of mutation.

4n sexual reproduction%two parents produce offspring that have uni'uecombinations of genes inherited from the two parents.

!


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produce a )aryotypedisplay.

! The two chromosomes in a pair have the same length, centromere position, andstaining pattern.

! These homolo"ous chromosomepairs carry genes that control the same inheritedcharacters.

Two distinct sex chromosomes%the = and the ), are an exception to the generalpattern of homologous chromosomes in human somatic cells.

! The pattern of inheritance of the sex chromosomes determines an individuals sex.

! 7uman females have a homologous pair of = chromosomes 5==6> males haveone = and one ) chromosome 5=)6.

! -ost of the genes carried on the = chromosome are not found on the tiny )chromosome, which has genes that are not found on the = chromosome.

! Only small parts of the = and ) chromosomes are homologous.

The other 22 pairs of chromosomes are called autosomes!

The occurrence of homologous pairs of chromosomes is a conse'uence of sexual

reproduction.! ?e inherit one chromosome of each homologous pair from each parent.

! The 89 chromosomes in each somatic cell are two sets of 2@, a maternal set 5fromyour mother6 and a paternal set 5from your father6.

The number of chromosomes in a single set is represented by n.

&ny cell with two sets of chromosomes is called a diploid celland has a diploidnumber of chromosomes, abbreviated as 2n.

:perm cells or ova *"ametes+have only one set of chromosomesA22 autosomesand an = 5in an ovum6 or 22 autosomes and an = or a ) 5in a sperm cell6.

& gamete with a single chromosome set is a haploid cell%abbreviated as n.

&ny sexually reproducing species has characteristic haploid and diploid numbersof chromosomes.

! For humans, the haploid number of chromosomes is 2@ 5nB 2@6, and the diploidnumber is 89 52nB 896.

! 4n the fruit fly,Drosophila, 2nB 1 and nB 8.

! 4n the domestic dog, 2nB C1 and nB @D.

4n a cell in which $%& synthesis has occurred, all the chromosomes areduplicated. ach duplicated chromosome consists of two identical sister chromatids.

4t is crucial to understand the differences among homologous chromosomes, sisterchromatids, nonsister chromatids, and chromosome sets, as shown in Figure [email protected].

Lets discuss the behavior of chromosome sets in the human life cycle.

The human life cycle begins when a haploid sperm cell fuses with a haploidovum.

! The union of these gametes, culminating in the fusion of their nuclei, isfertili(ation.

The fertili#ed egg *(y"ote+is diploid because it contains two haploid sets of



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chromosomes bearing genes from the maternal and paternal family lines.

&s a person develops from a #ygote to a sexually mature adult, mitosis generatesall the somatic cells of the body.

! ach somatic cell contains a full diploid set of chromosomes.

"ametes, which develop in the gonads 5testes or ovaries6, are notproduced by

mitosis.! 4f gametes were produced by mitosis, the fusion of gametes would produce

offspring with four sets of chromosomes after one generation, eight after asecond, and so on.

4nstead, gametes undergo the process of meiosis%in which the chromosomenumber is halved.

! 7uman sperm or ova have a haploid set of 2@ different chromosomes, one fromeach homologous pair.

Fertili#ation restores the diploid condition by combining two haploid sets ofchromosomes.

Organisms display a variety of sexual life cycles. Fertili#ation and meiosis alternate in all sexual life cycles.

The timing of meiosis and fertili#ation varies among species.

These variations can be grouped into three main types of life cycles.

-ost animals, including humans, have the first type of life cycle, in whichgametes are the only haploid cells.

! "ametes do not divide but fuse to form a diploid #ygote that divides by mitosis toproduce a multicellular diploid organism.

(lants and some algae have a second type of life cycle called alternation of"enerations!

! This life cycle includes two multicellular stages, one haploid and one diploid.! The multicellular diploid stage is called the sporophyte!

! -eiosis in the sporophyte produces haploid spores.

!


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cycle.

Only diploid cells can undergo meiosis because haploid cells have a single set ofchromosomes that cannot be further reduced.

&lthough the three types of sexual life cycles differ in the timing of meiosis andfertili#ation, they share a fundamental feature ach cycle of chromosome halving

and doubling contributes to genetic variation among offspring.

Concept 13!3 Meiosis reduces the num$er of chromosome sets from diploid tohaploid!

-any steps of meiosis resemble steps in mitosis.

Goth meiosis and mitosis are preceded by the replication of chromosomes.

4n meiosis, there are two consecutive cell divisions, meiosis ,and meiosis ,,%resulting in four daughter cells.

! The first division, meiosis 4, separates homologous chromosomes.

! The second division, meiosis 44, separates sister chromatids.

! The four daughter cells at the end of meiosis have only half as manychromosomes as the original parent cell.

-eiosis 4 is preceded by interphase%in which the chromosomes are replicated toform sister chromatids.

! The two genetically identical sister chromatids make up one replicatedchromosome.

! The sister chromatids are closely associated all along their length. Thisassociation is calledsister chromatid cohesion.

4n contrast, the two chromosomes of a homologous pair are individualchromosomes that were inherited from different parents.

! 7omologous chromosomes appear to be alike, but they may have differentversions of genes, called alleles, at corresponding loci.

We can compare mitosis and meiosis.

-eiosis halves the total number of chromosomes, reducing the number of sets ofchromosomes from two 5diploid6 to one 5haploid6, with each daughter cell receivingone set. -itosis conserves the number of chromosome sets.

-eiosis produces cells that differ genetically from the parent cell and from eachother. -itosis produces daughter cells that are genetically identical to the parent celland to each other.

Three events uni'ue to meiosis occur during meiosis 4

E. Synapsis and crossin" over

o $uring prophase 4, replicated homologs pair up and become physicallyconnected along their lengths by a #ipper;like protein structure, thesynaptonemal complex, in a process called synapsis!

o Crossin" over, genetic rearrangement between nonsister chromatids,occurs during this stage.

o Following disassembly of the synaptonemal complex in late prophase, the



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two homologs pull apart slightly but remain connected by at least one =;shaped region called a chiasma5plural, chiasmata6.

o :ynapsis and crossing over do not occur during mitosis.

2. -li"nment of homolo"s on the metaphase plate

o $uring metaphase 4 of meiosis, pairs of homologous chromosomes 5ratherthan individual chromosomes, as in mitosis6 line up on the metaphase

plate.

@. Separation of homolo"s

o $uring anaphase 4 of meiosis, the replicated chromosomes of each

homologous pair move toward opposite poles, while the sister chromatids

of each replicated chromosome remain attached.

o 4n anaphase of mitosis, the sister chromatids separate.

$uring meiosis 4, the sister chromatids are attached along their lengths by proteincomplexes called cohesins.

4n mitosis, en#ymes remove the cohesins to allow the sister chromatids to move toopposite poles of the cell at the end of metaphase.

4n meiosis, sister chromatid cohesion is released in two steps.

! 4n metaphase 4, homologs are held together by cohesion between sister chromatidarms in regions where $%& has been exchanged.

! 4n anaphase 4, cohesins are cleaved along the arms, allowing homologs toseparate.

! 4n anaphase 44, cohesins are cleaved at the centromeres, allowing chromatids toseparate.

?hat prevents cohesin cleavage at the centromere while it occurs along sisterchromatid arms at the end of metaphase 4H

! & protein named +shugoshin 5Iapanese for +guardian spirit6 protects cohesinsfrom cleavage at the centromere during meiosis 4, keeping the sister chromatidsJoined.

-eiosis 4 is a reductional divisionbecause it halves the number of chromosomesets from two 5the diploid state6 to one 5the haploid state6. 4n contrast, meiosis 44 isan equational division.

The mechanism for separating sister chromatids is virtually identical in both meiosis44 and mitosis.

The molecular basis of chromosome behavior during meiosis continues to be a focus

of intense research interest.

Concept 13!. enetic variation produced in sexual life cycles contri$utes toevolution!

-utations are the original source of genetic diversity.

Once different versions of genes arise through mutation, reshuffling of alleles



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during meiosis and fertili#ation produce offspring with their own uni'ue set of traits.

exual life cycles produce genetic variation among offspring.

The behavior of chromosomes during meiosis and fertili#ation is responsible formost of the variation that arises in each generation.

Three mechanisms contribute to genetic variation arising from sexualreproduction independent assortment of chromosomes, crossing over, and randomfertili#ation.

,ndependent assortment of chromosomescontributes to genetic variability dueto the random orientation of homologous pairs of chromosomes at the metaphaseplate during meiosis 4.

! There is a fifty;fifty chance that a particular daughter cell from meiosis 4 will getthe maternalchromosome of a certain homologous pair and a fifty;fifty chancethat it will receive thepaternalchromosome.

! ach homologous pair of chromosomes segregates independently of every otherhomologous pair during metaphase 4.

! Therefore, the first meiotic division results inindependent assortment of maternal

and paternal chromosomes into daughter cells. The number of combinations possible when chromosomes assort independentlyinto gametes is 2n, where nis the haploid number of the organism.

! 4f nB @, there are 2@B 1 possible combinations.

! For humans with nB 2@, there are 22@, or more than 1 million possiblecombinations of chromosomes.

Crossin" overproduces recom$inant chromosomes%which combine genesinherited from each parent.

! *rossing over begins very early in prophase 4, as homologous chromosomes pairloosely along their lengths.

! ach gene on one homolog is aligned precisely with the corresponding gene onthe other homolog.

! 4n a single crossover event, specific proteins orchestrate an exchange of thecorresponding segments of two nonsisterchromatidsAone maternal and onepaternal chromatid of a homologous pair.

! 4n crossing over, homologous portions of two nonsister chromatids trade places.

! &s a result, individual chromosomes carry genes derived from two differentparents.

! For humans, this occurs an average of one to three times per chromosome pair.

4n humans and most other organisms, crossing over may be essential for the liningup of homologous chromosomes during metaphase 4.

! *hiasmata hold homologs together as the spindle forms in meiosis 4.

*rossing over, by combining $%& inherited from two parents into a singlechromosome, is an important source of genetic variation.

&t metaphase 44, nonidentical sister chromatids sort independently from oneanother, further increasing the number of genetic types of daughter cells that areformed by meiosis.

The random nature of fertili(ationadds to the genetic variation arising fromLecture Outline for *ampbell/0eeceBiology, 1thdition, 3 (earson ducation, 4nc. 13-7


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meiosis.

&ny sperm can fuse with any egg.

! The ovum is one of more than 1 million possible chromosome combinations.

! The successful sperm is one of more than 1 million possibilities.

! The resulting #ygote could contain any one of more than C trillion possible

combinations of chromosomes.! *rossing over adds even more variation to this.

ach #ygote has a uni'ue genetic identity.

&ll three mechanisms reshuffle the various genes carried by individual membersof a population.

!volutionary adaptation depends on a populations genetic variation.

*harles $arwin recogni#ed the importance of genetic variation in evolution.

! & population evolves through the differential reproductive success of its variantmembers.

! Those individuals best suited to the local environment leave the most offspring,transmitting their genes in the process.

! This natural selection results in adaptation, the accumulation of favorable geneticvariations in a specific environment.

4f the environment changes or a population moves to a new environment, newgenetic combinations that work best in the new conditions will produce moreoffspring, and these genes will increase.

! Formerly favored genes will decrease.

:ex and mutation continually generate new genetic variability.

&lthough $arwin reali#ed that heritable variation makes evolution possible, hedid not have a theory of inheritance.

"regor -endel, a contemporary of $arwins, published a theory of inheritancethat explained genetic variation and thus supported $arwins theory.

! -endels work was largely unknown until ED, after both $arwin and -endelhad been dead for more than EK years.


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