Chapter 13: Meiosis and Sexual Life Cycles

OverviewVariations on a Theme

Organisms reproduce their own kind Offspring resemble their parents Genetics = study of heredity and variation

o Heredity = transmission of traits from one generation to the nexto Variation = sons and daughters are not identical to their parents

13.1: Offspring acquire genes from parents by inheriting chromosomesInheritance of Genes

Parents endow their offspring with coded info/hereditary units = geneso Genetic link to our ancestors o Written in the language of DNA

Gametes reproductive cells that transmit genes Somatic cells = body cells

o Humans have 46 chromosomes in somatic cells Locus = a gene’s specific location on a chromosome

Comparison of Asexual and Sexual Reproduction Asexual reproduction = one organism is the sole parent and passes copies of

its genes to its offspringo Offspring are exact genetic copies = clone = genetically identical

Sexual reproduction = 2 organisms give rise to offspringo Offspring has unique combination of genes from both parents

13.2: Fertilization and meiosis alternate in sexual life cyclesSets of Chromosomes in Human Cells

Life cycle = sequence of stages in reproductive history of an organismo Conception reproduction

Karyotype = ordered display of chromosomes 2 chromosomes of each of the 23 types makes up 46 chromosomes in

somatic cellso Pair of chromosomes have the same

length, centromere position, and staining pattern = homologous chromosomes/homologs

o Non-sex chromosomes = autosomeso Inherit one chromosome pair from

each parent = one set of 23 from mother, one from father

o Single set = n diploid cell has 2n chromosomes (somatic)

o Haploid cells only have one set = n (gamete)

o Each species has a different diploid and haploid numbero Things that organize a Karyotype:

Length Centromere location Staining Types of genes

Sex chromosomes X and Y chromosomes are exceptions to homologous chromosomes

o Females have homologous pair XXo Males have XY

Behavior of Chromosome Sets in the Human Life Cycle Life cycle begins when haploid sperm from father fuses with haploid egg

from the mother = fertilizationo Union of gametes fusion of nucleio Resulting fertilized egg = zygote is diploid because it has 2 sets of

haploid chromosomeso Throughout development mitosis of the zygote generates all the cells

in the body Gametes are the only cells of the human body not produced by mitosis

o Develop from specialized germ cells in the gonads (ovaries/testes) through meiosis

o Reduces number of chromosome sets to one in the gametes to counterbalance the doubling during fertilization

The Variety of Sexual Life Cycles Timing of meiosis and fertilization depends on the species Humans and most other animals:

o Meiosis occurs in germ cells producing gametes after fertilization diploid zygote divides by mitosis producing multicellular diploid

Plants and some algae to alternation of generationso Includes diploid and haploid multicellular stageso Multicellular diploid stage = sporophyte

Meiosis produces haploid cells = spores Doesn’t fuse with another cell- divides mitotically

o multicellular haploid stage = gametophyte Produces gametes through mitosis Fusion of two haploid gametes at fertilization creates a diploid

zygote which develops into the next sporophyte generation Fungi and protists (some algae):

o After gametes fuse to form a diploid zygote, meiosis occurs without multicellular diploid offspring developing

o Produces haploids that divide my mitosis to create either unicellular or multicellular organisms

o Haploid organism does more mitosis to produce cells that develop into gametes

o Either haploid or diploid cells can divide by mitosis but only diploid cells can divide by meiosis

o All 3 types of life cycles share fundamental result: genetic variation

13.3: Meiosis reduces the number of chromosome sets from diploid to haploidThe Stages of Meiosis

Steps are similar to mitosis but involves two cell divisions, meiosis I and meiosis II, resulting in four daughter cells

For a single pair of homologous chromosomes in a diploid cell, both members of the pair are duplicated and the copies sorted into four haploid cells

Homologs appear alike but have different versions of genes = alleles at corresponding loci

Meiosis I: Separates homologous chromosomes Prophase I

o Chromosome begin to condenseo Homologs loosely pair along their lengths,

aligned gene by geneo Paired homologs become physically

attached to each other along their lengths = synapsis

Attached by a zipper-like protein structure = synaptonemal complex

o Crossing over = genetic rearrangement between non-sister chromatids involving the exchange of corresponding segments of DNA begins during pairing and synaptonemal complex formation, completed while homologs are in synapsis

o Chiasmata = x-shaped region that each homologous pair has one or more of, exists at the point where crossover occurred

Appears as a cross because sister chromatid cohesion still holds the two original sister chromatids together

o Microtubules from one pole attach to the two kinetochores homologous pairs move toward their metaphase plate

Metaphase Io Pairs of homologous chromosomes are now arranged at the

metaphase plate with one chromosome in each pair facing each poleo Both chromatids of one homolog are attached to kinetochore

microtubules from a pole, those of the other attached to microtubules from the opposite pole

Anaphase Io Breakdown of proteins responsible for sister chromatid cohesion

along chromatid arms, allowing homologs to separateo Homologs more toward opposite poles, guided by spindleo Sister chromatid cohesion persists at the centromere, causing

chromatids to move together toward the same pole Telophase I and Cytokinesis (occur simultaneously)

o Each half of the cell has a complete haploid set of duplicated chromosomes, composed of two sister chromatids with nonsister DNA

o Cleavage furrow or cell plate formso In some species, chromosomes decondense + nuclear envelopes form

Meiosis II: Separates sister chromatids Prophase II

o Spinde formso Chromosomes, each still composed of two chromatids, move toward

the metaphase II plate Metaphase II

o Chromosomes are positioned at the metaphase plate

o Kinetochores of sister chromatids are attached to microtubules extending from poles

Anapahse II o Breakdown of proteins holding the sister chromatids together at the

centromere, allowing for chromatids to separateo Chromatids more toward opposite poles as individual chromosomes

Telophase II and Cytokinesiso Nuclei formo Chromosomes begin decondensingo Cleavage furrow or cell plate formso Produces four genetically distinct daughter cells with haploid sets of

chromosomes

A Comparison of Mitosis and Meiosis Meiosis reduces the number of chromosome sets for diploid to haploid,

while mitosis conserves the number of chromosome setso Meiosis produces genetically different cells, while mitosis

produces genetically identical cells Unique events in miosis I

o Synapsis and crossing over (prophase I): Duplicated homologs pair up and synaptonemal complex

holds them in synapsis so crossing over can occuro Homologous pairs at the metaphase plate (metaphase I):

Chromosomes are positioned at the metaphase plate as homologs rather than individual chromosomes

o Separation of homologs (anaphase I):

Duplicated chromosomes of each homologous pair move toward opposite poles but sister chromatids remain attached, as opposed to sister chromatids separating

Sister chromatids stay together through meiosis I but detach during meiosis II

o Sister chromatids are attached by cohesion proteins o In mitosis the attachment lasts until the end of metaphase when

enzymes cleave themo In meiosis cohesion is released in two steps: at the start of

anaphase I and anaphase II In metaphase I homologs are held together by cohesion

between sister chromatid arms in points of crossing over Combo of sister chromatid cohesion and crossing over

forms a chiasma, holding homologs together At the beginning of anaphase I the release of cohesion along

sister chromatid arms allows homologs to separate At anaphase II release of sister chromatid cohesion at

centromeres allows sisters to separateo Meiosis I = reductional division because it halves number of

chromosome sets in the cello Meiosis II = equational division because it produces haploid cells

13.4: Genetic variation produced in sexual life cycles contributes to evolutionOrigins of Genetic Variation Among Offspring

Mutations are the source of genetic diversity create different alleleso Reshuffling alleles during sexual reproduction produces variation

In organisms that reproduce sexually, behavior of chromosomes during meiosis and fertilization is responsible for variation

Independent assortment of chromosomeso At metaphase I the homologous pairs, consisting of one maternal

and one paternal chromosome, are ate the metaphase plate and can orient with its maternal or paternal homolog closer to a given pole = orientation is random

o Random chance of which daughter cell gets the maternal and paternal chromosome

o Each pair sorts independently of every other pair = independent assortment

Number of possible combinations is 2n

Humans = 223 8.4 million combinations

Crossing Overo Crossing over produces recombinant chromosomes = individual

chromosomes that carry genes derived from two parentso Humans average 1-3 crossover events per chromosome pairo Crossover begins in prophase I as homologous chromosomes pair o Each gene is aligned precisely with the corresponding gene on the

other homologo DNA of nonsister chromatids (one maternal and one paternal) is

broken by proteins at specific corresponding points and the two segments beyond the crossover point are joined to the other chromatid

o At metaphase II chromosomes with one or more recombinant chromatids can be oriented in two alternative, nonequivalent ways with respect to other chromosomes, because their sisters aren’t identical

Different possible arrangements of nonidentical sister chromatids further increases number of types of daughter cells

Random fertilizationo Fusion of a male gamete with a

female gamete during fertilization will produce a zygote with 70 trillion (223 * 223) diploid combinations

The Evolutionary Significance of Genetic Variation Within Populations

A population evolves through differential reproductive success of its variant members

Natural selection results in the accumulation of genetic variations favored by the environment

o Those best suited to the local environment leave the most offspring, thus transmitting their better suited genes

o As the environment changes, the population can survive if some of its members cope effectively with new conditions

Mutations = original source of different alleles, mixed and matched during meiosis

o New and different combinations may work better ability of sexual reproduction to generate genetic diversity = explanation for evolutionary persistence of process

In a stable environment seems more advantageous because it ensures the perpetuation of successful combinations of alleles

o Is less energy expensive Sexual reproduction is almost universal among animals Animals that reproduce asexually are rare

o Ex: bdelloid rotifer but still have other mechanisms that increase genetic diversity: cell membranes can crack to allow other DNA in = horizontal gene transfer

Genetic variation is evolutionary advantageous