Chapter 13 Meiosis and Sexual Life Cycles. Heredity ▫Is the transmission of traits from one generation to the next Variation ▫Shows that offspring differ.

Chapter 13Meiosis and Sexual Life Cycles

•Heredity▫Is the transmission of traits from one generation

to the next•Variation

▫Shows that offspring differ somewhat in appearance from parents and siblings Two parents give rise to offspring that have unique

combinations of genes inherited from the two parents

Figure 13.1

•Oppose to asexual reproduction▫One parent produces

genetically identical offspring by mitosis

▫Ex: Binary Fission•Asexual reproduction results

in clones•Clone: a group of genetically

identical individuals Figure 13.2

Parent

Bud

0.5 mm

•Genetics▫Is the scientific study of heredity and

hereditary variation▫Studies the passing of chromosomes from

parents to offspring•Meiosis creates gametes, sex cells containing

half the species chromosome number •Meiosis and Fertilization

▫maintain a species chromosome number during a sexual life cycle

▫Increase variation of individuals within a species

•Offspring acquire genes from parents by inheriting chromosomes

Genes

Chromosomes

Nucleus

Cell

•Genes▫Are the units of heredity▫Are segments of DNA▫Contain specific sequence of nucleotides

•Genes program cells to synthesize specific enzymes and other proteins which create an organism’s inherited traits

•We inherit▫One set of chromosomes from our mother

and one set from our father

Specific location of a gene

• gametes transmit genes from on generation to the next

• one chromosome contains hundreds to a few thousand genes

• each gene has a specific sequence of nucleotides

Sets of Chromosomes in Human Cells• In humans

▫Each somatic cell has 46 chromosomes, made up of two sets

▫One set of chromosomes comes from each parent

• All 46 are visible during mitosis

• There are 23 “types” of chromosomes and every cell has a maternal version and a paternal version of each one

• The two chromosomes composing a pair of each type are called homologous chromosomes

5 µmPair of homologouschromosomes

Centromere

Sisterchromatids

Figure 13.3

Visualizing Homologous Chromosomes•A karyotype

▫Is an ordered, visual representation of the chromosomes in a cell

•Homologous chromosomes▫Are the two chromosomes composing a pair▫Have the same characteristics or gene loci

Therefore, every cell has two copies of every gene

Allele: version of a gene Ex: Hitchhikers thumb, Tongue rolling

All cells have two alleles for every gene – one maternal and one paternal

▫May also be called autosomes Autosomes: do not control an individuals sex –

sex is controlled by sex chromosomes

•Sex chromosomes▫Are distinct from each other in their

characteristics▫Are represented as X and Y

Only small sections of X and Y are homologous▫Determine the sex of the individual, XX being

female, XY being male

Figure 13.5

Key

Haploid (n)

Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

SpermCell (n)

MEIOSIS FERTILIZATION

Ovary Testis Diploidzygote(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

•The Human Life Cycle

• At sexual maturity▫ The ovaries and testes

produce haploid gametes by meiosis

• During fertilization▫ These gametes, sperm and

ovum, fuse, forming a diploid zygote

▫ This starts the human life cycle

• The zygote (fertilized egg)▫ Develops into an adult

organism▫ Generates all of the somatic

cells of the organism

Meiosis reduces the number of chromosome sets from diploid to haploid•Meiosis

▫Takes place in two sets of divisions, meiosis I and meiosis II

▫Results in 4 daughter cells with half the chromosome number of the parent

Overview of Meiosis

Figure 13.7

Interphase

Homologous pairof chromosomesin diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids Diploid cell with

replicatedchromosomes

1

2

Homologous chromosomes separate

Haploid cells withreplicated chromosomes

Sister chromatids separate

Haploid cells with unreplicated chromosomes

Meiosis I

Meiosis II

•Meiosis I▫Reduces the

number of chromosomes from one diploid cell to two haploid cells

•Meiosis II▫Two haploid

daughter cells become four haploid daughter cells

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/how_meiosis_works.html

Centrosomes(with centriole pairs)

Sisterchromatids

Chiasmata

Spindle

Tetrad

Nuclearenvelope

Chromatin

Centromere(with kinetochore)

Microtubuleattached tokinetochore

Tertads line up

Metaphaseplate

Homologouschromosomesseparate

Sister chromatidsremain attached

Pairs of homologouschromosomes split up

Chromosomes duplicateHomologous chromosomes

(red and blue) pair and exchangesegments; 2n = 6 in this example

INTERPHASE MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I

•Interphase and meiosis I▫2n = 6

Figure 13.8

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/how_meiosis_works.html

TELOPHASE I ANDCYTOKINESIS

PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II ANDCYTOKINESIS

MEIOSIS II: Separates sister chromatids

Cleavagefurrow Sister chromatids

separate

Haploid daughter cellsforming

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Two haploid cellsform; chromosomesare still doubleFigure 13.8

•Telophase I, cytokinesis, and meiosis II

A Comparison of Mitosis and Meiosis•Meiosis and mitosis can be distinguished

from mitosis▫By three events in Meiosis l that are unique

to meiosis

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/unique_features_of_meiosis.html

•Synapsis and crossing over▫Crossing Over: Homologous chromosomes physically

connect and exchange genetic information (sections of DNA) Synapsis: nonsister chromatids connection preceding

crossing over▫Occurs at the end of Prophase I

Nonsister chromatids remain connected via chiasmata Chiasmata: location on nonsister chromatids where DNA

exchange occurred

•Tetrads on the metaphase plate▫At metaphase I of meiosis, paired

homologous chromosomes (tetrads) are positioned on the metaphase plates

•Separation of homologues▫At anaphase I of meiosis, cohesins between

nonsister chromatids are cleaved and homologous pairs move toward opposite poles of the cell Sister chromatids remain attached

▫In anaphase II of meiosis, cohesins between sister chromatids are cleaved and the sister chromatids separate

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/the_function_of_cohesin.html

Figure 13.9

MITOSIS MEIOSIS

Prophase

Duplicated chromosome(two sister chromatids)

Chromosomereplication

Chromosomereplication

Parent cell(before chromosome replication)

Chiasma (site ofcrossing over)

MEIOSIS I

Prophase I

Tetrad formed bysynapsis of homologouschromosomes

Metaphase

Chromosomespositioned at themetaphase plate

Tetradspositioned at themetaphase plate

Metaphase I

Anaphase ITelophase I

Haploidn = 3

MEIOSIS II

Daughtercells of

meiosis I

Homologuesseparateduringanaphase I;sisterchromatidsremain together

Daughter cells of meiosis II

n n n n

Sister chromatids separate during anaphase II

AnaphaseTelophase

Sister chromatidsseparate duringanaphase

2n 2nDaughter cells

of mitosis

2n = 6

•A comparison of mitosis and meiosis

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/comparison_of_meiosis_and_mitosis.html

Origins of Genetic Variation Among Offspring•Three mechanisms that contribute to

genetic variation from sexual reproduction:

1.Independent assortment of chromosomes2.Crossing over3.Random Fertilization

Independent Assortment of Chromosomes

•Homologous pairs of chromosomes▫Orient randomly at metaphase I of meiosis▫Maternal and paternal sister chromatids can be closer to either pole▫This results in 2n possibilities

In humans, this means there are 223 or 8.4 million possible combinations of maternal and paternal chromosomes

• Independent assortment

Figure 13.10

Key

Maternal set ofchromosomesPaternal set ofchromosomes

Possibility 1

Two equally probable arrangements ofchromosomes at

metaphase I

Possibility 2

Metaphase II

Daughtercells

Combination 1 Combination 2 Combination 3 Combination 4

• Homologous pair DNA is not exclusively maternal or paternal though; due to crossing over

• Crossing over▫ Produces recombinant chromosomes that carry genes

derived from two different parentsProphase Iof meiosis

Nonsisterchromatids

Tetrad

Chiasma,site ofcrossingover

Metaphase I

Metaphase II

Daughtercells

Recombinantchromosomes

1. Prophase I: synapsis and crossing over occur, homologs separate slightly

2. Chiasmata and cohesin between nonsister chromatids hold homologs together; they move to the metaphase plate

3. Break down of proteins holding between nonsister chromatid arms together allow homologs with recombinant chromosomes to separate

Random Fertilization•The fusion of gametes

▫Will produce a zygote with any of about 64 trillion diploid combinations

▫This does not even account for variations produced by crossing over

Evolutionary Significance of Genetic Variation Within Populations• Genetic variation

▫Is the raw material for evolution by natural selection

• Darwin: a population evolves through the differential reproductive success of its variant members▫ Individuals best suited to their environment

reproduce more – leaving offspring behind to continue transmitting beneficial genes

•Mutations▫Are the original source of genetic variation