2007-2008

Meiosis & Sexual Reproduction

Cell division/Asexual reproduction•Mitosis

▫produce cells with same information identical daughter cells

▫exact copies clones

▫same amount of DNA same number of chromosomes same genetic information

Aaaargh!I’m seeingdouble!

Asexual reproduction•Single-celled eukaryotes

▫yeast (fungi)▫Protists

Paramecium Amoeba

•Simple multicellular eukaryotes▫Hydra

What are thedisadvantages ofasexual reproduction?

What are the advantages?

budding

budding

How about the rest of us?•What if a complex multicellular organism

(like us) wants to reproduce?▫joining of egg + sperm

•Do we make egg & sperm by mitosis?

46 46+ 92

egg sperm zygote

What if we did, then….

Doesn’t work!

No!

•Genetics▫Is the scientific study of heredity and 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

•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

•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

•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

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

• The two chromosomes

composing a pair of each type are called homologous chromosomes

Human female karyotype

46 chromosomes23 pairs

22 pairs of autosomes = chromosomes that do not control one’s sex

Sexchromosomes

Human male karyotype

46 chromosomes23 pairs

homologouschromosomes homologous = “same information”

single-stranded

•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

homologouschromosomes

sister chromatidssingle-stranded double-stranded

homologouschromosomes

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

haploid

diploid

23

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

2346

egg

sperm

46

meiosis 46

fertilization

23

23gametes

zygote

Overview of Meiosis

•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 cellshttp://highered.mcgraw-hill.com/sites/983409233

9/student_view0/chapter11/how_meiosis_works.html

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

DNA replication

homologous chromosomes separate

Sister chromatids separate

Overview of meiosis

interphase 1 prophase 1 metaphase 1 anaphase 1

telophase 1

prophase 2 metaphase 2 anaphase 2 telophase 2

2n = 4

n = 2n = 2

n = 2

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

synapsis

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

homologous chromosomes (tetrads) are positioned on the metaphase plates

tetrad

synapsis

•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

metaphase1

• 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 parents

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

prophase 1

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