Ch 13 Sexual Life Cycles and Meiosis

• Living organismsHave ability to reproduce

• HeredityTransmission of traits from one generation to the next

• Variation• Offspring often differ in appearance from parents and

siblings • Environmental and genetic influences

• GeneticsScientific study of heredity and variation

Reproduction and the Transmission of Traits

Progeny (or offspring)-Inherit chromosomes from parents

-DNA in chromosomes encode the genes thatspecify characteristics

For exampleeye color, blood type, predisposition to disease

What are the mechanisms by which chromosomes are passed onto (eukaryotic) offspring?

Asexual reproduction

Sexual reproduction

LE 13-2

Parent

0.5 mm

Bud

Asexual reproductionBud forms by mitosis of parent cellsBud genetically identical to parent

Sexual Reproduction

1. Meiosis: formation of haploid gametes (eggs, sperm) in parents

2. Fertilization: fusion of gametes from opposite parents--> diploid zygote-->mitotic growth-->embryo-->adult

LE 13-5

Key

Haploid (n)

Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

Spermcell (n)

TestisOvary

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

FERTILIZATIONMEIOSIS

Diploidzygote(2n = 46)

HumanSexualReproduction

Offspringgenetically uniquefrom parents

How many chromosomes are inherited by human zygotes?

46 chromosomes

or

2n= 46

n= the number of chromosomes in a set

23 from mother, maternal chromosomes23 from father, paternal “

LE 13-3

5 µmHuman Karyotype

Stained metaphasechromosomesfrom humansomatic cell

Why stain metaphase

chromosomes?

Compact &easier to see com-pared to interphase

LE 13-3

5 µmPair of homologouschromosomes

Sisterchromatids

Centromere

Can you tell the difference between a human male and female by the karyotype?

Humans have 22 pairs of autosomes (non-sex chromosomes) 1 pair of sex chromosomes

Female sex ch.: XX Male sex ch.: XY

LE 13-3

5 µmHuman Karyotype

Male or female?

If somatic cells have two sets of chromosomes (diploid), how do gametes end up with only one set (haploid)?

Diploid precursors to the egg and sperm undergo meiosis in the testis and ovary.

LE 13-7

Homologous pairof chromosomesin diploid parent cell

Interphase

Homologous pair of replicated chromosomes

Chromosomesreplicate

Meiosis I

Diploid cell withreplicatedchromosomes

Sisterchromatids

Meiosis II

Homologouschromosomesseparate

Sister chromatidsseparate

Haploid cells withreplicated chromosomes

Haploid cells with unreplicated chromosomes

Overview of meiosis

aka tetrad

Diploid

Haploid

LE 13-4

Key

Maternal set ofchromosomes (n = 3)

2n = 6

Paternal set ofchromosomes (n = 3)

Two sister chromatidsof one replicatedchromosomes

Two nonsister chromatids in a homologous pair

Pair of homologouschromosomes(one from each set)

Centromere

LE 13-8ab

Sisterchromatids

Chiasmata

Spindle

Centromere(with kinetochore)

Metaphaseplate

Homologouschromosomesseparate

Sister chromatidsremain attached

Microtubuleattached tokinetochore

Tetrad

MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I

Homologous chromosomes (red and blue) pair andexchange segmentsHomologous recombination (crossing-over)

Pairs of homologouschromosomes split up

Tetrads line up

LE 13-8b

Cleavagefurrow

MEIOSIS II: Separates sister chromatids

PROPHASE II METAPHASE II ANAPHASE IITELOPHASE I AND

CYTOKINESISTELOPHASE II AND

CYTOKINESIS

Sister chromatidsseparate

Haploid daughter cellsforming

Two haploid cellsform; chromosomesare still double

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

LE 13-9

Prophase

Duplicated chromosome(two sister chromatids)

Chromosomereplication

2n = 6

Parent cell(before chromosome replication)

Chromosomereplication

MITOSIS MEIOSIS

Chiasma (site ofcrossing over) MEIOSIS I

Prophase I

Tetrad formed bysynapsis of homologouschromosomes

Tetradspositioned at themetaphase plate

Metaphase IChromosomes positioned at themetaphase plate

Metaphase

AnaphaseTelophaseCytokinesis

Homologuesseparateduringanaphase I;sisterchromatidsremain together

Sister chromatidsseparate duringanaphase

Daughtercells of

meiosis I

Haploidn = 3

Anaphase ITelophase I

MEIOSIS II

Daughter cellsof mitosis

2n2n

n

Sister chromatids separate during anaphase II

n n n

Daughter cells of meiosis II

Mitosis producestwo genetically identical diploid cells

Meiosis producesfour genetically distinct haploid cells

Mechanisms of Genetic Variation Among OffspringCaused by Sexual Reproduction

1. Crossing over (homologous recombination)

(Prophase I)

2. Independent assortment of chromosomes

(Metaphase I & II)

3. Random fertilization (post-meiosis)

LE 13-11

Prophase Iof meiosis

Tetrad

Nonsisterchromatids

Chiasma,site of crossingover

Recombinantchromosomes

Metaphase I

Metaphase II

Daughtercells

Recombination

Chiasmata, pl

LE 13-10

Key

Maternal set ofchromosomes

Paternal set ofchromosomes

Possibility 1 Possibility 2

Combination 2Combination 1 Combination 3 Combination 4

Daughtercells

Metaphase II

Metaphase ITwo equally probable

arrangements ofchromosomes at

Independent Assortment

Random Fertilization

• Any sperm can fuse with any ovum (unfertilized egg)

• a (human) zygote has about 64 trillion diploid combinations

• Each zygote has unique genetic identity

I think they were talking about us. I hope they had it right!

If not maybe we can answer questions.

See meiosis animation

What is missing?