Meiosis

AP Biology

Hereditary Similarity and Variation

• Heredity is the transmission of traits from one generation to the next

• Variation shows that offspring differ in appearance from parents and siblings

• Genetics is the scientific study of heredity and variation

Genes

• Genes are the units of heredity• Genes are segments of DNA• Each gene has a specific locus on a certain

chromosome• One set of chromosomes is inherited from

each parent

Sexual/Asexual

• Sexual reproduction- two parents give rise to offspring that have unique combinations of genes inherited from the two parents

• Asexual reproduction- one parent produces genetically identical offspring by mitosis

Karyotype

• karyotype -ordered display of the pairs of chromosomes from a cell

• homologous chromosomes (homologues)• Both chromosomes in a pair. They carry

genes controlling the same inherited characteristics– Alleles-

LE 13-3

5 µmPair of homologouschromosomes

Sisterchromatids

Centromere

LE 13-4

Key

Maternal set ofchromosomes (n = 3)

2n = 6Paternal set ofchromosomes (n = 3)

Two sister chromatidsof one replicatedchromosomes

Two nonsister chromatids in a homologous pair

Pair of homologouschromosomes(one from each set)

Centromere

“Hey, that’s my chromosome!”

Homologous Chromosomes

• Each pair of homologous chromosomes includes one chromosome from each parent

• For humans, the diploid number is 46 (2n = 46) 23 Pairs.

• Gametes are haploid cells, containing only one set of chromosomes

Meiosis: twice as nice

Meiosis I

• Synapsis

• Crossing over

• Tetrad formation

Meiosis II

same as mitosissister chromatids split

4 unique cells result

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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

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 segments; 2n = 6in this example

Pairs of homologouschromosomes split up

Tetrads line up

• Beginning of telophase I- each half of the cell has a haploid set of chromosomes– each chromosome still consists of two sister

chromatids

• Cytokinesis usually occurs simultaneously, forming two haploid daughter cells

LE 13-8b

Cleavagefurrow

MEIOSIS II: Separates sister chromatids

PROPHASE II METAPHASE II ANAPHASE IITELOPHASE I ANDCYTOKINESIS

TELOPHASE II ANDCYTOKINESIS

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

Crossing Over

• Because of crossing over in Prophase I, the two sister chromatids of each chromosome are no longer genetically identical

• Anaphase I: Homologous chromosomes separate. sister chromatids stay together

• Anaphase II: sister chromatids separate• The sister chromatids of each chromosome

are now two newly individual chromosomes.

Four for the Price of One

• At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes

• Each daughter cell is genetically distinct from the others and from the parent cell

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Propase

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

AnaphaseTelophase

Homologuesseparateduringanaphase I;sisterchromatidsremain together

Sister chromatidsseparate duringanaphase

Daughtercells of

meiosis I

Haploidn = 3

Anaphase ITelophase I

MEIOSIS IIDaughter cells

of mitosis

2n2n

n

Sister chromatids separate during anaphase II

n n nDaughter cells of meiosis II

Genetic variation contributes to Evolution

• Mutations create different versions of genes• Reshuffling of different versions of genes

during sexual reproduction produces genetic variation

• Three mechanisms for variation in Sexual:– Independent assortment of chromosomes– Crossing over (prophase I)– Random fertilization

Independent Assortment

• Independent assortment-each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs (not all dad’s chroms to 1 daughter! And mom’s to the other)

• For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes

Crossing over to the other side

• Crossing over produces recombinant chromosomes, which combine genes inherited from each parent

• In crossing over, homologous portions of two nonsister chromatids trade places

• Prophase I

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Prophase Iof meiosis

Tetrad

Nonsisterchromatids

Chiasma,site of crossingover

Recombinantchromosomes

Metaphase I

Metaphase II

Daughtercells

• Random fertilization adds to genetic variation because any sperm can fuse with any ovum

• The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations

• Crossing over adds even more variation