Diploid Haploid gametes contain one copy of each chromosome

[Pages:3]Meiosis and genetic variation

Meiosis

? When and where does it occur?

? What is the purpose?

? What happens?

Meiosis produces haploid cells that are genetically unique via

? Independent assortment

? Crossing over



? Mendel and the rules of inheritance

Janurary 21, 2022

Sexual life cycle

Key

Haploid (n) Diploid (2n)

Meiosis only

occurs during gamete formation

MEIOSIS Ovary

Haploid

FERTILIZATION

Testis

Diploid

Mitosis

Multicellular diploid adults (2n = 46)

Diploid cells have two copies of each chromosome

? One maternal, one paternal

? Homologous chromosomes contain copies of the same genes

Haploid gametes contain one copy of each chromosome

Sperm and Egg

4

MEIOSIS I: Separates homologous chromosomes

Prophase I

Metaphase I

Anaphase I

Telophase I and Cytokinesis

Crossing over

Sister chromatids Remain attached

Pair of homoogous chromosomes

Homologous chromosomes separate

Haploid cells

MEIOSIS II: Separates sister chromatids

Prophase II Metaphase II Anaphase II

Telophase II and Cytokinesis

Sister chromatids Four haploid

separate

cells

1

? Mitosis produces an exact copy of the parent cell.

? Used for growth and asexual reproduction.

? Meiosis produces reduced (haploid) gametes, which are genetically unique.

? Necessary only for sexual reproduction.

Mitosis vs Meiosis

Not paired

diploid

Homologs pair in meiosis 1

haploid

Sister chromatids separate in meiosis 2

Meiosis produces genetic variation through

Independent Assortment Crossing over

Independent Assortment

? Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs

Key

Maternal set of chromosomes

Paternal set of chromosomes

Possibility 1

Two equally probable arrangements of chromosomes at metaphase I

Possibility 2

Metaphase II

Figure 13.10

Combination 1

Daughter cells

Combination 2

Combination 3 Combination 4

Crossing Over

? Produces Recombinant

Prophase I of meiosis

Chromosomes that carry genes

derived from two different

Tetrad

parents

Metaphase I

Nonsister chromatids

Chiasma, site of crossing over

Metaphase II

Daughter cells

Figure 13.11

Recombinant chromosomes

Spindle fibers from one pole attach to BOTH sister chromatids

Crossovers are essential for correct alignment at metaphase 1

Spindle fibers from the other pole attach to the other homolog

Chiasmata hold the pair together while Tug of war Aligns tetrads

2

Gregor Mendel

P Generation

Phenotype: Genotype:

Gametes:

Purple flowers White flowers

PP

pp

P

p

F1 Generation

Phenotype: Genotype: Gametes:

F2 Generation

Purple flowers Pp

1/2 P

1/2 p

Sperm

P

p

P

Eggs

PP

Pp

p

Pp

pp

3

1

3

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