Meiosis

Advanced Animal Science

Meiosis Type of cell divisioncell division

One germ cell makes 4 gametes with ½ the # of chrom.

Occurs only in germ cells in gonads ○ testes /ovaries

Occurs in flowers ○ ovary and anther

Sperm

surrounding an egg

This shows how only one single sperm gets to penetrate the egg, releasing its nucleus of 23 chromosomes to merge with the nucleus of the egg and its 23 chromosomes.

Homologous Homologous ChromosomesChromosomes

Pair of chrom. similar in shape , size, and types of genes. Each locus (location of the gene) in same position on chrom.

Humans have 23 pairs of homologueshomologues Housefly – 6 prs Housefly – 6 prs Chicken – 39 prs Chicken – 39 prs Apple – 17 prsApple – 17 prs Dog – 39 prsDog – 39 prs Cat – 19 prs Cat – 19 prs

This is a karyotype (an image of an organism’s chromosomes)

This is a karyotype of a normal human male

Homologous Homologous ChromosomesChromosomes

Paternal(from Dad)

Maternal (from Mom)

eye color locus

eye color locus

hair color locus

hair color locus

Prophase IProphase I

Longest and most complex phase (90%). Chromosomes condense. SynapsisSynapsis - - a process: when hom. chrom. hom. chrom.

come together, pair up, form a tetradtetrad.

Prophase IProphase I - - SynapsisSynapsis

Nonsister chromatids

sister chromatids sister chromatidsTetrad

Prophase I -Prophase I - Crossing Crossing OverOver

Crossing over may occur in the tetrad:between nonsister chromatids, ends break and reattach

Crossing Over - Provides Crossing Over - Provides

VariationVariation

nonsister chromatids

Chiasma: site of crossing over

variation

Tetrad

Metaphase IMetaphase I

Shortest phase; paired homologues align. INDEPENDENT ASSORTMENTINDEPENDENT ASSORTMENT occurs

pairs of homologues line up independently of other pairs’ orientation toward the poles -- random. Adds variation.

Anaphase IAnaphase I Homologous chromosomes separate towards

the poles (Tetrads separate)

Sister chromatids Sister chromatids remain attached

Telophase ITelophase I Each pole now has haploid set of chromosomes Each pole now has haploid set of chromosomes

(however – still doubled).(however – still doubled). CytokinesisCytokinesis occurs: two haploid daughter cells

formed.

Meiosis IIMeiosis II No interphase II No interphase II

( no more DNA replicationDNA replication) Remember: Meiosis II Remember: Meiosis II is similar to mitosismitosis

Prophase II / Metaphase II

Anaphase II

Telophase IITelophase II Same as telophasetelophase in mitosismitosis. Nuclei form. CytokinesisCytokinesis occurs (2nd time). Four haploid daughter cells produced (chromosomes now

back to single condition).gametes ~ sperm or egg; ovule or pollen grain

Gamete Formation in Animals Diff. bet. male and female gametes.

Male: spermatogenesisMale: spermatogenesis all 4 develop into sperm cells.

Female: oogenesisFemale: oogenesiscytokinesis in meiosis is uneven. most of cytoplasm goes into 1 of the 4 meiotic products

(forms large egg cell) 3 other cells are small “polar bodies”, break down (extra

chrom. lost).

SpermatogenesiSpermatogenesiss

2n = 46

humangerm cell in testes

diploid (2n)

n=23Still doubled chromosomes

n=23Still doubled chromosomes

meiosis I

n=23

n=23

n=23

n=23

sperm

haploid (n)

meiosis II

Oogenesis

2n = 46

human germ cellin ovary

diploid (2n)

n=23Still doubled

n=23Still doubled

meiosis I

23

23

23

Ovumn=23

Polar Bodies

meiosis II

Mitosis vs Meiosis Mitosis

Body (somatic cells) 2 daughter cells made

(identical) Each w/ same # & kind

of chrom. as parent cell 1 division process 1 cytokinesis No synapsis or

crossing over Are diploid (2n)

Meiosis Germ cells of gonads 4 gamete cells made

(all different) Each w/ ½ chrom. # as

parent cell 2 divisions 2 cytokineses events Synapsis &

crossing over occurs in Prophase 1

Are haploid (n)

Chromosome numbers: However many

“types” of chromosomes an organism has, that number is the “n” number of chromosomes it has.

Body cells (somatic cells) therefore have a“2n” # (diploid)

Gametes have “n” # (haploid)

For humans, 2n = 46n = 23

Nondisjunction

When the tetrad (in Anaphase I) or the sister chromatids (in Anaphase II) do not separate, creating an abnormal # of chrom. to occur in the gametes.

Lethal most of the time

Karyotype… male

Karyotype… female