Meiosis

• Mitosis Review animation

Asexual (vegetative) Reproduction-

• one parent produces an offspring– Uses only mitosis– Produces genetically identical offspring (each

offspring is an exact copy of the original organism)• Rapid- no need to find mates – No mechanism for genetic diversity- works well in

a stable, unchanging environment.

Sexual Reproduction-• the formation of a new individual by a combination of

two haploid sex cells– Uses Meiosis- a specialized type of cell division which halves

the # of chromosomes in a gamete• Female Gamete

– egg (ovum)• Male Gamete

– sperm

– Requires Fertilization- • the union of a sperm and an egg • (N + N = 2N)

– Makes a zygote- • the product of fertilization (has two sets of chromosomes)

– Causes changes in the genetic information • in order to increase the diversity in the offspring.

How are the Goals of MEIOSIS Different from the Goals of MITOSIS?

MITOSIS MEIOSIS

1)Produces identical daughter cells (genetically)

2) Produces daughter cells with the same number of chromo-somes as mother cell

Homologous chromosomes DO NOT

interact

1)Produces cells that are NOT genetically identical

2) Cuts chromosome number of mother cell in half

Homologous chromosomes pair up,

align and separate

How Meiosis Works:AnimationA few more Vocabulary Words:• Germ Cells-

– Cells that give rise to the gametes of organisms

– They are in the process of or have the potential to undergo meiosis.

• Somatic Cells- – cells that do not have the potential to form

gametes• Synapsis-

– The pairing of homologous chromosomes during prophase of meiosis I

• Tetrad-– a unit of 4 chromatids formed by a

synapsed pair of homologous chromosomes, each of which has two chromatids

• Crossing Over- – The process by which homologues

exchange segments with each other.• Genetic Recombination- – the reshuffling of genes on a

chromosome. Caused by breakage of DNA and its reunion with the DNA of a homologous chromosome.

Meiosis

• animation

How does the process of meiosis accomplish the previously mentioned goals?

• two meiotic divisions

without DNA replication in between them.

• Independent assortment and crossing over

MEIOSIS I– Prophase 1:

• Homologous sister chromatids find each other. Crossing Over can occur during the latter part of this stage.

– Metaphase 1: • Homologous chromosomes align at

the equatorial plate. Independent assortment happens here.

– Anaphase 1: • Homologous pairs separate with

sister chromatids remaining together.

– Telophase 1: • Two daughter cells are formed with

each daughter containing only one chromosome of the homologous pair.

Meiosis II– Prophase 2:

• DNA does not replicate. Sister Chromatids prep for division

– Metaphase 2: • Sister chromatids align at

the equatorial plate.

– Anaphase 2: • Centromeres divide and

sister chromatids migrate separately to each pole.

– Telophase 2: • Cell division is complete.

Four haploid daughter cells are obtained.

Summary• One parent cell produces four daughter cells. – Daughter cells have half the number of chromosomes

found in the original parent cell and with crossing over, are genetically different.

• Meiosis differs from mitosis primarily because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes.

Meiosis Square Dance

• animation

• Independent Assortment-– The process of random

segregation and assortment of chromosomes during anaphase I of meiosis resulting in the production of genetically unique gametes.

• Chiasmata- – A cross-shaped structure

commonly observed between homologous chromosomes during meiosis; the site of crossing-over.

• Random Fertilization- – A mating system in which there is an equal opportunity

for all male and female gametes to join in fertilization– This means that an egg cell representing one of eight million different genetic

possibilities will be fertilized by a sperm cell representing one of eight million genetic possibilities and the resulting zygote can have any of the possible 64 trillion diploid combinations of genes.

Making Gametes • Oogenesis- – The process of ovum

formation in female animals

• Spermatogenesis- – The process of sperm

formation in male animals