Meiosis

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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

Homologous Chromosomes• are similar in shape and content • they carry genes for the same

proteins. • This means that you have two copies

for any given gene on a non-sex chromosome (not X or Y).

• Remember, organisms with two copies of each gene (and/or chromosome) are called diploid

Encylopedia Analogy- • Imagine you get a set of encylopedias from

each parent.• Each has 23 volumes present similar

information in a similar format• Ex. Each has a listing for “kangaroo",

but may say slightly different things about “kangaroos".

• You have inherited two versions of “kangaroo".

Homologous Chromosomes work in a similar way:• Chromosome 19 from your dad will have the

same genes on it as Chromosome 19 from your mom

• the information on these genes may be slightly differentEx. On this chromosome, you might get a brown hair version of the gene from your dad and a blond hair version of the gene from your mom

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

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• 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 • Spermatogenesis-

– The process of sperm formation in male animals

– Happens in the testes– Produces 4 haploid cells which go on to

develop tails and are called “spermatozoa”• Oogenesis-

– The process of ovum formation in female animals

– Happens in ovaries– Cytoplasm divides unequally after Meiosis 1– This “ovum” grows into a mature egg cell

and carries a rich supply of nutrients for the developing egg.

– The other 3 cells become polar bodies and die