Meiosis Notes

Cell division to form the gametes, sperm (male gamete) and egg (female gamete).

Normal cells are diploid (2n): 2 copies of every gene.

Gametes are haploid (n): 1 copy of every gene

Number of Chromosomes

Overview of Meiosis 2 cell divisions. Starts with 2 copies of each

chromosome (homologous), each with 2 chromatids (copies of DNA).

In meiosis I, homologous chromosomes are separated into 2 cells.

In Meiosis II the chromatids are separated into 4 cells.

Meiosis I (PMAT I) PROPHASE I - The homologous

chromosomes pair together (Sometimes crossing over occurs).

METAPHASE I - The pairs of homologous chromosomes line up along the middle

ANAPHASE I - Homologous chromosomes are pulled apart.

TELOPHASE I - One cell becomes two cells with one chromosome of the pair (haploid)

Meiosis II (PMAT II)

PROPHASE II – Prepare to divide METAPHASE II – Chromosomes line

up in the middle ANAPHASE II – Chromatids (copies

of DNA) pull apart TELOPHASE II – The end result is

four cells with one copy of each gene.

InterphaseInterphase

Mother cell Stages Of Meiosis: Meiosis I

Meiosis IIMeiosis II

Prophase I:Tetrad formation/

crossing over

Prophase I:Tetrad formation/

crossing overMetaphase I Metaphase I

Telophase ITelophase I

Prophase I:Condensing

Chromosomes

Prophase I:Condensing

Chromosomes

Anaphase I Anaphase I

Telophase ITelophase I

Stages Of Meiosis: Meiosis II

Metaphase II Metaphase II

Anaphase II Anaphase II

Telophase II Telophase II

The products of meiosis are 4 haploid cells each with a unique set of chromosomes.

Prophase IIProphase II

Segregation

In humans meiosis starts with one cell containing 46 chromosomes (23 pairs) and results in four cells containing 23 chromosomes.

The copies of DNA are separated when gametes are formed.

Independent Assortment Homologous

chromosomes are positioned randomly so any copy can be passed to the gametes with any combination of other chromosomes

There are 2n combinations possible during meiosis with n the haploid number of chromosomes for the organism

How many combinations are possible in human meiosis?

Possible combinations: 2n

n=23 in humans

223=about 8,300,000 combinations

Crossing Over

During Prophase I, the exchange of genetic material between homologous chromosomes

Prophase I:Tetrad formation/

crossing over

Prophase I:Tetrad formation/

crossing over

Crossing Over

Anaphase I Anaphase I

Telophase II Telophase II

Metaphase I Metaphase I

Telophase ITelophase IBecause of crossing over, every gamete receives a unique set of genetic information.

Fertilization

The combination of a sperm and an egg which forms a zygote.

1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations!

EggHaploidnucleus

Fertilization Results In A Diploid Zygote

SpermHaploidnucleus

Sperm

Fertilization Results In A Diploid Zygote

Egg

Haploidnucleus

Haploidnucleus

Sperm

Fertilization Results In A Diploid Zygote

EggHaploidnucleus

Haploidnucleus

Sperm

Fertilization Results In A Diploid Zygote

EggHaploidnucleus

Haploidnucleus

From Zygote to Embryo

Zygote2n

Zygote

Diploid

Mitosis

From Zygote to Embryo

Mitosis

From Zygote to Embryo

Mitosis

From Zygote to Embryo

Mitosis

From Zygote to Embryo

From Zygote to Embryo

Twins

Monozygotic Twins (Identical) 1/3 of all twins are identical Twins that form from one

zygote (one egg fertilized by one sperm).

These twins have identical genes and must be the same sex.

Having identical twins is random, not genetic

Dizygotic Twins (fraternal)

2/3 of all twins are fraternal Twins that form from two

zygotes (two eggs fertilized by two sperm)

Can be the same sex or different sexes.

The ability to have fraternal twins is thought to be genetic.