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.

Stages Of Meiosis: Meiosis I Interphase Mother cell Prophase I: Condensing Chromosomes Prophase I: Tetrad formation/ crossing over Metaphase I Anaphase I Telophase I Meiosis II

Stages Of Meiosis: Meiosis II The products of meiosis are 4 haploid cells each with a unique set of chromosomes. Telophase I Prophase II Metaphase II Anaphase II Telophase 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

Crossing Over Prophase I: Metaphase I Anaphase I Telophase I Tetrad formation/ crossing over Metaphase I Anaphase I Telophase I Because of crossing over, every gamete receives a unique set of genetic information. Telophase II

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!

Fertilization Results In A Diploid Zygote Egg Haploid nucleus Sperm Haploid nucleus

Fertilization Results In A Diploid Zygote Egg Haploid nucleus Sperm Haploid nucleus

Fertilization Results In A Diploid Zygote Egg Haploid nucleus Haploid nucleus Sperm

Fertilization Results In A Diploid Zygote Egg Haploid nucleus Haploid nucleus Sperm

From Zygote to Embryo Zygote Diploid Zygote 2n

From Zygote to Embryo Mitosis

From Zygote to Embryo Mitosis

From Zygote to Embryo Mitosis

From Zygote to Embryo Mitosis

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.

The End