Meiosis: cell division in which gametes are produced. Chapter 10 Sexual Reproduction and Genetics 10.1 Meiosis Meiosis I Meiosis: cell division in which gametes are produced. When gametes combine in fertilization, the number of chromosomes is restored.

Every cell has 46 chromosomes (23 pairs) EXCEPT the sex cells (egg and sperm) These cells are called gametes Human gametes contain 23 chromosomes total.

Chromosomes and Chromosome Number Chapter 10 Sexual Reproduction and Genetics 10.1 Meiosis Chromosomes and Chromosome Number Human body cells (somatic cells) have 46 chromosomes Each parent contributes 23 chromosomes 23 from mom 23 from dad

Homologous chromosomes —one of two paired chromosomes, one from each parent

Same centromere position Chapter 10 Sexual Reproduction and Genetics 10.1 Meiosis Homologous Chromosomes and Chromosome Number Same length Same centromere position Carry genes that control the same inherited traits

Males- Spermatogenesis Females- Oogenesis Males produce sperm in their testicles. Special cells there called spermatogonia start the process. Males constantly make sperm. Remember 4 are made each time! Billions will be made in a lifetime. Females- Oogenesis Females produce eggs in their ovaries. Special cells there called oogonia start the process. Females make eggs according to their menstrual cycle. Remember only 1 viable egg is made each time meiosis occurs. The other three die off and are called Polar Bodies. About 400 eggs will be made in a lifetime.

Production of Sperm cells Meiosis in Males

INTERPHASE and PROPHASE 1 MEIOSIS INTERPHASE and PROPHASE 1 special division to produce gametes Chromosomes are in homologous pairs Cell is “diploid” - has 2 chromosomes of each type Cell is “2n” where n=the number of types of chromosomes DNA Replication takes place – makes cell “4n”

MEIOSIS METAPHASE 1 Chromosomes line up next to their homologous partners - . Homologous pairs in this alignment are called tetrads Randomly sorted (which goes on left & which on right)

MEIOSIS ANAPHASE 1. Connections between chromosome pairs break. Chromosomes move away from their homologous partners -. centromeres do not break. Mistakes at this stage may cause both chromosomes to go to the same side

MEIOSIS TELOPHASE 1. Division into 2 new cells. New cells have different chromosomes. New cells are “diploid” or “2n”

MEIOSIS METAPHASE 2 Cells line up in 1 line like in mitosis

MEIOSIS ANAPHASE 2. Centromeres break and chromosomes split into chromatids like in mitosis

MEIOSIS TELOPHASE 2 Cells each divide into 2 cells

MEIOSIS 4 GAMETES New cells are haploid gametes. . could be sperm, pollen, or eggs (in species that need many eggs)

Production of eggs Meiosis in Females

MEIOSIS (forming some egg cells). PROPHASE 1 Same as normal meiosis so far . Cell is called “Primary Oocyte”

MEIOSIS METAPHASE 1 Chromosomes line up near edge of cell otherwise like normal meiosis

MEIOSIS ANAPHASE 1 Cell divides nuclear material normally other cell contents are divided unevenly

MEIOSIS TELOPHASE 1 Small cell being formed is called “Polar Body” Large cell is called “Secondary Oocyte” Polar Body may die now or may live long enough to divide

MEIOSIS METAPHASE 2 Chromosomes line up near edge in Secondary Oocyte Polar Body’s purpose was to remove chromosomes without losing much cell content

MEIOSIS ANAPHASE 2 Cell contents divide unevenly again

MEIOSIS TELOPHASE 2 New small cell is “2nd Polar Body” This polar body dies also

MEIOSIS 1 EGG 1 Living Cell becomes “ovum” Ovum - haploid egg cell

Meiosis Mistakes and Crossing Over

Results of Nondisjunction Down's Syndrome, a common form of mental retardation, is caused by possession of three copies of chromosome 21, instead of the usual two. Turner's Syndrome, a condition that causes infertility and immature sexual development in women, is due to the possession of only one X chromosome, instead of the two always present in normal women.

Crossing Over Homologous chromosome tetrads in metaphase I may exchange portions of their chromatids Crossing-over produces new combinations of alleles. KEY POINT: CROSSING OVER INCREASES GENETIC VARIATION IN THE GAMETES Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over one another. (3) The crossed sections of the chromatids are exchanged.

MALES

FEMALES

Chromosome Pairing Embryo Fetus Baby

Comparing Meiosis and Mitosis   Meiosis Mitosis Number of cells produced 4 2 Type of cells produced Haploid Diploid Cells are genetically… Different The Same Crossing over… Occurs Does not occur Number of divisions 1 Type of cells… Sex Cells Everything else

http://www.youtube.com/watch?v=iCL6d0OwKt8