Meiosis Formation of Gametes (Eggs & Sperm)

Important Terminology Diploid (2N): “two sets”; a cell that has 1 set of chromosomes from its female parent and 1 set from its male parent (=2 copies of each chromosome) Homologous: each chromosome from one parent has a corresponding chromosome from the other parent. Haploid (N or 1N): the number of chromosomes in a gamete (sex cell – egg or sperm) Only one copy of each chromosome

Polypoidy What would this mean??? More than diploid (more than 2 sets of chromosomes) Triploid (3N) Tetraploid (4N) Not viable (“survivable”) in humans Seedless watermelons are triploid, formed from a cross between diploid and tetraploid watermelons. Because the 3 copies of the chromosomes won’t line up correctly during meiosis, meiosis is halted, so no gametes (seeds) are formed.

Definition of Meiosis The process of reduction division in which the number of chromosomes per cell is cut in half (reduction) through the separation (division) of homologous chromosomes in a diploid cell Homologous chromosomes (“homologs”): “matched” chromosomes (not necessarily identical); both before and after replication!

Facts About Meiosis Preceded by interphase which includes chromosome replication Two meiotic divisions --- Meiosis I and Meiosis II Called reduction division Original cell is diploid (2n) Four daughter cells produced are monoploid (1n) (AKA, haploid)

Facts About Meiosis Daughter cells contain half the number of chromosomes as the original cell. Produces gametes (eggs & sperm) Occurs in the testes in males (Spermatogenesis) ovaries in females (Oogenesis)

More Meiosis Facts Human cells start with 46 double stranded chromosomes (2n). After 1st division - 23 double stranded chromosomes (n) in each of 2 cells After 2nd division - 23 single stranded chromosomes (n) in each of 4 cells   Occurs in our germ cells that produce gametes

Why Do We Need Meiosis? It is the fundamental basis of sexual reproduction. Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote.

Fertilization – “Putting it all together”

Replication of Chromosomes Replication is the process of duplicating a chromosome Occurs prior to division (interphase) Replicated copies are called sister chromatids. Held together at centromere Occurs in Interphase Replication is the process of duplicating chromosome. The new copy of a chromosome is formed by DNA synthesis during S-phase. The chromosome copies are called sister chromatids. Sister chromatids are held together at the centromere.

A Replicated Chromosome Gene X Sister Chromatids (same genes, same alleles) Homologs (same genes, different alleles) Homologs separate in meiosis I and therefore different alleles separate.

Meiosis Forms Haploid Gametes Meiosis must reduce the chromosome number by half. Fertilization then restores the 2n number. from mom from dad child too much! meiosis reduces genetic content The right number!

Meiosis: Two Part Cell Division Sister chromatids separate Meiosis I Meiosis II Homologs separate Diploid Diploid Haploid