Meiosis Formation of Gametes Eggs Sperm 1 Important

Meiosis Formation of Gametes (Eggs & Sperm) 1

Important Terminology ü Diploid (2 N): “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) ü Homologous: each chromosome from one parent has a corresponding chromosome from the other parent. ü Haploid (N or 1 N): ü the number of chromosomes in a gamete (sex cell – egg or sperm) ü Only one copy of each chromosome 2

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

Definition of Meiosis üThe 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 üHomologous chromosomes (“homologs”): “matched” chromosomes (not necessarily identical); both before and after replication! 4

Facts About Meiosis üPreceded by interphase which includes chromosome replication üTwo meiotic divisions --- Meiosis I and Meiosis II üCalled reduction division üOriginal cell is diploid (2 n) üFour daughter cells produced are monoploid (1 n) (AKA, haploid) 5

Facts About Meiosis üDaughter cells contain half the number of chromosomes as the original cell. üProduces gametes (eggs & sperm) üOccurs in the ütestes in males (Spermatogenesis) üovaries in females (Oogenesis) 6

More Meiosis Facts ü Human cells start with 46 double stranded chromosomes (2 n). üAfter 1 st division - 23 double stranded chromosomes (n) in each of 2 cells üAfter 2 nd division - 23 single stranded chromosomes (n) in each of 4 cells ü Occurs in our germ cells that produce gametes 7

Why Do We Need Meiosis? üIt is the fundamental basis of sexual reproduction. üTwo haploid (1 n) gametes are brought together through fertilization to form a diploid (2 n) zygote. 8

Fertilization – “Putting it all together” 2 n = 6 1 n =3 9

Replication of Chromosomes ü Replication is the process of duplicating a Occurs in chromosome Interphase ü Occurs prior to division (interphase) ü Replicated copies are called sister chromatids. ü Held together at centromere 10

A Replicated Chromosome Gene X Homologs (same genes, different alleles) Sister Chromatids (same genes, same alleles) Homologs separate in meiosis I and therefore different alleles separate. 11

Meiosis Forms Haploid Gametes ü Meiosis must reduce the chromosome number by half. ü Fertilization then restores the 2 n number. from mom from dad child too much! meiosis reduces genetic content The right number! 12

Meiosis: Two Part Cell Division Sister chromatids separate Homologs separate Meiosis II Diploid Haploid 13