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 includeschromosome replication • Twomeiotic divisions --- Meiosis I and Meiosis II • Called reduction division • Original cell is diploid (2n) • Four daughter cellsproduced are monoploid (1n) (AKA, haploid)
Facts About Meiosis • Daughter cellscontain half the number of chromosomesas the original cell. • Producesgametes(eggs & sperm) • Occurs in the • testes in males (Spermatogenesis) • ovaries in females (Oogenesis)
More Meiosis Facts • Human cells start with 46 doublestranded 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” 2n = 6 1n =3
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
SisterChromatids (same genes, same alleles) Homologs (same genes, different alleles) A Replicated Chromosome Gene X Homologs separate in meiosis I and therefore different alleles separate.
from mom from dad child too much! meiosis reduces genetic content Meiosis Forms Haploid Gametes • Meiosis must reduce the chromosome number by half. • Fertilization then restores the 2n number. The right number!
Meiosis I MeiosisII Meiosis: Two Part Cell Division Sister chromatids separate Homologs separate Diploid Diploid Haploid