MEIOSISMEIOSIS

Structure of Chromosome

Diploid Vs. Haploid• Diploid - two sets of chromosomesDiploid - two sets of chromosomes• Haploid - one set of chromosomesHaploid - one set of chromosomes• Diploid cells (2n or 2 sets of Diploid cells (2n or 2 sets of

chromosomes) are made into chromosomes) are made into haploid (1n or 1 set haploid (1n or 1 set chromosomes) during meiosischromosomes) during meiosis

• Examples of diploid cells: almost all your cells! Most plant & animal cells are diploid (2n)

• Examples of haploid cells: gametes, egg & sperm (n)

How Many Homologous How Many Homologous Chromosomes? Chromosomes?

• Homologous pairs carry genes controlling the same inherited traits.

• Humans have 23 pairs of homologous homologous chromosomes.chromosomes.a. 22 pairs of autosomesautosomesb. 1 pair of sex sex chromosomeschromosomes

Sex ChromosomesSex Chromosomes

XX chromosome - female XY chromosome - male

Homologous Chromosomes• Each organism that is produced via sexual

reproduction has 2 copies of autosomes, one comes from each parent

Homologous chromosomes (Homo=same) chromosomes that are the same size & shape; carry genes for the same traits

History of Life

Review Mitosis

Meiosis• Meiosis: the form of cell divisioncell division where

sex cells are produced (gametesgametes). • Purpose: to produce sex cells (gametes) for

sexual reproduction

• Sex cells (sperm or egg) have halfhalf the # of chromosomes chromosomes as a normal body cell

Diploid to Haploid

Spermatogenesis & Oogenesis

FertilizationFertilization

• The fusion of a spermsperm and eggegg to form a zygotezygote.

n= 23egg

sperm n = 23 2n = 46

zygote

Question:Question:• A cell containing 20 chromosomes20 chromosomes (diploid)(diploid) at

the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

• ANSWER = 10 chromosomes (haploid)

• Animation http://www.johnkyrk.com/mitosis.html

ComparisonMitosis Meiosis