Biology
Class Notes
5-2
Aim: Why is meiosis important?
 Sexual reproduction involves 2 parents—there is increased genetic variation
because of the different sets of DNA
 Parent sex cells—gametes—are haploid: this means that they have one set of
chromosomes
 The daughter cells that are produced are diploid: this means that they have 2
sets of chromosomes—one from each parent
(A) MEIOSIS
 Meiosis: form of cell division that reduces the chromosome number by
half when forming gametes—it consists of two nuclear divisions
 Just as in mitosis, the DNA must replicate itself before the nuclear
divisions begin.
DIVISION I
 During the first division of meiosis three very important
things occur
 First, the homologous chromosomes (chromosomes that
have the same sequence of genes) pair up
 Once the homologous chromosomes have paired up,
crossing over occurs. Crossing over is when pieces of the
DNA are exchanged between chromatids.


The homologous chromsomes then separate. THIS
REDUCES THE CHROMOSOMES NUMBER BY HALF.
The end result of the first division is two daughter cells with
different genetic information.
DIVISION II
 During the second division the two daughter cells will divide again
 THERE IS NO REDUCTION OF CHROMOSOME NUMBER IN
THE SECOND DIVISION
 The end result of the second division is 4 daughter cells will
different DNA
(B) GENETIC VARIATION
 Meiosis helps to increase genetic variation in three ways:
1. independent assortment
o random distribution of homologous
chromosomes during meiosis
2. crossing over:
o DNA is exchanged between two chromatids
o Also known as genetic recombination
3. Random Fertilization:
o Random joining of two gametes during fertilization
(C) GAMETOGENESIS
o Formation of gametes
MALES (spermatogenesis)
Formation of sperm
Testes
4 functional haploid cells
What
Where
End result
FEMALES (oogenesis)
Formation of sperm,
Ovaries
1 functional haploid egg
cell and three polar bodies
(D) COMPARISION
MITOSIS
2 genetically identical diploid cells are
produced
Occurs in all body cells
No crossing over occurs
One nuclear division
Non reduction in chromosome number
MEIOSIS
4 genetically different haploid cells are
produced
Occurs in sex cells only
Crossing over occurs
2 nuclear division
Reduction by half in chromosome number