1. LIFE SCIENCES
MS MDLULI NF
MEIOSIS

2. INTRODUCTION
 The nucleus of cells contains an entangled mass of threads
called the chromatin network.
 This chromatin network unwinds to form chromosomes which
contain DNA.
 Each body cell or somatic cell of every has its own distinct
number of chromosomes: the table below indicates somatic
cell of different organisms

3. Organism Number of chromosomes in body cells
Humans 46
Horse 66
Dog 78
Cat 38
Rat 42
Goldfish 94
Rice Plants 24

4.  Somatic cells are all the cells in the plant or animal body,
besides the sex cells.
 In somatic cells the chromosomes are are actually arranged
in pairs.
 E.g in humans, there are 22 different kinds of chromosomes
and these are numbered as chromosome number 1,2,3 up to
chromosome number 22, and there are two of each.

5.  The pairs of chromosomes making up chromosomes 1 to 22
are identical to each other and are referred to as
autosomes.
 The remaining two chromosomes are called Sex
chromosomes.
 In females these two are identical and are referred to as XX
chromosomes. In males, one of these is an X and the other a
Y chromosome.

6.  Therefore the somatic cells of humans have 22 pairs of
autosomes and pairs of autosomes and a pair of sex
chromosomes.
 Since there are really two sets of 23 different kinds of
chromosomes, we say that cells are diploid (2n) number of
chromosomes, and in humans the diploid or 2n number is 46.

7.  The two chromosomes making up each pair of chromosomes
are identical.
 One member of each pair is of maternal origin and the other
is of paternal origin, is called a homologous pair of
chromosomes.
 The sex chromosome of females are XX and in males the are
XY. The Y chromosome is of paternal origin, but the X
chromosome maybe of maternal or paternal origin.

9. THE NEED FOR MEIOSIS
 Plants and animals undergo sexual reproduction during
which a male gamete fuses with a female gamete to form a
single cell, the zygote.
 This zygote undergoes mitosis several times to produce
millions of identical cells.
 Some of these cells undergo physical and chemical changes
in order to perfom different functions.

10.  Some of these cells become sex cells or gametes.
 If the male and female gametes have the same number of
chromosomes as the adult from which they arise, the
chromosome number of the zygote will double. If the zygote
grows, producing more cells by mitosis, then all the cells of
the offspring will double.

11.  This can not be allowed to happen since, as explained
earlier, each species has a set of number of chromosomes.
 To get back the number of chromosomes characteristic of
each species, the number of chromosomes must be halved.
 This halving of the chromosomes number occurs by the cell
division referred to as meiosis.

12. THE PROCESS OF MEIOSIS
 We study the first division of meiosis in five phases.
 These are: Interphase 1, Prophase 1, Metaphase
1, Anaphase 1 and Telophase 1.
 The “1” after the name of each phase tells us that we are
referring to the first nuclear division.

28. REFERENCES
Cohen, P. (Nov 05, 2012). Meiosis.
Rachellraines. (May 04, 2010). Meosis.
Sherif, S. (Feb 13, 2011). Cell cycle and division.
Swan, J. (Jan 10, 2011). Chapter 13 Presentation.
tas11244. (July 06, 2011). Meiosis gamete production with
turning point.