LIFE SCIENCESMS MDLULI NF

MEIOSIS

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

 Organism 

 Number of chromosomes in body cells

Humans HorseDogCatRatGoldfishRice Plants

46667838429424

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.

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.

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.

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.

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.

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.

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.

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.

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.