Chromosomes are structures located in the nucleus of the cell. They are made of DNA and protein. Chromosomes are long thin threads called chromatin until cell division occurs. Then they become visible as rod-like chromosomes. Chromosomes are composed of genes. All the genes of an organism make up the organism’s genome. Genes control the physical characteristics of a species. All organisms of the same species contain the same number of chromosomes in their nuclei.
HAPLOID AND DIPLOID CELLS
Haploid cells contain 1 set of chromosomes in their nuclei. Diploid cells contain 2 sets of chromosomes in their nuclei. The human species contain 46 chromosomes in their nuclei. This is the diploid (2n) number. The sex cells of the species have 23 chromosomes in their nuclei. This is the haploid (n) number. When fertilisation takes place the 23 chromosomes (n) from the father (called Paternal chromosomes) and the 23 chromosomes (n) from the mother (called the Maternal chromosomes) combine to form the diploid (2n=46) number of chromosomes in the fertilised egg cell.
THE STAGES OF THE CELL CYCLE
INTERPHASE This is the part of the cell’s life process when it does not divide. In this phase the cell grows, proteins and enzymes are made, and it increases the number of cytoplasmic organelles. Near the end of interphase chromosome duplication occurs. When this occurs the single strand chromosome becomes a double strand. Each strand has identical genes.
MITOSIS-DIVISION OF THE NUCLEUS:Mitosis is the division of the nucleus. Two daughter nuclei, genetically identical the original nucleus, are formed. The 2 cells formed by mitosis are called daughter cells.
STAGES OF MITOSIS
PROPHASE As seen on the diagram below, the chromatin condenses forming chromosomes. Each chromosome is composed of two identical sister chromatids connected at the centromere. At this stage the nucleolus and the nuclear membrane break down and the spindle fibres form.
ANAPHASE As seen on the diagram below, the spindle fibres shorten (contract) and split the pair of chromosomes at the centromeres. The 2 sets of chromosomes are pulled to the opposite sides (poles) of the cell.
TELOPHASE As seen on the diagram below, each chromosome group becomes a nucleus when a nuclear membrane is formed around it. The chromosomes uncoil to become chromatin and the nucleolus reforms.
MEIOSIS
The Significance if mitosis:
HAPLOID AND DIPLOID CELLS
THE STAGES OF THE CELL CYCLE
INTERPHASE This is the part of the cell’s life process when it does not divide. In this phase the cell grows, proteins and enzymes are made, and it increases the number of cytoplasmic organelles. Near the end of interphase chromosome duplication occurs. When this occurs the single strand chromosome becomes a double strand. Each strand has identical genes.
MITOSIS-DIVISION OF THE NUCLEUS:Mitosis is the division of the nucleus. Two daughter nuclei, genetically identical the original nucleus, are formed. The 2 cells formed by mitosis are called daughter cells.
STAGES OF MITOSIS
PROPHASE As seen on the diagram below, the chromatin condenses forming chromosomes. Each chromosome is composed of two identical sister chromatids connected at the centromere. At this stage the nucleolus and the nuclear membrane break down and the spindle fibres form.
METAPHASE
As seen on the diagram below, the chromosomes line up along the middle of the cell. Each chromosome is connected to both sides of the cell by spindle fibres attached to the centromeres. 
ANAPHASE As seen on the diagram below, the spindle fibres shorten (contract) and split the pair of chromosomes at the centromeres. The 2 sets of chromosomes are pulled to the opposite sides (poles) of the cell.
TELOPHASE As seen on the diagram below, each chromosome group becomes a nucleus when a nuclear membrane is formed around it. The chromosomes uncoil to become chromatin and the nucleolus reforms.
MEIOSIS
A human body cell contains 23 pairs of chromosomes. The gametes - sperm or eggs - contain half this number of chromosomes, which is why meiosis is sometimes called 'reduction division'. (For more on chromosomes go to DNA and genes)
Before meiosis begins, the chromosomes are copied exactly. The DNA of each chromosome is replicated to form two chromatids. They then arrange themselves into homologous pairs (both coding for the same characteristics), and prepare for cell division. At this point maternal and paternal chromatids can exchange bits of DNA to recombine their genetic material and increase the potential for variation.
The homologous pairs of chromosomes then separate and move to the poles of the parent nucleus. For each of the 23 pairs there is a 50-50 chance as to which pole the paternal or maternal pair of chromatids goes. With over 8 million possibilities there are many opportunities for variation.
The nucleus now divides to form two daughter nuclei, each with a mixture of paternal and maternal chromosomes but with half the full complement of genetic material (and no pairs at all). This division is called Meiosis 1.
Finally the two daughter nuclei themselves divide to form gametes. This second division - Meiosis 2 - works just like mitosis. The chromosomes (really pairs of chromatids) split apart to form the genetic material of the four new cells. The end result is four sex cells each with a complete but single set of 23 chromosomes.
On fertilisation the nuclei of the sperm and the egg join to form a new nucleus, called the zygote. The zygote contains 23 pairs of chromosomes - 23 single chromosomes from the sperm, and 23 single chromosomes from the egg.The Significance if mitosis:
- it produces two identical daughter cells that have the same number of chromosomes
- genetic information on the chromosomes are identical
- some organisms can reproduce asexually by mitosis. (bacteria)
- growth and repair
- meiosis reduces the chromosome number in the cell by half
- chromosome diploid number is restored on fertilisation
- reproduction in some organisms, production of gametes
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