All of the characteristics expressed by an organism are due to different proteins. It is the order of the amino acids in a protein which determine its structure and function.
Within a cell it is the genes that control the proteins that are made. Genes are short sections of DNA (deoxyribonucleic acid), and build up chromosomes. The DNA itself is made up of four bases (A, T, C and G). These are complimentary - A is always paired with T; C is always paired with G.
Genes are divided into triplets, which are sequences of 3 bases that code for one particular amino acid. It is in this way that the DNA controls the amino acid sequence.
Mutations occur when sections of the DNA change. This means that the triplets can become different, and so code for amino acids that do not give the correct protein. Most mutations no effect on the organism, but there are mutations that are harmful and others that are beneficial. Certain external factors can increase the likelihood of mutations occurring. These include: ionising radiation (x-rays, UV light, α (alpha), β (beta) and γ (gamma) radiation), and also some chemicals. Cancer cells are cells in which a mutation causes them to divide uncontrollably.
Chromosomes are found in the nucleus of eukaryotic cells. They are tightly wound lengths of DNA that contains between a few hundred and several thousand genes. In humans, there are 23 homologous pairs of chromosomes. These are chromosomes that are the same size and carry genes that code for the same characteristics. One chromosome of each pair comes from each parent, and usually carry different alleles (versions of the same gene). The X shaped chromosomes are two chromatids that are joined at a centromere. They are only X shaped after the DNA has been replicated (copied), just before cell division.
Mitosis is the process by which normal body cells are made for growth or repair. It is also the process used in asexual reproduction. In order to divide, the cells first have to copy their genetic material - because of this, the resulting daughter cells are genetically identical (clones). Once the DNA has been replicated (copied), the chromosomes become supercoiled and compact enough to see with a light microscope. They appear in the traditional X shape - two chromatids joined at the centromere. The nuclear membrane breaks down, and the chromosomes align along the equator of the cell. Fibres form which attach to the centromeres and pull the chromatids to opposite poles of the cell. A new nuclear membrane forms around each set of chromatids, the cytoplasm divides and the cell membrane pinches inwards to give two daughter cells.
Meiosis is the process that forms the gametes (sex cells) necessary for sexual reproduction. Sexual reproduction gives variation within a species due to the random mixing of genetic information - half from each parent. Gametes are said to be haploid because they contain half a set of chromosomes. The zygote formed at fertilisation is diploid, with a full set of genetic information. The ovaries and testes contain the only cells in the human body which divide by meiosis.
The first stage of meiosis is DNA replication, but rather than lining up singly, the chromosomes arrange themselves as 23 homologous pairs (chromosomes that contain genes for the same chromosomes, but different alleles). One chromosome from each pair moves to either end of the cell, and the cytoplasm divides. The 2 daughter cells formed contain 23 chromosomes each.
The second stage of meiosis is similar to mitosis, but without the DNA replication. The 23 chromosomes align along the equator of the cell and fibres form which split the centromeres and pull one chromatid from each chromosome towards each pole. A new nuclear membrane forms, the cytoplasm divides and the cell membrane pinches. Four daughter cells are formed at the end of this process.