There are millions of cells in an organism and each cell normally possesses a nucleus, which contains many strands of chromosome. Chromosomes are composed of protein and deoxyribonucleic acid. Gene, the basic unit of genetic contents, is composed of DNA which stores genetic information that determines the protein synthesis of cells. This would in turn determine traits of animals and plants.

Different organisms have different number of sets of chromosomes. Cells of the same organism have the same number of sets of chromosome (except of gametes). Taking human as an example, every human cell has 23 pairs of chromosomes (except gametes). In a chromosome pair, one comes from the father and another comes from the mother. The two sets of chromosome come together during fertilization. The cell that forms by fertilization would divide into 2 identical cells. Both cells inherit the unique and new combination of chromosome. The cells of the embryo would go on dividing into identical cells.

DNA is the most important substance on the chromosome. There is a series of bases on every DNA strand, and the sequence of the bases determines what type of protein is to be synthesized. These in turn determine the types of enzymes to be synthesized, influencing the chemical reactions in the cell. This functional determination of the cell would influence the traits of the organism. Thus, DNA is often described as the blueprint of the build-up of organisms. Altering DNA would in turn alter the cellular genetic information. This would in turn alter the synthesis of protein or enzymes, and thus alter the traits .

The main component of gene is DNA (Deoxyribonucleic acid), a double helix found in virtually chromosomes of all cells, which is responsible for inheritance. There are 4 kinds of bases on the DNA molecules. The bases are abbreviated as A, G, C and T ：

1. Adenine	2. Guanine
3. Cytosine	4. Thymine

The sequences of the 4 bases determine the genetic codes of the organism. The four bases are divided into two groups, namely purines (A, G) and pyrimidines (T, C). A purine must match with a pyrimidine, which specifically means that A must match with T, and C must match with G. Genetic code refers to the sequence of A, T, G, C on a DNA strand. This sequence would in turn determine the sequence of amino acid in the protein synthesized. Sequences of amino acids on protein molecule synthesized are determined by sequence of codons, each of which is composed of 3 bases.

How is the use of 20 types of amino acids for proteins synthesis controlled by the 4 kinds of bases? Hypothesis that one base is responsible for one amino acid means that 4 kinds of bases can only be responsible for 4 kinds of amino bases. Therefore, it is guessed that a combination of several bases should be responsible for one amino acid. Hypothesis that a combination of 2 bases is responsible for the use of one amino acid means that 16 amino acids can be used for protein synthesis. Similarly, if an amino acid is made up of three bases is the case, then 64 amino acids can be used for protein synthesis. 64 are much more than the kinds of amino acids found in a human body. M.W.Nirenberg and T.H.Matthaei probed into these theories in 1961 experimentally. At the same time, the first genetic codon was discovered. It was found that the tribasic group UUU was the codon that was responsible for using phenylalanine for protein synthesis. In 1965, the 64 codons were all decoded, and a hereditary dictionary was compiled. This was the landmark on molecular genetics.

History of Genetics

Conclusion

The knowledge of genetic codes nowadays is that: (1) a triplet of bases is the code for the incorporation of one amino into a polypeptide chain. A given amino acid may be coded for by more than one codon; (2) readings of codons are unidirectional; (3) there are neither boundary nor overlapping between codons. There are also nonsense codons, like UAA, UAG and UGA, which do not code for any amino acids. However, it is thought that these nonsense codons may be responsible for terminating the incorporation of amino acids on a peptide chain, thus this kind of codon is also called the termination codon. So far experiments on genetic codes have been done on phages, which are organisms even simpler than bacteria. However, it is believed that the pattern of genetic codes of higher plants and animals are generally similar. Codons of different organisms may even be identical. DNA acts as the blueprint of life. Genes contain all unique information about the species and they work together interactively. While some may have a canceling effect on others, some may act as cooperative genes, being involved as a key in a particular chemical reaction.