BACKGROUND

The life form on our planet is primarily based on a molecule that we call DNA. It carries within its structure the heredity information that determines the structure of proteins and the instructions that direct cells to grow and divide. So are the messages that bring about the differentiation of fertilized eggs into the multitude of specialized cells that are necessary for the successful functioning of higher plants and animals.

A nucleotide is one of the the four molecules:

A : Adenine

C : Cytosine

G : Guanine

T : Thymine

In a gene, each group of three successive nucleotides is called a codon. Each codon is is responsible of synthesizing an amino acid (one of the table on the previous page).

Although a gene is responsible of synthesis of a protein, it has parts which are `garbage' and parts which are `functional'. These parts are called introns and exons, respectively. So, it is actually the exon zones which produces the protein and introns can practically be omitted from the sequence.

When it is time to produce a protein, a kind of a mask is generated from the gene. This mask is called the messenger RNA (mRNA)which also consists of only four kind of nucleotides. These are

A : Adenine

C : Cytosine

G : Guanine

T : Urasil

It is only the useful parts that are masked, namely the exons. The introns are simply ignored. This mask generation is called transcription. Each nucleotide in the gene is represented by a one-to-one correspondence. Here is the table of transcription:

ORIGINAL TRANSCRIPTED INTO

DNA NUCLEOTIDE mRNA NUCLEOTIDE

A U

C G

G C

T A

After the transcription is completed, the codons of the mRNA are used to manufacture the protein. Since there are 4 nucleotides, as you might have realized already, there are $4\times 4\times 4$ possibilities for them to group in four. That means there are 64 codons. But as we know they synthesize only 20 amino acids and some codons stop the process. So, evidently, some codons must synthesize the same amino-acid. That is true, and is called degeneracy.

Here is the table of which mRNA codon is synthesizing which amino acid.

FIRST THIRD

POSITION 4cSECOND POSITION POSITION

U C A G

PHE SER TYR CYS U

PHE SER TYR CYS C

LEU SER stop stop A

LEU SER stop TRP G

LEU PRO HIS ARG U

LEU PRO HIS ARG C

LEU PRO GLN ARG A

LEU PRO GLN ARG G

ILE THR ASN SER U

ILE THR ASN SER C

ILE THR LYS ARG A

MET THR LYS ARG G

VAL ALA ASP GLY U

VAL ALA ASP GLY C

VAL ALA GLU GLY A

VAL ALA GLU GLY G