The Genetic Code
The sequence of nucelotides in DNA forms a code that determines the sequence of amino acids in the proteins of an organism.
In eukaryotic cells the DNA is stored in the nucles, however sythnesis of proteins takes place in the cytoplasm of cells; therefore the DNA code needs to be transcribed in to a single-stranded molecule called ribonucleic acid (RNA). There are several types of RNA, one that transfers the DNA code from the nucleus to the cytoplasm (called mRNA). mRNA is small enough to leave the nucleus through nuclear pores in the envelope and to enter the cytoplasm, where the code that it contains is used to determine the sequence of amino acids in the proteins which are synthesised there.
It is the sequence of nucleotide bases on mRNA that is referred to as the genetic code. The mRNA code is derived from DNA but it is not IDENTICAL, it is COMPLEMENTARY. The term codon refers to the sequence of three bases (triplet) on mRNA that codes for a single amino acid.
The Genetic Code- Features
The main features of the genetic code are:
- Each amino acid in a protein is coded for by a sequence of three nucleotide bases on mRNA.
- A few amino acids have only a SINGLE codon.
- The code is a degenerative code. This means most amino acids have more than one codon. E.G the amino acid leucine has six different codons (UUA, UUG, CUU, CUC)
- There are three codons that do not code for any amino acids. These are called stop codons and mark the end of a polypeptide chain.
- the code is non-overlapping, each base in the sequence is read only once. Thus, six bases numbered 123456 are read as codons 123, 456.
- It is a UNIVERSAL code. The same codon codes for the same amino acid in all organisms (with a few minor exceptions).
DNA is composed of two nucleotide chains wound around each other (double helix). We shall now look at the structure of a related molecule that is usually made up of a single nucleotide chain: ribonucleic acid (RNA).
Ribonucleid Acid (RNA) Structure:
Ribonucleic acid is a polymer made up of repeating mononucelotide sub-units. It forms a single strand which each nucleotide is made up of:
-The pentose sugar RIBOSE.
- One of the organic bases, A,C,G,U (U instead of T),
-A phosphate group.
The two types of RNA that are important in protein synthesis are:
-Messenger RNA (mRNA)
- Transfer RNA (tRNA)
Messenger RNA (mRNA)
mRNA consists of thousands of mononucleotides and is a long strand that is arranged in a SINGLE helix. It is manufacture when DNA forms a mirror copy of part of one of its two strands, and so there is a variety of different types of mRNA. Once formed, mRNA leaves the nucleus via pores in the nuclear envelope and enters the cytoplasm, where it associates with the ribosomes. It then acts as a template upon which proteins are built. Its structure is suited to this function because it posesses the correct sequences of the many triplets of organic bases that code for specific polypeptides. It is also easily broken down and so only exists when it is needed to manufacture a given protein.
Transfer RNA (tRNA)
Transfer RNA (tRNA) is a small molecule made up of around 80 nucleotides. It is a single-stranded chain folded into a clover-leaf shape, with one of the chains extending beyond the other. This is the part of tRNA to which amino acids can easily attach. There are several types of tRNA each able to carry a single amino acid. At the opposite end of the tRNA molecule is a sequence of three other organic bases, known as the anticodon. For each amino acid there is a different sequence of organic bases on the anticodon.
The organic bases in DNA will pair up in a precise way. These are complementary pairs. In RNA however, the base T is always replace with a similar base known as uracil (U). RNA can link with both DNA and other RNA molecules. The complementary base pairings that RNA forms are therefore:
- G & C
-A & U
During proteinsynthesis, the anticodon pairs with the three complementary bases that make up the triplet of bases on mRNA. The tRNA structure, with its end chain for attaching amino acids and its anticodon for pairing with the codon of the mRNA, is structurally suited to its role of lining up amino acids on the mRNA template during protein synthesis.
Diagram of Transfer RNA (tRNA)