RNA and protein synthesis

RNA
The second type of nuclein acid is called ribonucleic acid (RNA). It is a large molecule similar in structure to a single strand of DNA but containing the base uracil in place of thymine, and ribose sugar in place of deoxyribose.

Structure of RNA

Adenine (A)
Guanine (G)
Uracil (U)
Cytosine (C)

Two types of RNA are messenger RNA (mRNA) and transfer RNA (tRNA).

Protein synthesis

1. Transcription of DNA into mRNA

The appropriate region of a DNA molecule temporarily splits open to expose its bases. A molecule of mRNA is formed (transcribed) from one of the DNA strands using free nucleotides present in the nucleus. Complementary base pairing (G-C, C-G, T-A and A-U) determines the sequence of the bases on the mRNA molecule. Each triplet of bases of mRNA is called a codon.
When the mRNA molecule is complete, it leaves the nucleus and enters the cytoplasm.

2. tRNA

In the cytoplasm there are tRNA molecules which each bear a triplet (anticodon) of bases corresponding to a particular amino acid. Each tRNA molecule picks up the appropriate amino acid from the cytoplasm. Every cell has as many types of tRNA as there are types of amino acid.

3. Translation of RNA into protein

The mRNA molecule becomes attached to a ribosome. Its triplet code is read and tRNA anticodon links with each complementary codon of bases on the mRNA molecule.
The completed protein (consisting of very many amino acids) is then released into the cytoplasm.
Each tRNA molecule becomes attached to another molecule of its amino acid ready to repeat the process. The mRNA is often reused to produce further molecules of the same protein.
Thus each gene codes for one protein (or polypeptide) with the order of the amino acids in the protein being determined by the sequence of the bases in the gene`s DNA. It is by this means that genes control an organism`s phenotype. Variation amongst the members of a species results from differences in the sequence of the DNA`s bases. These are brought about by mutations.