Genes, Transcription & Translation

Gene - A length of DNA that codes for one (or more) polypeptides.

Polypeptide - A polymer consisting of a chain of amino acids, joined together by peptide bonds.

Genome - The entire DNA sequence (nucleotide base pairs) of an organism.

Protein - A large polypeptide (usually 100 or more amino acids). Some consist of more than one polypeptide chain e.g. Haemoglobin.

Locus - A specific position on a chromosome, occupied by a specific gene.

In the human genome, there are about 25,000 genes. A few are in the mitochondria, but most genes are found on the chromosomes within the nucleus. Each gene occupies a specific locus on the chromosome. The DNA in the chromosomes is associated with histone proteins.

Remember each chromosome consists of one molecule of DNA, and each gene is just a part of a DNA molecule.

Genes code for polypeptides such as:

• structural proteins e.g. collagen and keratin
• haemoglobin
• immunoglobulins (antibodies)
• cell surface receptors
• antigens
• actin and myosin in muscle cells
• tubulin proteins in the cytoskeleton
• channel proteins (found in cell membranes)
• electron carriers
• enzymes

Since genes code for enzymes, they are involved in the control of all metabolic pathways, and the synthesis of all non-protein molecules found in cells.

The Genetic Code:                                                                                                                     The sequence of nucleotide bases on a gene provides a code, with the instructions for making a polypeptide or protein. The genetic code has these characteristics:

• It is a triplet code. A sequence of 3 nucleotide bases code for an amino acid. There are four bases (A, T, C & G) which can be arranged in groups of three (e.g. AAT, CTG), so the number of possible triplet combinations is 4 cubed or 64. As there are only 20 amino acids used for protein synthesis, this is more than enough.
• It is a degenerate code. All amino acids except methionine have more than one code (e.g. TGT and TGC both code for Cysteine)
• Some triplets don't code for an amino acid, but instead indicate 'stop' - the end of the polypeptide chain.
• It is widespread but not universal. For example, the base sequence TCT codes for the amino acid serine in any organism. This is useful for genetic engineering, as genes can be transferred from one organism into another, and it will usually produce the same protein. However there are some variations:
  • In mammalian mitochondria: there are 2 codes for methionine (whereas there is usually only 1 code), and one of the standard stop codes, codes for tryptophan.
  • In ciliated protoctists, two of the standard stop codes code for glutamic acid.

TRANSCRIPTION

Transcription - The creation of a single-stranded mRNA copy of the DNA coding strand.

Transcription is the first stage of protein synthesis. A messenger RNA (mRNA) molecule is made. For this, one strand (the template strand) of the length of DNA…