What is a gene?
- A gene is a sequence of nucleotide bases found in DNA that codes for one (or more) polypeptide.
- Genes code for polypeptides which have structural and metabolic roles e.g. keratin, collagen, channel proteins, etc.
The genetic code
- Is...the sequence of nucleotide bases on the gene providing a code for the sequence of amino acids in a polypeptide chain.
- There are 4 different types of nucleotide bases found in DNA: A,C,T,G. The genetic code is a triplet code which means 3 bases code for 1 amino acid. This results in 4x4x4 = 64 different triplet codes. As there are only 20 amino acids (used in protein synthesis) the code is described as degenerate - as most amino acids have more than one code.
- Some codons (a line of 3 bases) correspond to 'stop' codons which indicate the end of the polypeptide chain.
1. A gene unwinds and unzips due to the H bonds between complementary bases being broken.
2. Complementary RNA nucleotides bind to the exposed bases on the template strand. A-U, T-A, C-G.
3. Transcription stops at a stop triplet, e.g. TAG.
4. A complimentary copy of the base sequence of the template strand of the DNA has been made, called mRNA.
5. The mRNA is small enough to leave the nuclear envelope via a nuclear pore to a ribosome.
1. A molecule of mRNA binds with a ribosome.
2. Each ribosome has 2 sites to hold 2 tRNA molecules, each with an attached amino acid.
3. Each set of 3 bases on a mRNA molecule is called a codon. The first codon is always AUG, coding for Methionine.
4. A tRNA with the complimentary anticodon is activated using ATP, and is attracted to the complementary codon by H bonds.
5. A second tRNA carrying a different amino acid then…