OCR A2 Biology F215: Cellular Control

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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


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