The genetic code

Gene

A short section of DNA that codes for a specific polypeptide.

What are the key properties of the genetic code

Triplet code, non-overlapping, degenerate, universal

What is the significance of a triplet code

A sequence of 3 bases in DNA (or mRNA) codes for one specific amino acid.

How many possible triplet codes are there

4³=64

What is the significance of the genetic code being non-overlapping

3 bases codes for 1 amino acid, the next 3 bases codes for the next amino acid. Each base is part of 1 triplet. Order of triplets in DNA (or mRNA) therefore determines the order of amino acids in a polypeptide.

What is the significance of the degenerate nature of the genetic code

Genetic code is redundant (more triplet codes than amino acids) therefore most amino acids are coded for by more than 1 triplet code. Helps reduce the potential harmful effects of point mutations.

What is the significance of the genetic code being universal

Almost all organisms use the same genetic code. Allows gene transfer between organisms (genetic engineering). Provides evidence for a common ancestor.

What is the start codon

AUG

What are the stop codons

UAA, UGA, UAG

DNA template (antisense) strand

Directs the synthesis of the mRNA, The mRNA is complementary in sequence to the DNA template strand.

DNA coding (sense) strand

Has the same base sequence as the mRNA strand (except uracil is replaced by thymine).

What are the 2 stages of gene expression

Transcription and translation.

Transcription

A single stranded messenger RNA (mRNA) copy of a gene is synthesised by RNA polymerase in the nucleus, using one DNA strand as a template.

Translation

The order of bases in mRNA determines the order in which amino acids are joined together at a ribosome, in the cytoplasm, to form a polypeptide.

How does mRNA leave the nucleus for translation

Via a nuclear pore