Download 3.5.5 Explain the relationship between one gene

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3.5.5 Explain the relationship between one gene and one polypeptide
A gene is a sequence of DNA which encodes a polypeptide sequence
A gene sequence is converted into a polypeptide sequence via the processes of transcription
(making an mRNA transcript) and translation (polypeptide synthesis)
Translation uses tRNA molecules and ribosomes to join amino acids into a polypeptide chain
according to the mRNA sequence (as read in codons)
The universality of the genetic code means all organisms show the same relationship between
genes and polypeptides (indicating a common ancestry and allowing for transgenic techniques to
be employed)
Some proteins may consist of a number of polypeptide chains and thus need multiple genes (e.g.
hemoglobin consists of four polypeptide subunits encoded by two different genes)
When a gene is mutated it may lead to the synthesis of a defective polypeptide, hence affecting
protein function
The 'One Gene - One Polypeptide' Rule
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There are two exceptions to the 'one gene - one polypeptide' rule:
Genes encoding for tRNA and rRNA do not code for polypeptide sequences (only mRNA
sequences code for polypeptides)
A single gene may code for multiple polypeptides if alternative splicing occurs (the removal of
exons as well as introns)