Protein synthesis

The genetic code specifies the amino acids that are assembled to make polypeptides. The way that DNA codes for polypeptides is central to our understanding of how cells and organisms function.

1. State that a polypeptide is coded for by a gene and that a gene is a sequence of nucleotides that forms part of a DNA molecule

• A polypeptide is coded for by a gene
• A gene is a sequence of nucleotides that forms part of a DNA molecule
• A DNA molecule consists of many genes

2. State that a gene mutation is a change in the sequence of nucleotides that may result in an altered polypeptide

• A gene mutation is a change in the sequence of nucleotides that may result in an altered polypeptide
• For example the different sequence of a triplet code of nucleotides cause the substitution of an amino acid on the surface of the beta chain in haemoglobin
• Which causes sickle cell anaemia which is a disease which disrupts the normal functions of a haemoglobin
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3. Describe the way in which the nucleotide sequence codes for the amino acid sequence in a polypeptide with reference to the nucleotide sequence for HbA (normal) and HbS (sickle cell) alleles of the gene for the β-globin polypeptide

• DNA controls protein structure by determining the exact order in which amino acids join together
• The sequence of nucleotide bases in a DNA molecule is a code for the sequence of amino acids in a polypeptide
• The nucleotide sequence codes for amino acids by triplet codes
• This means that a sequence of three nucleotides codes for one amino acid
• The sequence is always read in the same direction from one of the two DNA strands, called a sense strand
• The complementary strand is called the anti-sense strand
• 
• The amino acid sequence coded from the HbA (normal) allele of the gene is Val-His-Leu-Thr-Pro-Glu-Glu-Lys-
• While the amino acid sequence coded from the HbS (sickle cell) allele of the gene is Val-His-Leu-Thr-Pro-Val-Glu-Lys-
• This type of mutation is called substitution, where an amino acid is substituted with another
• 
• This mutation takes place…