1. The role of DNA in protein
synthesis
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2. DNA controls protein synthesis
• The shape and behaviour of a protein molecule
depends on the exact sequence of the amino
acids (primary structure)
• DNA controls protein structure by determining
the exact order in which the amino acids join
together when proteins are made in a cell
• The triplet code:
– The sequence of bases or nucleotides in a DNA
molecule is a code for the sequence of amino acids in
a polypeptide
– The code is a three-letter or triplet code
– Each sequence of 3 bases stands for 1 amino acid
– Sequence read in 1 direction and on 1 strand of DNA
molecule
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4. Protein synthesis
• DNA molecule unwinds and unzips
• Free activated RNA nucleotides pair up with
exposed bases from one strand
• Forms sugar-phosphate backbone
• Single stranded molecule – mRNA
(messenger RNA ) leaves nucleus
• Free amino acids and transfer RNA
(tRNA ) in cytoplasm. Each tRNA bonds
with a particular amino acid
• mRNA attaches to ribosome small subunit
and codon exposed to the large subunit
(first 3 bases always AUG)
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5. • tRNA molecule with complementary anticodon
forms hydrogen bonds. This tRNA has the amino
acid attached to it
• A second tRNA (bringing a different amino acid)
bonds with the next 3 exposed bases. The 2 amino
acids are held closely together and peptide bond is
formed between them (reaction catalysed by
peptidyl transferase – found in small subunit of
ribosome)
• Ribosome moves along mRNA ‘reading’ next 3
bases. Several ribosomes work on the same mRNA
strand at the same time (polyribosomes ). First
tRNA leaves as the third one joins to the second
one
• Polypeptide chain continues to grow until a ‘stop’
codon (UAA, UAG or UGA) is exposed on the
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8. Genes and genome
• Gene – a part of a DNA molecule which
codes for just one polypeptide. One DNA
molecule contains many genes.
• Genome – total set of genes in a cell/total
information in one cell/genetic code of
organism
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