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The genetic code
The genetic code as base triplets in mRNA which code for specific amino acids.
The genetic code is universal, non-overlapping and degenerate.
Universal, non-overlapping, degenerate, base triplet code.…read more

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The structure of molecules of and transfer (tRNA).…read more

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Candidates should be able to compare the structure and composition of DNA, mRNA and tRNA.
Features DNA mRNA tRNA
Shape Double stranded- held Single stranded Single stranded
together by hydrogen
Sugar Deoxyribose sugar Ribose sugar Ribose sugar
Bases A,T,C,G A,U,C,G A,U,C,G
Where its found Mostly in the nucleus Manufactured in Manufactured in
nucleus but found nucleus but found
throughout the cell throughout the cell
Chemical Very stable Chemically unstable Chemically more stable
that mRNA but less
than DNA.…read more

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Polypeptide synthesis
Transcription as the production of mRNA from DNA. The role of RNA polymerase.…read more

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Steps in transcription
1. RNA helicase unwinds the DNA
2. The promoter region in the DNA
molecule is recognised and RNA
polymerase binds to this section of DNA
3. RNA polymerase lines up RNA
nucleotides along the strand. It moves
along the strand using it as a template
for mRNA synthesis. Specific base
pairing means that the resulting Mrna
molecule is complementary copy of the
DNA strand.
4.…read more

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The splicing of pre-mRNA to form mRNA in eukaryotic cells.…read more

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Translation as the production of from the
of carried by mRNA. The role of and tRNA.
1. The RNA binds to the ribosome
2. The Trna with an anticodon
complementary to the first codon on
Mrna attaches itself to the Mrna by
specific base pairing
3. The peptide bond forms between the
amino acids carried by the two tRNA
4. The first tRNA molecule moves away
leaving behind its amino acid
5.…read more

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Gene mutation
Gene mutations might arise during DNA replication. The deletion and substitution of bases.…read more

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Gene mutations occur spontaneously. The mutation rate is increased by mutagenic
agents. Some mutations result in a different amino acid sequence in the encoded
polypeptide. Due to the degenerate nature of the genetic code, not all mutations
result in a change to the amino acid sequence of the encoded polypeptide.…read more


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