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6.2.26 · Nucleic Acids and Protein Synthesis

The Universal Genetic Code — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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6.2.2 The Universal Genetic Code — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain what is meant by the genetic code being triplet, non-overlapping, degenerate and universal, and use a codon table to predict amino acid sequences.

  • Sense codons
  • Start codon
  • Stop codons
  • Gene boundaries
  • Third base
  • Silent mutation

Section 1 of 8Properties of the genetic code

Genetic code
Genetic code the set of rules by which a base sequence is translated into the amino acid sequence of a polypeptide.
Triplet
  • Triplet three bases (one codon) specify one amino acid.
Non-overlapping
  • Non-overlapping each base belongs to only one codon, so mRNA is read three bases at a time from a fixed starting point with no reuse of bases.
Degenerate
  • Degenerate there are 64 codons but only 20 amino acids, so most amino acids have more than one codon, for example six codons for leucine.
Universal
  • Universal almost all organisms use the same code, so the same codon specifies the same amino acid in bacteria, plants, fungi and animals.
Sense codons
  • 61 of the 64 codons specify amino acids, an average of about three per amino acid.

Section 2 of 8Start and stop codons

Start codon
  • AUG codes for methionine and sets the reading frame at which translation begins.
Stop codons
  • UAA, UAG and UGA code for no amino acid, and when one reaches the ribosome translation ends and the polypeptide is released.
Gene boundaries
  • Gene boundaries these signals show the cell where genes start and stop.

Section 3 of 8Why degeneracy matters

Third base
  • Third base the codons for one amino acid often differ only in the third base.
Silent mutation
  • Silent mutation a change in that base may give a different codon for the same amino acid, so the polypeptide is unchanged.
Buffer
  • Buffer degeneracy therefore reduces the damaging effect of some mutations.

Section 4 of 8Why universality matters

1Genetic engineering
  • Genetic engineering a gene from one species can be transcribed and translated in another.
  • The human insulin gene inserted into Escherichia coli gives human insulin.
  • Plants with a bacterial pest-resistance gene and sheep making human clotting factors rely on the same principle.
2Evolution
  • Evolution a shared code is good evidence for a single common ancestor.
3Comparing sequences
  • Comparing sequences codons mean the same across species,
  • so similarities in protein sequences such as cytochrome c, or in homologous genes, show how closely related organisms are.
4Exceptions
  • Exceptions minor ones exist, for instance in mitochondria and some single-celled organisms, so the code is nearly universal.

Section 5 of 8Using a codon table

1
If given a DNA template strand, write the complementary mRNA first.
2
Find the start codon AUG on the mRNA.
3
Read three bases at a time.
4
Look up each codon in the table to name the amino acid.
5
Stop at a stop codon.
Length
  • Length a coding region of 300 bases between start and stop codons codes for a polypeptide of about 100 amino acids.
Bases
  • Bases use U, not T, when reading an mRNA codon table.
Table pattern
  • Table pattern the second base often indicates the chemical type of the amino acid.

Section 6 of 8More from the specification

1Hydrogen bonds stabilizing codon-anticodon pairings
  • Hydrogen bonds stabilizing codon-anticodon pairings specific hydrogen bonds between complementary mRNA codons and tRNA anticodons ensure accurate translation function.
2Nucleotide
  • The unit of DNA made of a phosphate, a deoxyribose sugar and a base.
3Intron
  • A non-coding section of a gene removed from the mRNA.
4Protein synthesis
  • Making proteins from amino acids using the DNA code.
5Hydrogen bond
  • The bond between paired bases that holds the two strands together.
6Location
  • Location transcription of the code occurs in the nucleus, while translation into protein occurs in the cytoplasm at ribosomes.

Section 7 of 8Key terms

TermMeaning
Genetic codeThe rules relating base sequences in mRNA to the amino acid sequence of a polypeptide.
DegenerateDescribes a code in which most amino acids are specified by more than one codon.
UniversalDescribes a code that is the same in nearly all organisms.
Non-overlappingEach base belongs to only one codon and is read once.
Start codonAUG, which sets the reading frame and codes for methionine.
Stop codonUAA, UAG or UGA; a codon that signals the end of translation.
Reading frameThe way a base sequence is divided into codons, determined by the start codon.

Section 8 of 8Exam technique

1
Saying degenerate means the code is faulty — it means several codons can code for one amino acid.
2
Saying universal means every organism has the same genes — it means the same codons mean the same amino acids.
3
Forgetting that stop codons do not code for amino acids.
4
Reading overlapping codons — the code is non-overlapping and read in successive triplets.
5
Giving DNA bases (T) when the mRNA codon table is needed.

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