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All 12 Flashcards — The genetic code and codon tables
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Question
What is a codon?
Answer
A group of **three** consecutive bases on mRNA that codes for **one amino acid** (or a start/stop signal).
Question
How many bases are read at a time when decoding mRNA?
Answer
**Three** — the genetic code is a **triplet** code, read in non-overlapping groups of three.
Question
What is the genetic code?
Answer
The set of rules linking each mRNA **codon** to the **amino acid** it specifies; it is nearly the same in all living things.
Question
How many possible codons are there, and why?
Answer
**64** — there are 4 bases and codons are 3 bases long, so **4 × 4 × 4 = 64**. This covers 20 amino acids plus start/stop.
Question
What does it mean that the genetic code is universal?
Answer
Almost **all organisms** use the **same codons** for the same amino acids.
Question
Why does universality make genetic engineering possible?
Answer
Because the code is shared, a gene from one species can be **read correctly** by another (e.g. a human gene placed in bacteria).
Question
What does it mean that the genetic code is degenerate (redundant)?
Answer
**More than one codon** can code for the **same amino acid** (e.g. UUU and UUC both code for phenylalanine).
Question
Give an example of degeneracy.
Answer
**UUU and UUC** both code for **phenylalanine**; **UCU and UCC** both code for **serine**.
Question
What is a silent mutation?
Answer
A base substitution that changes a codon to another codon for the **same amino acid**, so the **protein is unchanged**.
Question
How does degeneracy allow silent mutations?
Answer
Because several codons code for the same amino acid, a base change can give a different codon that still specifies the **same** amino acid.
Question
How do you read an mRNA sequence using a codon table?
Answer
**Split it into non-overlapping threes**, look up each codon in turn, and keep the amino acids in **order**.
Question
What is the difference between 'universal' and 'degenerate'?
Answer
**Universal** = the same code in all organisms (between species); **degenerate** = several codons for one amino acid (within the code).
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Topic 4.2 hub
Protein synthesis
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