Lecture 7

Question 1

What is the start codon and the end codon?

Answer 1

AUG (methionine) - start codon

UAA, UGA & UAG - stop codons

Question 2

How many amino acids does a codon specify?

How many codons does and amino acid have?

Answer 2

One codon - one amino acid

One amino acid usually has more than one codon

Question 3

How many possible codons are there and how many amino acids are there?

Answer 3

61 codons for 20 amino acids
3 are stop codons
64 in total

Question 4

Features of mRNA

Answer 4

Copy of DNA - complementary to one strand only
Synthesised in the nucleus (transcription)
Carries the message to the cytoplasm for translation
- the message specifies the sequence of amino acids in the protein
- the message is decoded (translated) by the ribosomes during synthesis of the protein

Question 5

Features of tRNA

Answer 5

Adapters for the genetic code
They are the link between the codons in the mRNA and the amino acids in the polypeptide
- amino acids cannot recognise codons directly
- each tRNA matches its amino acid to its codon
- the part of the tRNA that matches the codon is called the anticodon
Every cell must contain at least 30 tRNAs
- need one or more for each amino acid
- do not need a different one for every codon
- usually there’s at least 32 different tRNAs

Question 6

Structure of tRNA?

Answer 6

Primary structure
Single stranded
Around 80 nucleotides long
Many nucleotides are modified
Secondary structure 
Common structure 
Base paired
Anticodon at one end; amino acid site at the other

Question 7

How many nucleotides sequence an amino acid?

What direction are codons read?

Answer 7

Triplet code - 3 nucleotides specify the amino acid

5’-3’ direction