Translation - Gene Expression

Question 1

what is a codon

Answer 1

A codon is a DNA or RNA sequence of three nucleotides (a trinucleotide) that forms a unit of genomic information encoding a particular amino acid

Question 2

4 key features of codons

Answer 2

• 61 of 64 possible 64 codons specify an amino acid
• most amino acids have more than one codon
• 3 codons specify stop: UAG, UAA & UGA
• 1 codon specifies start AUG (methionine)

Question 3

what is the adapter molecule

Answer 3

the adapter molecule is a small RNA molecules which is a transfer RNA (tRNA)
it has a region which can bind an amino acid and a region which can interact with mRNA

Question 4

explain ‘charging’ a tRNA

Answer 4

an enzyme (aminoacyl-tRNA synthetase) recognises both a specific amino acid and the correct tRNA for this amino acid and joins them together

Question 5

what is translation

Answer 5

the synthesis of proteins by ribosomes using mRNA as a set of instructions

Question 6

what are the 3 stages of translation

Answer 6

1. initiation
2. elongation
3. termination

Question 7

explain the initiation phase of translation

Answer 7

1. a specific initiation tRNA (carrying methionine) binds to a small ribosomal subunit
2. the small ribosomal subunit/initiator tRNA complex identifies the 5’ G cap and attaches to mRNA
3. the small ribosomal subunit/initiator tRNA complex moves along the mRNA (5’-3’) until it finds the initiation codon AUG
4. the complex stops, with the initiator tRNA carrying the first methionine positioned in the p site
5. large ribosomal subunit attaches itself

Question 8

explain the elongation phase of translation

Answer 8

1. a charged tRNA with an anticodon complementary to the A site codon lands in the A site
2. a- the ribosome will break the bond that binds the amino acid to the tRNA in the P site, transfer the amino acid to the newly arrived amino acid ( that attaches to the tRNA in the A site) and form a peptide bond between them
   a - LEAVES a tRNA with the growing amino acid in A site AND an empty/uncharged tRNA in the P site
   b - while the tRNA’s are bound to the mRNA (P&A sites) the ribosomes moves 3 nucleotides down the mRNA, the tRNA with growing amino acid chain moves to the P site (was in A) so the chain of amino acids can exit through the tunnel in P, the uncharged tRNA moves to E site (was in P)
3. in the E site, the anticodon of the tRNA detaches from the mRNA codon and the uncharged tRNA is expelled
4. a new charged tRNA with an anticodon complementary to the next A site codon enters the ribosome at the A site
   PROCESS REPEATS

Question 9

explain the termination phase of translation

Answer 9

1. when the ribosome reaches a stop codon, a protein (release factor) enters the A site
2. the release factor breaks the bonds between the P site tRNA (using water) and the final amino acid. this causes the polypeptide chain to detach from its tRNA and the newly polypeptide chain is released
3. the small and large ribosomal subunits dissociate from the mRNA and each other

Question 10

what is a genotype

Answer 10

set of genes and/or DNA combinations that are responsible for a particular trait
determined at a DNA level

Question 11

what is a phenotype

Answer 11

the physical expression, or characteristics of a genotype (can be observed)
defined by what happens at a protein level together with environmental influences