Gene Expression 2

Question 1

What does mRNA consist of?

Answer 1

A leader sequence, a reading frame sequence and a trailer sequence

Question 2

Why do sequences of mRNA vary?

Answer 2

because amino acid coding sequences (reading frames) differ and so do leader and trailer sequences

Question 3

Are the leader and the trailer sequences translated?

Answer 3

No

Question 4

Where does the trailer sequence start?

Answer 4

After the stop codon

Question 5

What is a ribosome made up of?

Answer 5

A small subunit and a large subunit

Question 6

What do both the subunits contain?

Answer 6

rRNA and ribosomal proteins

Question 7

What does the large subunit contain?

Answer 7

28S rRNA
5.85S rRNA
55rRNA
Around 50 ribosomal proteins

Question 8

What does the small subunit contain?

Answer 8

18S rRNA

Around 30 ribosomal proteins

Question 9

What is the P site on a ribosome?

Answer 9

Peptidyl-tRNA binding site

Question 10

What is the A site on a ribosome?

Answer 10

Aminoacyl- tRNA binding site

Question 11

Describe a rough overview of translation

Answer 11

The mRNA passes through the ribosome, where the codons are recognised by tRNAs carrying specific amino acids

Question 12

What is at the 3’ end of tRNA and what is it’s role?

Answer 12

An acceptor stem (CCA) which binds to the amino acid

Question 13

Why does the anticodon loop vary?

Answer 13

It depends on the amino acid at the acceptor stem

Question 14

What is the role of the anticodon loop?

Answer 14

Reads the matching codon on mRNA

Question 15

What enzyme is used to add the right amino acid to each tRNA?

Answer 15

aminoacetyl-tRNA synthase

Question 16

How many aminocetyl-tRNA syntheses are there?

Answer 16

20 (one for each amino acid)

Question 17

What is charging?

Answer 17

When an amino acid binds to the acceptor stem

Question 18

How is an amino acid added to tRNA?

Answer 18

1. The amino acid and ATP bind to aminoacyl-tRNA synthase.
2. ATP looses two phosphate groups and binds to the amino acid as AMP, this exposes a binding site.
3. Uncharged tRNA binds to the complex
4. AMP is lost which causes the binding of tRNA to the amino acid
5. Enzyme returns to original shape

Question 19

Describe how translation is initiated.

Answer 19

1. Small ribosomal subunit binds to mRNA at the start of the coding region.
2. Met-tRNA binds to the AUG start codon
3. The large ribosomal subunit binds and completes the initiation complex
4. The initiation complex is as follows, Met-tRNA is bound to AUG on mRNA in the P site of the ribosome, the next codon is positioned on the A site.

Question 20

Describe how the peptide chain is elongated

Answer 20

Begins by the appropriate aminoacyl-tRNA biding to the codon in the A site. Each time an amino acid is in the A site a peptide bond will be formed with adjacent amino acids.

Question 21

Describe the termination of translation.

Answer 21

1. At a stop codon a release factor reads the triplet and polypeptide synthesis ends
2. The polypeptide is released from the tRNA into the cytoplasm where it will fold.
3. The tRNA is released into the ribosome
4. The two ribosomal subunits separate from the mRNA

Question 22

What is a polysome?

Answer 22

When several ribosomes translate mRNA at the same time (producing many polypeptides simultaneously from a single mRNA)