5: Transcription & Translation

Question 1

What are the stages in transcription?

Answer 1

Initiation; elongation & termination

Question 2

What 3 components are essential for initiation to occur and why?

Answer 2

(1) Transcription factors (auxiliary proteins); (2) sequence motifs (e.g. TATA box) which is found in the promotor region upstream to transcription start site & is recognised by (3) RNA polymerase

Question 3

Why is RNA processing required, and what are the steps involved?

Answer 3

Because the RNA produced in transcription is not functional (has to be processed before leaving the nucleus to be transcribed)
- 5’ capping = addition of 7-methylguanosine cap to 5’ end to prevent degradation & aid in export and translation. Signifies mRNA
- Polyadenylation of 3’ tail = signifies mRNA (3’ end); prevents degradation & aids in export and translation
- Splicing of introns = introns removed (non coding regions). Remaining exons = expressed sequences

Question 4

What is alternative splicing and why is it important?

Answer 4

Alternative splicing is key to protein diversity, as it is the selective splicing/removal of exons, depending on the tissue. Leads to (production of different forms of mRNA) different numbers of exons in the same gene in different tissues so they can perform different functions.

Question 5

Describe the process of pre-mRNA splicing

Answer 5

A bubble containing intron sequence (= lariat) is formed between 2 exon sequences, due to the recognition of certain sites.
Lariat site is recognised and released, leaving only exons which are joined back up = mRNA molecule

Question 6

Many diseases are caused by frameshift mutations. What does this mean?

Answer 6

Insertions or deletions of nucleotides that disrupt reading frame. Rarely generate functional protein

Question 7

Outline the steps in translation (protein synthesis)

Answer 7

1. Activation: coupling correct amino acid to appropriate tRNA molecule
2. Initiation: assembly of ribosome subunits : methionine; tRNA; mRNA
3. Elongation: amino acids added to nascent polypeptide chain in mini cycle
4. Termination: release of polypeptide chain from tRNA and dissociation of ribosomes

Question 8

Describe activation (in translation)

Answer 8

Aminoacyl-tRNA Synthetase is the enzyme that adds correct amino acid to cognate tRNA
Creates a high energy ester bond between amino acid & tRNA
ATP is hydrolysed in the process