Topic 8: Gene Expression

Question 1

Define what a gene is

Answer 1

A stretch of DNA, chromosomal locus, codes for protein

Question 2

Describe main differences between DNA and RNA

Answer 2

DNA has thymine base while RNA has urasil base. DNA is double stranded but RNA is single stranded. DNA has deoxyribose sugar while RNA has sugar ribose.

Question 3

Describe the 3 main types of RNA

Answer 3

1. mRNA(messenger): transition molecule between gene and protein
2. rRNA(ribosomal): protein translation
3. tRNA(transfer): read the triplet code to synthesise a protein

Question 4

Describe the mechanisms behind DNA replication

Answer 4

3 stages: Initiation, elongation, termination
Using helicase to unwind DNA, DNA polymerase and DNA ligase to seal. Needs activated deoxyribonucleotide triphosphate(dNTP) and DNA template

Question 5

Describe the mechanisms behind making RNA- transcription

Answer 5

3 stages: initiation, elongation, termination
Needs RNA polymerase, nucleotide triphosphates(ntp), and DNA template.
Template strand read in 3’ to 5’ direction. Promoter intiates transcription, permits RNA polymerase to bind, controls timing of transcription. Enhancers control location and level of gene expression. Elongation occurs. Genes contain introns- dont code for genes but controls timing and level of gene expression. Progression from pre to mature mRNA: capping, polyadenylation occurs, then splicing.

Question 6

What occurs when moving from pre to mature RNA phase?

Answer 6

1. Capping: 5’ cap protects against degradation
2. Tailing/ polyadenylation: addition of polyA tail at 3’ end, protects against degradation
3. Splicing: in middle to remove introns

Question 7

What is genetic translation?

Answer 7

When mRNA is translated into a protein sequence. Mature RNA leaves nucleus for ribosomes in cytoplasm.
The amino acid chains that make up a protein are sythesised by ribosomes. Reading code can be degenerate, changes can lead to mutation. Requires tRNA.

Question 8

Describe the role of tRNA in translation

Answer 8

Carried out by polysome( mutiple ribosomes) that bind to mRNA and work in parallel to translate it.
tRNA acts as carrier, brings amino acids to ribosome. One side of unique tRNA recognises base pair code and other side picks amino acids to link to chain. Note that amino acids must be charged before addition to chain, and some tRNA have wobble positions.

Question 9

Explain the role of A and P sites in elongation in translation

Answer 9

New amino acid attached to tRNA arrives at P site(LHS). Moves to A site and attaches to previous amino acid, creating peptide bond formation. Empty P site moves away, and A replaces it, leaving a new empty A site.

Question 10

Name the start codon

Answer 10

Methionine- AUG/ATG

Question 11

Is the start codon of a polypeptide sequence at C or N terminal?

Answer 11

N terminal

Question 12

Why are there more types of mRNA compared to RNA?

Answer 12

A single gene an make different mRNA molecules, and can be spliced differently. The use of different transcription intiation sites will also lead to greater variety of mRNA.

Question 13

What are synonymous codons?

Answer 13

Codons that code for the same amino acids. This is why changes in 3rd position of base pair sequences do not cause much damage compared to 1st and 2nd.

Question 14

What is a deletion and frame shift alteration?

Answer 14

When one base of codon is deleted, and other bases shift down. This changes reading frame of entire protein- may mess up stop codon

Question 15

State difference in structure of RNA and DNA

Answer 15

RNA- 2 OH groups
DNA- 1 OH group on C3, and 1 H group