5 Gene Expression

Question 1

Genes produce proteins

Answer 1

This idea pre-dates DNA structure —> Beadle and Tatum (19410 - mutations in Neurospora crassa can be localised to particular point on chromosome —> correlate with defects in particular enzymes)

Central dogma (Crick) = image below

Question 2

Genetic Code

Answer 2

Genetic code - no punction / non-overlapping / universal

DNA adapted - changing the 3rd base in a codon often has no effect on the AA coded for

Genetic code is non-random —> adapted for resistance to change but is now fixed

First position changes result in similar chemistry

Question 3

Bacterial DNA

Answer 3

Packaged by supercoiling and wrapping round some proteins / fairly open structure / default position is that genes are on - transcribed / circular DNA

Question 4

RNA polymerase

Answer 4

• Core polymerase —> sigma proteins —> holoenzyme

Question 5

Genes and promoters

Answer 5

RNA polymerase recognised promoter, unwinds due to the A-T binds (weaker than c-G becayse AT onky have 3 H bonds) sites and starts RNA resytnetisis at +1

Synthesising 5’-3’ that’s complementary to template stand —> unwinds 15 bases at a time roughly

Question 6

Control of gene expression - simplest way for a repressor protein to work

Answer 6

bind to DNA and prevent RNA polymerase transcription which is negative regular

Question 7

Control of gene expression - positive regulators

Answer 7

enhance the amount of transcription occurring

Question 8

Control of gene expression - tRNA adapters

Answer 8

tRNA adapters - Anticodon - recognises codon by complimentary base pairing —> attach relative AA

Question 9

Ribosome structure

Answer 9

half of its mass is ribosomal RNA and it’s AT is made of RNA = ribozyme

Ribosome has 2 subunits and 3 possible binding sites

Question 10

Chemistry of translation

Answer 10

Adjacent slots, bring together 2 tRNAs and therefore 2 AA corresponding to the codons, theyre close enough that the enzyme can then catalyse the reaction between the amino group of 1 AA to the carboxyl group of another AA which is attached to the tRNA which eliminates the tRNA and the 2 join together —> peptide bond

When stop codons reached a different protein binds

Question 11

Eukaryotic gene expression

Answer 11

Transcription (nucleus) and translation (cytoplasm) occur in separate places + separate times

Message is processed —> capping, poly-A tail, splicing

Chromatin structure - 1st level of control

Question 12

Chromatin organisation

Answer 12

DNA wrapped around histones to make nucleosomes / wrapped into 30nm fibre / most DNA wrapped in fibres for a majority of the time —> major level of control based on how tightly packaged the genes are = DNA accessibility

Question 13

Eukaryotic gene expression - DNA accessibility

Answer 13

if theres a methyl group on the CG that’s a marker that the DNA should stay packaged (marker of inactive DNA)

Question 14

Eukaryotic gene expression - histone modifications

Answer 14

methylation (tighter) and acetylation (less tight)

Question 15

Eukaryotic gene expression - chromatin remodelling

Answer 15

ATP dependant / nucleosome sliding / displacement = exposes part of DNA

Question 16

When does eukaryotic transcription occur

Answer 16

Only present when certain proteins are present (highly regulated)

Question 17

Eukaryotic message processing

Answer 17

3’ the end gets truncated —> helps with recognition (sacrificial extension that can be removed without damaging mRNA)

Question 18

Splicing

Answer 18

snRNPs serve to find the boundaries between exons and introns by base pairing and facilitate the removal of introns - in nucleus

Question 19

Alternative splicing

Answer 19

Genes can produce multiple related proteins

90% human genes may be alternatively spliced