MGD Session 5

Question 1

What are the three stages of transcription?

Answer 1

Initiation, Elongation, Terminatoin

Question 2

Where does transcription occur?

Answer 2

In the nucleus

Question 3

What happens in the initiation stage of transcription?

Answer 3

The initiation code is recognised (TATA). Transcription factors bind (upstream of the gene) and attract RNA polymerase to start mRNA production.

Question 4

What does RNA polymerase do?

Answer 4

It separates the DNA strands for the RNA nucleotides to bind along the template strand.

Question 5

What are the 5 kinds of RNA?

Answer 5

mRNA (messenger)
rRNA (ribosomal)
tRNA (transfer)
miRNA (micro)
noncoding RNA

Question 6

What happens in the elongation stage of transcription?

Answer 6

RNA polymerase travels along the template strand, picking up base pairs and copying them onto a complimentary RNA strand to produce one stranded mRNA.

Question 7

What happens in the termination stage of transcription?

Answer 7

The mRNA adds a methyl-guanine “cap” to the 5’ end and the 3’ end is polyadenylated (tailing)

Question 8

How is the methyl-guanine cap bonded and why?

Answer 8

With a 5’ - 5’ triphosphate linkage bond to stabilise the mRNA

Question 9

What is splicing?

Answer 9

The removal of introns from pre-mRNA to make mature mRNA.

Question 10

What is the difference between an endonuclease and an exonuclease?

Answer 10

An endonuclease breaks within the polynucleotide but an exonuclease degrades the polynucleotide from one of the ends.

Question 11

On which chromosome is the gene for PKU located?

Answer 11

Chromosome 12

Question 12

What mutation causes PKU?

Answer 12

Single base substitution disrupting splicing and causing intron skipping.

Question 13

What are the key features of mRNA?

Answer 13

It uses RNA Polymerase II
It makes up about 2% of our total RNA
There are 100,000s of kinds of mRNA
We only have a few copies of each kind

Question 14

What are the key features of rRNA?

Answer 14

It uses RNA Polymerase I
It makes up >80% of our total RNA
There are only a few kinds
We have many copies of each kind

Question 15

What is the difference between eukaryotic rRNA and prokaryotic rRNA?

Answer 15

Eukaryotic rRNa has 40s and 60s subunits making it 80s

Prokaryotic rRNA has 30s and 50s subunits making it 70s

Question 16

What are the key features of tRNA?

Answer 16

It uses RNA polymerase III
It makes up about 15% of our RNA
There are about 100 kinds
There are many copies of each kind of tRNA

Question 17

What is Ionosine?

Answer 17

It is an aspecific nucleotide (it complements A, C, G and U). It can be referred to as the wobble position.

Question 18

When tRNA has a bound amino acid, what is it known as?

Answer 18

aminoacyl-tRNA

Question 19

The genetic code is degenerate, non-overlapping and universal. What does this mean?

Answer 19

Degenerate = more than one code for each amino acid.
Non-overlapping = it is read 123, 456 not 123, 234...
Universal = it is the same in every organism

Question 20

What is the code for the ‘start codon’?

Answer 20

AUG (methionine)

Question 21

What are the 3 stages of translation?

Answer 21

Initiation, Elongation, Termination

Question 22

What occurs in the intiation phase of translation?

Answer 22

At the 5’ cap, the 40s subunit with Met-tRNA binds as the starting codon is recognised (AUG). The 60s subunit then binds.

Question 23

What happens in the elongation phase of translation?

Answer 23

More aminoacyl-tRNAs enter the ribosome to extend the protein.

Question 24

Explain the significance of P sites and A sites in translation (elongation).

Answer 24

The P site is the site holding the peptide chain, the A site accepts the tRNA. The tRNA binds to the A site using GTP and forms a peptide bond with the tRNA in the P site, making it uncharged. The tRNA then leaves the P site and the ribosome moves along, moving the new tRNA into the P site from the A site (translocation).

Question 25

Explain termination in translation.

Answer 25

A stop codon is read on the mRNA. No tRNA can bind to this codon so the peptide and tRNA are hydrolysed to release the protein into the cytoplasm.

Question 26

Define gene.

Answer 26

A unit of heredity.
A transcription unit.
A length of DNA that contains the code for a protein.

Question 27

List the main reactions involved in RNA maturation. (name 3)

Answer 27

Capping
Polyadenylation (tailing)
Splicing

Question 28

How can substitutions in the genetic code affect a protein?

Answer 28

They alter the primary sequence so the tertiary structure can be affected.

Question 29

How does a mutation affecting a stop codon affect a protein?

Answer 29

It can lead to a longer, malfunctional polypeptide chain.

Question 30

How are bacterial cells different to eukaryotes in terms of gene expression?

Answer 30

Bacteria have:

• simpler promoters
• different transcription factors
• only one kind of RNA polymerase
• transcription and translation are coupled
• no post-transcriptional processing (capping, tailing, splicing)
• short lived mRNAs
• simpler ribosomes
• distinctive translation factors

Question 31

How does the fact that bacterial cells have simpler ribosomes help us clinically?

Answer 31

It means that the 30s or 50 subunits can be attacked by antibiotics so they are bacteria-specific and don’t harm eukaryotes

Question 32

What happens to the protein if a mutation causes a premature stop codon?

Answer 32

The protein is shorter and may not function.

Question 33

Is a mutation from Glutamate to Aspartate more or less likely to have a profound affect than a mutation from Glutamate to Valine?

Answer 33

Less likely as glutamate and aspartate are both negatively charged amino acids.

Question 34

What happens if a promoter region has a mutation?

Answer 34

Transcription factors may not bind so gene expression will be affected.