MGD S5 - Transcription, Translation and the Genetic code

Question 1

Explain the process of Initiation of transcription

Answer 1

Initiation code upstream of gene (5’TATA) recognised and transcription factors bind to this code

Transcription factors attract RNA polymerase

RNA polymerase separates the DNA helix and allows free nucleotides to bind to complementary bases

Question 2

Describe the process of Elongation in transcription

Answer 2

RNA polymerase moves down the template strand picking up base pairs and copying them onto the complementary mRNA strand

Question 3

Describe the process of termination in transcription

Answer 3

When gene fully transcribed the mRNA adds a methyl-guanine cap to 5’ (with a 5’-5’ triphosphate bond) end to stabilise the mRNA

At the 3’ end there is a stop codon (AAUAA) that prompts cleavage of mRNA

The 3’ end is then polyadenylated (a ~200 adenine tail is added)The product is called pre-mRNA

Question 4

Describe the process of splicing in transcription

What is produced at the end of this process?

Answer 4

Introns are removed from the pre-mRNA

Mature RNA is produced

Question 5

What is the function of endonuclease and exonucleases?

Answer 5

Endonuclease breaks bonds within the polynucleotide

Exonuclease breaks bonds at the 5’ or 3’ end of a polynucleotide

Question 6

What are the three type of RNA?

Answer 6

mRNA
rRNA
tRNA

Question 7

Describe the pertinent features of mRNA

Hint: Produced by? How many kinds? What proportion of overall RNA?

Answer 7

Produced by RNA polymerase II

~2% of overall RNA

100,000s of kinds

Few copies of each kind

Question 8

Describe the pertinent features of rRNA

Hint: Produced by? How many kinds? What proportion of overall RNA?

Answer 8

Produced by RNA polymerase II

> 80% overall RNA

Few kinds

Many copies of each

Question 9

What is rRNA used for?

Answer 9

To bind to the mRNA and provide location for tRNA

Facilitates reading of genetic code from mRNA

Question 10

What is the difference in the structure of eukaryotic and prokaryotic ribosomes?

Answer 10

In eukaryotes ribosomes are 80s, comprised of 60s and 40s subunits

In prokaryotes ribosomes are 70s, comprised of 50s and 30s subunits

Question 11

How is the genetic code from mRNA interpreted?

Answer 11

Read in triplets with no overlapping or gaps

Question 12

Describe the pertinent features of tRNA

Hint: Produced by? How many kinds? What proportion of overall RNA?

Answer 12

Produced by RNA polymerase III

~15% of overall RNA

~100 kinds

Very many copies of each

Question 13

How are tRNA molecules specific to certain amino acids?

Answer 13

tRNA anticodon determines the complementary amino acid that that particular tRNA will bind to

Question 14

How does the inclusion of Ionosine in tRNA anticodons allow multiple codons coding for the same amino acid to be recognised.

Answer 14

Ionosine is aspecific and will bind to A, C and U

Where ionosine binds to a nucleotide of a codon may be referred to a the ‘wobble base pair’.

Wobble base pairs occur in the wobble position (3rd nucleotide of a codon) where binding between nucleotides is weaker, allowing for non-standard pairings.

tRNA anticodons that includes ionosine can therefore recognise multiple codons

Question 15

How does amino acid binding affect a tRNA molecule?

Answer 15

tRNA is uncharged when unbound

bound tRNA is charged and referred to as an aminoacyl-tRNA

Question 16

Describe the process of Initiation in translation

Answer 16

The the 5’ cap the 40s subunit with Met-tRNA binds

Moves along until start codon (normally 5’AUG) is recognised, 5’CAU is the anticodon for this.

The 60s subunit then binds to allow elongation to begin

Question 17

What are the two tRNA binding sites in a ribosome and their general functions?

Answer 17

P site for holding peptide chain

A site for accepting new tRNA

Question 18

Describe stage 1 and 2 of elongation in translation

Answer 18

Met-tRNA occupies P site and another aminoacyl-tRNA with anticodon complementary to next codon enters A site (this requires GTP).

Question 19

Describe stage 3 and 4 of elongation in translation

Note: Don’t forget the enzyme involved

Answer 19

The methionine from Met-tRNA forms a peptide bond with the next aminoacyl-tRNA in the A site.

This binding requires peptidyl-transferase tRNA in P site now uncharged and leaves ribosome

Ribosome moves along to open up A site for the next aminoacyl-tRNA (with complementary anticodon)

Rinse and repeat the process building a new protein

Question 20

Describe the process of termination in translation

Answer 20

Stop codon enters A site

Release factor binds to A site

Peptide and tRNA in P site are hydrolysed releasing the protein into the cytoplasm

Question 21

List the three processes involved in post transcription mRNA maturation

Answer 21

Capping
Polyadenylation
Splicing

Question 22

there are 64 possible codons and only ~20 amino acids to code for, what does this mean for coding of mRNA/proteins from DNA?

Answer 22

Triplet code is degenerative, there is more than one codon for each amino acid as well as assorted start and stop codons.

Question 23

What are the implications of the degenerative triplet code to disease/mutation

Answer 23

Substitutions in the genetic code can lead to a different primary sequence of proteins.

A mutation that affects a stop codon can lead to longer malfunctional polypeptide production

Question 24

List the 9 ways bacterial gene expression differs from mammalian gene expression.

Answer 24

Simpler promoters
Different transcription factors
Different translation factors
Single RNA polymerase
Coupled transcription/translation
No post transcriptional processing
Short lived mRNA
Simpler ribosomes
Distinctive translation initiation mechanism

Question 25

How might differences between bacterial and mammalian gene expression be exploited clinically?

Give an example.

Answer 25

Targeted disruption of processes specific to bacteria. Hence harming bacteria and leaving the human host you are treating unaffected

Eg. Attacking the 30s subunit of ribosomes specific to bacteria

Question 26

How might we predict the effects of different gene mutations?

Answer 26

If an amino acid is substituted for another of similar properties as a result of mutation it is unlikely to produce a large effect.

If an amino acid is substituted for another of different properties (Eg Sickle Cell Disease) we might predict a large effect.

A delayed or premature stop codon will cause truncated or elongated proteins that are most likely malfunctional.

Question 27

How can mutations outside of exons affect gene expression?

Answer 27

Mutations in promoter regions where transcription factors bind can activate or deactivate the gene.