Session 5 - Transcription and Translation

Question 1

Define transcription

Answer 1

The process by which DNA is transcribed into an RNA message

Question 2

What are the four stages of transcription

Answer 2

Initiation, Elongation, Termination and Splicing

Question 3

What occurs at initiation stage of transcription? (4)

Answer 3

• Intitiation code is recognised 5’TATA3’
• Transcription factors (regulating proteins) bind at this code, upstream of the fene
• RNA polymerase is attracted and starts mRNA production
• RNA polymerase ‘separates’ the DNA strands for the RNA nucleotides to bind

Question 4

What does RNA polymerase do during process of elongation?

Answer 4

Travels along template stand, picking up base pairs and copying them onto a complimentary RNA strand.

Question 5

How long does process of elongation continue?

Answer 5

Until genetic sequence is ‘transcribed’ onto the mRNA molecule

Question 6

What two things happen at termination stage of transcription?

Answer 6

Methyl-guanine ‘cap’ added to 5’ end

Polyadenylation occurs

Question 7

How is the methyl-guanine ‘cap’ added to mRNA, and what is its function?

Answer 7

Methyl-guanine bonded with 5’ - 5’ triphosphate linkage to stabilise the mRNA

Question 8

What is polyadenylation of mRNA and what is its function?

Answer 8

The addition of lots of adenine nucelotides added at the 3’ end of mRNA. Improves stability.

Question 9

What happens during splicing, and how does this process occur?

Answer 9

Introns are removed. Endonucleases remove introns within polynucleotide and exonucleases degrade polynucleotide for 5’ or 3’ end.

Question 10

Define translation

Answer 10

The process by which the RNA message is translated into amino acid code

Question 11

Answer following for mRNA
What type of polymerase used?
Wht percent of all RNA does it make up?
How many different kinds are there?
How many copies of each present?

Answer 11

RNA polymerase II
-2%
100,000s of kinds
A few copies of each present

Question 12

Answer following for rRNA
What type of polymerase used?
Wht percent of all RNA does it make up?
How many different kinds are there?
How many copies of each present?

Answer 12

RNA polymerase I
>80%
Few kinds
Many copies of each

Question 13

Answer following for tRNA
What type of polymerase used?
Wht percent of all RNA does it make up?
How many different kinds are there?

Answer 13

RNA polymerase III
15%
100 kinds
Very many copies of each

Question 14

What is Ribosomal RNA used for?

Answer 14

To bind to mRNA and provide the location for tRNA

Question 15

What is the differnce between eukaryotic and prokaryotic ribosomes?

Answer 15

Eukaryotes - Ribosome 80s (made up of 60s and 40s subunits)

Prokaryotes 70s - (made up of 50s and 30s subunits)

Question 16

How is the genetic code read in a ribosome?

Answer 16

In triplets, with no overlapping and no gaps

Question 17

What is meant by saying that DNA is degenerate?

Answer 17

Some amino acids can be coded for by several different codons

Question 18

When is tRNA uncharged, and what is it referred to when it is charged?

Answer 18

When no amino acid is bound to it. Is referred to as an aminoacyl-tRNA when amino acid bound and it is charged

Question 19

Give following for DNA replication
Enzyme
Activated substance
Template
3 stage process?

Answer 19

DNA polymerase
dNTPs
DNA
yes

Question 20

Give following for making mRNA (transcription)
Enzyme
Activated substance
Template
3 stage process?

Answer 20

RNA polymerase
NTPs
DNA
yes

Question 21

Give following for making a polypeptide (translation)
Enzyme
Activated substance
Template
3 stage process?

Answer 21

ribosome
Amino acid
mRNA
yes

Question 22

Outline following for ribosomes in prokaryotes
Number of rRNAs
Protein number
Subunits
Size of ribosome

Answer 22

3 rRNAs
56 proteins
30s, 50s
70s

Question 23

Outline following for ribosomes in eukaryotes
Number of rRNAs
Protein number
Subunits
Size of ribosome

Answer 23

4 rRNAs
82 proteins
40s, 60s
80s

Question 24

Name three stages of translation

Answer 24

Initiation
Elongation
Termination

Question 25

What occurs at initiation stage of TRANSLATION

Answer 25

- 40s subunit of ribosome binds with Met-tRNA attached binds at the 5' cap end of the mRNA - the 60S subunit then binds and elongation occurs

Question 26

What starting codon must be recognised for translation to begin? What amino acid does it code for? What is the anticodon found on tRNA?

Answer 26

5' AUG, which is a specific codon to one amino acid - methionine. 5' CAU is the anticodon required.

Question 27

What are the two ribosomal sites at which tRNA binds, and what goes where?

Answer 27

P site - For the site holding the peptide chain | A site - for the site accepting the tRNA

Question 28

What are the first two steps in the process of translational elongation, and what do they require?

Answer 28

1 - Met-tRNA occupies the P site | 2-Another aminoacyl-tRNA enters the ribosome to occupy the A site (requires GTP)

Question 29

What are the third and fourth steps of translational elongation?

Answer 29

3- Methionine forms a peptide bond with the next aminoacyl - tRNA, making original tRNA in the P sit now uncharged 4 - The uncharged tRNA now leaves and the ribosome translocates (moves along)

Question 30

What is the binding of two amino acids catalysed by?

Answer 30

Peptidyl Transferase

Question 31

How does the termination of translational elongation occur?

Answer 31

Requires a stop codon to be read on the mRNA.

Question 32

Give three stop codons, and explain why they halt process of translation

Answer 32

5'UAA, 5'UAG and 5'UGA. No tRNAs that can bind to these codons, so the peptide and tRNA are hydrolysed and release the protein into the cytoplasm.

Question 33

Define the term "gene"

Answer 33

A unit of heredity and a length of DNA on a chromosome that contains the code for a protein (or RNA) as well as sequences necessary for its expression, such as promoter and terminator sequences and introns

Question 34

What are the three major reactions involved in the processing of Eukaryote processing

Answer 34

Capping, polyadenylation and splicing

Question 35

What is the 3' molecule in a DNA triplet called a wobble position

Answer 35

Change in third base more likely to cause change in amino acid

Question 36

What does 5' to 3' template provide?

Answer 36

N to C polypeptide chain extension

Question 37

Give three features of DNA triplet code

Answer 37

Degenerative Non overlapping Read in 5' to 3' prime direction

Question 38

What can substitutions in genetic code lead to?

Answer 38

Different primary amino acid sequence, which can affect the tertiary structure of a protein

Question 39

What can be a large effect of a single base substitution?

Answer 39

Can occur in a stop codon, leading to longer polypeptide chain that will be malfunctional

Question 40

Give the nine key differences between mammalian and bacterial gene expression

Answer 40

``` Prokaryotes have - Simpler promoters - Different transcription factors - Single RNA polymerase - Coupled transcription - translation - No post-transcriptional processing - Short lived mRNAs - Simpler ribosomes (exploited by attacking 30s subunit - Distinctive translation initiation mechanism -Different translation factors ```

Question 41

What does the severity of a mutation depend on?

Answer 41

The amount of difference in the protein caused

Question 42

How can mutations outside the coding region affect gene expression?

Answer 42

Mutations to promoter regions where transcription facts bind can affect gene expression, to either constituively activate or deactivate it