Transcription and translation

Question 1

What are the inhibitors of transcription and translation

Answer 1

transcription: Alpha-amanitin (death-cap mushroom) and Rifampicin

translation: Ricin(castor bean seeds) and Chloramphenicol

Question 2

What is required for transcription

Answer 2

• ribonucleoside triphosphates (NTPs) to make a new strand
• a template to copy
• RNA polymerase for polymerisation
• an energy source

Question 3

How does transcription start and stop

Answer 3

• RNA polymerase (RNAP) generates RNA strands using a DNA template and ribonucleotides
• the promoter region has specific sequences to recruit RNA polymerase and regulatory proteins
• the transcription termination region contains specific sequences to indicate stopping transcription
  (a hairpin structure plus a u-rich region pauses and releases RNA polymerase, promoters in prokaryotes contain conserved consensus sequences at -10 and -35. RNA polymerase binds via the sigma subunit

Question 4

What are promoter regions

Answer 4

transcription factors that help initiate transcription
- positive regulation: activating transcription factor proteins recruit RNA polymerase to promoter region
- negative regulation: gene is expressed unless repressor protein is present which prevents RNA polymerase from binding (blocks access to RNA polymerase)

Question 5

How does RNA polymerase carry out transcription

Answer 5

RNAP uses one strand of DNA as a template to make a complementary copy, RNA moves along the template in 3’ to 5’ direction

RNA uses complementary base pairing (A,U,G,C), nucleotides are added to the 3’ end of the RNA chain, RNA synthesis is described as going in the 5’ to 3’ direction as added to 3’ end if growing chain but it is actually moving along in 3’ to 5’

DNA helicase opens up DNA strand to expose one template strand in 3’ to 5’ direction

hydrolysis of pyrophosphate releases energy to drive transcription

Question 6

What are the different types of RNA

Answer 6

mRNA
- codes for proteins

non-coding RNA
- regulatory and structural

Question 7

What do protein coding genes look like

Answer 7

start
- eukaryotic mRNAs are modified by adding 7 methylguanosine to the 5’ end (5’ cap - essential)

then region to translate the open reading frame which is translated

end
- eukaryotic mRNAs are modified by adding around 250 adenosine nucleotides (poly A tail)

Question 8

What are the modifications of RNA

Answer 8

• export from proteins
• aid stability
• protection from degradation

RNA decay pathways target destruction and proteins can vary their stability and have degradation mechanism which prevents completed gene expression

Question 9

Why is splicing good

Answer 9

multicellular eukaryotic genes are spliced removing the introns and joining the exons

pre-mRNAs can be spliced in alternative ways

splicing allows for greater protein diversity

Question 10

What are the basic features of translation

Answer 10

• A,U,G,C must be translated into the 20 amino acids that code for proteins
• each degenerate triplet code, codes for a specific amino acid or STOP

Question 11

How is genetic code read

Answer 11

amino acids are attached to specific tRNAS, the anticodon of the ‘charged’ tRNA base pairs with a complementary codon on the mRNA

aminoacyl-tRNA synthetase enzymes attach amino acids to specific tRNAs , there are two binding sites. one specific for tRNA for one amino acid and the other for a specific amino acid

Question 12

What is the mechanism of translation

Answer 12

• charged tRNAs create a link between mRNA and protein
• protein synthesis occurs at the ribosomes (RNA + protein)
• ribosomal RNA provides the catalytic function for translation

Question 13

What are the three sites in ribosomes

Answer 13

Exit site - tRNA exit
Peptidyl site - has growing polypeptide chain
Aminoacyl site - new rRNA with amino acids attached enter

ribosome translates 5’ to 3’

Question 14

How is translation initiated in prokaryotes and eukaryotes

Answer 14

prokaryotes
- a sequence in 5’ untranslated region, Shine-Dalgarno, pairs with 16s rRNA and places AUG in P site of ribosome. N-formylmethonine is the initiating amino acid carried by tRNA

eukaryotes
- the small ribosomal subunit and initiating tRNA - Met binds to the 5’ cap and scans the mRNA until the first AUG is reached

Question 15

What are the steps in translation elongation

Answer 15

i. a ‘charged’ aminoacyl tRNA enter the A site pairing codon with anticodon, elongation factors act as transporters bringing aminoacyl tRNA
ii. as peptide bond formation occurs, the covalent bond between amino acid and tRNA at P site is broken
iii. mRNA translocates by three bases, tRNA in P site exits via E, tRNA with chain is in P and A is free
iv. peptide bond formation moves from A to P
v. E moves along to continue the chain

Question 16

What are the steps in translation termination

Answer 16

i. a STOP codon enters A site, release factors that mimic tRNA shape recognise STOP codon at A site and enter A site to stimulate hydrolysis of bond linking polypeptide chain to tRNA at P site
ii. the polypeptide is released, ribosomal subunits dissociate and mRNA and tRNA are released

Question 17

What are the categories of mutation in translation

Answer 17

• Missense mutation
• Silent mutation
• Frameshift mutation
• Nonsense mutation

Question 18

What is a missense mutation

Answer 18

a mutation that results in an amino acid change but does not change the reading frame of the protein

Question 19

What is a silent mutation

Answer 19

a mutation that does not result in a change in amino acid

Question 20

What is a frameshift mutation

Answer 20

a mutation involving insertion or deletion of bases that is not a multiple of three. this shifts the reading frame of all codons following the site of mutation

Question 21

What is a nonsense mutation

Answer 21

a mutation that changes a codon specifying an amino acid into a STOP/termination codon