Transcription

Question 1

what bases are used in mRNA?

Answer 1

Adenine, Guanine, Cytosine and uracil

Question 2

what types of RNA are there?

Answer 2

rRNA - not translated - used in ribosome

tRNA - not translated- covalent binding of amino acids and their transport to the ribosome

mRNA - translated - contains instructions for protein synthesis

Question 3

transcription is catalysed by what enzyme?

Answer 3

by RNA polymerase

Question 4

where does transcription start?

Answer 4

it begins at the promoter and ends at the terminator

Question 5

describe the genes tructure

Answer 5

• promoter region
• exons/introns
• terminator region

Question 6

what are the 3 RNA polymerase enzymes?

Answer 6

RNApolymerase 1= makes rRNA

RNApolymerase 2= makes mRNA

RNA polymerase 3= makes tRNA and 5s rRNA

Question 7

describe the three stages in the transcription process

Answer 7

1. initiation -RNA polymerase binds to DNA and unwinds a segment of the promoter
2. elongation - RNA polymerase moves along the template synthesising RNA until it reaches the terminator region
3. termination - transcription continues beyond the proteincoding region (3’ untranslated region ) - the termination process is imprecise, within the transcribed sequence = polyadenylation signal which recruits endonuclease an enzyme that cleaves mRNA beyond the sequence

Question 8

how is transcription controlled in eukaryotic cells?

Answer 8

Activators - proteins that bind to gene sites known as enhancers and speed up the rate of transcription

repressors - these proteins bind to selected sets of genes at sites known as silencers and thus slow transcription

coactivators - these adapter molecules integrate signals from activators and perhaps repressors

basal transcription factors- in response to injunctions from activators - these factors position RNA polymerase at the start of transcription and initiate the transcription process

Question 9

what are the modifications of mRNA post transcription?

Answer 9

• 5’ capping - adding 7 methyl guanosine residue catalyzed by guanyltransferase
• splicing - out with the introns - necessary b/c eukaryotic genes are split into different regions - carried out by spliceosome (consiting of snRNA and protein)
• 3’poly A tailing - polyA polymerase adds 40-350 adenine residues to the cleaved 3’ end - stabilizes the mRNA molecules
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Question 10

what type of RNA is associated with the spliceosome?

Answer 10

small nuclear ribonucleoproteins (snRNPs)

Question 11

how do we make a number of different proteins from the same gene sequnce?

Answer 11

Differential splicing of mRNA precursors -

Exons: retained/skipped

introns: excised/retained

shift splice sites: exon size increase or decrease

or by RNA editing

occurs via the introduction of a new stop codon into the RNA sequence by deaminating a C to U (by deaminase enzyme)