Gene expression Flashcards
What is transcription?
Producing mRNA using DNA
Where does transcription occur?
Nucleus
What are the stages of transcription?
Inititation
Elongation
Termination
What happens in initiation of transcription?
Transcription factor binds to promoter site
RNA polymerase is recruited
What is a common sequence in the promoter site of genes in eukaryotes?
TATA
What is the base in front of RNA polymerase marked as?
+1
What is the direction of the +1 base called?
Downstream
What direction does RNA polymerase move along the DNA strand in?
3’ to 5’
What makes RNA polymerase move along the DNA strand 3’ to 5’?
The binding of the transcription factor sets the direction
What happens in elongation of transcription?
RNA polymerase reads DNA strand 3’ to 5’
forms pre-mRNA 5’ to 3’
RNA processing
What is the DNA strand that RNA polymerase reads called?
DNA template strand/non-coding strand
What reaction occurs repeatedly in elongation of transcription?
(rNMP)n + rNTP —–> (rNMP)n+1 + PPi
What is the purpose of RNA processing?
Convert pre-mRNA into mature mRNA
What are the different processes involved in RNA processing?
Capping
Tailing
Splicing
What happens in capping?
At 5’ end of pre-mRNA
methylated guanine is added by 5’-5’ linkage
What is the purpose of capping?
Protect mRNA against degradation by exonucleases
Role in translation???????
What happens in tailing?
At 3’ end of pre-mRNA
many A bases are added on
What is the enzyme repsonsible for tailing?
polyA polymerase
What is the purpose of tailing?
Protect mRNA against degradation by exonucleases
What happens in splicing?
Introns are splices out of pre-mRNA
exons are joined together
What enzyme is responsible for splicing?
Spliceosome enzyme complexes
What is alternative splicing?
Refers to how some introns can be kept, some exons removed
or exons joined together in different order
to create a new protein
What are introns?
Base sequences that are not expressed
What are exons?
Base sequences that are expressed
What happens in termination of transcription?
Termination sequence in pre-mRNA is recognised
cleaved by endonuclease
What is the name of the DNA strand that mRNA resembles?
Coding strand
Where does translation occur?
Cytoplasm
What organelle is responsible for translation?
Ribosomes
What is a polyribosome?
Multiple ribosomes translating mRNA at same time
What are ribosomes made up of?
rRNA
Proteins
What type of ribosome is present in prokaryotes? What are its subunits?
70S ribosomes
50S and 30S subunits
What type of ribosome is present in eukaryotes? What are its subunits?
80S ribosomes
60S and 40S subunits
What are the types of RNA?
ribosomal RNA, rRNA
messenger RNA, mRNA
transfer RNA, tRNA
Are there more types of mRNA or tRNA or rRNA?
More types of mRNA
then tRNA
least types of rRNA
Are there more copies of mRNA or tRNA or rRNA?
More copies of tRNA
then rRNA
least copies of mRNA
What are the features of the genetic code?
Triplet code
Degenerate
Non-overlapping
What is the triplet code?
3 nucleotide bases code for 1 amino acid
What is the adapter molecule of the genetic code?
tRNA
What is meant by the genetic code being degenerate?
More than one triplet code
can all code for the same amino acid
What is meant by the genetic code being non-overlapping?
Each base belongs to one triplet code only
not more than one
5’ to 3’ mRNA produces a polypeptide chain in what order?
N to C
What is the structure of tRNA?
Clover leaf shape
Stem loops
Anti-codon
What are stem loops?
Anti-parallel RNA
Base pairs formed in the middle
What is an anti-codon?
Three bases at a specific point on the tRNA molecule
What does the anticodon do?
Recongises mRNA codon with complementary base sequence
base pairs form
How do some mRNA codons code for the same amino acid?
Third base in anticodon/mRNA codon is wobble base
can form base pairs with multiple bases
What may be at the 3’ end of the tRNA molecule?
Amino acid
What is tRNA called when there is no amino acid at the 3’ end? And when there is an amino acid?
No amino acid - uncharged tRNA
Amino acid present - amino acyl tRNA
What is amino acid activation?
Refers to adding amino acid to tRNA molecule 3’ end
What happens in amino acid activation?
ATP is hydrolysed to release energy
this energy is used to combine amino acid to tRNA 3’ end
What is the enzyme responsible for amino acid activation?
Amino acyl tRNA synthetase
What are the stages of translation?
Inititation
Elongation
Termination
What happens in initiation of translation?
Ribosome recruited to 5’ cap of mRNA
Methionyl tRNA recruited to 5’ cap
How is the ribosome recruited to the 5’ cap of mRNA in initiation of translation?
Requires energy
What does methionyl tRNA recognise?
Start codon AUG
What happens in elongation of translation?
Ribosome moves along mRNA
tRNA molecules enter and leave ribosome
Polypeptide chain grows
What direction does the ribosome move along the mRNA?
5’ to 3’
Where do tRNA molecules leave the ribosome from?
P site
Where do tRNA molecules enter the ribosome into?
A site
How does the polypeptide chain grow in elongation of translation?
Peptidyl transferase moves amino acid from tRNA in P site to tRNA in A site
forms peptide bond between the two
and hence a polypeptide chain
What is required for elongation of translation?
Requires energy
What happens in termination of translation?
Stop codon enters A site
polypeptide chain is hydrolysed off tRNA at its C terminus
What does UTR stand for?
Untranslated region
Where are UTRs located?
5’ and 3’ end of mRNA
What is meant by a gene being expressed?
Gene is switched on
the protein it codes for is produced
Why are there many types of mRNA?
Because there are lots of genes
Alternative splicing
Why are there few copies of mRNA?
Because many ribosomes can translate one mRNA molecule producing lots of protein molecules from it
Why are there few types of rRNA?
All have same function
