DNA Transcription & Translation Flashcards
(14 cards)
Which enzyme carries out transcription?
RNA polymerase
What is a TATA box?
DNA sequence around 25bps upstream from the start site of transcription.
Transcription factors bind to this TATA box to help position RNA polymerase and initiate transcription.
How and why is mRNA modified after transcription?
5’ end is capped, this makes translation easier and protects the chain from degradation by enzymes.
3’ end is cleaved and a polyA tail is added, helps to prevent degradation.
Introns are spliced, this forms mature mRNA.
How does splicing work?
Introns are removed by a spliceosome complex which recognises specific splice sequences.
Exons are brought together and the intron loop is released.
This turns pre-mRNA into mature mRNA.
What is alternative splicing?
Some exons are removed which means that different mRNAs are formed from the same pre-mRNA.
This means that a single gene can code for many different proteins depending on the combination of exons.
Identify the stop codons.
UAA
UAG
UGA
What is the codon for Methionine?
AUG
Why are some tRNAs able to recognise more than one amino acid?
Phenomenon called ‘wobble’.
The 1st nucleotide of the anticodon on a tRNA can recognise different bases in the 3rd base on mRNA.
How is protein synthesis initiated?
Initiator tRNAs carrying Met bind to the small subunit of a ribosome that travels along the mRNA when it reaches a suitable AUG.
These initiator tRNAs are different to normal tRNAs.
(Bacterial proteins start with N-formyl methionine (fMet) instead).
What is the function of aminoacyl tRNA synthetase?
‘Charges’ a tRNA by attaching its complimentary amino acid using ATP.
How does a ribosome synthesise a protein from mRNA?
Ribosomes have 3 sites: E, P, A
- Used tRNA released from E site. An amino acid is added to the chain at the P site. New charged tRNA enters the A site.
- The peptidyl chain is transferred to A site.
- Large subunit moves along 3 bases, so A site is empty.
- Small subunit moves along too.
- Process repeats.
How is translation terminated?
When the ribosome reaches a stop codon: UAA, UAG, UGA
release factors bind and the polypeptide chain is released from the ribosome.
The mRNA coding region is normally covered in ribosomes, what is the advantage of this?
Protects the chain from degradation.
Forms several protein chains from the same mRNA.
What is nonsense-mediated decay?
This is where a mutation creates a premature STOP codon.
This section of mRNA downstream from the new stop codon is exposed since there are no ribosomes attached, so it will be degraded by nucleases.
