MGD Session 5 Flashcards
What are the three stages of transcription?
Initiation, Elongation, Terminatoin
Where does transcription occur?
In the nucleus
What happens in the initiation stage of transcription?
The initiation code is recognised (TATA). Transcription factors bind (upstream of the gene) and attract RNA polymerase to start mRNA production.
What does RNA polymerase do?
It separates the DNA strands for the RNA nucleotides to bind along the template strand.
What are the 5 kinds of RNA?
mRNA (messenger) rRNA (ribosomal) tRNA (transfer) miRNA (micro) noncoding RNA
What happens in the elongation stage of transcription?
RNA polymerase travels along the template strand, picking up base pairs and copying them onto a complimentary RNA strand to produce one stranded mRNA.
What happens in the termination stage of transcription?
The mRNA adds a methyl-guanine “cap” to the 5’ end and the 3’ end is polyadenylated (tailing)
How is the methyl-guanine cap bonded and why?
With a 5’ - 5’ triphosphate linkage bond to stabilise the mRNA
What is splicing?
The removal of introns from pre-mRNA to make mature mRNA.
What is the difference between an endonuclease and an exonuclease?
An endonuclease breaks within the polynucleotide but an exonuclease degrades the polynucleotide from one of the ends.
On which chromosome is the gene for PKU located?
Chromosome 12
What mutation causes PKU?
Single base substitution disrupting splicing and causing intron skipping.
What are the key features of mRNA?
It uses RNA Polymerase II
It makes up about 2% of our total RNA
There are 100,000s of kinds of mRNA
We only have a few copies of each kind
What are the key features of rRNA?
It uses RNA Polymerase I
It makes up >80% of our total RNA
There are only a few kinds
We have many copies of each kind
What is the difference between eukaryotic rRNA and prokaryotic rRNA?
Eukaryotic rRNa has 40s and 60s subunits making it 80s
Prokaryotic rRNA has 30s and 50s subunits making it 70s
What are the key features of tRNA?
It uses RNA polymerase III
It makes up about 15% of our RNA
There are about 100 kinds
There are many copies of each kind of tRNA
What is Ionosine?
It is an aspecific nucleotide (it complements A, C, G and U). It can be referred to as the wobble position.
When tRNA has a bound amino acid, what is it known as?
aminoacyl-tRNA
The genetic code is degenerate, non-overlapping and universal. What does this mean?
Degenerate = more than one code for each amino acid. Non-overlapping = it is read 123, 456 not 123, 234... Universal = it is the same in every organism
What is the code for the ‘start codon’?
AUG (methionine)
What are the 3 stages of translation?
Initiation, Elongation, Termination
What occurs in the intiation phase of translation?
At the 5’ cap, the 40s subunit with Met-tRNA binds as the starting codon is recognised (AUG). The 60s subunit then binds.
What happens in the elongation phase of translation?
More aminoacyl-tRNAs enter the ribosome to extend the protein.
Explain the significance of P sites and A sites in translation (elongation).
The P site is the site holding the peptide chain, the A site accepts the tRNA. The tRNA binds to the A site using GTP and forms a peptide bond with the tRNA in the P site, making it uncharged. The tRNA then leaves the P site and the ribosome moves along, moving the new tRNA into the P site from the A site (translocation).
Explain termination in translation.
A stop codon is read on the mRNA. No tRNA can bind to this codon so the peptide and tRNA are hydrolysed to release the protein into the cytoplasm.
Define gene.
A unit of heredity.
A transcription unit.
A length of DNA that contains the code for a protein.
List the main reactions involved in RNA maturation. (name 3)
Capping
Polyadenylation (tailing)
Splicing
How can substitutions in the genetic code affect a protein?
They alter the primary sequence so the tertiary structure can be affected.
How does a mutation affecting a stop codon affect a protein?
It can lead to a longer, malfunctional polypeptide chain.
How are bacterial cells different to eukaryotes in terms of gene expression?
Bacteria have:
- simpler promoters
- different transcription factors
- only one kind of RNA polymerase
- transcription and translation are coupled
- no post-transcriptional processing (capping, tailing, splicing)
- short lived mRNAs
- simpler ribosomes
- distinctive translation factors
How does the fact that bacterial cells have simpler ribosomes help us clinically?
It means that the 30s or 50 subunits can be attacked by antibiotics so they are bacteria-specific and don’t harm eukaryotes
What happens to the protein if a mutation causes a premature stop codon?
The protein is shorter and may not function.
Is a mutation from Glutamate to Aspartate more or less likely to have a profound affect than a mutation from Glutamate to Valine?
Less likely as glutamate and aspartate are both negatively charged amino acids.
What happens if a promoter region has a mutation?
Transcription factors may not bind so gene expression will be affected.
