Flashcards in L11 Molecular Biology of the Gene and Gene Expression Deck (35):
Almost all human genes possess a set of similar characteristics. Which of the following are common characteristics of genes?
b) Transcription 'start' and 'stop' signals
c) Exons and introns
d) Upstream regulatory regions
e) All of the above?
e) All of the above
Not all genes are expressed in all cells, all the time. How does a cell control which genes are expressed or not?
- Gene expression is driven by RNA polymerase II
- Transcription factors bind in and around the promoter region.
- Activators bind to enhancer sequences on DNA to significantly increase expression
Which enzyme drives gene expression?
RNA polymerase II
Where do transcription factors bind?
Promoter region of the gene
Where to activators bind on a gene to fine tune and enhance it's expression?
Upstream enhancer sequence
How many types of RNA polymerase are there?
Type I, II, and III
Which RNA polymerase is responsible for the production of the large ribosomal RNA?
RNA polymerase I
Which RNA polymerase is responsible for the production of mRNA?
RNA polymerase II
Which RNA polymerase is responsible for the production of tRNA and the small ribosomal RNA molecules?
RNA polymerase III
What is the role of RNA polymerase I?
Production of large ribosomal RNA
What is the role of RNA polymerase II?
Production of mRNA
What is the role of RNA polymerase III?
Production of tRNA and the small ribosomal RNA moleules
At which stage of DNA replication does the creation of mRNA, tRNA and rRNA take place? Which enzyme is responsible for producing these molecules?
During transcription RNA polymerase produces these molecules.
There are 3 types of RNA polymerase.
__1__ molecules collide randomly with DNA in the __2__ and bind with specific DNA sequences called __3__ (i.e. the __4__ sequence or __5__ box).
The RNA polymerase then opens up a short length of the DNA double helix, exposing a specific section of DNA on each strand.
One strand acts as a __6__ for base pairing with the incoming __7__.
1) RNA polymerase
7) RNA nucleotides
__1__ switch genes off in two ways:
1. They may bind next to __2__, thus blocking its function, or;
2. Their binding site may __3__ that of the __4__, preventing it from binding in the first place.
1) Transcriptional repressors
2) transcriptional activators
Wilm's tumour protein (WT1) is an example of a transcriptional [activator / repressor]. In the developing kidney, WT1 binds to the __2__ region of the EGR-1 gene (a transcriptional activator), switching __3__ expression.
Mutation of the gene that encodes WT1 can lead to uncontrolled expression of EGR-1 and can lead to development of kidney tumours early in life.
In this respect, the WT1 gene is considered to be a tumour suppressor gene.
What happens at the 5' end of the mRNA molecule during transcription?
5' end is capped by addition of methylated G nucleotide by removal of a phosphate by a phosphatase, addition of a GMP via a guananyl transferase, and the addition of a methyl group via a methyl transferase.
What happens at the 3' end of the mRNA molecule during transcription?
The 3’ end is cleaved at a specific site and a poly-A tail of up to 200 nucleotides is added by poly-A polymerase.
Are all of the codons that are transcribed into mRNA used?
No. There are coding regions (exons) and non-coding regions (introns). The introns are spliced out of the sequence.
What is the translational start codon?
This codes for methionine.
What are the translational stop codons?
What are the 3 stages of translation?
How is the half-life of mRNA related to the control of gene expression?
In cytoplasm, mRNA associates with ribosomes which translate the RNA sequence into a polypeptide chain (protein)
Many ribosomes attach to each mRNA (multiple bubbles)
In time mRNA is degraded (half-life)
The half-life of mRNA is a way in which the cell can regulate gene expression levels
What is EGR-1?
A transcriptional activator (promotes gene transcription)
Which transcriptional repressor switches off EGR-1 in the kidneys?
Wilms Tumour Protein
In which direction does nucleic acid synthesis occur?
5' to 3'
Nucleotides are only able to be added to the 3' end.
Where does translation take place?
On the ribosomes, mRNA provides the template
What provides the template for genetic translation?
How is genetic translation initiated?
An initiator tRNA carrying methionine associates with a small ribosomal unit in association with eukaryotic initiation factor 2 (eIF2).
This small ribosomal unit-tRNA complex recognises the 5' end of a mRNA (capped with eIF4E and eIF4G) and scans along the mRNA for a start codon. This allows the large subunit to bind, initiating translation.
There are three binding sites within the ribosome, these are the __1__ site, the __2__ site, and the __3__ site.
What are the 4 stages of translation elongation?
1) An aminoacyl – tRNA binds to the A site and the tRNA molecule at the E site is released
2) Carboxyl end of the polypeptide chain is uncoupled from the tRNA in the P site and joined by a peptide bond to the amino acid attached to the new tRNA molecule in the A site via a peptide transferase enzyme.
3) Large ribosomal sub unit steps one codon along the mRNA
4) Small ribosomal subunit steps one codon along so that the new peptidyl –tRNA in the A site moves to the P site as and the tRNA that occupied the P site of the ribosome moves 1 codon along the mRNA molecule and becomes the new E –site. This generates a new A site.
What mnemonic is helpful for remembering the order of the binding sites in translation elongation?
The elongation APE?
APEs have long arms?
Which molecules drive elongation?
Elongation factors EF1 and EF2
How is translation terminated?
When the ribosome encounters a stop codon (UAA, UAG, UGA), cytoplasmic release factors bind to the stop codon and free the carboxyl end of the growing polypeptide chain.