Protein synthesis Flashcards
1
Q
What does UTR stand for?
A
• UTR = untranslated region
2
Q
Describe the coding regions and what the UTRs do
A
- The coded region of the mRNA is subdivided into segments of 3 (codons)
- In the middle of the mRNA structure, there is the middle which is the coding region but on either sides are untranslated regions (UTRs), so there is a 5’ and 3’ untranslated region.
- These UTRs do not code for anything are very important: the 5’ UTR determines the rate at which the protein is synthesised and the 3’ UTR tends to affect the stability of the RNA (how long before it degrades.)
3
Q
How are the 5’ cap and 3’ poly-A tail produced?
A
• Then there are the 5’ cap and 3’ polyadenylated tail, which are not coded for by the genes but are done through enzymatic processes, on the primary transcript.
4
Q
Codons specify for anticodons. These anticodons correspond
A
tRNA molecules that carry amino acids
5
Q
Describe the genetic code
A
- As triplet codons are used there are four possible bases it means 4x4x4 combinations = 64 possible codons. However, there are only 20 amino acids. This means the genetic code is degenerate.
- There are also stop codons which tell the ribosome to stop translating
- There is a great amount of generality in the genetic code (with few notable variations), which supports evolution. As this genetic code is used by most biological organisms.
6
Q
Describe ribosomes
A
- Ribosomes are made from rRNA (gold) and protein (grey). They are ribonucleoprotein complexes.
- They can be found free in the cytosol or bound to the endoplasmic reticulum, forming the RER.
- Protein synthesis occurs at the cleft between large and small subunits
- Ribosomes adds 20 amino acid chains to polypeptide chain per second
7
Q
Describe the components of a ribosome
A
- Mammalian ribosomes consist of 40S (small sub unit) and 60S (large subunit) together being 80S ribosome (S: is a unit of sedimentation, not linear, hence why not 100S, it is a measure of density). These assemble in the initiation stage of protein synthesis.
- 40S:18S rRNA (1900 nucleotides) and 33 proteins
- 60S: 5S, 5.8S and 28S (120, 160 and 4700 nucleotides and 49 proteins)
8
Q
Describe the 80S ribosome
A
- There are 3 parts of the subunit: exit, peptide and aminoacyl parts
- As the polypeptide is synthesised it gets extruded out of the peptide groove in the ribosome. The amino acyl groove is where the next amino acid comes in and sits. The exit site is where the consumed tRNA leaves (lost AA) leaves.
9
Q
Compare eukaryotic and prokaryotic ribosomes
A
On image
10
Q
Describe the antibiotics that block protein synthesis in ribosomes
A
on image
11
Q
Describe the stages in protein synthesis
A
- Initiation – assembly of the ribosome on mRNA together with the first initiator tRNA: aminoacyl tRNA
- Elongation – charged tRNAs are delivered to the ribosome and addition of amino acids one at a time building the polypeptide
- Termination – encounter with a stop codon and end of protein synthesis and release of polypeptide. Also involved in the dissociation of ribosome from RNA.
12
Q
Describe initiation
A
- Initiation factors bind to cap and polyA tail (PolyAiBP binds to AAAA…). 40S ribosomal subunit is associated with EiF3, Dissociates with EiF2-GTP (GTP is bound), which is associated with met-tRNAi (initiator). These elements combine on the ribosomal unit forming 43S RSU. 43S RSU binds to [mRNA + IF] to form a 48S pre-initiation complex, RNA is bound and associated with small RSU, with all IF (Including met- tRNAi). Met-tRNAi joins P site on the ribosome.
- RNA translocates along RSU, as a consequences ATP is consumed and broken down into ADP + Pi.
- Every time the ribosome translocates along a single base on the RNA, a single molecule of ATP is consumed.
- Scanning -> ribosomes start at cap site along RNA until at some point the met-tRNAi recognises the start codon and scanning.
- met- tRNAi occupies P site and RNA is ready for next stage of initiation.
- EiF2 hydrolyses GTP forming GDP + Pi and EiF3.
- This allows the 60S RSU to join with the 48S making 80S.
- Initiation of elongation during translation.
- The second aminoacyl tRNA joins ribosome enters the A site with elongation factor 1. This forms a peptide bond between two amino acids this results in translocation occurring along the 80S RSU.
13
Q
Describe elongation
A
- Polypeptide chain has elongated by a single amino acid shift of RNA through the ribosome.
- tRNA occupies the p site which has attached the elongated polypeptide chain.
- Leaves from e site of the spent tRNA.
- tRNA must be recharged for it to be used, aminoacyl tRNA synthase recharges this. This uses ATP producing a recharged tRNA + ADP +Pi
- The corresponding tRNA that corresponds to a specific codon in A site can now enter ribosomes, facilitated by EF1 and GTP hydrolysis allowing the transfer of amino acids to polypeptide chain using peptidyl transferase, translocation occurs via translocase, using EF2 and hydrolysing GTP.
- Spent tRNA in e site and the new attached tRNA in p site.
- In order for the ribosome to produce sufficient protein for the cell to survive, a lot of protein needs to be produced from a single message therefore the translocation of ribosome along RNA. As it progresses we get elongation of a polypeptide chain. Multiple initiation processes occur, not only one ribosomes carries out protein synthesis, many ribosomes carrying out protein synthesis on ribosomes
- The association of RNA with multiple ribosomes is called a polysome (in electron micrograph image.)
14
Q
Describe termination
A
- Stop codon enters A site and is recognized by release factor protein rather than Trna. Binds to A site so protein synthesis stops. This stimulates hydrolysis of terminal peptidyl trna and the release of polypeptide chain from the ribosome.
- This uses energy so it hydrolyses GTP making GDP and Pi and new formed protein and trna and release factor
- At termination of translation the 2 ribosomal units dissociate from RNA (break up into two subunits).
