Protein synthesis Flashcards
What specifies the particular order of amino acids in the polypeptide chain?
The codons in coding region on mRNA specifies the sequence of amino acids
How many possible combinations of triplet codon can there be?
64 codons
If 3 bases form a codon and there are 4 possible codons then 4x4x4 = 64 codons
If there are 64 codons then why are there only 20 amino acids?
This means more than one codon can code for the same amino acid.
There’s some redundancy within the genetic code.
Why is there some redundancy within the genetic code?
protects against deleterious effects of mutations and so makes them “silent mutations.” Silent mutations are mutations that don’t have an observable effect on an organisms phenotype.
What are the 2 populations of ribosomes that carry out protein synthesis?
one population bound to endoplasmic reticulum while the other population is free within cytosol.
Where exactly does protein synthesis occur within the ribosome?
occurs within cleft between the 2 ribosomal subunits
How long does it take for a ribosome to add an amino acid to a polypeptide?
ribosome able to add 20 amino acids every second
Why does a cell need multiple ribosomes performing protein synthesis ate the same time?
although protein synthesis is fast the turn over of proteins within cell is also rapid. This means to avoid proteins that the cell needs from being lost the cell needs multiple ribosomes performing translation.
What is the composition of the 80S ribosome?
Contains a 60S subunit and a 40S subunit
What is the composition of the 60S ribosomal subunit?
Contains 3 different rRNA’s: 5S, 5.8S and 28S
also contains 49 different proteins
What is the composition of the 40S ribosomal subunit?
Contains 18S rRNA also contains 33 different proteins
What are the 3 sites formed within the cleft of the 80S ribosome once it has been formed?
- Peptidyl binding site (P site)
- Aminoacyl tRNA binding site (A site)
- Exit site (E site)
What are the 3 stages of translation? briefly describe what occurs during each stage
Initiation: Assembly of the ribosome on mRNA together with the first initiator tRNA (met-tRNAi)
Elongation: Charged tRNAs delivered to the ribosome
and add amino acids one at a time building the polypeptide
Termination: Ribosome encounters a stop codon causing end of protein synthesis and release of the polypeptide
Describe the process of Initiation
- Cap binding proteins associate with 5’ cap of mRNA while poly A binding proteins associate with poly A tail. This forms a circular complex.
- 40S ribosomal subunit becomes associated with multiple initiation factors, e.g. eIF3
- eIF2 associates with GTP-bound met-tRNAi (met-tRNA initiator).
- 40S ribosomal subunit and met-tRNA combine together to form the 43S ribosomal subunit
- 43S ribosomal subunit then combines with circular mRNA complex to form the 48S pre-initiation complex. (met-tRNA binds to the P site on the ribosome).
- Starting at Cap site, ribosome translocates one codon along mRNA. The met-tRNA initiator scans for start codon.
- Once start codon recognised, scanning stops.
- eIF2 then hydrolyses GTP to form GDP and Pi. At the same time eIF3 is released.
- This allows 60S ribosomal subunit to join 48S pre-initiation complex to form the 80S ribosome.
- Elongation factor 1 then brings 2nd Aminoacyl tRNA (carrying correct amino acid) to the 80S ribosome where it then enters A site
- Peptide bond forms between the 2 amino acids and the 80S ribosome then translocates one codon along the mRNA.
Describe the process of elongation
- “Spent tRNA” exits the ribosome through the E site
- Once it has exited the spent tRNA is then recharged. The reaction that does this is catalysed by an enzyme called Aminoacyl tRNA synthase. Enzyme consumes ATP so the reaction produces AMP and pyrophosphate (PPi) as well as the recharged tRNA. (Recharged tRNA is one that has amino acid bound to it).
- Recharged Aminoacyl tRNA then binds to the A site of the 80S ribosome.
- Polypeptide chain is then transferred to new amino acid via formation of a peptide bond. The enzyme that catalysts this bond formation is Peptidyltransferase
- mRNA then is translocated one codon along. As well as this the tRNA molecules are also translocated one along so that the spent tRNA is at the E site of the ribosome. The enzyme that facilitates this is Translocase.
