Lecture 8: proteins and how they're made Flashcards
Name the three processes involved in processing mRNA
Capping, tailing and splicing
where does splicing occur
In the spliceosome: a large complex of proteins and small RNAs
How does splicing occur and what short base pair sequences determine splicing ?
Introns are removed from the transcript and then the exons are rejoined to form mature mRNA. Donor and acceptor sequences near the ends of the introns determine splicing
How can so many proteins be coded for by so little genes?
Alternative splicing allows for multiple gene products from the same gene.
mutations in the splice sites can affect
generation of gene products
What determines the final structure - therefore function
Protein sequence
How many possible codons, with how many code for amino acids. but why are there only ~40 trnas?
64, 61. tRNAs can base pair to more than one codon- wobble hypothesis
What are the three main steps of translation
Initiation, elongation and termination.
Translation initiation requires energy for assembly. (GTP) Describe the assembly
- Small ribosomal unit with initiator tRNA already bound binds 5’ cap of mRNA. 2. Small ribosomal subunit scans downstream to find translation start site.
- H bonds form between initiator anticodon and mRNA. Large ribsomal subunit then binds.
What is the amino acid on the initiator tRNA
methionine
Translation elongation is a 3 step cycle, what is the 3 steps
Codon recognition, peptide bond formation, translocation.
If the mRNA chain is has the 5’ to 3’ in this direction on the ribosome, what is the order of the sites?
EPA
Which two steps of elongation require GTP as an energy source
codon recognition and translocation
describe translation elongation: codon recognition,
The initiator RNA sits in the P site and the then the anticodon of another tRNA recognises the next codon, slotting in to A site.
describe peptide bond formation in translation elongation
A peptide bond forms between the two amino acids catalysed by the ribosome, and removes the amino acids from the tRNA in the P site to the one in the A site.
describe translocation in translation elongation
tRNA in P site moves to E and is released. tRNA dragging polypeptide chain moves from A to P site
What enzyme ‘reloads’ empty tRNAs in the cytoplasm
Aminoacyl-tRNA synthetase
Describe the termination of translation
- Ribosome reaches a stop codon on mRNA which results in the binding of a release factor. 2.This promotes the hydrolysis of the bond between the p-site tRNA and the last amino acid, releasing the polypeptide.
- 2 GTP is used to dissociate recyclable ribosomal units
Describe the basic structure of an amino acid and what determines the properties of the amino acid.
It has a central carbon bonded to an amino group, H and carboxyl group with the last being the R group. These side chains determines the properties of the amino acids.
What are the different properties a side chain can confer to the amino acid
hydrophobic, hydrophilic (electrically charged/ polar)
describe primary structure of protein
linear chain of amino acids. monomers forming polymers
secondary structure of protein
H- bonding between atoms of the polypeptide backbone. weak bonds over a large region result in a repeating folding pattern.
two main secondary structures?
alpha helix and beta sheets
describe tertiary structure of protein
Overall 3D shape of a polypeptide stabilised by weak interactions between side chains (H or ionic bonds/ hydrophobic interactions) reinforced by strong COVALENT disulphide bonds
describe quartenary structure of protein
two or more polypeptides associate with each other to achieve a single functional macromolecule. Maybe be homo : same subunits or hetero.
protein folding is complex and may require the assistance of
chaperone proteins
how many amino acids are coded for
20
Secretory proteins or membrane anchored proteins are processed in a different way to others. Describe the process of their peptide translation
- Polypeptide synthesis begins at a cytoplasmic ribosome.
- When the signal peptide emerges it is recognised by signal recognition particle which docks to SP and stops elongation.
- The SRP then binds to receptor protein on ER membrane.
- SRP detaches and polypeptide synthesis resumes. 4. The Signal cleaving enzyme cuts off the signal peptide.
Step 5
What is the fancy name for the start of a protein chain
N terminus
What is step 5 of the processing of secretory proteins
Completed polypeptide folds into final conformation. signal sequence cleaved solubilises the protein in the ER lumen.
What is step 5 of the processing of membrane proteins
Completed polypeptide folds into final conformation. One or more hydrophobic segments of the polypeptide chain anchor it in the bilayer ER membrane.
After the signal peptides involvement in processing proteins. Where do the proteins go before going to the plasma membrane or targetted by lysosomes?
Both types of proteins are moved via transport vesicles to the Golgi where they are further processed and carbohydrates may be added. The proteins are packaged into vesicles at the Golgi trans face
When proteins are targeted by lysosomes what do they do then?
They serve the degradation of nutrients or faciliate the renewal of the cell own macromolecules.
What are post translation modifications, where do they occur.
occurs after folding, in the golgi or the cytosol. Makes the protein functional with the addition of groups like biotin, phosphate, methyl, carbohydrate. Can be cleavage or ubiquitination.
What is the purpose of post translation modifications
They can confer activity, ability to interact with other molecules or direct to a particular location.
This provides another level of temporal control-> only modify when needed because it needs it to be functional
Mutations can affect the structure and function of a protein. Where are two places it happens and what scale
Germline can affect many cells, local during cell division. Large scale: chromosomal rearrangements. Small scale alterations: one or few nucleotides altered.
two types of point mutations
substitution insertion/deletion
silent mutations are when
nucleotide is swapped but it still codes for the same amino acid
missense mutation
This is when substitution means a different amino acid is coded for so major affect if this residue is critical for function
nonsense mutation
substitution that codes for UAG codon. Protein is truncated, effect can be major if functional regions now absent/ misfolded.
frameshift mutation via insertion / deletion
protein altered from point of frameshift, can have major effect because all amino acids afterwards altered.
3bp deletion
causes the removal of one amino acid.
What are other errors that cause non functional proteins not point mutations
mutations in the donor and acceptor regions of mRNA can affect splicing. modification errors
transcription errors.
What is an exon and what is an intron
An exon is a coding region whereas an intron is random noncoding region.
