Lecture 30 - Gene Expresion: RNA Translation and Packaging Flashcards
Mention the three RNA processing steps and explain briefly their roles.
5’ capping: addition of the 7-methyl guanosine cap protects RNA from degradation by nucleases and is recognised by the translation machinery
3’ polyadenylation: Poly(A) tail addition to enhance RNA stability and regulates nucleus-to-cytoplasm transport
RNA splicing: removal of introns (intervening sequence) and joining of exons (expressed sequence)
How is it possible that different mRNAs can be produced from the same set of genes?
Some genes allow for alternative splicing, which allows slightly different mRNAs (hence proteins) to be made from the same gene.
Mention 2 examples of alternative splicing.
Exon skipping: leave out an exon
Mutually exclusive exon: multiple slight different exon options
alternative 5’ splice donor sites: multiple options of splice start
alternative 3’ splice acceptor sites: multiple options of splice end
Explain how alternative splicing plays a role in the sex determination of Drosophila flies.
Numerator dimers are present in females due to the excess in gene products of the X chromosome. The presence of these dimers allow for the transcription of the sex-lethal protein in females, not males. Sxl proteins leads to alternative splicing in the tra (transformer) gene sequence, bypassing the initial stop codon. In male, since no Sxl protein is present, the transcription is stopped prematurely, hence no expression of the tra protein.
Tra protein itself leads to another alternative splicing for the transcription of the DSX-protein which causes female differentiation in Drosophila.
What is the genetic code? What does it mean to be synonymous codons and wobble?
The genetic code is a set of instructions that direct the translation of DNA into 20 amino acids.
Synonymous codons are codons that encode the same amino acids, while wobble of the code describes codons where the last base is irrelevant.
What are the stop codons?
UAA, UAG, UGA
Why does non-overlapping genetic codes allow for a greater flexibility in amino acid options?
Overlapping genetic codes means the next set of codons would have its first two base sequence determined, leading to less variability in the options. Non-overlapping sequence allow for the code to be read in repeats of three, hence allow a much more higher variability.
Briefly explain the 3 key processes of translation.
Initiation: Ribosome finds and binds to start codon on mRNA
Elongation: polypeptide chain grows by addition of individual amino acids carried to the ribosome by tRNA
Termination: translation process reaches the stop codon and it dissociates
Mention the three sites involved in the process of translation elongation. Explain their function.
P site (Peptidyl-tRNA binding site): codon recognition/matching between anticodon and codon
A site(Aminoacyl-tRNA binding site): peptide bond formation and amino acid translation
E (Exit Site): exit of uncharged of tRNA and translation machinery progress
What are the different types of gene expression regulation?
- DNA accessibility
- Transcription factors (inhibition/promotion)
- Processing
- Transport into the cytoplasm
- mRNA stability or interference RNA (siRNA or miRNA)
- Translation initiation
- Protein folding, stability, activity
Differ between prokaryotic and eukaryotic translation initiation.
In prokaryotes, the initiation factor and small ribosome unit finds and binds to the ribosome binding site (Shine-Dalgarno sequence). fMet-tRNA then binds to the start codon, after which the initiation factor dissociate allowing binding of large ribosomal unit and translation to start.
In eukaryotes, the pre-initiation complex finds and binds to 5’ cap, and THEN move along the mRNA to find the consensus Kozak sequence. Finally, the initiation factor dissociate allowing the binding of the large ribosome unit (forming initiation complex)
