Flashcards in Protein Synthesis Deck (20):
What is pre-mRNA?
mRNA where the introns have yet to be spliced out to form functional mRNA.
Why do phospholipids from a bilayer?
They are cylindrical in shape, so it's the most effective way to avoid water (cholesterol is more of a wedge so slots in).
Why is one of the fatty acids in the membrane phospholipids unsaturated?
Unsaturated fatty acids have a lower melting point so the core of the bilayer is liquid, allowing substances to diffuse across.
What is the difference between euchromatin and heterochromatin?
Euchromatin is light in colour because it is open and unwrapped and used for transcription. Heterochromatin is dark in colour because it is tightly packaged and not being used for transcription. Different cells use different genes and have different areas of euchromatin and heterochromatin.
Which way does RNA polymerase move along the antisense strand of DNA?
3' to 5'.
(Carbon 3 has a hydroxyl group attached, carbon 5 has a phosphate group attached.)
The sense strand goes 5' to 3', and DNA polymerase moves 5' to 3'.
What is the general organisation of a gene?
At the 3' end on the antisense strand there will be a promoter, which is a DNA sequence that defines where transcription by DNA polymerase will begin.
The gene is made up of introns (non-coding regions) and exons (regions that code for the polypeptide).
What is required for transcription of the gene to be initiated?
Must be euchromatin.
Necessary transcription factors and DNA polymerase must bind to the promoter.
Lipid-soluble ligands and their cytosolic receptors can act as transcription factors.
What is the difference between the sense and antisense strand of DNA?
The sense strand contains codons, and the antisense strand contains anticodons.
How is mRNA stabilised?
1) The 5' end is capped.
2) The is polyadenylation of the 3' end - 100 to 300 adenine are added to form a poly-A tail, by the enzyme poly-A polymerase.
What regulates the amount of mRNA, and therefore protein (how is mRNA destabilised)?
microRNA (small, non-coding RNA) destabilise mRNA and so reduce the level of protein.
What is it called when mRNA is destabilised by microRNA?
Why is the DNA code a degenerate code?
There's more than one codon for each amino acid - means not all mutations will result in a change in the amino acid sequence. This makes the code robust as a mutation might not produce a physiological effect.
What is the function of a ribosome?
The large subunit catalyses the formation of a polypeptide.
(Ribosome moves from 5' to 3'.)
Where does the ribosome read from and to, and what is that region called?
From start codon (AUG) to stop codon = open reading frame (ORF).
What makes the ribosome fall off at the stop codon?
The stop codon is recognised by a releasing factor which causes the ribosome complex to fall apart and terminate translation.
Why can antibiotics stop protein synthesis (transcription inhibitors) in bacterial cells without harming animal cells?
Bacterial ribosomes are structurally and functionally different (70S not 80S).
How are proteins modified in the Golgi body?
Glycosylation (addition of sugar residues).
Ubiquitination (addition of ubiquitin which marks an abnormal protein for degradation).
How are proteins localised?
Proteins that need to move across the nuclear envelope are given a nuclear localisation signal peptide in their sequence.
Proteins that are either secreted from the cell, or inserted into the plasma membrane are given an endoplasmic reticulum signal sequence.
What is the start codon?