Session 5 - Transcription and Translation Flashcards
Define transcription
The process by which DNA is transcribed into an RNA message
What are the four stages of transcription
Initiation, Elongation, Termination and Splicing
What occurs at initiation stage of transcription? (4)
- Intitiation code is recognised 5’TATA3’
- Transcription factors (regulating proteins) bind at this code, upstream of the fene
- RNA polymerase is attracted and starts mRNA production
- RNA polymerase ‘separates’ the DNA strands for the RNA nucleotides to bind
What does RNA polymerase do during process of elongation?
Travels along template stand, picking up base pairs and copying them onto a complimentary RNA strand.
How long does process of elongation continue?
Until genetic sequence is ‘transcribed’ onto the mRNA molecule
What two things happen at termination stage of transcription?
Methyl-guanine ‘cap’ added to 5’ end
Polyadenylation occurs
How is the methyl-guanine ‘cap’ added to mRNA, and what is its function?
Methyl-guanine bonded with 5’ - 5’ triphosphate linkage to stabilise the mRNA
What is polyadenylation of mRNA and what is its function?
The addition of lots of adenine nucelotides added at the 3’ end of mRNA. Improves stability.
What happens during splicing, and how does this process occur?
Introns are removed. Endonucleases remove introns within polynucleotide and exonucleases degrade polynucleotide for 5’ or 3’ end.
Define translation
The process by which the RNA message is translated into amino acid code
Answer following for mRNA What type of polymerase used? Wht percent of all RNA does it make up? How many different kinds are there? How many copies of each present?
RNA polymerase II
-2%
100,000s of kinds
A few copies of each present
Answer following for rRNA What type of polymerase used? Wht percent of all RNA does it make up? How many different kinds are there? How many copies of each present?
RNA polymerase I
>80%
Few kinds
Many copies of each
Answer following for tRNA
What type of polymerase used?
Wht percent of all RNA does it make up?
How many different kinds are there?
RNA polymerase III
15%
100 kinds
Very many copies of each
What is Ribosomal RNA used for?
To bind to mRNA and provide the location for tRNA
What is the differnce between eukaryotic and prokaryotic ribosomes?
Eukaryotes - Ribosome 80s (made up of 60s and 40s subunits)
Prokaryotes 70s - (made up of 50s and 30s subunits)
How is the genetic code read in a ribosome?
In triplets, with no overlapping and no gaps
What is meant by saying that DNA is degenerate?
Some amino acids can be coded for by several different codons
When is tRNA uncharged, and what is it referred to when it is charged?
When no amino acid is bound to it. Is referred to as an aminoacyl-tRNA when amino acid bound and it is charged
Give following for DNA replication Enzyme Activated substance Template 3 stage process?
DNA polymerase
dNTPs
DNA
yes
Give following for making mRNA (transcription) Enzyme Activated substance Template 3 stage process?
RNA polymerase
NTPs
DNA
yes
Give following for making a polypeptide (translation) Enzyme Activated substance Template 3 stage process?
ribosome
Amino acid
mRNA
yes
Outline following for ribosomes in prokaryotes Number of rRNAs Protein number Subunits Size of ribosome
3 rRNAs
56 proteins
30s, 50s
70s
Outline following for ribosomes in eukaryotes Number of rRNAs Protein number Subunits Size of ribosome
4 rRNAs
82 proteins
40s, 60s
80s
Name three stages of translation
Initiation
Elongation
Termination
What occurs at initiation stage of TRANSLATION
- 40s subunit of ribosome binds with Met-tRNA attached binds at the 5’ cap end of the mRNA
- the 60S subunit then binds and elongation occurs
What starting codon must be recognised for translation to begin? What amino acid does it code for? What is the anticodon found on tRNA?
5’ AUG, which is a specific codon to one amino acid - methionine. 5’ CAU is the anticodon required.
What are the two ribosomal sites at which tRNA binds, and what goes where?
P site - For the site holding the peptide chain
A site - for the site accepting the tRNA
What are the first two steps in the process of translational elongation, and what do they require?
1 - Met-tRNA occupies the P site
2-Another aminoacyl-tRNA enters the ribosome to occupy the A site (requires GTP)
What are the third and fourth steps of translational elongation?
3- Methionine forms a peptide bond with the next aminoacyl - tRNA, making original tRNA in the P sit now uncharged
4 - The uncharged tRNA now leaves and the ribosome translocates (moves along)
What is the binding of two amino acids catalysed by?
Peptidyl Transferase
How does the termination of translational elongation occur?
Requires a stop codon to be read on the mRNA.
Give three stop codons, and explain why they halt process of translation
5’UAA, 5’UAG and 5’UGA.
No tRNAs that can bind to these codons, so the peptide and tRNA are hydrolysed and release the protein into the cytoplasm.
Define the term “gene”
A unit of heredity and a length of DNA on a chromosome that contains the code for a protein (or RNA) as well as sequences necessary for its expression, such as promoter and terminator sequences and introns
What are the three major reactions involved in the processing of Eukaryote processing
Capping, polyadenylation and splicing
What is the 3’ molecule in a DNA triplet called a wobble position
Change in third base more likely to cause change in amino acid
What does 5’ to 3’ template provide?
N to C polypeptide chain extension
Give three features of DNA triplet code
Degenerative
Non overlapping
Read in 5’ to 3’ prime direction
What can substitutions in genetic code lead to?
Different primary amino acid sequence, which can affect the tertiary structure of a protein
What can be a large effect of a single base substitution?
Can occur in a stop codon, leading to longer polypeptide chain that will be malfunctional
Give the nine key differences between mammalian and bacterial gene expression
Prokaryotes have - Simpler promoters - Different transcription factors - Single RNA polymerase - Coupled transcription - translation - No post-transcriptional processing - Short lived mRNAs - Simpler ribosomes (exploited by attacking 30s subunit - Distinctive translation initiation mechanism -Different translation factors
What does the severity of a mutation depend on?
The amount of difference in the protein caused
How can mutations outside the coding region affect gene expression?
Mutations to promoter regions where transcription facts bind can affect gene expression, to either constituively activate or deactivate it
