Lecture 6 Flashcards
What is a gene?
A DNA sequence that is transcribed
What does mRNA do?
Carries the genetic message to the site of protein synthesis
Features or mRNA
Encodes protein sequence
Most are rapidly degraded by nucleases
Single stranded
Same “language” as DNA
Thymine is replaced with uracil however
Several mRNA strands made as transcription proceeds
Short half life - any mRNA has to be renewed if protein is continuously needed
Features of rRNA
Ribosomal RNA
Makes up much of the ribosome
Site of protein synthesis
Very stable, majority of cellular RNA
Features of tRNA
Carries amino acids to ribosomes for translation
Very stable molecules
There is at least one, usually more, for each amino acid
How is transcription carried out?
One strand of ds DNA is transcribed
- anti sense (template) strand copied 3’-5’ direction
- sense (non-template) strand is not copied
The RNA strand is synthesised in the 5’-3’ direction
Sequence of the new RNA strand is - complimentary and anti parallel to the antisense strand of DNA
- identical to that of the sense strand except for T in DNA and U in RNA
What is RNA polymerase?
Enzyme that catalyses DNA-directed RNA synthesis
- this is transcription
- enzyme reads DNA to make RNA
- processive elongation of RNA chain (one nucleotide at a time)
Part of larger transcription complex
- assembles when transcription is initiated
DNA is unwound and rewound in the process
- RNA polymerase does the unwinding
- different from DNA replication
What is the function of the following parts of RNA polymerase Alpha x2 Beta Beta' W
Alpha x2 - scaffolding and interaction with other proteins that regulate transcription; involved in assembly of the enzyme and possibly catalysis
Beta - catalytic site
Beta’ - DNA binding
W- not well characterised; may be involved for assembly of the multi subunit structure of RNA polymerase
Difference between RNA polymerases in prokaryotic and eukaryotic cells
Prokaryotic - single polymerase for all functions
Eukaryotic - 5 different polymerases
- pol I - makes 5.8S, 18S and 28S rRNA
- pol II- makes mRNA
- pol III - makes tRNA and 5S RNA
- mitochondrial and chloroplast - resemble prokaryotic RNA polymerases
What does the RNA polymerase need?
Substrates
Template
Does not need a primer
Where does transcription start?
Promoter - start sequence on DNA that is not transcribed - upstream 5' of coding region - contains consensus sequences - place where proteins bind to DNA RNA polymerase - regulators of transcription initiation
What should a promoter sequence be? How and why do promoters vary?
Similar to the consensus
Strong promoters - match consensus closely
Weak promoters - match consensus sequences poorly
Promoter strength rises - transcription frequency rises
Why - some proteins are needed in greater abundance
Features of transcription start site
Starts at specific nucleotide
- nucleotide denoted as +1
- +1 is a purine in 90% of genes
- G is more common than A
- bases on either side are often C or T (CGT, CAT)
What are the three stages of transcription?
Initiation - the chain is started
Elongation - the chain gets longer
Termination - the chain is finished
How is initiation of transcription carried out?
RNA polymerase holoenzyme binds to promoter
DNA unwinds - DNA helix unwind at the AT rich promoter and transcription bubble is formed
The chain starts with a GTP/ATP
- RNA polymerase catalyses the stepwise addition of each ribonucleotide
The first 9 ribonucleotides are added - at this point anortion rates are high
- the holoenzyme subunit leaves the complex
