Coding Life: Reading the instructions of the cell Flashcards
functions of RNA
Store information
Act as an enzyme
Properties of DNA and proteins
RNA world hypothesis
RNA was the first information-storage unit
Used in DNA replication, transcription and translation
Evolved overtime to act as catalysts
RNA structure
Polymer of nucleotides linked by phosphodiester bonds
3’ OH
5’ phosphate
Ribose sugar (extra OH-) makes it unstable
Base uracil replaces thymine
5’ is usually triphosphate not monophosphate
Usually shorter
RNA in the cell are single stranded (form 3D structures by folding for stability)
how does template DNA make complementary RNA? RNA polymerase and the non-template strand
DNA is unwound
One strand is used as a template for the synthesis of an RNA transcript (complementary)
Happens by polymerisation of ribonucleotide triphosphate
RNA polymerase: carries out polymerisation
Adds successive nucleotides to the 3’ end
Called Pol II for transcription of protein-coding genes in eukaryotes
non-template strand: not used
Strands grow 5’ to3’ (3’ direction)
Runs opposite direction to the template
initiation in transcription- promoters, terminators, housekeeping genes, sigma factor, general transcription factors, transcriptional activator protein, mediator complex and initiation
Promotors: RNA polymerase and associated proteins bind.
Often a TATA box sequence (5’ TATAA 3’)
First nucleotide is is 25 base pairs from TATA box
RNA polymerase moves 3’ to 5’
Terminators: transcription stops here
Transcript is released
Housekeeping genes: required all the time. Transcription is not stopped.
Sigma factor: mediates promoter recognition in bacteria.
General transcription factors: involved in promotor recognition in eukaryotes.
At least 6 proteins acting on the promotor
Necessary for transcription to occur (also requires transcriptional activator proteins)
Transcriptional activator protein: bind to a specific DNA sequence known as an enhancer and help the cell control when transcription occurs.
Bind to enhancers in or near the gene and also bind with proteins that allow transcription to begin
Controls gene expression
Mediator complex: attracted or recruited by transcriptional activator proteins to then recruit RNA polymerase to the promotor.
A loop in DNA may be needed as enhances can be anywhere
Initiation: once the mediator complex and Pol II are in place, transcription begins.
elongation in transcription
Elongation: the adding of successive ribonucleotides
Takes place in a bubble where the DNA duplex strands are separated and RNA-DNA duplex forms
Bubble is 14 base pairs in bacteria and RNA-DNA duplex is 8
Bonds between phosphates are phosphate bonds - when cleaved provides energy for phosphodiester bond to form
Cleavage makes the polymerisation reaction irreversible
RNa polymerase - what it is, what it does, incorrect
Multi-protein complex
Transcription occurs inside it
Separates the strands, adds the nucleotides, restores the helix etc.
1 incorrect nucleotide in 10000
primary transcript compared to messenger RNA
Primary transcript: RNA transcript that comes off the template DNA strand.
Contains info ribosome needs
Messenger RNA: the molecule that ribosomes combine with.
mRNA in prokaryotes
Prokaryotes:
Primary transcript is mRNA
Ribosomes can bind to the 5’ end as the 3’ end is being synthesised
No nuclear envelope separating tasks
Polycistronic mRNA: can code for multiple proteins
RNA in eukaryotes and processing
Eukaryotes:
RNA processing occurs in the nucleus which converts the primary transcript into mRNA
RNA processing:
5’ cap is added (7-methylguanosine) - essential for translation as Ribosome uses it to recognise mRNA and attach
Polyadenylation - addition of 250 A ribonucleotides forming a poly A tail - role in export
First two help with stabilisation
Introns (non-coding) are removed (RNA splicing) and exons (coding) remain - spliceosome does this
In splicing, after one cut, a lariat (or loop) forms. After the second cut it is freed to be broken down in the cytoplasm
alternative splicing
Presence of introns allows for this
Primary transcripts for the same genes are spliced to yield different mRNA and therefore different proteins
RNA transcript processing
Not all become mRNA
Often produced by a RNA polymerase different to Pol II
rRNA and tRNA are most common - needed to synthesis proteins
other types of RNA
rRNA: found in ribosomes that aid in translation. Genes and transcripts for rRNA are in the nucleolus in eukaryotes.
tRNA: carries individual amino acids in translation.
snRNA: found in eukaryotes and involved in spicing, polyadenylation and other processes.
siRNA and miRNA: can inhibit translation or cause destruction of an RNA transcript.
