DNA Transcription Flashcards
Transcription
Catalysed by RNA polymerase.
Separate strands of duplex DNA- acres to template (non-coding strand).
Needs nucleotides as substrate.
Energy comes from hydrolysis of phosphate groups.
Initiated when RNA polymerase binds to dsDNA.
dsDNA is transient my unwound to make the transcription bubble. Newly formed RNA Makes a hybrid DNA- RNA molecule- new RNA is release from the template at the transcription bubble moves
Transcription promoters
RNA polymerase binding site in the dsDNA is the promoter. RNA polymerase moves from the 3’ to 5’ along the template strand and the new RNA is synthesised from the 5’ to 3’ end.
RNA polymerase binds at the promoter sequence.
Binding is between DNA and a sigma subunit, different sigma subunits recognise different promoter sequences.
Promoters share some common sequences called consensus sequences.
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Factors impacting the effectiveness of transcription
Transcription efficiency is controlled by how well RNA polymerase binds to its promoters.
Tight binding if RNA pol = strong promoter.
Closer the sequence of the two regions -35 region and the -10 region to the consensus sequence, the stronger the binding of RNA oil
Mutations in the -10 or -35 region leads to decreased binding.
Mechanisms of transcription: initiation
Binding- DNA is in a closed complex when the RNA pol binds to the dsDNA through the signs subunit.
Open complex- FNA unwinds into an open complex from -10 to +2
Elongation- RNA synthesis now begins
Promoter clearance- RNA pol passed the promoter and the sigma dissociates
Mechanisms of transcription: elongation
The core complex (sigma removed) binds more tightly once sigma is removed and moves along the non-coding template strand of the DNA from 3’ to 5’. RNA polymerase has no proofreading, and so has an error rate of 1 bad pair per 10000, so RNA is degraded and replaced rapidly.
Mechanisms of transcription: termination
Occurs when RNA polymerase pause at a specific sequence. The sequence of the new RNA makes a stem-loop structure which causes the complex to pause.
RNA processing
Protein synthesis can occur even before transcription is complete.
A single RNA is the template for a number of different proteins. All mRNA’s have a 5’ leader sequence which does not code for an amino acid sequence. It lies upstream (5’ end) from the coding sequence and includes a binding element for the ribosome. The coding sequence begins with an AUG triplet.
Post transcriptional processing in eukaryotes
The 5’ end of a new mRNA is capped with a methyl guanosine nucleotide through a 5’ linkage.
mRNA which lack a polyA tail are rapidly degraded. An enzyme complex recognises the consensus sequence AAUAAA at 3’ end of an mRNA transcript and truncated the mRNA. Then polyadenylate polymerase adds 80-250 A bases to 3’ end.
Splicing- eukaryotic genes are discontinuous. Introns are removed from primary RNA transcript to produce mature mRNA ready for translation.
Post transcriptional processing in eukaryotes
The 5’ end of a new mRNA is capped with a methyl guanosine nucleotide through a 5’ linkage.
mRNA which lack a polyA tail are rapidly degraded. An enzyme complex recognises the consensus sequence AAUAAA at 3’ end of an mRNA transcript and truncated the mRNA. Then polyadenylate polymerase adds 80-250 A bases to 3’ end.
Splicing- eukaryotic genes are discontinuous. Introns are removed from primary RNA transcript to produce mature mRNA ready for translation.
Splicing
Either:
Catalysed by slices ones which are small nuclear RNA’s in complex with small nuclear ribonucleoproteins.
Self splicing (introns cleave themselves out). Usually found in mitochondrial and chloroplast rRNA, tRNA and mRNA. Splicing is catalysed by RNA (either the intron or the snRNA). Multiple mRNA's can be produced from the same transcript when there are multiple processing sites for splicing.
Regulating genes
Constitutive genes- expressed throughout the life of a cell, also known as housekeeping genes
Regulated genes- expression (inducible genes) allows levels of mRNA and protein to rise and fall in response to eg environment, cell cycle.
Expression can be regulate via post transcriptional processing, mRNA degradation, translation etc, it’s in the cells best interest to conserve energy and control gene expression from the very beginning.
Transcriptional level regulation
Proteins regulate the initiation of transcription:
- specificity factors- alters specificity of RNA polymerase for a promoter. The sigma subunit of RNA polymerase is one. Makes contact with the promoter, recognising and binding to the promoter sequence: sigma 70 recognises most common consensus promoter.
- repressors- impress access of RNA polymerase to the promoter. Repressor proteins bind to DNA sequences called operators near the promoter. DNA loops up, prevents access by RNA pol. And so prevents transcription.
- activators enhance the RNA polymerase-promoter interaction, thus increasing levels of transcription. -ve signal: molecular signal causes activator dissociation. +Ve signal: molecular signal causes activator binding.
Bi- directional
Replication begins at one point in DNA and moves outwards in nth directions. Replication bubble starts at one spot and keeps getting bigger as the DNA is synthesised. Junction where DNA splits from being double-stranded to single-stranded is called the replication fork. Topoisomerases nick one DNA strand to release torsional stress and then rejoin it again.
