Lecture 2.1: Transcription Flashcards
What is a Gene?
A gene is a unit of hereditary information/ section of DNA that codes for a specific mRNA or protien
Exon & Intron
Exon: A coding section of DNA
Intron: A non-coding section of DNA
Transcription Steps (3)
Initiation: Binding of RNA polymerase to promoter region
Elongation: RNA polymerase synthesises mRNA/ RNA transcript
Termination: RNA polymerase reaches termination site, detaches from DNA and then RNA transcript is released
Transcription in Prokaryotic Cells (Initiation)
Starts with binding of RNA polymerase to promoter region in DNA
Prokaryotic promoters are characterised by 2 sets of sequences located 10 and 35 base pairs upstream of the transcription start site
Prokaryotic RNA Polymerase: structure and what it does during Initiation
It is a holoenzyme consisting of 6 subunits
The role of the sigma part of the RNA polymerase is to bind specifically to the -35 and -10 sequences leading to the transcription
The polymerase then unwinds the DNA around the initiation site, forming an open promoter complex transcription is initiated by polymerisation of free NTs
Transcription in Prokaryotic Cells (Elongation)
The sigma subunit of RNA polymerase then dissociates from the core polymerase which migrates along the DNA and elongates the growing RNA chain
Transcription in Prokaryotic Cells (Termination)
RNA synthesis continues until the polymerase encounters a termination signal, then transcription stops, the RNA is released from the polymerase, and the enzyme dissociates from its DNA template
The most common type of termination signal consists of a symmetrical inverted repeat of a GC-rich sequence followed by approximately seven A residues, that forms a stable stem-loop structure in the RNA, terminating transcription
Types of RNA molecules: Name 5
rRNA, mRNA, tRNA, snRNA, siRNA, miRNA, snoRNA, piRNA, IncRNA
PART 1
Transcription in Eukaryotic cells (Initiation): RNA polymerase II
RNA Polymerase II requires transcription factors (TFs) to initiate transcription
General TFs are involved in transcription from most polymerase II promoters
Additional, specialised (gene-specific) transcription factors bind to DNA sequences that control expression of individual genes and are responsible for regulating gene expression
PART 2
Transcription in Eukaryotic cells (Initiation): RNA polymerase II
[Promotor Region Table]
Typical Eukaryotic promoter region:
BRE G/C G/C G/A C G C C TFIIB
TATA T A T A A/T A A/T TBP
INR C/T C/T A N T/A C/T C/T TFIID
DPE A/G G A/T C G T G TFIID
———————————————————————————-
For most RNA Polymerase II transcription start points, only 2/3 of the sequences are present
PART 3
Transcription in Eukaryotic cells (Initiation): RNA polymerase II
TFIID recognises and binds to the TATA box through its TBP subunit, this enables the binding of TFIIB
The rest of the general TFs (TFIIE, TFIIH) and RNA polymerase II then assemble at the promoter
TFIIH contains DNA helicase as one of its subunits
TFIIH also phosphorylates RNA pol II –> conformational change (in CTD) –> RNA pol II released from TFs –> elongations starts
PART 4
Transcription in Eukaryotic cells (Initiation): RNA polymerase II
The phosphorylated CTD the binds to additional proteins to facilitate elongation including “chromatin remodelling factors”
These then displace nucleosomes during transcription
What type of enzymes mediate phosphorylation?
Kinases
PART 5
Transcription in Eukaryotic cells (Initiation): RNA polymerase II
[Additional Protiens]
Refer to SLIDE 21 of Lecture 2.1 of Molecules, Genes and Diseases
Transcription in Eukaryotic cells (Elongation): RNA polymerase II
RNA Polymerase elongation rate 6-70nt/sec
Makes sugar-phosphate backbone
Phosphodiester Bonds
Transcription in Eukaryotic cells (Termination): RNA polymerase II
Termination less understood in eukaryotes
Involves cleavage of new transcript followed by template-independent polyadenylation at 3’ end
Transcription in Eukaryotic cells: RNA polymerase I
RNA polymerase I is devoted solely to the transcription of ribosomal RNA genes
Transcription in Eukaryotic cells: RNA polymerase III
(pre) RNA processing
5’ end capping with 7-methylguanosine (m7G)- protects nascent mRNA from degradation and assists in ribosome binding
Splicing by spliceosomes- to remove introns
3’ end is modified via polyadenylation (polyA tail)- protects mRNA from degradation, aids in export of mature mRNA into cytoplasm, assists in ribosome binding
Why is splicing important?
Transcripts of many eukaryotic genes (95% human ones) are spliced in more than 1 way
Alternative splicing increases diversity of proteins
It enables eukaryotes to increase the coding potential of their genomes
What happens to mature RNA?
They are guided through nuclear pore complexes (NPCs)
Aqueous channels in nuclear membrane leading to cytosol
Which DNA strand is transcribed?
The antisense strand is the template for mRNA
As sense strand is the coding strand that contains the true nucleotide sequence of the gene the mRNA needs
