Gene organisation, transcription and regulation Flashcards
DNA vs RNA; RNA classes; Gene transcription; mRNA processing; Gene regulation by small RNAs
What are genes?
Units of inheritance
Code for proteins and fRNAs
What is the purpose of gene expression?
Leads to cellular specialisation
What do blood cells develop from?
Common progenitor cells
Differences result from differences in gene expression
What is RNA?
Initial product of gene expression Single stranded ribose sugars Uracil instead of thymine Functional (tRNA, rRNA) or protein translation template (mRNA)
What is transcription?
The process of DNA info being transcribed to RNA
DNA strands unwind
Ribonucleotide bases pair with bases on antisense (template) strand
Bases are joined by RNA polymerases to form phosophodiester bonds
What are transcription factors?
Special gene regulatory proteins
Expression determined by cell lineage
Activators - activate gene expression (more copies)
Repressors - suppress gene expression
What are the functions of the different types of RNA polymerase?
I - rRNA
II- mRNA
III- tRNA/5s RNA
What is the initiation step and why is it needed?
Complex assembles at gene promotor
Level of transcription regulated by transcription factors
Required to build a transcription complex before RNA synthesis occurs
What sequence specifies the initiation point for transcription?
TATA - gene promoter
After this, RNA Pol II will begin transcription
What comprises the basal transcription complex?
TF IID (TATA binding protein) binds to minor groove, beginning to unwind DNA
TF IIA and IIB bind next
RNA Pol II binds to TF IIB with TF IIF attached
TF IIE H and J bind to Pol II
How do transcription factors effect chromatin?
Recruit proteins with enzymatic activities to modify histones
Hyperactylation - genes expressed as DNA exposed
Hypoactylation - genes repressed as DNA hidden
What are the key features of E.coli transcription?
Only one type of RNA polymerase, which has 4 subunits - 2 alpha and 2 beta
Gene promoter = TATAAA
What are the key features of bacterial transcription?
Sigma factors bring RNA polymerase to different classes of genes
Occurs in the cytoplasm ∴ RNA made in same location as it is translated and can be used straight away
No introns ∴ bases contained in one continuous sequence
What is pre-mRNA?
Primary transcript
contains introns and exons
Why must pre-mRNA be spliced before being exported to the cytoplasm?
To remove introns
To add a nucleotide cap at 5’ end and add a poly-A tail to the 3’ end
What are introns?
Non coding regions
begin with GU and end in AG
What are exons?
Coding regions
end in AG
Where is the splice donor site?
Located between the end 2 bases of the exon and GU of the intron
Where is the splice acceptor site?
Comes after 15 pyramidine bases, any base, then CAG
Pyr15NCAG
What are snRNPs and what is their function?
Small ribonuclear proteins produced from snRNA
Numbered and form the spliceosome
Where do snRNPs bind and what do they form?
U1 binds to splice donor site
U5 binds to splice acceptor site
U2,4,6 bind in the middle
U1,2,4,5,6 form splicing complex used to cleave splice donor sequence
How is the pre-mRNA spliced?
Cleaved at splice donor site
An A residue in the intron is used as the branch point to which UG binds
Phosphodiester bond between G and exon cleaved at splice acceptor site
Exons are joined by RNA ligase
How is the ‘cap’ added to the 5’ end of mRNA?
Terminal phosphoanhydride bond is cleaved to form a diphosphate
GTP has two phosphates removed to form GMP
GMP attaches to the diphosphate = 5’-5’ linkage
7’ carbon in guanine is methylated to produce:
7-methylguanylate cap
Why is a cap added to mRNA?
Prevent exonuclease degradation
Allow recognition by ribosomes
What is polyadenylation?
The addition of the poly-A tail to pre-mRNA
Added one base at a time by poly-A-polymerase
Adds around 10-200 bases 11-30 bases after AAUAAA
Why is a poly-A tail added to pre-mRNA?
Provides stability
Length allows monitoring of translation
Prevents translation of damaged mRNA
What are micro RNAs (miRNAs)?
18-26 nucleotides in length
Involved in regulation of mRNAs
How are genes regulated through RNA interference?
dsRNA produced then separated and divided into small ss-siRNA (small interfering RNA) by DICER
siRNA guides endonuclease activity to remove a ‘passenger’ strand
RNA induced silencing complexes are formed and they recognise and cleave target mRNA that is complimentary to the siRNA
How do miRNAs act in gene regulation?
They degrade target mRNAs or inhibit their translation
What are non-coding RNAs?
Any non translated RNA (not mRNA)
What can long ncRNA affect?
epigenetic expression of genome
key for x inactivation in female cells
What does antisense RNA do?
If complimentary, it will hybridise with mRNA more strongly than DNA binds
Prevents translation
What is the purpose of the RNA induced silencing complex?
Acts in RNA interference
Incorporated siRNA or miRNA and uses it as a template for recognising a complementary mRNA target
complementary target binding triggeres RISC RNase which cleaves it
