Gene Expression Flashcards
(30 cards)
Basal transcription factors
Initiate transcription
Match RNA polymerase with appropriate DNA promoter region
Analogous to sigma function in bacteria
Promoter
Contains a TATA box
5’-TATAAAA-3’
TATA box about 30bp from transcription START codon
Transcription initiation complex
Distal control elements make up an enhancer on the DNA strand before the promoter region
Activator molecules bind the enhancer
The activator/enhancer complex allows a DNA bending protein to bind to the DNA and bend the strand over so the activators are close to the TATA box
A group of mediator proteins binds to the activators and promoter region
Transcription factors and RNA polymerase II join onto the mediator proteins and form a transcription initiation complex
Transcription initiation
Upstream of the gene (5’ on non-template strand) is the promoter which includes a TATA box
After binding to the START codon, the RNA polymerase II moves 3’-5’ along template strand and manufactures new mRNA strand in 5’-3’ direction
Transcription elongation
RNA polymerase II moves along DNA and unwinds about 10-20 base pairs at a time
mRNA is elongated and detaches from DNA
Transcription termination
RNA polymerase II transcribes polyadenylation signal in DNA
10-35 base pairs downstream of signal mRNA transcript detaches from RNA polymerase II
5’ cap added to 5’ transcript end
3’ tail added to 3’ end
5’ cap
Modified guanine residue Protects from degradation Aids stability Promotes export of mRNA from nucleus Assists with translation
3’ tail
Poly-A tail
50-250 adenine nucleotides
The more adenine nucleotides means more stability and less degradation
Splicing
Excision of introns and joining of exons
snRNPs - small nuclear RiboNuclear Proteins
snRNPs join with proteins to make a spliceosome
mRNA has splice sites which bind to complementary snRNA sequence
Spliceosome excises and degrades introns and joins exons together
Aminoacyl tRNAs
Aminoacyl tRNA synthetases catalyse addition of amino acids to tRNAs
Active site on aminoacyl tRNA synthetase binds ATP and amino acid
AMP and amino acid bonding release two phosphate groups from the enzyme
Enzyme complex is activated
Activated amino acid transferred from synthetase to specific tRNA
Aminoacyl tRNA created
Cloverleaf secondary structure
CCA sequence at 3’ end of tRNA is the amino acid binding site
Triplet loop at opposite end is the anticodon that pairs with mRNA codon
tRNA structure folds over to produce L shaped tertiary molecule
Wobble hypothesis
tRNA anticodons can successfully bind codons whose third position requires a non-standard base pairing
e.g. CCG tRNA can bind GGA even though the third pairing is unorthodox
tRNAS can bind more than one type of codon
Ribosomes
Separated into large and small subunit
Large subunit forms peptide bonds
Small subunit holds mRNA in place
tRNA is found at 3 sites: A, P and E
‘A’ site of ribosome
Acceptor
tRNA binds mRNA codon
‘P’ site of ribosome
Peptide bond
tRNA adds amino acids to polypeptide chain
‘E’ site of ribosome
Exit
Formerly bound tRNAs leave ribosome
Translation initiation
Small ribosomal subunit binds first amino acid tRNA which is carrying formyl methionine
Small ribosomal subunit scans downstream to translation START site in mRNA 5’-AUG-3’
Large ribosomal subunit binds
Translation begins
Translation elongation
tRNA anticodon hydrogen bonds to mRNA codon in acceptor site
Large ribosomal subunit catalyses peptide bond formation
Growing polypeptide chain at C terminus transferred to tRNS in acceptor site
Translation termination
Release factor binds at acceptor site, not tRNA
Polypeptide-tRNA bond in peptide bond site is hydrolysed
Polypeptide is released
2 GTP molecules are added
Primary protein structure
Unbranched polypeptide chain held together by polypeptide bonds formed in the large subunit of the ribosomal canal
Secondary structure
Hydrogen bonding between amino acid side chains forms alpha helices or bets sheets
Alpha helix
Primary chain is coiled into a spiral structure and stabilised by hydrogen bonds
Beta sheets
Primary chain zig zags back and forth to form a pleated sheet with adjacent strands held together by hydrogen bonds
Tertiary protein structure
Hydrophobic collapse forms hydrophilic external faces
van der Waals interactions form between non-polar amino acid side chains
Hydrogen bonds form between polar amino acid side chains
Ionic bonds form between charged amino acid side chains
Disulphide bridges form between cysteine molecules
