Prokaryotic Transcription
- in cytoplasm
- coupled transcription and translation
- no definite phase of occurrence
- one RNA polymerase
- no initiation factors
Eukaryotic Transcription
- in nucleus
- occurs in G1 and G2
- three RNA polymerases
- multiple general transcription factors
What is a control element?
- non-coding DNA segments
- regulate transcription by binding transcription factors
What is a promoter?
a sequence of DNA bound by the first components of the pre-initiation complex
What is an enhancer?
- distal control elements
- short nucleotide segments that influence the rate of transcription
- may be thousands of nucleotides away from the promoter
- may be bound by silencer/repressor proteins
What is an intron?
- interrupt the coding sequence
Define transcription
the process by which the information in a strand of DNA is copied into a new molecule of messenger RNA (mRNA)
Different RNA polymerases
- RNA polymerase I; produces pre-rRNA
- RNA polymerase II; produces pre-mRNA
- RNA polymerase III; produces pre-tRNA
Four steps of transcription
1) Initiation
2) Elongation
3) Termination
4) Processing
INITIATION
Unwinding of DNA
- RNA polymerase binds to the template strand via the pre-initiation complex
- chromatin remodelling exposes the promoter
- pre-initiation complex binds to recognition site on promoter
ELONGATION
- RNA polymerase moves along the template strand, synthesising a pre-mRNA molecule
- pre-mRNA is processed; capping, splicing & polyadenylation
TERMINATION
- addition of additional adenine nucleotides at the 3’ end of the RNA transcript
- POLYADENYLATION
PROCESSING
- introns are removed and exons are spliced together to form a mature mRNA molecule one single protein-coding sequence
Pre-mRNA processing
- cap added at 5’ end
- tail added at 3’ end
- glutynaltransferase attaches methylated GTP cap to 5’ end
- splicing; removal of introns by spliceosomes
Purposes of 5’ end capping
- Regulation of nuclear export
- Prevention of degradation by exonucleases
- Promotion of translation
Poly-A tail addition
- polyadenylation signals; bind cleavage factors
- poly-A polymerase adds about 100-250 adenine nucleotides to cut 3’ end
- forms a poly-A tail
- stabilises the transcript, aids in transcription termination and exportation from nucleus to cytosol
Translation overview
- mRNA moves into the cytoplasm
- becomes associated with ribosomes
- tRNA with correct anticodon carries amino acid to mRNA
- anticodon-codon complementary base pairing occurs
- peptide chain is transferred from resident tRNA to incoming tRNA
-tRNA departs and will pick up another AA
Three steps of translation
- initiation
- elongation
-termination
Structure of the ribosome
- Large ribosomal subunit (50s) contains three slots for tRNAs
- Ribosome slides from 5’ to 3’ end
- A —> P —-> E
- aminoacyl-tRNA, peptidyl-tRNA & exit
- small ribosomal subunit (30s) has the mRNA binding site
Structure of tRNA
- acceptor stem at 3’ end has amino acid attachment site
- anticodon read 3’-5’ antiparallel to codon mRNA
tRNA wobble
- “Wobble position” at 5’ end of anticodon
- allows the same tRNA to recognise multiple codons for amino acid it carries
- allows for fewer tRNA molecules
Loading tRNA with correct amino acid
- Enzymes called aminoacyl-tRNA synthases specific for each AA
- recognise the AA and the suitable tRNA molecule
- uses ATP
INITIATION; translation
- small ribosomal subunit and initiator tRNA binds to the 5’ end cap
- complex moves along in the 3’ direction
- tRNA molecule binds to the codon via it’s anticodon
What is the sequence of initiator tRNA?
UAC
