Transcription And Translation Flashcards
Transcription
DNA is transcribed into an RNA copy
Translation
mRNA is translated into a polypeptide at the ribosome
Prokaryotic Gene Structure
Promoter, Regulatory Sequence, Coding Region, Terminator
Eukaryotic Gene Structure
Enhancer, Basal Promoters (CCAAT -100, TATA -25), Exons and Introns, termination
Basic Transcription Steps
- Initiation
- Elongation
- Termination
Initiation
Recruitment of RNA-Polymerase (also protein sigma factor for prokaryotes)
RNA polymerase separates DNA strands to form template strand
Elongation
RNA polymerase moves in a 5’-3’ direction unwinding the DNA as it goes and using template strand to synthesize the RNA.
Pyrophosphate (provides the energy for the process) is cleaved off.
Prokaryotic vs Eukaryotic transcription
Prokaryotic: All genes transcribes by the same core RNA polymerase. Direct binding by polymerase to promoter via sigma factor.
Eukaryotic: 3 polymerases used. RNA polymerases cannot bind directly to promoter but need general transcription factors.
RNA processing in Eukaryotes
RNA polymerase makes a pre-mRNA which needs to undergo processing to produce a final mature RNA.
RNA processing steps in Eukaryotes
Transcription
Capping
Splicing
Tailing
Capping
Methylguanosine cap added at 5’ end of mRNA
Limits degradation, initiate translation
Tailing (Polyadenylation)
3’ poly A tail added
Limits degradation, important for translation, required for transport out of nucleus
Splicing
Introns removed
Step 1- break 5’ phosphodiester linkage from preceding exon. Lariat loop formed
Step 2- break linkage of last nucleotide of intron from first nucleotide of the following exon. Lariat loop is maintained when broken off (eventually degrades away).
mRNA
Contains the information for the sequence of amino acids in a polypeptide according to the genetic code
tRNA
A molecule with two functional sites; the anticodon and the 3’ single stranded region where an appropriate amino acid attaches.
Translation initiation in prokaryotes
- 16S rRNA in small subunit undergoes base pairing with Shine Delgarno sequence in 5’ UTR of mRNA
- Initiator tRNA binds to start codon in mRNA
- Large ribosomal subunit binds
Translation prokaryotes: Initiator tRNA
tRNA charged with N-formyl methionine
Translation prokaryotes:
AUG codons that occur after the initial start codon use tRNA charged with N-formyl methionine?
False, they use different tRNA charged with methionine
Translation initiation in eukaryotes
- 40s binds 5’ end of the cap structure
- 40s migrates on the 5’-UTR till the optimum AUG
- 60s joins 40s subunit
- Ribosomal complex starts translation
Translation Eukaryotes: Initiator tRNA
tRNA methionine
Translation: Elongation
- A charged tRNA binds to the A site
- Peptidyl transfer reaction: bond forms between the polypeptide chain and amino acid in the A site. The polypeptide is transferred to the A site.
- Translocation and release of tRNA: the ribosome translocates one codon to the right. The uncharged tRNA is released from the E site. Process is repeated until stop codon is reached.
Translation: Termination
- A release factor binds to the stop codon at the A site.
- The polypeptide is released from the tRNA in the P site. The tRNA is then released
- The mRNA, ribosomal subunits, and release factors dissociate.
Location of translation in Prokaryotes
Cytoplasm
Location of translation in Eukaryotes
Cytosol
