General Biology/Genetics/Gene Expression
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Flow of genetic information
- DNA → mRNA → polypeptide
- Transcription: DNA → mRNA
- RNA polymerase
- Nucleus in eukaryotes
- Transcription also makes rRNA and tRNA
- Translation: mRNA → polypeptide
- Ribosomes: protein and rRNA
- Genetic code and tRNA
“Central Dogma” edit
The Genetic Code edit
- Triplet codon
- 64 triplet codons (43)
- Experimentally deciphered in 1961
- Nearly universal
- Implies common ancestor to all living things
- Minor exceptions: certain ciliates, mitochondria, chloroplasts
- Still evolving
Transcription edit
- RNA polymerase
- NTP substrates
- Synthesizes single stranded RNA complementary to template strand of DNA
- New nucleotides to 3’ end
- Begins at promoter site
- no primer necessary
- Ends at terminator site
- Much posttranscriptional modification in eukaryotes
Transcription bubble edit
Promoter site
- Prokaryotes
- -10 nt, TATA box
- -35 nt, additional signal
- Eukayotes
- -25, TATAAA box
- Additional signals upstream
- Promoters may be strong or weak
- In eukaryotes, access to promoter depends upon state of chromatin coiling
Eukaryote mRNA edit
- Synthesized as pre-mRNA, processed in nucleus
- 5’ end: GTP cap placed in inverted position
- Essential for ribosome recognition
- 3’ end: poly-A tail; non-templated addition of ~50-250 A nucleotides; stability
- Introns: intervening sequences removed
Translation edit
- Requires:
- mRNA
- tRNA
- ribosomes
- translation factors (various proteins)
- In prokaryotes, takes place on growing mRNA
- In eukaryotes, in cytoplasm on free ribosomes and RER
- AUG start codon to stop codon
Translation in bacteria edit
tRNA
- Transfer RNA
- Two important parts
- Anticodon
- Hydrogen bonds with mRNA codon
- 3’ end
- Accepts amino acid (using energy of ATP)
- Aminoacyl-tRNA synthetase
Aminoacyl tRNA synthase edit
- Enzyme used to bind amino acid from the cytoplasm to tRNA, which then transfers that amino acid to the ribosome for polypeptide formation
Ribosome structure edit
Large ribosome subunit edit
Translation edit
- Initiation complex
- Small ribosomal subunit
- mRNA
- fMet-tRNA (prokaryotes only; met-tRNA in eukaryotes)
- Initiation factors
- Elongation
- Ribosome
- mRNA
- tRNAs
- Elongation factors
Initiation complex edit
Elongation, translocation edit
- incoming tRNA enters the A site
- rRNA catalyzes peptide bond formation. Note that growing peptide is attached to what was incoming tRNA at P site after translocation.
- empty tRNA leaves via E site; recycled
- A site ready for next charged tRNA
Introns/exons edit
- In eukaryotes, coding regions of gene may be interrupted by introns, noncoding regions of DNA (RNA)
- Introns
- 22- >10,000 nt in length
- 5’ GU … 3’ AG removal sequence
- Not essential to genes
- May constitute >90% of gene
- removed from pre-mRNA to form mRNA
- Exon: often codes for functional domain of protein
- translatable mRNA
References edit
This text is based on notes very generously donated by Paul Doerder, Ph.D., of the Cleveland State University.