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 | edit source]The Genetic Code
[edit | edit source]- 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 | edit source]- 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 | edit source]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 | edit source]- 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 | edit source]- 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 | edit source]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 | edit source]- 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 | edit source]Large ribosome subunit
[edit | edit source]Translation
[edit | edit source]- 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 | edit source]Elongation, translocation
[edit | edit source]- 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 | edit source]- 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 | edit source]This text is based on notes very generously donated by Paul Doerder, Ph.D., of the Cleveland State University.