Chapter 15 Flashcards
Archibald Garrod
1902 Recognized that alkaptonuria is inherited by recessive allele. Patients lacked enzyme
Connected genes to enzymes
Beadle and Tatum 1941
Deliberately created mutations in chromosomes. Used x rays to damage DNA and looked for nutritional mutations. Used minimal media. Looked for cells lacking specific enzymes. One-gene/one-enzyme
Media vs Rich and Minimal
Media is a place that have materials to grow
Rich Medium- all AA’s, chem, salt, soil etc. Everything needed to grow.
Minimal Media - Few things need to grow, sees what organism produces itself
One-gene/One-enzyme
Modified to One gene/one-polypeptide. Operation on one gene=one protein. NOT How it actually works
Central Dogma of Biology
DNA makes RNA makes Protein
First described by Francis Crick
Retroviruses violate this order using reverse transcriptase to convert RNA genome into DNA
Transcription
DNA to RNA
Uses single strand of DNA to make RNA and then goes back together. DNA template strand
mRNA used to direct synthesis of polypeptides
Translation
RNA to protein (Polypeptide)
Synthesis of polypeptides. Takes place at ribosome, requires several kinds of RNA
mRNA
rRNA
tRNA
mRNA makes protein
rRNA component of ribosomes (Translation)
tRNA Link between mRNA and protein (translation) AA’s brought in.
Francis Crick and Sydney Brenner
Genetic Code
Determined how the order of nucleotides in DNA encoded amino acid order. Codons. Insertions of deletions (Frameshift mutations) Indicates importance of reading frame.
Genetic code is strongest evidence of common ancestory
Codon
Block of 3 DNA nucleotides corresponding to an amino acid
Marshall Nirenberg
Identified the codons that specify each AA
Stop Codon
UAA, UGA, UAG
Start Codon
AUG
Degenerate codon
Some AA’s are specified by more than one codon
Prokaryotic Transcription
Single RNA polymerase only goes 5’ to 3’ as ribonucleotides are added.
Initiation of mRNA synthesis does NOT REQUIRE a primer
Termination is marked by “stop” with a Hairpin (U and A bonds together, weaken DNA/RNA binding and forces enzyme to “Fall off”.)
Requires a Promoter Site, Start Site, Termination Site
Only in prokaryote is transcription and translation coupled (No nucleus)
Spaced vs Unspaced Codons
Spaced - Codon sequence in a gene is punctuated (Incorrect)
Unspaced - Condon adjacent to each other
Promoter Site
Forms a recognition and binding site for RNA polymerase found upstream of start site -35 sequences and -10 sequences
Transcription bubble
Small region of DNA unwinds by RNA polymerase to break H bonds and read DNA. Starts at -10 sequence
Core Enzyme vs Holoenzyme
Core - 2 alphas, Beta, Beta ‘
Holoenzyme 2 alphas, Beta, Beta ‘ and Sigma subunit
Eukaryotic Transcription
3 different RNA polymerases
RNA poly I transcribes rRNA
RNA poly II transcribes mRNA and some snRNA.
RNA poly III transcribes tRNA and some small RNA’s
Each RNA poly recognizes its own promoter
mRNA modifications - addition of 5’ cap (Guanine cap) to protect from degradation involved in translation initiation. Addition of 3’ poly-A tail (AAAAAA) to protect from degradation. Removal of non coding sequence (introns)
Eukaryotic Initiation
Requires series of transcription factors and get the RNA poly II enzyme to promoter for gene expression. RNA Polymerase forms initiation complex
Introns vs Exons
Introns are non-coding sequences
Exons are sequences that will be translated. snRNP’s recognize intron-exon boundaries and clusters with proteins to form spliceosome to remove introns
tRNA and charging reaction
(Aminoacyl-tRNA synthetases- adds AA to the acceptor stem of tRNA) (Anticodon loop contains 3 nucleotides complementary to mRNA codons)
Each aminoacyl-tRNA synthetase recognizes only 1 AA but several tRNA’s
Charged tRNA has an AA added using ATP energy. Can undergo peptide bond formation without additional energy. Ribosomes do not verify AA attached to tRNA
P site
A site
E site
P site - binds the tRNA attached to the growing peptide chain
A site - binds the tRNA carrying the next AA
E site - binds the tRNA that carried the last AA and exits tRNA from ribosome
First tRNA goes into P site, then all others first go into A then P then E
Ribosomes 2 primary functions
1 Decode the mRNA using anticodons and codons
2 Form peptide bonds between amino acids
Peptidyl transferase - enzymatic component of ribosome and forms peptide bonds between AA
Prokaryote vs Eukaryote
Translation
Pro - initiation complex initiator tRNA charged with N-formylmethionine, mRNA strand and small ribosomal subunit. Ribosome binding sequence (RBS) of mRNA positions small subunit correctly, large subunit added, initiator tRNA bound to P site with A site empty
Euk - Similar except initiation AA is methionine, more complicated initiation complex, LACKS RBS, small subunit binds to 5’ cap of mRNA.
2nd charged tRNA can bind to empty A site and requires elongation factors. 2 AA’s can form peptide bonds. Translation may occur in either cytoplasm or RER. Signal sequences at beginning of polypeptide sequence bind to SRP (Signal recognition particle). SRP recognized by RER receptor proteins. Docking holds ribosome to RER and begins protein-trafficking pathway
Wobble
Pairing allows less stringent pairing between 3’ base of codon (third letter) and the 5’ base of the anticodon, this allows tRNA’s to accommodate all codons
Point Mutations
Alter single base
Base substitution
Substitute one base for another
Silent Mutation
Same amino acid inserted with different bases
Missense mutation
Changes amino acid inserted.
Transitions or transversions
Nonsense mutation
Changed to stop codon before it is suppose to stop.
Frameshift Mutation
Addition or deletion of a single base, much more profound consequences. Alter reading frame downstream. Triplet repeat expansion mutation - repeated unit is expanded and repeated
Chromosomal Mutation
Change the structure of a chromosome. Deletions - Part of chromosome is lost
Duplication - Part of chromosomes is copied
Inversion - part of chromosomes in reverse order
Translocation - part of chromosomes is moved to a new location or different chromosome.
