Gene Expression

Flow of genetic information
Crick: 1 gene = 1 protein, 1 gene = 1 enzyme
Transcription: info. In DNA used to synthesize mRNA
Translation: ribosome uses mRNA to make polypeptide (in ER and folded)
RNA composition/types
uracil(U): pyrimidine (pairs with adenine)
mRNA: messenger, encodes AA seq.
rRNA: ribosomal, part of ribosome
tRNA: transfer, carries AA to ribosome, reads 3 nucleotides and has cognate
Genetic code
Codon: 3 nucleotides mRNA, each triplet = 1 AA, UUU = phenylalanine
Read in sets of 3
Mutation: 1 letter deleted
Universal genetic code
Unambiguous, degenerate (redundant), start codon = AUG, 3 stop codons (UAG/UGA)--protein
Transcription (DNA -> RNA) -> translation (RNA -> protein)
Translation
Function: tRNA binds to AA and brings to mRNA in ribosome
Structure: RNA molecule
Anticodon: complementary/antiparallel to codon
Enzymes: aminoacyl: tRNA synthetase -> 20 different enzymes
Links carboxyl group of AA to 3’ end of tRNA before reaching active site of rRNA
Ribosomes: enzyme complex binds to mRNA template/catalyzes peptide bonds
Large and small subunit
Large has binding site for tRNA interactions with mRNA

Replication
Transcription (DNA -> RNA)
Translation (RNA -> protein)
Location
Nucleus
Nucleus
Cytoplasm
Initiation
-starts at origin of replication, replication bubble
-DNA helicase unzips mol., breaks H-bonds
-SSBP keep strands apart
-topoisomerase relaxes helix/prevents from breaking
-DNA polymerase lays down complementary bases by attaching to primer
-RNA primer needed
-RNA primase puts down primer on lagging strand which is later removed and replaced with DNA
-synthesized in 5’ -> 3’
-DNA is template, 1 strand template and other is not transcribed
-RNA polymerase unzips molecule and binds to promoter (specific DNA seq on transcribed strand @ start of gene @ 3’ end)
-NO PRIMER NEEDED
-begins transcribing mRNA 5’ -> 3’
-rRNA units disassembled
-mRNA arrives with loaded tRNA from cytoplasm
-tRNA reads start codon AUG and adds anticodon that forms H-bonds
-initiation complex formed

Elongation
-nucleoside triphosphate added along with phosphodiester bonds (poly), release pyrophosphate
-5’ phosphate attached to 3’ OH
-DNA poly adds nucleotides to 3’ OH side
-leading strand: continuously synthesized towards replication fork
-lagging strand: synthesized in short fragments (Okazaki fragments) away from fork
-adds complementary RNA bases and nucleoside triphosphates
-template read 3’ -> 5’
-A site: where tRNA enters with AA; ribozyme catalyzes peptide bond
-P site: holds growing polypeptide
-E site: tRNA leaves
-each cycle adds 1 AA
-translocation: ribosome moves down 1 codon

Termination
-RNA removed
-strands are glued together with DNA ligase
-2 identical daughter molecules of DNA
-reads termination sequence and transcript falls off
-mRNA processing (Eukaryotes only):
-pre-mRNA: not in correct form to give info for protein synthesis, modified
-mature mRNA: alternation of pre-mRNA ends + RNA splicing
-introns (not translated) spliced out and extrons (expressed, coding regions) glued together
-5’ cap (modified guanine with 3 phosphates) and poly A tail added
-for export from nucleus, protect from degradation, help ribosome attach
-stop codon read by releasing factors and binds to stop codon in A site and protein is released
-rRNA parts disassemble
-tRNA goes back to be recharged