central dogma -RNA

chapter 7

central dogma• “central dogma of molecular biology”

• originally coined by Francis Crick• not actually dogma• describes how DNA replicates and codes for proteins

• protein coding• DNA unwinds

• strands separate• transcription• translation

amplification• transcription

• many RNA copies can be made• translation

• many polypeptide copies can be made from each mRNA

transcription• initiation

• strands separate• promoter region

• elongation• RNA polymerase

• terminationRNA

polymerase

direction of transcription

free nucleotides

template

transcription• initiation

• strands separate• promoter region

• elongation• RNA polymerase

• termination

eukaryotic transcription• more complex than prokaryotes

• 3 different polymerases• require general transcription factors• genes spread out along DNA

• large variety of regulatory sequences• more compact chromatin

eukaryotic transcription• initiation (PIC complex)

• general transcription factors1. TFIID

• binds to TATA box (~25bp from start) or Inr2. TFIIB 3. TFIIF / RNA polymerase II4. TFIIE - TFIIH

TFIID

TFIIB

TFIIFTFIIH

TFIIE

• messenger RNA - • carries message

about polypeptide• contains a sequence

of codons• three base

sequence • codes for an

amino acid

RNA

• messenger RNA - • carries message

about polypeptide• contains a sequence

of codons• three base

sequence • codes for an

amino acid

RNA eukaryotic genomes• processing of mRNAs

• prokaryotes• mRNA immediately attaches

to ribosome• eukaryotes

• capping• splicing• polyadenylation

pre-mRNA splicing• splicing pre-mRNAs

• introns - intervening sequences• exons - expressed sequences• RNA splicing

• introns must be removed• introns contain splicing signals • carried out by spliceosome

• snRNPs (small nuclear ribonucleoproteins)

• splicing can affect protein produced

• same gene may produce different proteins

exporting mRNA• mRNA export

• poly-A-binding proteins• cap-binding complex• exon junction complex

• proteins bound to sites of connected exons• nuclear pore complex

• properly tagged mRNA is exported

RNA• transfer RNA

• present in the cytoplasm• carry the 20 amino acids• charged by aminoacyl-tRNA synthetases• anticodon

• ribosomes• large subunit

• 3 tRNA binding sites• 2x as big (mass)

• small subunit• mRNA binding site

E P A

assembly

G G G U U U U GC UG C

translation• three phases

• initiation• elongation

• codon recognition• peptide bond formation• translocation

• termination

• three phases• initiation• elongation

• codon recognition• peptide bond formation• translocation

• termination

translation

ribosomes• polyribosomes

• often multiple ribosomes act on mRNA• multiple copies at once

• bound ribosomes• polypeptide signals binding

ribosomes• polyribosomes

• often multiple ribosomes act on mRNA• multiple copies at once

• bound ribosomes• polypeptide signals binding

digesting proteins• controlling amounts of proteins

• proteases• proteolysis • different proteases activated at different times

• proteasomes• digest short-lived or malfolded proteins• cytosol and nucleus• protease core