Transcript From Gene to Protein

From Gene to Protein
Chapter 17
Gene Expression
 The process by which DNA directs the synthesis of proteins
 2 stages: transcription and translation
 Detailed flow of info from gene to protein
 Explain how genetic mutations affect organisms through their
proteins
 Central Dogma (Francis Crick)
DNA
transcribed
RNA
translated
 Proteins link DNA to phenotype
protein
Decoding Genes to Phenotype
 Archibald Garrod
 Inborn error of metabolism
 Inherited disease from inability to make a particular enzyme
 Suggested that genes dictate phenotype through enzymes that catalyze
specific reactions
 George Beadle and Edward Tatum
 One gene – one enzyme hypothesis
 A single gene specifies synthesis of a single enzyme in the body
 Shared Nobel Prize
 Additional Scientists
 Not all proteins are enzymes and many proteins are 1+ polypeptides
 Revised to one gene – one polypeptide
 Each gene codes for 1 polypeptide of a protein
From DNA to Protein
 Genes instruct, but don’t
build
 Nucleotides and amino
acids are different
‘languages’
 RNA connects them
 Transcription: same
language
 Translation: different
language
 Occurs in all organisms
DNA
 Sugar is deoxyribose
 Has –H
RNA
 Sugar is ribose
 Has -OH
 Bases are A,C, G, and T
 Bases are A, C, G, and U
 Double-stranded helix
 Single-stranded
 Only in nucleus
 Not confined to nucleus
 Modified only by mutations
 Lots of processing and
 1 type
modifications
 3 types (mRNA, tRNA, rRNA)
Reviewing DNA and RNA
Summary of Protein Synthesis
 Genes determine the sequence
of bases along an mRNA
 Only template strand is
transcribed
 Similar to DNA replication
 mRNA is complementary to
template strand
 Non-template strand is
‘identical’ to mRNA = coding
strand
 Sequence of mRNA as codons
translated to amino acids
Transcription
 In the nucleus
 Initiation
 RNA polymerase binds to
promoter
 Many work at once = efficiency
 Elongation
 Builds 5’
3’ = downstream
 Unstable complex so mRNA
immediately released and DNA
rejoins
 Termination
 Terminator reached = releasing
transcript (pre-mRNA)
mRNA Processing
 Before mRNA leaves the nucleus
 Alteration of 5’ and 3’ ends
 5’ cap, modified G, directs ribosome attachment for protein synthesis
 Poly-A tail, addition of 5-250 adenines (A), inhibits degradation as leaves
nucleus
 RNA splicing
 Pre-mRNA transcript contains exons and introns
 RNA sequences and DNA sequences that encode them
 Spliceosomes splice out introns and rejoin exons = true mRNA
Translation
 In the cytoplasm within ribosomes
 mRNA as codon message from
DNA
 Translated by tRNA
 Anticodons and amino acid ends
 Ribosomes facilitate addition of
tRNA to mRNA
 3 steps like transcription
Ribosomes
 Facilitate coupling of tRNA
anticodons and mRNA
codons
 Large and a small subunits
 Functional when subunits
join with mRNA
 3 unique binding sites
facilitate
 More than 1 can bind to a
single mRNA
Initiation
 Small ribosome subunit binds to mRNA and moves to start codon
 1st tRNA enters the P site carrying the AA met
 Anticodon is what?
 Large subunit binds
 Initiation factors facilitate and GTP supplies energy
Elongation
 2nd tRNA molecule
enters the A site
 Once matched the
growing polypeptide
chain binds to the new
AA
 Ribosome shifts 5’
3’,
changing P site tRNA to
E site, A site to P site,
and freeing A site =
translocation
Termination
 Stop codon sequence that signifies the end of a polypeptide chain
 Sequences are UAG, UAA, and UGA
 Don’t code for AA’s
 Polypeptide cleaved from last tRNA (P site) and leaves the
ribosome
 Folds into quaternary structure = protein
Decoding Codons
 Only 4 nucleotide bases to
specify 20 amino acids
 Genetic instructions are
based on codons
 42 = 16 (not enough);
43 = 64 (plenty)
 Demonstrates redundancy,
but not ambiguity
 3rd base is wobble base
 Nearly universal across
species
Mutations
 Changes to the genetic information of a cell, or virus
 Ultimate source of diversity because ultimate source of new
genes
 Chapter 15 was large scale mutations which effect long DNA
segments
 What were the 4 types?
 Point mutations change single nucleotides and causes changes
in single specific codons
 Base-pair substitution
 Frameshift mutations result from altering the reading frame
 Base-pair insertions/deletions
Base-pair Substitutions
 Replaces 1 nucleotide pair with another
 Effect depends on particular codon change and location
 Silent mutations occur at wobble bases so have no effect on
the encoded protein
 Missense mutations change 1 AA to another
 Nonsense mutation codes for a stop codon rather than an AA
 Stops translation early
 Can result in wrong AA being added, abnormal protein
shape, or shortened proteins
 Alters active site and can make non-functional
Frameshift Mutations
 Insertions and deletions add or remove nucleotide pairs

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
of a gene
Often more deleterious when changes don’t occur in groups
of three
Any downstream bases will be affected
Almost always causes protein to be nonfunctional
THE CAT ATE THE RAT
THE ATA TET HER AT (delete C)
THE CAT GAT ETH ERA T (insert G)