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Chapter 17: From Gene to Protein
One gene  one polypeptide
Overview
 Transcription
o DNA  RNA
o DNA acts as a template for the new RNA molecule
o Carries DNA “message” to protein-synthesizing machine (mRNA = messenger
RNA)
o Occurs in nucleus
 Translation
o RNA polypeptide
o Translate base sequence to amino acid sequence
o Occurs at the ribosomes
Genetic Code
 4 nucleic acids
43 = 64 combinations
 20 amino acids
 in transcription, one strand of DNA is the template strand  mRNA is complementary
to the template
 in RNA code, Uracil takes the place of Thymine
o ex: template strand of DNA has ACC … mRNA complement is UGG
 codon= triplet of nucleotides that codes for one amino acid
o # nucleotides on an mRNA is 3x the number of amino acids
o see chart on p. 299
o start codon is AUG
 also codes for Met
 polypeptide starts being made here
o stop codons are UAA, UAG, UGA; these indicate the end of translation
o reading frame: grouped sequence in which the code is read
 ALL LIVING THINGS SHARE THE SAME GENETIC CODE!
Transcription (transcribe = copy)
 RNA polymerase
o first pulls 2 DNA strands apart
o also adds new RNA nucleotides (to 3’ end only)
o transcription unit = piece of DNA that is transcribed onto RNA (the whole DNA
molecule isn’t transcribed at once!!)
 Stages
1. Initiation
 Promoter region
o Initial site of RNA polymerase attachment
o Includes start codon and several dozen nucleotide pairs “upstream”
 Transcription factors
o Mediate binding of RNA polymerase and initiation of transcription
o Transcription factors + RNA polymerase = transcription initiation
complex
 TATA box
o Transcription factor binds here
TATAAAA
ATATTTT
2. Elongation
 RNA polymerase moves down DNA sequence, unwinding it 10 – 20 bases at a
time
 Rate of 60 nucleotides / second
 Several molecules of polymerase may work on transcribing the same gene at the
same time in order to speed things up
3. Termination
 Terminator = RNA sequence that signals the end of transcription
 Transcription produces a precursor to the polished RNA molecule called pre-RNA
 Modifications to RNA
o Alterations of ends

5’ cap = modified G is added to 5’ end, protecting RNA from being
degraded by enzymes and serving as a signal for ribosome attachment

poly (A) tail = 30-200 amino acids on the 3’ end that prevents enzyme
degradation and helps ribosome attachment; also helps transport RNA
out of nucleus
o RNA splicing

Exons are coding regions of the RNA that are eventually expressed

Introns are noncoding regions in between the exons that must be
removed

Introns are removed from pre-RNA sequence before export to the
cytoplasm

Sequences at the end of introns signal snRNP’s (small nuclear
ribonucleoproteins) to splice at those locations

snRNP’s work with other proteins to cut and paste together the exons
o ribozymes

intron RNA catalyzes its own removal

NOT ALL CATALYSTS ARE PROTEINS!!
Translation
 tRNA (transfer RNA) translates the mRNA code by transferring amino acids within the
cytoplasm to the ribosome
o each type of tRNA links a specific codon wth its corresponding amino acid
o anticodon: sequence of tRNA complementary to mRNA codon
o made in the nucleus during transcription
o strand of 80 nucleotides folded into cloverleaf-like shape
o 45 different tRNA’s
 the base-pairing rules relax a bit on tRNA molecules; this is known as wobble
o ex: U in 3rd position of anticodon can pair with A or G
o ex: I (inosine) in 3rd position of anticodon can pair with U, C, or A
o aha! This is why 1 amino acid may correspond with multiple codons, only
differing in the last letter
 aminoacyl-tRNA synthetase joins amino acids to their respective tRNA’s (20 types)
 ribosomes consist of 2 subunits (large and small)
o both subunits are made in nucleolus and combine in cytoplasm
o
proteins + rRNA (ribosomal RNA)
o binding sites

P site: holds tRNA carrying growing polypeptide

A site: holds tRNA carrying the on-deck amino acid

E site: where tRNA molecules exit the ribosome
 Stages (same as transcription)
1. Initiation
 Requires GTP (source of energy)
 mRNA, tRNA, 2 ribosomal subunits come together
2. Elongation
 Requires GTP
 Amino acids added single file, with peptide bonds forming in between
 Translocation: tRNA in A site moves to P site, then E site
3. Termination
 Release factor binds to the site where the stop codon is “read”
 Causes H2O to bind to pp, hydrolyzing pp from ribosome
 Everything dissasembles, leaving completed pp behind
Mutations
 Changes in genetic material
 Point mutations can change only a tiny piece in the DNA and may or may not have
dramatic results
 Base pair substitutions
o Replacement of nucleotide pair with another pair
o Silent mutations do not change which amino acid is synthesized
o Missense mutations still codes for an amino acid, but the wrong one
o Nonsense mutations code for stop codon, causing production of
nonfunctional protein
 Insertions and Deletions
o + or – one or more nucleotide pairs
o usually have a more profound effect than bp substitutions
o frameshift mutation occurs if # of nucleotides added or deleted  3

alters all codons located “downstream”