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Chapter 15
Vocabulary:
One gene / one polypeptide hypothesis
Central Dogma
Transcription
Translation
mRNA
tRNA
rRNA
snRNA
SRP
microRNA
Codon
Intron
Exon
Point mutation
Frameshift mutation
Translocation
Outline
15.1 Nature of Genes
Garrod examined diseases that seemed to run in families. He hypothesized that a
mutation in a gene affected an enzyme (a gene product) and that these mutations could be
passes on.
Beadle and Tatum induced DNA damage that altered the functionality of their enzyme
product. They were then able to verify that these mutations could be passed on in a
Mendelian fashion. Since they observed that single gene mutations affected single
enzymes involved in a metabolic pathway this lead to the one gene / one polypeptide
hypothesis.
The central dogma of molecular biology explains how the genotype gives rise to the
phenotype of an organism.
Simplistically DNA  RNA  proteins
Transcription translation
Modified with discovery of reverse transcriptase (found in retroviruses)
DNA ↔ RNA  proteins
Transcription uses the template strand of DNA to make a mRNA strand that has the same
sequence as the coding strand (except that there are U’s in RNA and T’s in DNA)
Translation uses a ribosome to read the mRNA and synthesize proteins
RNA’s
mRNA – carries the code
rRNA – component of ribosomes
tRNA – carries amino acids used to build proteins
snRNA – involved in processing mRNA transcript
SRP – allows for translation on the roughER
MicroRNA – regulatory function
siRNA – regulatory function
15.2 Genetic Code
Crick and Brenner discovered that a triplet of nucleotides, codon, encodes for a single
amino acid by inducing mutations that deleted single, double and triple nucleotides from
DNA and observing the protein products.
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Genetic code is degenerate (redundant)
Stop codons: UAG, UAA, UGA
Start codon: AUG (methionine)
Wobble effect at third position
Near universal (exceptions in organelles and ciliates (prokaryotes also use Nformylmethionine instead of methionine to initiate translation)
15.3 Prokaryotic Transcription
Promoter – sequence within DNA
Docking site for RNA polymerase
Signifies start of a gene
Infers directionality of the gene
Elongation uses RNA polymerase to add ribonucleotides that are complementary to the
template strand of DNA
RNA polymerase has a subunit that recognizes the promoter with accuracy and another
subunit that synthesizes mRNA. RNA polymerase does not require a primer
Most common mechanism for termination is the formation of a hairpin structure
In prokaryotes transcription and translation happen simultaneously.
Operons are multiple genes under the control of the same promoter
15.4 Eukaryotic Transcription
Promoter - -10sequence is a TATA box
More initiation factors involved
Elongation occurs in the same fashion, but eukaryotes have multiple RNA polymerases
RNA polymerase I – transcribes rRNA
RNA polymerase II – transcribes mRNA and snRNA
RNA polymerase III – transcribes tRNA and other small RNA
Termination sites are not well defined
mRNA processing:
5’ 7-methyl guanosine cap is added
Allows for binding to the ribosome
Protects end from degradation
3’ poly A tail is added
Protects end from degradation
Regulates how many products can be made from a single transcript
Intron splicing
15.5 Eukaryotic pre-mRNA splicing
Introns
Exons (1-1.5% of the genome)
snRNA recognizes intron exon junction and form a splicesome
(introns are tagged by their sequence)
Cleavage occurs at 5’ end of intron and a lariat is formed
Free 3’ end of exon is used to displace the intron and join exon to exon
Alternate splicing allows a single transcript to be translated into different proteins. (Very
common in the immune system – VDJ recombination allows for the creation of endless
variety in antibodies.)
15.6 Structures of tRNA and Ribosomes
tRNA has two ends
anticodon loop
acceptor end to which an amino acid is bound by aminoacyl-tRNA synthetases
Once tRNA has bound amino acid it is “charged”
Ribosomes are composed of large and small subunits
Small subunit binds mRNA
Large subunit has enzyme activity that will bond amino acids
Ribosome has 3 sites:
A – accepts new charged tRNAs with anticodons that are complementary to the
mRNA
P – site where the formation of a peptide bond between amino acids is catalyzed
E – site where empty tRNA is released from ribosome
15.7 Process of Translation
Prokaryotic initiation
Small subunit binds mRNA at conserved sequence
Initiation factors associate
Initiator tRNA enters ribosome at start codon
N-formymethionine is first amino acid added.
Eukaryotic initiation
Small subunit binds to 5’ cap
Methionine is first amino acid added
Elongation
tRNA with anticodon that can bind codon enters A site (wobble effect)
peptide bond formed in P site
empty tRNA leaves ribosome
translocation
repeat
Termination
Stop codon is reached
Release factors enter ribosome instead of tRNA
Direction to roughER for translation involved signal sequence in beginning of
polypeptide. SRP bind polypeptide and roughER receptor and translation occurs into
roughER.
15.8 Summarizing Gene Expression
15.9 Mutations: Altered Genes
Mutation – any change in the DNA sequence of an organism
Point mutation – insertion, deletion, substitution
Substitution
Silent
Missense
Transitions
Transversions
Nonsense
Insertion and deletion
Frameshift
Triplet repeat expansion mutations
Chromosomal mutations
Deletions – portion lost
Duplication – portion is copied
Inversions – portion excised, inverted, inserted
Translocation – portion moved from one locus to another
Problems with regulation
Problems with meiosis
This is really about 3 chapters in one chapter; transcription, translation and them
mutations. Similar to DNA replication, the overall process is very important for them to
understand. Any specific questions will be about eukaryotes rather than prokaryotes, but
pointing out differences is useful.