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Chapter 15
Genes and How They Work
15.1 Nature of Genes
one gene / one polypeptide hypothesis.
The central dogma of molecular biology
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
rRNA
tRNA
snRNA
SRP RNA
MicroRNA
15.2 Genetic Code
43 = 64 codons
Codon Table (Table 15.1, p. 283)
Genetic code is degenerate
Stop codon
Start codon
Wobble effect at third position
Near universal
15.3 Prokaryotic Transcription
Begins at a promoter  transcribes the transcription unit  ends at the terminator
Promoter – sequence within DNA
Elongation uses RNA polymerase to add ribonucleotides that are complementary to the
template strand
Most common mechanism for termination is the formation of a hairpin structure
In prokaryotes transcription and translation happen simultaneously.
Operons
15.4 Eukaryotic Transcription
Promoter (differs for different polymerases)
Elongation occurs in the same fashion, but eukaryotes have multiple RNA polymerases
RNA polymerase I
RNA polymerase II
RNA polymerase III
Transcription factors
Termination sites not well defined
mRNA processing:
5’ cap
3’ tail
Intron splicing
15.5 Eukaryotic pre-mRNA splicing
Introns
Exons
snRNA recognizes intron exon junction and form a splicesome
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
15.6 Structures of tRNA and Ribosomes
tRNA has two ends
anticodon loop
acceptor end to which an amino acid is bound
aminoacyl-tRNA synthetases
Ribosomes are composed of large and small subunits
3 sites:
A
P
E
Ribosomes have enzymatic functions
15.7 Process of Translation
Initiation
Initiation factors associate
Initiator tRNA enters ribosome
Small subunit of ribosome binds mRNA
Large subunit of ribosome binds small subunit
Elongation
tRNA with anticodon that can bind codon enters A site
peptide bond formed in P site
empty tRNA leaves ribosome
translocation
repeat
Termination
Stop codon is reached
Release factors enter ribosome
Direction to roughER via SRP
15.8 Summarizing Gene Expression
15.9Mutations: Altered Genes
Mutation
Point mutation – insertion, deletion, substitution
Substitution
Silent
Missense
Transitions
Transversions
Nonsense
Insertion and deletion
Frameshift
Chromosomal mutations
Deletions
Duplication
Inversions
Translocation