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Genes and How They Work
Chapter 15/16
The Nature of Genes
Beadle and Tatum proposed the one gene –
one enzyme hypothesis.
Today we know this as the one gene – one
polypeptide hypothesis.
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The Nature of Genes
The central dogma of molecular biology
states that information flows in one
direction:
DNA
RNA
protein
Transcription is the flow of information from
DNA to RNA.
Translation is the flow of information from
RNA to protein.
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The Genetic Code
Deciphering the genetic code required
determining how 4 nucleotides (A, T, G, C)
could encode more than 20 amino acids.
Francis Crick and Sydney Brenner
determined that the DNA is read in sets of
3 nucleotides for each amino acid.
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The Genetic Code
codon: set of 3 nucleotides that specifies a
particular amino acid
reading frame: the series of nucleotides
read in sets of 3 (codon)
– only 1 reading frame is correct for
encoding the correct sequence of amino
acids
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The Genetic Code
Marshall Nirenberg identified the codons
that specify each amino acid.
RNA molecules of only 1 nucleotide and of
specific 3-base sequences were used to
determine the amino acid encoded by
each codon.
The amino acids encoded by all 64 possible
codons were determined.
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The Genetic Code
stop codons: 3 codons (UUA, UGA, UAG)
in the genetic code used to terminate
translation
start codon: the codon (AUG) used to
signify the start of translation
The remainder of the code is degenerate
meaning that some amino acids are
specified by more than one codon.
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Gene Expression Overview
template strand: strand of the DNA double
helix used to make RNA
coding strand: strand of DNA that is
complementary to the template strand
RNA polymerase: the enzyme that
synthesizes RNA from the DNA template
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Gene Expression Overview
Transcription proceeds through:
– initiation – RNA polymerase identifies
where to begin transcription
– elongation – RNA nucleotides are added
to the 3’ end of the new RNA
– termination – RNA polymerase stops
transcription when it encounters
terminators in the DNA sequence
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Gene Expression Overview
• Translation proceeds through
– initiation – mRNA, tRNA, and ribosome
come together
– elongation – tRNAs bring amino acids to
the ribosome for incorporation into the
polypeptide
– termination – ribosome encounters a
stop codon and releases polypeptide
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Gene Expression Overview
Gene expression requires the participation
of multiple types of RNA:
messenger RNA (mRNA) carries the
information from DNA that encodes
proteins
ribosomal RNA (rRNA) is a structural
component of the ribosome
transfer RNA (tRNA) carries amino acids to
the ribosome for translation
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Eukaryotic Transcription
RNA polymerase I transcribes rRNA.
RNA polymerase II transcribes mRNA and
some snRNA.
RNA polymerase III transcribes tRNA and
some other small RNAs.
Each RNA polymerase recognizes its own
promoter.
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Eukaryotic Transcription
Initiation of transcription of mRNA requires a
series of transcription factors
– transcription factors – proteins that act
to bind RNA polymerase to the promoter
and initiate transcription
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Eukaryotic pre-mRNA Splicing
In eukaryotes, the primary transcript must be
modified by:
– addition of a 5’ cap
– addition of a 3’ poly-A tail
– removal of non-coding sequences
(introns)
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Eukaryotic pre-mRNA Splicing
The spliceosome is the organelle
responsible for removing introns and
splicing exons together.
Small ribonucleoprotein particles (snRNPs)
within the spliceosome recognize the intronexon boundaries
– introns – non-coding sequences
– exons – sequences that will be translated
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tRNA and Ribosomes
tRNA molecules carry amino acids to the
ribosome for incorporation into a
polypeptide
– aminoacyl-tRNA synthetases add
amino acids to the acceptor arm of tRNA
– the anticodon loop contains 3
nucleotides complementary to mRNA
codons
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tRNA and Ribosomes
The ribosome has multiple tRNA binding
sites:
– P site – binds the tRNA attached to the
growing peptide chain
– A site – binds the tRNA carrying the
next amino acid
– E site – binds the tRNA that carried the
last amino acid
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tRNA and Ribosomes
The ribosome has two primary functions:
– decode the mRNA
– form peptide bonds
peptidyl transferase is the enzymatic
component of the ribosome which forms
peptide bonds between amino acids
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Translation
Elongation of translation involves the
addition of amino acids
– a charged tRNA binds to the A site if its
anticodon is complementary to the
codon at the A site
– peptidyl transferase forms a peptide
bond
– the ribosome moves down the mRNA in
a 5’ to 3’ direction
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Translation
There are fewer tRNAs than codons.
Wobble pairing allows less stringent pairing
between the 3’ base of the codon and the
5’ base of the anticodon.
This allows fewer tRNAs to accommodate all
codons.
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Translation
Elongation continues until the ribosome
encounters a stop codon.
Stop codons are recognized by release
factors which release the polypeptide from
the ribosome.
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Mutation: Altered Genes
Point mutations alter a single base.
– base substitution mutations – substitute
one base for another
• transitions or transversions
• also called missense mutations
– nonsense mutations – create stop codon
– frameshift mutations – caused by
insertion or deletion of a single base
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Mutation: Altered Genes
triplet repeat expansion mutations involve
a sequence of 3 DNA nucleotides that are
repeated many times
triplet repeats are associated with some
human genetic diseases
– the abnormal allele causing the disease
contains these repeats whereas the
normal allele does not
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Mutation: Altered Genes
Chromosomal mutations change the
structure of a chromosome.
– deletions – part of chromosome is lost
– duplication – part of chromosome is
copied
– inversion – part of chromosome in
reverse order
– translocation – part of chromosome is
moved to a new location
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Mutation: Altered Genes
Too much genetic change (mutation) can be
harmful to the individual.
However, genetic variation (caused by
mutation) is necessary for evolutionary
change of the species.
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Control of Gene Expression
• Controlling gene expression is often
accomplished by controlling transcription
initiation.
• Regulatory proteins bind to DNA to
either block or stimulate transcription,
depending on how they interact with RNA
polymerase.
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Control of Gene Expression
• Prokaryotic organisms regulate gene
expression in response to their
environment.
• Eukaryotic cells regulate gene expression
to maintain homeostasis in the organism.
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Regulatory Proteins
• Gene expression is often controlled by
regulatory proteins binding to specific DNA
sequences.
– regulatory proteins gain access to the
bases of DNA at the major groove
– regulatory proteins possess DNAbinding motifs
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Posttranscriptional Regulation
• Introns are spliced out of pre-mRNAs to
produce the mature mRNA that is
translated.
• Alternative splicing recognizes different
splice sites in different tissue types.
• The mature mRNAs in each tissue possess
different exons, resulting in different
polypeptide products from the same gene.
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Protein Degradation
• Proteins are produced and degraded
continually in the cell.
• Proteins to be degraded are tagged with
ubiquitin.
• Degradation of proteins marked with
ubiquitin occurs at the proteasome.
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