How Genes Work
Chapter 9
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DNA or Protein?
Mendel’s work left a key question unanswered:
What is a gene?
The work of Sutton and Morgan established that
genes reside on chromosomes
But chromosomes contain proteins and DNA
So which one is the hereditary material
Several experiments ultimately revealed the
nature of the genetic material….DNA
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9.3 Discovering the Structure of DNA
DNA is made up of nucleotides
Each nucleotide has a central sugar, a
phosphate group and an organic base
The bases are of two main types
Purines – Large bases
Adenine (A) and Guanine (G)
Pyrimidines – Small bases
Cytosine (C) and Thymine (T)
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Fig. 9.3 The four nucleotide subunits that make up DNA
Nitrogenous
base
5-C sugar
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Fig. 9.4 The DNA
double helix
Erwin Chargaff made
key DNA observations
that became known as
Chargaff’s rule
Purines = Pyrimidines
A = T and C = G
The two
possible
basepairs
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In 1953, James Watson and Francis Crick deduced
that DNA was a double helix
They came to their conclusion using Tinkertoy
models and the research of Chargaff and Franklin
Fig. 9.4
James Watson
(1928)
Francis Crick
(1916-2004)
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9.4 How the DNA Molecule Replicates
The two DNA strands are held together by weak
hydrogen bonds between complementary base pairs
A and T
C and G
If the sequence on one strand is
The other’s sequence must be
ATACGCAT
TATGCGTA
Each chain is a complementary mirror image of the
other
So either can be used as template to reconstruct the other
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There are 3 possible methods for
DNA replication
Fig. 9.5
Daughter DNAs
contain one old
and one new
strand
Original DNA
molecule is
preserved
Old and new
DNA are
dispersed in
daughter
molecules
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How DNA Copies Itself
The enzyme helicase first unwinds the double helix
The enzyme primase puts down a short piece of RNA termed the primer
DNA polymerase reads along each naked single strand adding the
complementary nucleotide
Fig. 9.8
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Transcription & Translation
Gene expression is the use of information in DNA to direct the production
of proteins
The path of genetic information is often called the central dogma
DNA
RNA
Protein
Fig. 9.10
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transcription
translation
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9.5 Transcription
The transcriber is
RNA polymerase
It binds to one DNA
strand at a site
called the promoter
It then moves along
the DNA pairing
complementary
nucleotides
It disengages at a
stop signal
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Fig. 9.11
Transcription & Translation
A cell uses three kinds of RNA to make proteins
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Ribosomal RNA (rRNA)
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9.6 Translation
Translation converts the order of the nucleotides of
a gene into the order of amino acids in a protein
The rules that govern translation are called the
genetic code
mRNAs are the “blueprint” copies of nuclear genes
mRNAs are “read” by a ribosome in threenucleotide units, termed codons
Each three-nucleotide sequence codes for an
amino acid or stop signal
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Fig. 9.12
What happened to Thymine (T)?
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Making the Protein
mRNA binds to the
small ribosomal
subunit
The large subunit
joins the complex,
forming the
complete ribosome
mRNA threads
through the
ribosome producing
the polypeptide
Fig. 9.16
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Transfer RNA
tRNAs bring amino
acids to the ribosome
They have two
business ends
Anticodon which is
complementary to
the codon on
mRNA
3’–OH end to
which the amino
acid attaches
Hydrogen
bonding causes
hairpin loops
3-D shape
Fig. 9.14
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Fig. 9.15 How translation works
The process continues until a stop codon enters the A site
The ribosome complex falls apart and the protein is released
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transcription
translation
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9.7 Architecture of the Gene
In eukaryotes, genes are fragmented
They are composed of
Exons – Sequences that code for amino acids
Introns – Sequences that don’t
Eukaryotic cells transcribe the entire gene,
producing a primary RNA transcript
This transcript is then heavily processed to
produce the mature mRNA transcript
This leaves the nucleus for the cytoplasm
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Fig. 9.17 Processing eukaryotic mRNA
Protect from
degradation
and facilitate
translation
Different combinations of exons can generate different
polypeptides via alternative splicing
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6. The polypeptide chain
grows until the protetin is
completed.
7. Phosphorylation or other
chemical modifications can
alter the activity of a protein
after it is translated.
Amino
acid
Completed
polypeptide
tRNA
5’
Ribosome moves
toward 3’ end
Cytoplasm
Fig. 9.18 How
protein synthesis
works in
eukaryotes
Ribosome
5. tRNAs bring their amino
acids in at the A site of the
ribosome. Peptide bonds
form between amino acids at
the P site, and tRNAs exit the
ribosome from the E site.
4. tRNA molecules
become attached to
specific amino acids
with the help of
activating enzymes.
Amino acids are
brought to the
ribosome in the order
dictated by the mRNA.
DNA
Nuclear
membrane
3’
3’
RNA
polymerase
1. In the cell nucleus, RNA
polymerase transcribes
RNA from DNA
3’
Poly-A
tail
5’
5’
5’
3’
Primary
RNA transcript
Exons
Cap
Small
ribosomal
subunit
Nuclear
pore
5’
Cap
Large
ribosomal
subunit
mRNA
Poly-A
tail
Introns
mRNA
3’
2. Introns are excised from the RNA
transcript, and the remaining exons are
spliced together, producing mRNA
3. mRNA is transported out of the
nucleus. In the cytoplasm, ribosomal
subunits bind to the mRNA
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9.9 Mutation
The genetic material can
be altered in two ways
Recombination
Change in the
positioning of the
genetic material
Mutation
Change in the
content of the
genetic material
Bithorax mutant
Fig. 9.22
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9.9 Mutation
Mutation and recombination provide the raw material
for evolution
Evolution can be viewed as the selection of particular
combinations of alleles from a pool of alternatives
The rate of evolution is ultimately limited by the
rate at which these alternatives are generated
Mutations in germ-line tissues can be inherited
Mutations in somatic tissues are not inherited
They can be passed from one cell to all its
descendants
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Kinds of Mutation
Mutations are caused in one of two ways
Errors in DNA replication
Mispairing of bases by DNA polymerase
Mutagens
Agents that damage DNA
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Kinds of Mutation
The sequence of DNA can be altered in one of two
main ways
Point mutations
Alteration of one or a few bases
Base substitutions, insertion or deletion
Frame-shift mutations
Insertions or deletions that throw off the
reading frame
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Fig. 9.23
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Kinds of Mutation
The position of genes can be altered in one of two
main ways
Transposition
Movement of genes from one part of the
genome to another
Occurs in both eukaryotes and prokaryotes
Chromosomal rearrangements
Changes in position and/or number of large
segments of chromosomes in eukaryotes
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Mutation, Smoking and Lung Cancer
Agents that cause cancer are called carcinogens
These are typically mutagens
The hypothesis that chemicals cause cancer was
first advanced in the 18th century
Many investigations since then have determined
that chemicals can cause cancer in both animals
and humans
For example, tars and other chemicals in
cigarette smoke can cause cancer of the lung
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