Biology

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Chapter 20

• Biology
• Sixth Edition
• Raven/Johnson
• (c) The McGraw-Hill Companies, Inc.

20.2 and 20.3 only
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Mutations are rare (a particular gene typically
mutates in about 1 in a million gametes) but
important. One method of evolution. More
important (in terms of evolution) in germ-line
cells than in somatic cells. Why?
Mutation in the bithorax a gene important in a
critical stage of development.
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Change in the content of a genetic message
I love Biology 155 so much, I wish I could take
it again. I live Biology 155 so much, I wish I
could take it again. I take Biology 155 so much,
I wish I could love it again.
Point mutation
Genetic Recombination
Genetic recombination alters gene location and
can occur by gene transfer or reciprocal
recombination
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DNA Alteration by Mutation

• Base substitution Spontaneous pairing errors
• Chemical modification a base is chemically
  altered by a mutagenic chemical
• DNA breaks Ionization radiation can cause
  double strand breaks in DNA
• Slipped mispairing frameshift mutation
• Triplet expansion 3-base sequence repeated
  several times

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Changes in gene position Chromosomal
rearrangements -translocations -inversions Inser
tional inactivation -transposons
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Cancer is unregulated cell growth and results in
a tumor caused by damaged genes.
Cells that leave a tumor and form new tumors at
distant sites are called metastses.
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Sarcoma tumors arising from connective tissue,
bone, or muscle. Carcinoma tumors arising from
epithelial tissue such as skin. Mutagen
(mutagenic) an agent responsible for causing a
mutation in DNA. Carcinogen (carcinogenesis
theory) an agent thought to cause cancer.
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The more colonies the more potent the carcinogen
- Has enzymes that can convert carcinogens to
mutagens
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Cancer Alley??
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Many cancers can be avoided just by altering our
behavior!
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Some Tumors are Caused by Chemicals - and many
of these chemicals are common (see table 20.3)
Benzene, diesel exhaust, mineral oils,
pesticides, cigarette smoke!
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Mutations in proto-oncogenes genes
Proto-oncogenes are responsible for initiating
cell division when the correct external signal is
received (Simon says). When they mutate to
oncogenes, they initiate cell division without
receiving external signals. Thus, this cell
continues to divide without regulation (doesnt
wait for Simon says).
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Mutations in tumor-suppressor genes
Tumor-suppressor genes are responsible for
blocking cell division (Simon didnt say). When
they mutate to, they are no longer able to block
cell division. Thus, this cell continues to
divide without regulation (doesnt wait for
Simon says).
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Mutations in tumor-suppressor genes
p16 binds with Cdk, blocking cyclins, so E2F is
never released, reinforcing the G1 checkpoint.
Phosphorylation of Rb releases E2F and stimulates
cells division
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Damage to p53 stops this process.
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It usually takes about four mutations within a
cell to initiate cancerous growth. That is why
cancer is more prevalent among the elderly.
Exposure to carcinogens can greatly speed this
process!
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1) Receiving the signal to divide. 2) Relay
switch. 3) Amplifying the signal. 4) Releasing
the brake. 5) Checking that everything is
ready. 6) Telomerase Also - Preventing
Angiogenesis and Metastasis
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The End.
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Plasmid extrachromosomal DNA segments
Conjugation
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When chickens are infected with this virus, the
src viral gene doesnt need the chicken promoters
to initiate cell division.