Download Chapters 10a and 11 PowerPoint

Professor E. U. Gene
Chapters 10 & 11
Mendel, DNA & Genes
Gregor Mendel
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Austrian monk, gardener,
scientist
First acknowledged to
study heredity – the
passing on of
characteristics from
parents to offspring
Traits – characteristics
that are inherited
Father of genetics – the
branch of biology that
studies heredity
The Peas
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Mendel chose garden peas
They reproduce sexually, so
they have both male and
female sex cells, called
gametes
Pollen and egg unite in a
process called fertilization
Fertilization results in a
fertilized cell, called a zygote,
that develops into a seed
The Peas Mate
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Pollination is the transfer of
pollen grains from a male
reproductive organ to a
female productive organ
Both male and female organs
are close together in the
same pea flower
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As a result, peas normally selfpollinate
This is what Mendel wanted in
most cases
Mendel also removed the
male organ and dusted pollen
on flower of another plant
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Called cross-pollination
Mendel’s Peas
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Mendel’s peas had
been self-pollinating
for a long time
This meant that the
tall ones had been tall
for a long time and
the short ones had
been short for
generations
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Called purebreds
Mendel’s Monohybrid Crosses
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Mendel performed crosspollination with a tall pea
plant (6 foot purebred) and
a short pea plant (2 foot
purebred) – these are the
parental generation (P1)
Hybrid – offspring of
parents that have different
forms of a trait
All of the offspring grew as
tall as the tall parent – first
filial generation (F1)
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The “short trait” seemed to
disappear
Monohybrid Crosses
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Mendel let his F1
plants self pollinate
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Second filial
generation (F2)
He counted over 1000
plants
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About 75% were tall
About 25% were short
Ratio of 3:1
Monohybrid Crosses
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Mendel did these same
types of crosses for 7
traits
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Seed shape: round vs.
wrinkled
Seed color: yellow vs.
green
Flower color: purple vs.
white
Flower position: axial vs.
terminal
Pod color: green vs. yellow
Pod shape: inflated vs.
constricted
Plant height: tall vs. short
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back
The Rule of Unit Factors
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Mendel concluded that each organism has two factors
that control each of its traits
We now know these factors are genes and that they
are located on chromosomes
Alternate forms of genes are called alleles
Each of Mendel’s traits had two forms of genes – two
alleles
 One comes from mother, one from father
The Rule of Dominance
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Some alleles are dominant over
recessive alleles
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In genetics, capital letters are used
to express dominant alleles and
lower case letters are used to
express recessive alleles
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Ex: “T” for tall allele, “t” for short
allele
So what will a “TT” plant look like?
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Dominant alleles cover recessive
alleles
A Tt plant?
A tt plant?
Earlobes, Forelock, Dimples,
Straight thumb, Bent Pinky, Middigit hair
The Law of Segregation
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Mendel’s first law of
heredity
Every individual has
two alleles of each
gene and when
gametes are
produced, each
gamete receives one
of these alleles
So how did this work
in Mendel’s F2
generation ?
Phenotypes
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Two organisms can
look alike on the
outside, but have
different allele
combinations
Phenotype is the way
an organism looks
and behaves
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ex: yellow seeds can
be TT or Tt
Genotypes
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The allele combination an
organism contains is known
as its genotype
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An organism is homozygous
for a trait if the two alleles for
the trait are the same
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You can’t always see this
because of dominiance
TT and Tt are different
genotypes
Ex: TT or tt
An organism is heterozygous
for a trait if the two alleles for
the trait are different
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Ex: Tt
Mendel’s Dihybrid Crosses
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Mendel crossed peas that differed from
each other in two traits
He crossed plants that were homozygous
for round yellow seeds (RRYY) with plants
that were homozogous for green wrinkled
seeds (rryy)
F1 generation
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All plants produced round, yellow seeds
What was dominant?
The Second Generation (F2)
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Let F1 plants selfpollinate (were
heterozygous for
two traits)
He got all four
combinations in a
certain ratio
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Round, yellow – 9
Round, green – 3
Wrinkled, yellow – 3
Wrinkled, green - 1
The Law of Independent
Assortment
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Mendel’s second law of
heredity
Genes for different traits
are inherited
independently of each
other
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Inheritance of one trait has
no influence on another trait
Instead of a ratio of
9:3:3:1, what would the
dihybrid cross have looked
like?
Punnett Squares
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1905, Reginald Punnet,
English biologist
created a shorthand
way of finding
EXPECTED proportions
of possible genotypes
in the offspring of a
cross
Monohybrid crosses
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See overhead
Dihybrid crosses
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See overhead
Probability
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Reality is rarely like a Punnett square
When you toss a coin, what’s the
likelihood it will be heads?
You toss 20 heads in a row, what’s the
likelihood the next will be heads?
TOSS COINS
ASSIGNMENT
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P. 262, Problem-Solving Lab
P. 276, Connection to Math
Section 1 Review
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What structural features of pea plants made them
suitable for Mendel’s genetic studies?
What are genotypes of a homozygous and a
heterozygous tall pea plant?
One parent is homozygous tall and the other is
heterozygous. How many offspring will be
heterozygous?
How many different gametes can an RRYy parent
form? What are they?
What is the law of segregation?
What is the law of independent assortment?
What is the rule of dominance?
In garden peas, the allele for yellow peas is dominant
to the allele for green peas. Suppose you have a plant
that produces yellow peas, but you don’t know
whether it is homozygous dominant or heterozygous.
What experiment could you do to find out?
Genetic Variation
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Crossing over during meiosis provides
variability
How many different kinds of gametes can
a pea plant produce?
Each cell has 7 pairs of chromosomes
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Each can line up at the equator in two
different ways and separate by segregation
2n = 27 = 128 possible combinations without
crossing over
When you include the egg, 128 x 128 =
16,384 different combinations of offspring
Genetic Variation
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In humans, how many possible combinations
are there in a single sperm or egg?
223 = 8,388,608 combinations
How many possible combinations with
fertilization
8,388,608 x 8,388,608 = 7.04 x 1013 (over 70
trillion)
Genetic Recombination
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With crossing over, additional variation is
added providing an almost endless
amount of variation possible
This reassortment of chromosomes and
the genetic information they carry, either
by crossing over or by independent
segregation of homologous chromosomes
is called genetic recombination
Variation is the raw material that forms
the basis for evolution to act on
Nondisjunction
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Go back to anaphase I
The failure of homologous chromosomes to separate
properly during meiosis is called nondisjunction
In one type of nondisjunction, two kinds of gametes
result
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One with an extra chromosome (trisomy)
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Trisomy 21 – Down syndrome
One missing a chromosome (monosomy, usually don’t survive)
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One exception – Turner syndrome, female is XO
Nondisjunction
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The other type of nondisjunction
involves a total lack of separation
of homologous chromosomes
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Not uncommon in plants – often
larger and healthier
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Gamete inherits a complete diploid set
Zygote has polyploidy
chrysanthemum (tetraploid, 4n)
Wheat (hexaploid, 6n)
Apples (3n)
There are even chemicals that help
plant breeders do this artificially
Gene Linkage
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Genes that are close
together on a
chromosome are often
inherited together
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Crossing over rarely
works for just one gene
These genes are said to
be linked
So chromosomes, not
genes follow Mendel’s
independent
assortment
Chromosome Mapping
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Crossing over occurs
Geneticists use the
frequency of crossing
over to map the
relative position of
genes on a
chromosome
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Genes that are further
apart are more likely
to have crossing over
occur
Chromosome Mapping
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Suppose there are 4 genes on a chromosome –
A, B, C, D
Frequencies of recombination as follows:
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Between
Between
Between
Between
A
A
B
C
& B: 50% (50 map units)
& D: 10% (10 map units)
& C: 5% (5 map units)
& D: 35% (35 map units)
These give a relative distance between genes
A -10 units- D -35units- C -5 units- B (whole
thing is 50 units)
Section 2 Review
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How are the cells at the end of meiosis different from
the cells at the beginning of meiosis?
What is the significance of meiosis to sexual
reproduction?
Why are there so many varied phenotypes within a
species such as humans?
How does meiosis support Mendel’s law of
independent assortment?
What is crossing over?
How can you use crossing over to map a chromosome?
What are linked genes?
What is DNA?
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DNA contains the complete
instructions for
manufacturing all the
proteins for an organism
DNA achieves its control by
determining the structure of
proteins
The moral - DNA codes for
PROTEINS
Let’s start with an animated
tour
Alfred Hershey & Martha Chase
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For a long time, many
scientists believed protein
was the genetic material
 Protein is more complex
than DNA
1952, Hershey & Chase
experimented with
radioactively labeled viruses
(bacteriophages) made only
of protein & DNA
 DNA was labeled with
one isotope
 Protein with a different
isotope
Hershey & Chase
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Bactriophages inject genetic material into bacteria,
causing the bacteria to create more bacteriophages
Hershey & Chase “followed” the radioactively labeled
viruses
 Discovered that DNA was the genetic material being
injected into the bacteria
Structure of Nucleotides
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DNA is a polymer of
nucleotides
Nucleotides have 3 parts
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A simple sugar (deoxyribose in
DNA)
A phosphate group
A nitrogenous base
4 possible nitrogenous
bases in DNA
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Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)
Watson & Crick
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In 1953, published a paper
proposing that DNA is made of
two chains of nucleotides held
together by nitrogenous bases
 Hydrogen bonds hold
strands together (weak)
 A only bonds with T, C only
bonds with G
DNA is a double helix
Also correctly described now
DNA replicates
Won the Nobel Prize
DNA- Common Thread
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DNA is the same among ALL organisms
with DNA
The differences come with the different
sequences of the four bases
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Like different words meaning different things,
even though the letters are the same
The more alike the sequences, the more related
he organisms are
Replication of DNA
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Without replication, new
cells (after mitosis)
would only have half
the DNA of their parents
All organisms undergo
DNA replicaton
How DNA Replicates
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Due to complementary base
pairing, “knowing” the
sequence of one strand helps
you predict what the other will
look like
Replication begins when an
enzyme breaks the hydrogen
bonds between bases
 Called “unzipping” the DNA
Cells have stockpiles of
nucleotides and these bond
with the newly exposed bases
 Done by a set of enzymes
called DNA polymerases
How DNA Replicates
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Each DNA strand has 2
ends, labeled the 5’ and
and the 3’ end.
 Replication proceeds in
the 5’ 3’ direction
Each new strand formed is
a complement of one of
the parents strands
 Half old, Half new called semiconservative
replication
Genetic continuity is
maintained
Section 1 Review
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Describe the structure of a nucleotide.
How do the nucleotides in DNA bond with each
other within a strand? How do they bond with
each other across strands?
Explain why the structure of a DNA molecule is
often described as a zipper.
How does DNA hold information?
The sequence of nitrogenous bases on one
strand of a DNA molecule is GGCAGTTCATGC.
What would be the sequence of bases on the
complementary strand?
In general, describe the steps that occur during
DNA replication.
Genes & Proteins
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The information in DNA is put to work
through the production of proteins
Some proteins become important structures
Other proteins (enzymes) control chemical
reactions
Remember that proteins are polymers of
amino acids
The sequence of nucleotides
determines the string of amino
acids and the protein
RNA
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RNA differs from DNA in
3 ways:
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RNA is single stranded
(DNA is double)
The sugar in RNA is ribose
(DNA is deoxyribose)
Instead of thymine, RNA
contains the base uracil
3 types of RNA
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Messenger RNA (mRNA)
rRNA
tRNA
Transcription
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In the nucleus (if cell is
eukaryotic), enzymes make an
RNA copy of a portion of a DNA
strand
 This process is called
transcription
Begins as enzymes unzip the
molecule of DNA in region of
gene being transcribed
RNA polymerases pair free
nucleotides with their
complementary strands
The mRNA strand breaks away
RNA Processing
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Genes usually contain long non-coding
sequences of bases, called introns
Regions that contain coding sequences
are called exons
Enzymes in the nucleus cut out the
intron segments from the mRNA
strand and then paste it back together
Genetic Code
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A code is needed to convert the
language of mRNA (4 letters) into the
language of proteins (20 amino acids)
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How big do the words have to be??????
A codon is a group of 3 nitrogenous
bases in mRNA that codes for an amino
acid
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1st one found was UUU - coding for
phenalynine
64 possible codons, so some amino acids
correspond to multiple codons
Genetic Code
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Some codons don’t
code for an amino acid
- they code for
“instructions”
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UAG - codon for “stop”
AUG is the “start”
codon as well as coding
for methionine
The code is universal
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Used in ALL organisms
the same
Translation
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Translation is the
process of converting
the information in a
sequence of
nitrogenous bases in
mRNA into a sequence
of amino acids in a
protein
Takes place at the
ribosomes in the
cytoplasm
Transfer RNA
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Transfer RNA (tRNA) molecules
bring the amino acids (which are
in the cytoplasm) to the
ribosome
 Each tRNA molecule attaches
to only one type of amino acid
There is a sequence of 3
nucleotides on the opposite side
of the tRNA molecule from the
amino-acid attachment site
 It is complementary to a
specific codon, thus it is called
an anti-codon
At the Ribosome
• tRNA attaches to
mRNA at the ribosome
• mRNA strand “slides
along” and another tRNA
attaches at second
bonding site
• two amino acids bond
together with a peptide
bond
• mRNA “slides along”
again, allowing another
tRNA to come on in
• stops at certain codon
and is released
Section 2 Review
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How does the DNA nucleotide sequence
determine the amino acid sequence in a protein?
What is a codon, and what does it represent?
What is the role of tRNA in protein synthesis?
Compare DNA replication and transcription.
You have learned that there are stop codons that
signal the end of an amino acid chain. Why is it
important that a signal to stop translation be part
of protein synthesis?
In general, sequence the steps involved in protein
synthesis from the production of mRNA to the
final translation of the DNA code.
Mutations
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Any change in the DNA
sequence is called a mutation
Can be caused by errors in
replication, transcription, cell
division, or by external agents
Most mutations are
detrimental - create
disfunctional proteins, etc.
Occasionally, they may have
positive effects
Mutations
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Mutations in reproductive cells are passed on
to offspring
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Happen to a sperm or egg
All of offspring’s cells have this trait
Most embryos do not survive
Mutations in body cells effect only that
individual
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Usually only one cell or a few cells at first
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Passed on if continue to divide
Point Mutations
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A change in a single
base pair in DNA
Will change one amino
acid
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Affects depend on
which amino acid is
changed
Example
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THE DOG BIT THE CAT
THE DOG BIT THE CAR
Frameshift Mutation
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A mutation in which a
single base is added
or deleted from the
DNA
Every codon after the
mutation would be
different
Example
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THE DOG BIT THE
CAT
THE DOB ITT HEC AT
Chromosomal Alterations
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Structural changes in chromosomes
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Especially common in plants
Often happens in meiosis
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Parts of chromosomes break off
Parts of chromosomes switch places
Most zygotes fail to grow
Non-disjunction is an example
Chromosomal Mutations
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4 kinds
Deletion: part of a chromosome is
left out
Insertion: part of a chromatid
breaks off and attaches to sister
chromatid (duplication of segment)
Inversion: part of a chromosome
breaks off and reattaches
backwards
Translocation: part of a chromosme
breaks off and attaches to nonhomologous chromosome
Causes of Mutations
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Spontaneous mutations occur as
a mistake during DNA
replication (probably - we don’t
know for certain)
Mutagens are environmental
agents that can cause a change
in DNA
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X-rays, UV light, nuclear radiation
Dioxins, asbestos, benzene,
formaldehyde
Repairing DNA
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Cells have repair
mechanisms for fixing DNA
These enzymes “proofread”
new DNA strands during
replication and replace
incorrect nucleotides with
correct ones
 Work well, but not
perfect
 The more exposure to a
mutagen, the more likely
the mutation will not be
fixed
Section 3 Review
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What is a mutation?
Describe how point mutations and frameshift
mutations affect the synthesis of proteins.
Explain why a mutation in a sperm or egg cell has
different consequences than one in a heart cell.
How are mutations and cancer related?
Name and describe the four kinds of chromosomal
mutations.
The chemicals in cigarette smoke are known to
cause cancer. Propose a series of steps that could
lead to development of lung cancer in a smoker.