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ISCI 2001
Chapter 16
GENETICS
“From DNA to Protein”
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What Is a Gene?
• A gene is a section of DNA
that contains instructions
for making a protein.
• An organism’s genetic genotype.
• The traits an organism
exhibits - phenotype.
• Genes are translated into
proteins
– Proteins, other
macromolecules expressed
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Chromosomes
A chromosome
• Single strand of DNA
• DNA wrapped around ‘Histone
proteins’
Chromosomes are ‘condensed’
– Chromatin condenses into
chromosomes
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Genes are located at the
same place in homologous
chromosomes
Pair of
homologous
chromosomes
Location of Genes
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Linked Genes
Chromosomes – Karyotype
1. Chromosomes
are paired up!
2. Homologous
23-Pairs
3. Similar in: Size;
Shape;
Centromere
Location, etc.
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Chromosomes
• 23rd pair
– sex chromosomes —determines the sex of the person.
• Males have one X and one Y chromosome.
• Females have two X chromosomes.
• All the other chromosomes - autosomal.
– Pairs 1-22
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By 1950, scientists knew DNA was the genetic material, but
they did not know the structure of DNA.
In 1953, Watson and Crick built a model of DNA that was
consistent with available evidence.
They used X-ray photos of DNA taken by Franklin and Wilkins
as part of their research.
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The Structure of DNA
A molecule of DNA
– double helix.
The “side” of the ladder
consists of alternating
molecules of
deoxyribose sugar and
phosphate. The “rungs”
are a series of paired
nitrogenous bases.
Built of Nucleotides
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The Structure of DNA
Only four bases are used in DNA:
• Adenine (A)
• Guanine (G)
• Cytosine (C)
• Thymine (T)
A only binds with T, and G only binds with C.
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DNA Replication
During replication,
• The two strands of the DNA molecule unzip
– Helicase enzyme
• Each strand serves as a template for making a new
partner, following the base-pairing rules
• Each new DNA molecule contains one old strand and
one new strand
– Semi-conservative replication
• Each new DNA molecule is identical to the original
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Animation
• DNA REPLICATION
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How Cells Reproduce
In mitosis, one parent cell divides into two daughter cells
that have the same genetic information as the parent.
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How Cells Reproduce
Mitosis is part of the cell cycle.
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Cell Division and the Cell Cycle
• The Cell Cycle
– Period of time between cell divisions
• Interphase
– G1 , G2 , S
– Longest cell cycle phase
• Mitosis
– Division of the cell’s nucleus
– Cytokinesis – division of the cytoplasm
of the cell
• Phases of Mitosis
– Prophase is the longest phase of
mitosis!
– Figure 3.32
– End product: duplicate daughter cells
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How Cells Reproduce
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Animations
• 1. Cell Cycle
• 2. Mitosis and Cytokinesis
• 3. Mitosis (Alt)
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Meiosis: Genetic Diversity
Meiosis is a special form of cell division used
to make haploid cells, such as eggs and
sperm.
Meiosis
– one diploid parent divides into four haploid
daughter cells.
During sexual reproduction,
– sperm and egg join to restore the diploid
chromosome number.
– Zygote (one-cell)
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Meiosis
Meiosis occurs in two stages:
Meiosis I and Meiosis II. Each
stage includes several phases.
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Meiosis I – Major Events
• Crossing Over – Exchange of Genetic Material
during Meiosis I
• Reduction and Division
• Genetic Variation:
– Crossing Over
– Random Fertilization
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Meiosis II – Significant Events
• Similar to mitosis
• 4 – Haploid cells produced
– 23 chromosomes
• Sperm – Spermatogenesis
– All 4-sperm are ‘viable’
• Egg – Oogenesis
– Only 1 of 4 are viable
– Polar Bodies
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Meiosis
As a result of the independent separation of
homologous chromosomes and crossing over, no
two eggs or sperm produced by a single individual
are alike.
The genetic diversity produced during meiosis is
crucial to evolution.
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ANIMATION
• MEIOSIS
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Transcription and Translation
DNA provides instructions for cells to build proteins
through the processes of transcription and
translation.
In transcription, DNA is used as a template for
making RNA.
In translation, this RNA is used to assemble a
protein.
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Transcription and Translation
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Transcription
In eukaryotes, transcription occurs in the cell nucleus.
The two strands of DNA separate. One strand serves as
a template for constructing the RNA transcript.
– mRNA – messenger RNA
RNA uses uracil (U) instead of thymine (T).
RNA polymerase adds the free nucleotides to the
growing RNA molecule.
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Transcription
Once this process is complete, the DNA zips back up.
The RNA molecule made during transcription is called
messenger RNA (mRNA).
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Translation
Translation occurs at ribosomes.
mRNA is translated into protein by “reading” triplets of
nucleotides called codons.
Each codon represents an amino acid. These are
strung together to make a protein.
Translation uses transfer RNA (tRNA) to transfer amino
acids to the protein being assembled.
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Translation
The tRNA’s anticodons bind to the mRNA’s codons.
The process repeats until a stop codon is reached.
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Translation
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PROTEIN SYNTHESIS ANIMATION
• PROTEIN SYNTHESIS
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Gregor Mendel’s work explained hereditary patterns.
His experiments breeding pea plants demonstrated
the existence of dominant and recessive traits.
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Mendel postulated that heritable factors (genes) that
determine traits consist of two separate alleles.
Half of the sex cells an individual produces carry
one allele, and the other half carry the second
allele.
This is Mendel’s principle of segregation.
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When two homozygotic (WW or ww) pea
plants are bred, the offspring inherit one
W (round pea) allele and one w (wrinkled
pea) allele.
These Ww plants are called heterozygotes.
All of the heterozygotes express
dominant characteristics—they are round
pea plants.
In the second generation, self-fertilizing Ww
plants produce a 3:1 ratio of round pea
to wrinkled pea plants.
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By crossing plants with two
different traits, Mendel showed
that the inheritance of one trait
was independent of the
inheritance of the other.
This is Mendel’s principle of
independent assortment.
ANIMATION
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In incomplete dominance, the combination of two
alleles in a heterozygote produces an intermediate
trait.
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Inheritance: Beyond Mendelian Genetics
In codominance, the combination of two alleles in a
heterozygote results in the expression of both
traits.
Blood type is an example of codominance. Three
alleles—A, B, and O—can result in blood types A, B,
AB, or O. The AB blood type shows codominance.
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Polygenic traits are determined by more than one
gene. They tend to show more of a continuum than
traits determined by a single gene.
Examples: human eye color, skin color, and height
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Inheritance: Beyond Mendelian Genetics
Pleiotropy occurs when a single gene affects more
than one trait.
Examples:
• sickle cell anemia in humans
• a single allele in cats that leads to white fur, blue
eyes, and deafness
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Sex-linked traits are determined by genes found on
sex chromosomes (X and Y in humans).
Sex-linked genes occur more often on the X
chromosome. Consequently, men, who have only
one X chromosome, are more likely than women to
exhibit recessive sex-linked traits such as red–
green color-blindness and hemophilia.
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The Human Genome
The human genome consists of 23 pairs of
chromosomes.
The Human Genome Project sequenced the entire
human genome.
Over 99.9% of the 3 billion base pairs in the human
genome are identical in all humans.
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The Human Genome
Humans have about 30,000 genes.
Most of the bases in the genome consist of “junk
DNA” with no known function.
The chromosomes differ in the number of genes they
carry.
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Genetic mutations:
• Occur when the sequence of nucleotides in an
organism’s DNA is changed
•
May result from errors in DNA replication or from
exposure to mutagens (UV light, X-rays,
chemicals)
•
May have no effect, some effect, or very dramatic
effects
•
In egg or sperm cells may be passed down to
offspring
•
Are the ultimate source of genetic variation
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A point mutation occurs when one
nucleotide is substituted for
another.
A nonsense mutation creates a stop
codon in the middle of a proteincoding sequence.
A frameshift mutation occurs when
nucleotides are deleted or inserted,
shifting the reading frame of the
amino acid sequence.
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Radioactivity and Genetic Mutation
When ionizing radiation strikes
electrons in the body with sufficient
energy, they free the electrons from
the atoms they were attached to.
These free electrons may strike and
damage DNA directly.
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Radioactivity and Genetic Mutation
Frequently dividing cells have less time to repair their
DNA damage and thus are more vulnerable to
radiation damage.
Examples: cells in bone marrow, lining of the
gastrointestinal tract, testes, and developing fetus
Because cancer cells also divide frequently, radiation
is sometimes used to treat tumors.
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• A mutation in a single gene is never enough
to cause cancer—mutations in many key
genes are required.
• Over a lifetime, mutations build up until a
combination of mutations in a single cell
allows uncontrolled cell division.
• Further mutations expand the tumor cells’
ability to divide and spread.
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Cancer is most likely to strike:
• Older people
• People who have been exposed to mutationcausing agents
• People who have inherited mutations in cancerrelated genes
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Genes that have been implicated in cancer:
• Proto-oncogenes: when mutated, they become
oncogenes that stimulate abnormal cell division.
• Tumor-suppressor genes: prevent cancer by
inhibiting cell division. Mutations in both alleles of
a tumor-suppressor gene are necessary to destroy
the protective effect.
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Cancer: Genes Gone Awry
Metastasis: the ability of tumor cells to spread
around the body and give rise to secondary tumors.
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ANIMATIONS
 BIOTECHNOLOGY
 DNA LIBRARY
 DNA Fingerprinting
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