Download Lyons/Hewitt/Suchocki/Yeh, CONCEPTUAL INTEGRATED SCIENCE

Hewitt/Lyons/Suchocki/Yeh
Conceptual Integrated
Science
Chapter 16
GENETICS
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This lecture will help you
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What Is a Gene?
Chromosomes
The Structure of DNA
DNA Replication
Transcription and Translation
Meiosis: Genetic Diversity, Meiosis I, Meiosis II
Mendelian Genetics
Inheritance: Beyond Mendelian Genetics
The Human Genome
Genetic Mutations
Radioactivity and Genetic Mutation
Cancer: Genes Gone Awry
History of Science: Discovery of the Double Helix
Science and Society: Genetic Counseling
Technology: Biotech
Science and Society: DNA Fingerprinting
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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 makeup is its
genotype.
• The traits an organism exhibits are known
as its phenotype.
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Chromosomes
A chromosome consists of a
single long strand of DNA
and small proteins called
histones around which the
DNA is wrapped. Genes are
sections of chromosomes.
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Chromosomes
Most cells have two of each kind of
chromosome. These cells are diploid, and
their matched chromosomes are called
homologous chromosomes.
Sperm and egg cells contain only one of
each kind of chromosome. These cells are
haploid.
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Chromosomes
• Humans have 46 chromosomes (23 pairs).
• One pair—the 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 are autosomal
chromosomes.
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Chromosomes
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The Structure of DNA
A molecule of DNA consists of two strands and
looks like a spiraling ladder. It is often called a
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.
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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
• 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
• Each new DNA molecule is identical to the
original
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DNA Replication
DNA replication always begins at fixed spots
within chromosomes. In eukaryotes,
replication begins simultaneously at many
points.
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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.
RNA uses uracil (U) instead of thymine (T).
Otherwise, transcription follows the same basepairing rules as DNA replication.
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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Transcription
Once the mRNA molecule has been
transcribed:
• A cap and a tail are added
• Irrelevant stretches of nucleotides—called
introns—are removed from the mRNA.
The sections that
remain are called
exons.
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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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Meiosis: Genetic Diversity
Meiosis is a special form of cell division used to make
haploid cells, such as eggs and sperm.
In meiosis, one diploid parent divides into four haploid
daughter cells.
During sexual reproduction, sperm and egg join to restore
the diploid chromosome number.
At the start of meiosis, the cell has already duplicated its
genetic material and has four of every chromosome.
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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
• Prophase I—chromosomes condense, nuclear
membrane breaks down, spindle apparatus
appears.
• Metaphase I—homologous chromosomes line
up at the equatorial plane. Crossing over occurs.
This results in recombination, the
production of new combinations of
genes that are different from the
parents’ combinations of genes.
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Meiosis I
• Anaphase I—chromosome pairs are
separated by the spindle apparatus.
• Telophase I—chromosomes move to the
poles of the cell.
• Cytokinesis occurs, producing two
daughter cells each with two sets of
chromosomes.
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Meiosis II
• Prophase II—chromosomes move to the center
of the cell and a new spindle apparatus forms.
• Metaphase II—chromosomes line up along the
equatorial plane.
• Anaphase II—sister chromatids separate.
• Telophase II—sister chromatids move to
opposite poles of the cell.
• Nuclear membranes then reform and cytokinesis
produces four haploid daughter cells.
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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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Mendelian Genetics
Gregor Mendel’s work explained hereditary
patterns.
His experiments breeding pea plants
demonstrated the existence of dominant
and recessive traits.
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Mendelian Genetics
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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Mendelian Genetics
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, selffertilizing Ww plants produce a 3:1
ratio of round pea to wrinkled pea
plants.
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Mendelian Genetics
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.
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Inheritance: Beyond Mendelian
Genetics
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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Inheritance: Beyond Mendelian
Genetics
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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Inheritance: Beyond Mendelian
Genetics
Linked genes are genes that are often
inherited together. The closer two genes
are to each other on a single
chromosome, the more likely they are to
be inherited together.
Examples: body color and wing size in fruit
flies
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Inheritance: Beyond Mendelian
Genetics
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 sexlinked traits such as red–green colorblindness 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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The Human Genome
There are more than 3 million places in the
human genome where the base pair
sequence differs among humans.
These single nucleotide polymorphisms
(SNPs) may help scientists identify genes
related to human diseases.
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Genetic Mutations
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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Genetic Mutations
A point mutation occurs when one
nucleotide is substituted for another.
A nonsense mutation creates a stop
codon in the middle of a protein-coding
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.
Indirect damage occurs when the
freed electron hits a water molecule
in the cell, producing a free radical,
a group of atoms that has an
unpaired electron and is therefore
unstable and highly reactive. The
free radical then reacts with DNA
and damages it.
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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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Cancer: Genes Gone Awry
• 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: Genes Gone Awry
Cancer is most likely to strike:
• Older people
• People who have been exposed to
mutation-causing agents
• People who have inherited mutations in
cancer-related genes
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Cancer: Genes Gone Awry
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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History of Science: Discovery of the
Double Helix
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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Science and Society: Genetic
Counseling
A pedigree is a family tree that shows which
relatives are and are not affected by a particular
genetic disease.
Medical tests can determine if a person is a
carrier of a disease allele.
Amniocentesis and chorionic villus sampling
can determine if a fetus has a genetic disease.
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Technology: Biotech
Biotechnology applies knowledge of
genetics and molecular biology to
• Diagnose and treat disease
• Engineer specialized plant varieties
• Develop organisms for specific
applications (such as spider silk produced
in goats’ milk)
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Science and Society: DNA
Fingerprinting
Only identical twins have identical DNA.
Forensic scientists use restriction
fragment length polymorphisms
(RLFPs) to match DNA to suspects.
Another method involves comparing ACGT
sequences for variable regions of the
genome to available samples.
Because DNA evidence is very reliable, it is
not controversial among scientists.
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