Ch16 genetics

Chapter 16:
Genetics
Principles of Science II

This lecture will help you understand:
• What Is a Gene?
• Chromosomes: Packages of Genetic Information
• The Structure of DNA
• DNA Replication
• How Proteins Are Built
• Genetic Mutations
• How Radioactivity Causes Genetic Mutations
• Meiosis and Genetic Diversity
• Mendelian Genetics
• More Wrinkles: Beyond Mendelian Genetics

This lecture will help you understand:
• The Human Genome
• Cancer: Genes Gone Awry
• Environmental Causes of Cancer
• Transgenic Organisms and Cloning
• DNA Technology—What Could Possibly Go Wrong?
• History of Science: Discovery of the Double Helix
• Technology: Gene Therapy
• Science and Society: Genetic Counseling
• Science and Society: DNA Forensics

What Is a Gene?
• A gene is a section of DNA that contains the
instructions for building a protein.
• An organism's genes make up its genotype.
• The traits of an organism make up its phenotype.

Chromosomes: Packages of Genetic
Information
• A chromosome consists of a
long DNA molecule wrapped
around small proteins called
histones. Genes are sections
of chromosomes.

Information
• Most cells have two of each kind of chromosome.
These cells are diploid, and their matched
chromosomes are called homologous
chromosomes.
• Sperm and eggs contain only one of each kind of
chromosome. They are haploid.

Information
• 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 autosomes.

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 "sides" of the ladder consist of alternating
molecules of deoxyribose sugar and phosphate. The
"rungs" are a series of paired nitrogenous bases.

The Structure of DNA
• Four nitrogenous bases are used in DNA:
– Adenine (A)
– Guanine (G)
– Cytosine (C)
– Thymine (T)
• A binds with T, and G binds with C.

DNA Replication
• During replication:
– DNA's two strands are
separated.
– Each strand serves as a
template for building a new
partner, following the base-
pairing rules.
– Each new DNA molecule
includes one old strand and one new strand.
– Each new DNA molecule is identical to the
original.

How Proteins Are Built
• RNA, or ribonucleic acid,
plays a key role.
• RNA differs from DNA in
several ways:
– Single-stranded instead
of double-stranded
– Uses ribose instead of
deoxyribose sugar
– Uses the nitrogenous base
uracil (U) instead of thymine (T)

• DNA provides instructions for cells to build proteins
through the processes of transcription and
translation.
• During transcription, DNA is used as a template for
making an RNA molecule.
• During translation, this RNA molecule is used to
assemble a protein.

• Transcription
– In eukaryotes, transcription occurs in the cell
nucleus.
– The two strands of DNA separate, and one strand
serves as a template for building the RNA
transcript.
– Transcription follows the usual base-pairing rules
except that RNA uses uracil (U) instead of
thymine (T).
– RNA polymerase adds the free nucleotides to the
growing RNA molecule.

• RNA processing
– Introns are removed.
– Exons remain.
– A cap and a tail are
added.
– The result is an mRNA
molecule ready for
translation.

• Translation
– Translation occurs at ribosomes in the cytoplasm.
– Codons, sets of three nucleotides, are "read"
from the mRNA.
– Most codons represent a single amino acid to be
added to the growing protein.
– Stop codons tell the ribosome that no more
amino acids should be added and that translation
is complete.

• The genetic code

• A tRNA molecule has a set
of three nucleotides, called an
anticodon, and carries a single,
specific amino acid.
• A tRNA's anticodon binds to
the mRNA's codon.

Genetic Mutations
• Occur when the sequence of nucleotides in an
organism's DNA is changed
• May result from errors during DNA replication or
from exposure to things that damage DNA (UV light,
X-rays, chemicals, etc.)
• May have no effect, some effects, or huge effects
• In eggs or sperm, may be passed down to offspring
• Are the ultimate source of all genetic diversity and
provide the raw materials for evolution

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 gene.
• A frameshift mutation occurs when nucleotides are
inserted or deleted, shifting the codons that are
"read" during translation.

How Radioactivity Causes Genetic
Mutations
• Ionizing radiation strikes electrons
in the body, freeing them from the
atoms they were attached to.
• The free electrons may hit and
damage DNA directly.
• Free electrons may hit a water
molecule, producing a free radical,
a group of atoms that has an
unpaired electron and is highly
reactive. The free radical may then
react with DNA and damage it.

How Radioactivity Causes Genetic
Mutations
• Frequently dividing cells have less time to repair
DNA damage before passing on mutations and so
are more vulnerable to radiation damage.
– Examples: cells in the bone marrow, lining of the
digestive tract, testes, and developing fetus
• Because cancer cells also divide frequently,
radiation is sometimes used to treat tumors.

Meiosis and Genetic Diversity
• Meiosis is a form of cell
division used to make haploid
cells, such as eggs and sperm.
• In meiosis, one diploid cell
divides into four haploid cells.
• During sexual reproduction,
sperm and egg join to
restore the normal diploid
chromosome number.

• At the beginning of meiosis, the diploid cell has
already copied its DNA.
• Meiosis takes place in two steps: meiosis I and
meiosis II.

• During prophase I of meiosis,
crossing over occurs: Chromosomes
exchange parts with their homologous
chromosomes.
• The chromosomes in the dividing cell
are now different from those in the
original cell.
• Crossing over results in recombination,
the production of new combinations of
genes different from those found in the
original chromosomes.

• How does meiosis result in genetic diversity?
1. Crossing over
2.Independent separation of homologous
chromosomes
• The genetic diversity produced during meiosis is
crucial to evolution.

Mendelian Genetics
• Gregor Mendel's experiments breeding pea plants
explained many hereditary patterns.
• Mendel demonstrated the existence of dominant
and recessive traits.

Mendelian Genetics
• Mendel postulated that the genes that determine
traits consist of two separate alleles. One allele is
inherited from each parent.
• Mendel's principle of segregation: When an
individual makes sex cells (sperm or eggs), half the
sex cells carry one allele, and the other half carry
the other allele.

Mendelian Genetics
• Mendel bred two pea plants
that varied in a single trait
--for example, round peas (RR)
and wrinkled peas (rr).
• The offspring inherited
one R (round pea) allele
and one r (wrinkled pea)
allele. They were Rr.
• All of the offspring expressed
the dominant characteristic—
they had round peas.
• In the second generation, self-fertilizing the Rr plants
resulted in a 3:1 ratio of round-pea plants to wrinkled-
pea plants.

Mendelian Genetics
• Mendel's principle of
independent assortment:
The inheritance of one
trait is independent of
the inheritance of a
second trait.
• Mendel demonstrated
this by crossing plants
with two different traits.

More Wrinkles: Beyond Mendelian Genetics
• In incomplete dominance, there are two alleles and
neither is dominant. The heterozygote has an
intermediate trait.
• Example: snapdragon color

• In codominance, a heterozygote expresses the traits
of both alleles.
• Example: human blood type

• 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

• Pleiotropy occurs when a single
gene affects more than one trait.
• Example: sickle cell anemia in
humans

• Linked genes are often inherited together. The
closer two genes are to each other on a
chromosome, the more likely they are to be
inherited together.
• Example: body color and wing size in fruit flies are
linked

• Sex-linked traits are determined by genes found on
the X chromosome. Men, who have only one X
chromosome, need only one recessive allele to
express a recessive sex-linked trait. These traits are
more common in males than females.
• Examples: red-green color-blindness, hemophilia

The Human Genome
• A genome is the total genetic material of an
organism.
• The Human Genome Project determined the DNA
sequence of the entire human genome.
• Over 99.9% of the 3.2 billion nucleotide pairs in the
human genome are identical in all humans.

The Human Genome
• Humans have about 22,000 genes.
• Many human genes give rise to RNA transcripts that
are processed in different ways. So, one gene can
provide the instructions for building multiple
proteins.
• The function of more than half of our genes is still
unknown.

The Human Genome
• Single-nucleotide polymorphisms (SNPs) are
locations in the genome where the nucleotide
sequence differs among humans.
• More than 3 million SNPs are known.
• SNPs may help scientists identify genes related to
human diseases.

Cancer: Genes Gone Awry
• Cancer occurs when cells in the body divide out of
control.
• Mutations in the genes that control cell division
result in cancer.
• A mutation in a single gene is not 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.
• Cancer is most likely to strike older people, those
who have been exposed to mutation-causing
agents, and those who have inherited mutations in
cancer-related genes.

• Genes that have been implicated in cancer:
– Proto-oncogenes: When mutated, they become
oncogenes that stimulate abnormal cell division.
– Tumor-suppressor genes: They prevent cancer
by inhibiting cell division.
• Metastasis is the ability of tumor cells to spread
around the body and give rise to secondary tumors.
Cancer is much harder to treat once metastasis has
occurred.

Environmental Causes of Cancer
• A person's environment is responsible for about
80%–90% of the mutations that result in cancer.
• Environmental risk factors:
– Smoking
– Diet
– Radiation
– Ultraviolet light
– Chemicals
– Infection by certain viruses and bacteria

Transgenic Organisms and Cloning
• A transgenic organism is one that contains a gene
from another species.
• Typical process for developing transgenic bacteria

• Examples of transgenic organisms:
– Bacteria that produce insulin and other important
products
– Plants that
• produce medicines
• have resistance to pests, diseases, or
herbicides
• are drought-resistant or able to grow in salty
soils
– Animals that produce products:
• Sheep with increased wool production
• Pork with higher levels of omega-3 fatty acids
• Salmon that grow faster

• Cloning is the creation of an organism that is
genetically identical to one that already exists.
• In mammals, cloning is done through the process of
nuclear transplantation.
• Potential uses of cloning:
– A routine part of agriculture
– Could generate herds of identical animals with
desirable traits
– Cloning of endangered species could help
increase their numbers
– Cloning of deceased pets

DNA Technology – What Could Possibly Go
Wrong?
• Some bacteria and viruses are a danger to human
health or to natural habitats.
– How likely is an accidental release?
• Potential dangers of genetically modified (GM)
plants and animals:
– Is the safety of GM food adequately tested?
– Should GM foods be labeled?

DNA Technology – What Could Possibly Go
Wrong?
• Potential dangers of GM plants and animals
(continued):
– Plants that are toxic to pests also harm nontarget
species --for example, Monarch butterflies
– May lead to the evolution of resistant
"superweeds" that can be controlled only with
very toxic chemicals
– Contamination of natural habitats or populations
by transgenic plants and animals or their genes
– Cost of GM seeds and products
• Effects on human societies

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.
• Watson and Crick used X-ray photos of DNA taken by
Franklin and Wilkins as part of their research.

Technology: Gene Therapy
• Many genetic diseases
occur when people do not
have a working gene for
making a key protein.
• Gene therapy attempts to
introduce DNA for the
normal, working gene into
a person's cells.
• Some large setbacks have
occurred in gene therapy,
but there are some recent
promising developments also.

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
whether a person is a carrier of a
disease allele.
• Amniocentesis and chorionic villus
sampling can determine whether a
fetus has a genetic disease.

Science and Society: DNA Forensics
• Forensic scientists use short tandem repeat (STR) analysis
to determine whether DNA samples match.
• Between 1989 and 2011, DNA evidence exonerated 272
people who were imprisoned for crimes they did not
commit.
• DNA forensics was used to identify the victims of the 2001
World Trade Center terrorist attacks.
• DNA forensics can be used to establish paternity and trace
familial relationships.
• DNA forensics can be used to identify disease-causing
microorganisms or endangered species.
• Ethical concerns – DNA contains a wealth of private
information about family relationships, susceptibility to
diseases, and so on.

Ch16 genetics

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