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 along DNA molecule wrappedaround small proteins calledhistones. Genes are sectionsof chromosomes.


• 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.


• 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 mRNAmolecule ready fortranslation.


• 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 ananticodon, and carries a single,specific amino acid.

• A tRNA's anticodon binds tothe 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.

Genetic Mutations

How Radioactivity Causes Genetic Mutations

• Ionizing radiation strikes electronsin 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 thenreact 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 celldivision used to make haploidcells, such as eggs and sperm.

• In meiosis, one diploid celldivides into four haploid cells.

• During sexual reproduction,sperm and egg join to restore the normal diploidchromosome 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: Chromosomesexchange parts with their homologouschromosomes.

• The chromosomes in the dividing cellare now different from those in theoriginal cell.

• Crossing over results in recombination,the production of new combinations ofgenes different from those found in theoriginal chromosomes.


• How does meiosis result in genetic diversity?1. Crossing over2.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 plantsthat varied in a single trait --for example, round peas (RR)and wrinkled peas (rr).

• The offspring inheritedone R (round pea) alleleand one r (wrinkled pea)allele. They were Rr.

• All of the offspring expressedthe 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 ofindependent assortment:The inheritance of onetrait is independent ofthe inheritance of a second trait.

• Mendel demonstratedthis by crossing plantswith 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 singlegene 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 nothave a working gene formaking a key protein.

• Gene therapy attempts to introduce DNA for thenormal, working gene intoa person's cells.

• Some large setbacks haveoccurred in gene therapy,but there are some recentpromising developments also.

Science and Society: Genetic Counseling

• A pedigree is a family tree thatshows which relatives are andare not affected by a particulargenetic disease.

• Medical tests can determine whether a person is a carrier of adisease allele.

• Amniocentesis and chorionic villussampling 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.