Hewitt/Lyons/Suchocki/Yeh Conceptual Integrated Science

1. Hewitt/Lyons/Suchocki/YehConceptual Integrated Science Chapter 16 GENETICS

2. This lecture will help you understand: • 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

3. 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.

4. 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.

5. 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.

6. 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.

7. Chromosomes

8. 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.

9. 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.

10. 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

11. DNA Replication DNA replication always begins at fixed spots within chromosomes. In eukaryotes, replication begins simultaneously at many points.

12. 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.

13. Transcription and Translation

14. 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 base-pairing rules as DNA replication. RNA polymerase adds the free nucleotides to the growing RNA molecule.

15. Transcription Once this process is complete, the DNA zips back up. The RNA molecule made during transcription is called messenger RNA (mRNA).

16. 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.

17. 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.

18. Translation The tRNA’s anticodons bind to the mRNA’s codons. The process repeats until a stop codon is reached.

19. Translation

20. 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.

21. Meiosis Meiosis occurs in two stages: Meiosis I and Meiosis II. Each stage includes several phases.

22. 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.

23. 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.

24. 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.

25. 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.

26. Mendelian Genetics Gregor Mendel’s work explained hereditary patterns. His experiments breeding pea plants demonstrated the existence of dominant and recessive traits.

27. 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.

28. 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, self-fertilizing Ww plants produce a 3:1 ratio of round pea to wrinkled pea plants.

29. 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.

30. Inheritance: Beyond Mendelian Genetics In incomplete dominance, the combination of two alleles in a heterozygote produces an intermediate trait.

31. 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.

32. 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

33. 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

34. 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

35. 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 sex-linked traits such as red–green color-blindness and hemophilia.

36. 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.

37. 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.

38. 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.

39. 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

40. 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.

41. 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.

42. 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.

43. 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.

44. 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

45. 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.

46. Cancer: Genes Gone Awry Metastasis: the ability of tumor cells to spread around the body and give rise to secondary tumors.

47. 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.

48. 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.

49. 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)

50. 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.