Genetics chapter 3 part 1


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  • 2.6 Diploid eukaryotic cells have two sets of chromosomes. (a) A set of chromosomes from a female human cell. Each pair of chromosomes is hybridized to a uniquely colored probe, giving it a distinct color. (b) The chromosomes are present in homologous pairs, which consist of chromosomes that are alike in size and structure and carry information for the same characteristics. [Part a: Courtesy of Dr. Thomas Ried and Dr. Evelin Schrock.]
  • 2.7 Each eukaryotic chromosome has a centromere and telomeres.
  • 2.8 Eukaryotic chromosomes exist in four major types based on the position of the centromere. [Micrograph by L. Lisco, D.W. Fawcett/Visuals Unlimited.]
  • Table 2.1 Features of the cell cycle
  • 2.12 The number of chromosomes and the number of DNA molecules change in the course of the cell cycle. The number of chromosomes per cell equals the number of functional centromeres, and the number of DNA molecules per cell equals the number of chromatids.
  • Table 2.2 Major events in each stage of meiosis
  • Genetics chapter 3 part 1

    1. 1. Cell Division CHAPTER 3, PART 1
    2. 2. Cell Division; Mitosis • Mitosis produces two identical daughter cells that are exact replicas of the parental cell • Most body cells are somatic cells (nonreproductive), usually with chromosomes present in pairs, the number of chromosomes is the diploid number (2n)
    3. 3. Cell Division (Reproductive Cells); Meiosis • Meiosis produces gametes that have half the number of chromosomes as the original cell: haploid (n) • The gametes are not identical to one another • Basis for sexual reproduction; genetic diversity is the adaptive advantage of sex! Aids evolution!
    4. 4. Homologous Pair • Diploid cells carry two sets of genetic information. • Where are they coming from? • Haploid cells carry one set of genetic information.
    5. 5. Locus; location of specific gene
    6. 6. Homologous vs. Non-homologous Chromosomes Non-homologous Chromosomes Homologous Chromosomes / Homologs Homologous chromosomes (homologs) = members of a chromosome pair that are identical in the arrangement of genes they contain (but might have different alleles) – i.e. 2 copies of chromosome #1. Homologs pair during meiosis! Non-homologous chromosomes = chromosomes that contain different genes and do not pair during meiosis
    7. 7. Gene Order on Homologous Chromosomes Homologous chromosomes contain the same genes in the same order Gene A Gene D Gene B Gene E Gene C Gene F Are the DNA sequences of homologous completely identical? No! can have different alleles!
    8. 8. Chromosome Structure Overview • Centromere: attachment point for spindle microtubules • Telomeres: tips of a linear chromosome. Provide chromosomal stability • Limits Cell Division; over time telomeres become shorter • Aging and Cancer • 2009 Nobel Prize awarded to E. Blackburn • Origins of replication: where the DNA synthesis begins
    9. 9. Chromosomal Classification and the Position of The Centromere
    10. 10. What is a possible difference between two homologs? 17% 17% 17% 17% 17% 17% Di ffe re nt Di ge ffe ne Di re s ffe nt re le Di nt ng ffe lo th re ci s nt fo ra ce nt lle ro le s m er ep Di os ffe ... re nt al Al le lo le ft s he ab ov e A. Different genes B. Different lengths C. Different loci for alleles D. Different centromere positions E. Different alleles F. All of the above
    11. 11. 3.1 Mitosis Divides Somatic Cells • Mitosis is the process of cell division that produces two genetically identical daughter cells from one original parental cell • It is precisely controlled to prevent either an excess or insufficient number of cells • Rate of division is important • Too slow: failure to develop, morphological abnormalities • Too fast: growth of structures beyond boundaries (cancer!) • Both: Death!
    12. 12. Stages of the Cell Cycle • Cell division is regulated by control of the cell cycle, a cycle of DNA replication and division • Cell cycles of all eukaryotes are similar • The two principal phases of the cell cycle are M phase, the short time during which the cells divide and a longer interphase, the time between M phases
    13. 13. Interphase G zero Ex. Neurons, eye cells, certain bone cells
    14. 14. Interphase • During the Gap 1 (G1) phase of interphase, all proteins needed for normal cell function are transcribed and translated; the duration of G1 varies • DNA is replicated during S phase or synthesis phase, which follows G1 • Two sister chromatids are produced! • A small number of cells enter G0 after G1; cells in G0 never progress through the cell cycle • The completion of S phase leads into G2 or Gap 2 phase, during which the cells prepare for division
    15. 15. DNA Replication The chromosomes are replicated prior to cell division 1 chromosome Circle 1 chromosome after replication Why? Homologous Chromosomes What do you call these two identical strands? The two strands are completely identical
    16. 16. Sister Chromatids Non-sister chromatids Sister Chromatids Sister Chromatids: The 2 subunits of a replicated chromosome. - They should be identical. Non-sister Chromatids: chromatids from different chromosomes Find a pair of non-sister chromatids
    17. 17. Sister Chromatids are IDENTICAL! A B c Homologous before replication Homologous after replication A B c A B c a b C a a b C What alleles will be on each chromatid? b C If 1 sister has “A”, the other sister will too, etc
    18. 18. Chromosomes During Mitosis • Cells at the beginning and the end of mitosis are diploid (2n) • Progressive condensation of chromosomes begins in prophase and reaches a maximum in metaphase • Centromeres, specialized sequences where sister chromatids are joined together, become visible in prophase; centromeres bind protein complexes called kinetochores DNA in blue Microtubules in green Kinetochores in pink
    19. 19. Substages of M Phase • M phase is divided into • Prophase • Prometaphase • Metaphase • Anaphase • Telophase • M phase accomplishes karyokinesis, partitioning of DNA into daughter cell nuclei and cytokinesis, the partitioning of the cytoplasm
    20. 20. Study Figure 3.2!
    21. 21. Chromosome Distribution • In animal cells, two centrosomes appear, which migrate to form the opposite poles of the dividing cell • Centrosomes are the source of microtubules; microtubules have a minus (-) end at the centrosome and a plus (+) end that grows away from the centrosome • The spindle fibers emanate from the centrosomes in a pattern called the aster
    22. 22. Types of Microtubules in Cells 1. Kinetochore microtubules embed in the kinetochore at the centromere of each chromatid, and are responsible for chromosome movement 2. Polar microtubules extend toward the opposite pole of the centrosome and contribute to cell elongation and cell stability 3. Astral microtubules grow toward the membrane of the cell, and contribute to cell stability
    23. 23. Metaphase Chromosomes • By the end of prometaphase, kinetochore microtubules are bound to each kinetochore • Metaphase chromosomes are 10,000-fold condensed compared to the onset of prophase; these chromosomes are pulled toward each centrosome by the kinetochore microtubules • The opposing forces align the chromosomes along the metaphase plate
    24. 24. Sister Chromatid Cohesion • Sister chromatid cohesion • Balances tension created by pull of kinetochore microtubules • Cohesin holds sister chromatids together, preventing their premature separation • 4-subunit protein • coats sister chromatids along their entire length • greatest concentration at the centromeres
    25. 25. Anaphase • Sister chromatids separate at anaphase and begin to move toward opposite poles in the cell • In anaphase A the sister chromatids separate due to the enzyme separase cleaving Scc1, the central component of cohesin • The separation of sister chromatids is called chromosome disjunction
    26. 26. Anaphase, continued • During anaphase, polar microtubules extend in length, causing an extended shape • The altered shape facilitates cytokinesis at the end of telophase, leading to formation of two daughter cells
    27. 27. Completion of Cell Division; Telophase • In telophase, nuclear membranes reassemble around the chromosomes at each pole • Decondensation returns chromosomes to their diffuse interphase state • Two identical nuclei occupy the elongated cell What’s Next?
    28. 28. Cytokinesis • In animal cells, a contractile ring of actin creates a cleavage furrow around the circumference of the cell; this pinches the cell in two • In plants, a new cell wall is constructed along the cellular midline • In both, cytokinesis divided the cytoplasm and organelles between the daughter cells
    29. 29. Mitosis Produces Identical Daughter Cells • Mitosis separates replicated copies of sister chromatids into identical nuclei, forming two genetically identical daughter cells • The diploid number of chromosomes (2n) is maintained throughout the cell cycle
    30. 30. # of chromatids
    31. 31. Cell Cycle Checkpoints • Common, genetically controlled signals drive the cell cycle • Cell cycle checkpoints are monitored by protein interactions for readiness to progress to the next stage • A common mechanism is carried out by protein complexes joining a protein kinase with a cyclin protein What happens if we What happens if we lose control of the lose control of the cell cycle? cell cycle?
    32. 32. Cyclins and Cdks • Protein kinase components of the complexes are activated by association with cyclins and so are called cyclin-dependent kinases (Cdks) • Multiple cyclin and Cdks form a variety of complexes • For example, cyclin BCdk1 is required to initiate M phase; the complex also activates an enzyme that degrades cyclin B
    34. 34. The RB1 Gene Is a Tumor Suppressor Gene • The unphosphorylated Retinoblastoma protein (pRB) acts like a brake on the cell cycle, preventing progression to S phase • It is one of many proteins known as tumor suppressors, with roles in blocking the cell cycle • The gene RB1, which produces pRB, is a tumor suppressor gene
    35. 35. Proto-oncogenes are the green light for the cell cycle! Proto-oncogenes are the green light for the cell cycle!
    36. 36. The Cyclin D1 Gene Is a Proto-Oncogene • The gene cyclin D1 leads to formation of the cyclin D1-Cdk4 complex that stimulates the cell cycle to enter S phase • Cyclin D1 is a protooncogene, defined as a gene that when expressed stimulates cell cycle progression
    37. 37. Cell Cycle Mutations and Cancer • Normal cells proliferate only when needed, in response to signals from growth factors • They are also responsive to neighboring cells; growth is moderated to serve the best interests of the whole organism • Cancer is characterized by out-of-control proliferation of cells that can invade and displace normal cells
    38. 38. Oncogenes are the gas pedal STUCK ON!  Oncogenes are the gas pedal STUCK ON! 
    39. 39. Mutations Related to Cancer Development • Cancer-causing mutations alter cyclin D1Cdk4 and pRB interactions • Some mutations increase the number of copies of cyclin D1, now an oncogene! • Higher-than-normal levels of cyclin D1 promote uncontrolled entry into S phase, due to constant phosphorylation of pRB
    40. 40. Mutations Related to Cancer Development • Another mutation affects RB1; it produces a pRB that binds weakly or not at all to E2F • Can lead to uncontrolled entry into S phase • This is loss of a tumorsuppressor gene! • Several types of cancers are associated with RB1 mutations, including retinoblastoma, and bladder, lung, bone, and breast cancers
    41. 41. AND NOW ON TO MEIOSIS…
    42. 42. 3.2 Meiosis Produces Gametes for Sexual Reproduction • Reproduction can be divided into two broad categories: • In asexual reproduction, organisms reproduce without mating and produce genetically identical offspring • In sexual reproduction, gametes (reproductive cells) are produced; these unite during fertilization
    43. 43. Multicellular Eukaryotes Reproduce Mainly Sexually • Males and females carry distinct reproductive tissues and structures • Mating requires the production of haploid gametes from both male and female • The union of haploid gametes produces diploid progeny
    44. 44. Meiosis versus Mitosis • Meiosis is distinguished from mitosis as it results in the production of four haploid gametes • Meiotic interphase is followed by two division stages called meiosis I and meiosis II. • No DNA replication between these stages!
    45. 45. Meiosis I vs. II In meiosis I Ihomologous In meiosis homologous chromosomes separate; chromosomes separate; reducing the diploid number reducing the diploid number of chromosomes to the of chromosomes to the haploid number haploid number In meiosis II, In meiosis II, sister sister chromatids chromatids separate to separate to produce four produce four haploid haploid gametes gametes
    46. 46. Meiosis I • Three hallmark events occur in meiosis I 1. Homologous chromosome pairing 2. Crossing over between homologous chromosomes 3. Segregation (separation) of homologous chromosomes, which reduces chromosomes to the haploid number
    47. 47. Stages of Meiosis I • Meiosis I is divided into prophase I, metaphase I, anaphase I, and telophase I • Pairing and recombination of homologs takes place in prophase I • Prophase I is subdivided into five stages: leptotene, zygotene, pachytene, diplotene, and diakinesis
    48. 48. Prophase I has five stages…. On to Pachytene….
    49. 49. Synaptonemal complex!
    50. 50. Synaptonemal complex: -occurs between nonsister chromatids of homologous chromosome -contains the recombination nodule, essential for crossing over of genetic material
    51. 51. Metaphase I • In metaphase I chiasmata between homologs are dissolved; this completes crossing over • Homologs align on opposite sides of the metaphase plate
    52. 52. Anaphase I • Anaphase I begins when homologs separate from one another and are pulled to opposite poles of the cell • Sister chromatids are firmly attached by cohesin
    53. 53. Telophase I and Cytokinesis • In telophase I the nuclear membranes reform around the separated haploid sets of chromosomes • Cytokinesis follows telophase I and divides the cytoplasm to create two haploid cells • Meiosis I is called the reductional division because the ploidy of the daughter cells is halved compared to the original diploid parent cell
    54. 54. Reduction Division!
    55. 55. Meiosis II • Meiosis II divides each haploid daughter cell into two haploid cells, by separating sister chromatids from one another • The process is similar to mitosis in a haploid cell • Four genetically distinct haploid cells are produced, each carrying one chromosome of a homologous pair
    56. 56. The Mechanistic Basis of Mendelian Ratios • Separation of homologs and sister chromatid in meiosis constitutes the mechanical basis of Mendel’s laws • For example, in an organism that is genotype Aa, the homologs bearing A and a separate from one another during anaphase I • At the end of meiosis, two gametes have the A allele and two have a; this generates the 1:1 ratio predicted by the law of segregation
    57. 57. Independent Independent Assortment Assortment
    58. 58. Questions?