Chapter 19 Chromosomes cell division and the cell cycle

• Cell Cycle


  • All cells arise from preexisting cells via cell division
  • Divided into cell division and interphase
• Cell Division


  Nucleus and cytoplasm divide, splitting the cell more or less equally forming 2 new cells
• Interphase


  Period of intense metabolic activity where DNA and organelles are replicated between cell divisions
• G1


  Begins immediately after cells divide, cell produces RNA, proteins, and other essential molecules, length varies dramatically
• G0


  Metabolically active state that does not include DNA replication, reserved for non-dividing cells
• Cells in G0


  • Nerve cells
• S Phase


  DNA (deoxyribonucleic acid) replicates, lasts ~6-8 hrs in mammals
• G2


  Mitochondria divide, precursors of spindle fibers form, DNA begins to condense into chromosomes, lasts ~4-6 hrs
• DNA replication occurs during S phase
• Cell Division


  Follows interphase, requires two separate but related processes: Mitosis (nuclear division) and Cytokinesis (cytoplasmic division)
• Somatic Cells


  All body cells except the germ cells, contain a full complement of chromosomes, are diploid
• Gametes


  Contain half the number of chromosomes of somatic cells, are haploid
• Human gametes contain 23 chromosomes
• Homologous Chromosomes


  One chromosome of each pair comes from the mother and the other from the father, contain genes that control the same inherited traits
• Homologous chromosome pairs are the result of sexual reproduction
• Chromosomal Condensation


  Chromosomes condense after replication, facilitates mitosis, chromosomes are metabolically inactive
• Histones


  Globular proteins that are important in controlling DNA's activities, DNA double helix is associated with them
• Chromosomal Condensation


  DNA double helix forms small clusters around histones, which align to form hollow tubules, which in turn form larger coils
• Chromosomal Condensation


  • Vital to cell replication and appropriate gene expression, enables geneticists to spot potential chromosomal defects in fetuses
• Amniocentesis


  Physicians extract fluid from the liquid-filled cavity surrounding the growing fetus through a long needle inserted through the mother's abdomen, to test for chromosomal abnormalities
• Chromatids and Chromosomes


  During G1, each chromosome consists of a single DNA molecule and associated protein (histones), during S phase DNA strands replicate so chromosomes now consist of two identical chromatids held together by a centromere, when the cell divides the double-stranded chromosomes are split in half with sister chromatids separating and going to each daughter cell
• Mitosis


  Divided into four stages: Prophase, Metaphase, Anaphase, Telophase
• Prophase


  Chromosomes condense, nuclear envelope disappears, centrioles divide and migrate to opposite poles, spindle fibers form and attach to chromosomes
• Metaphase


  Chromosomes line up on equatorial plate of the dividing cell, facilitates separation of chromatids
• Anaphase


  Chromosomes begin to separate
• Telophase


  Chromosomes migrate or are pulled to opposite poles via the centrioles, new nuclear envelope forms, chromosomes uncoil
• Centriole


  Consists of nine sets of microtubules with three in each set, found in the cytoplasm and replicate during interphase, migrate to opposite poles of the nucleus during mitosis
• Mitotic Spindle


  Made of microtubules, forms during prophase, chromosomal fibers connect to the chromosomes, polar fibers extend from a pole to the equatorial region, separates double-stranded chromosomes during mitosis
• Cytokinesis


  Division of the cytoplasm of a cell, begins in late anaphase or early telophase, results from the contraction of microfilaments lying beneath the plasma membrane
• Cell Cycle Control


  Controlled by chemical messages produced in the cytoplasm, and external controls such as hormones, growth regulators, and cell contact
• Cancer


  Cancer cells grow uncontrollably because they lose contact inhibition, consume large quantities of vital nutrients, weakening and eventually killing host cells
• Endocrine system


  Consists of numerous small glands scattered throughout the body that produce hormones transported in the blood to distant sites to influence many functions
• Functions influenced by hormones

  • Homeostasis
  • Growth and development
  • Reproduction
  • Energy production, storage, and use
  • Behavior
• Target cells

  Contain receptors for specific hormones, selectivity occurs because of protein receptors
• Types of hormone receptors

  • Embedded in the plasma membrane
  • Located in the cytoplasm
• Tropic hormones


  Stimulate other endocrine glands to synthesis and release of other hormones
• Nontropic hormones


  Activate cellular processes, act more directly than tropic hormones
• Types of hormones produced in the body

  • Steroids
  • Proteins and polypeptides
  • Amines
• Negative feedback control


  Hormone secretion is often controlled by negative feedback mechanisms, the end product of a biochemical process inhibits its own production
• Positive feedback loops are (rarely) seen in the endocrine system, the hormonal product of a cell or organ stimulates the production of another hormone