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AS Level Biology 9700

5. The mitotic cell cycle

Written by: Pranav I
Formatted by: Pranav I

Index

5.1 Growth and reproduction
5.2 Chromosomes
5.3 The cell cycle
5.4 Mitosis
5.5 The role of telomeres
5.6 The role of stem cells 
5.7 Cancers

5.1 Growth and reproduction

  • All cells (except gametes) in the bodies of multicellular organisms are genetically identical → all came from the zygote
  • Cells reproduce by dividing and passing on copes of their genes to daughter cells

5.2 Chromosomes

  • Sister chromatids → two identical chromatids of one chromosome held together by the centromere (position is characteristic for specific chromosomes)
  • Formed during interphase by the replication of the DNA strand
  • The genes on the chromatids are identical
  • Made of chromatin - a combination of DNA and proteins
  • Histones - DNA winds around the outside of these proteins; basic in nature (easily interact with acidic DNA)

Fig 5.1 The structure of a chromosome

5.3 The cell cycle

  • It is the sequence of events that takes place from one cell division until the next; it is made up of interphase, mitosis (nuclear division) and cytokinesis (cell division)
  • Interphase
    • G1 phase
      • The gap after cell division and before S phase
        • Cells make RNA, enzymes and other proteins needed for growth
    • S phase
      • Signal may be received that the cell should divide again
      • DNA in the nucleus replicates so that each chromosome consists of two identical chromatids
      • Relatively short phase
      • The centrosome replicates to form two centrosomes
      • Construction of kinetochores begins
    • G2 phase
      • Cell continues to grow
      • New DNA made during the S phase is checked (errors are usually repaired)
      • Preparations are made for division
      • Eg: Sharp increase in tubulin production to make microtubules for the mitotic spindle
  • Nuclear division (mitosis)
    • Growth stops temporarily
    • Nucleus divides into two identical nuclei
  • Cell division (cytokinesis)
    • Animal cells - constriction/infolding of cytoplasm between the two new nuclei
    • Plants cells - formation of a new cell wall (cell plate) between the two new nuclei
  • The length of the cell cycle is very variable
Fig 5.2 The mitotic cell cycle

5.4 Mitosis

  • Nuclear division that produces two genetically identical daughter nuclei, each containing the same number of chromosomes as the parent nucleus
  • Prophase
    • Chromosomes start to appear as the chromatin coils up (early prophase)
    • Nuclear envelope breaks up into small vesicles (disassembles)
    • Nucleolus forms part of several chromosomes (disappears)
    • Chromosomes are seen to contain two identical chromatids
    • Centrosomes move to the opposite ends of nucleus to form the poles of the spindle
  • Metaphase
    • Centrosomes reach the poles (to organize the production of spindle microtubules)
    • Chromosomes line up across the equator of the spindle
    • Attached to the spindle by their centromeres
  • Anaphase
    • Chromatids move to opposite poles (centromeres first; pulled by the spindle)
  • Telophase
    • Chromatids reach the poles of the spindle and begin to recoil
    • Nuclear envelope and nucleolus begin to re-form
    • Remains of spindle is broken down
  • Plants cells do not contain centrosomes
  • New cell wall forms between daughter nuclei after mitosis

Centrosomes, centrioles and centromeres

  • Each centrosome consists of a pair of centrioles surrounded by proteins
  • Centromeres are the site of attachment of spindle microtubules
  • Two kinetochores (protein) are present at the centromere, one on each chromatid
  • The pulling action is achieved by the shortening of the microtubules, both from the pole and the kinetochores

The importance of mitosis

  • Growth of multicellular organisms
    • Cloning of cells via mitosis allows for multicellular organisms to grow from unicellular zygotes
  • Replacement of damaged or dead cells
  • Repair of tissues by cell replacement
  • Asexual reproduction
    • Offsprings are genetically identical to the parents
    • Budding is used for multicellular organisms
  • Immune response
    • The cloning of B and T lymphocytes is dependent on mitosis

5.5 The role of telomeres

  • A short part of the DNA is not copied during replication → this information is lost → loss of vital genes results in cell death
  • Telomeres contain short repeated sequences of DNA (multiple repeat sequences)
  • Gradually get shorter as the cells replicate and age
  • Make DNA a bit longer and have no useful information
  • Extra bases are added to telomeres after each cell cycle to prevent loss from non-telomere DNA (telomerase enzyme)
  • Telomerase does not add bases to telomeres in specialized cells

5.6 The role of stem cells

  • Relatively unspecialized cells that retains the ability to divide an unlimited number of times, and which have the potential to become specialized cells
  • Totipotent, pluripotent and multipotent based on their potency (ability to different cell types)
  • The more specialized the cell, the lower its ability to divide (most cells in human adults do not divide)
  • Stem cells are present in the skin, bone marrow, gut and heart for growth and repair

5.7 Cancers

  • A result of uncontrolled mitosis → cancerous cells divide repeatedly to form a tumor (an irregular mass of cells)
  • Mutations in genes that control division result in oncogenes
  • Carcinogen is a substance or environmental factor that can cause cancer
  • Malignant tumors spread from their site of origin; benign tumors do not
  • Metastasis is the breaking off of cells from malignant tumors and spreading to other parts of the body
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