Cell Division - Growth, Repair, Reproduction, and Cancer

Cell Division

• Mitosis
  • Part of a larger cell cycle
  • Occurs in the nucleus of eukaryotic cells
  • Responsible for growth and repair
    • Replaces the entire lining of the small intestine
    • Liver cells only divide for repairing
    • Nerve cells do not divide
  • New cells arise by division of existing cells
  • Resulting cell is capable to divide again
• Meiosis
  • Cell divides twice to becomes a gamete
  • Will not divide any further until fertilization

Chromosomes

• Humans have 46 chromosomes
• Made up of 2 identical chromatids which are connected by a centromere
• Contain DNA
• Long and thin for replication and decoding
• Become short and fat prior mitosis → easier to separate due to compact form

Cell Cycle

• Interphase
  • Phase with highest metabolism (mitochondria have a high activity)
  • Muscles never complete the whole cycle
  • G1: Protein synthesis and growth (10 hours)
    • Preparation for DNA replication (e.g. growths of mitochondria)
    • Differentiation, only selected genes are used to perform different functions in each cell
  • S: DNA Replication (9 hours)
  • G2: short gap before mitosis to allow organelles and proteins for mitosis to be made (4 hours)
• G0: Resting phase (nerve cells)
• M-phase (mitosis)
  • Mitotic division of the nucleus (prophase, metaphase, anaphase, telophase)
  • Cytokinesis (division of the cytoplasm)

Mitosis

• Process of producing 2 diploid daughter cells with the same DNA by copying their chromosomes (clones)
• Chromosomes can be grouped into homologous pairs
• Mitosis occurs in
  • Growth
  • Repair
  • Replacement of cells with limiting life span (red blood, skin cells)
  • Asexual replacement
• Controlled process, cancers result from uncontrolled mitosis of abnormal cells
• Division of the nucleus (karyokinesis) and the cytoplasm (cytokinesis) are two processes of mitosis
• Division of cytoplasm after nucleus. Delayed if cells have more than one nucleus (muscle)
• Active process that requires ATP

Prophase

• Chromosomes become shorter and thicker by coiling themselves (condensation)
• This prevents tangling with other chromosomes
• Nuclear envelope disappears/breaks down
• Protein fibres (spindle microtubules) form
• Centrioles are moving toward opposite poles forming the spindle apparatus of microtubule

Metaphase

• Centrioles at opposite poles
• Chromosomes line up on the equator of the spindle
• Centromeres (kinetochores) attach to spindle fibres
• Kinetochores consist of microtubules and "motor" proteins which utilise ATP to pull on the spindle

Anaphase

• Spindle fibres separate and pull copies of chromatids to spindle poles
• Mitochondria around spindle provide energy for movement

Telophase

• Chromatid at the pole
• Sets of chromosomes form new nuclei
• Chromosomes become long and thin, uncoil!
• Nuclear envelope forms around the nucleus

Cytokinesis

• Region of constriction
• Actin molecules (ring) contract

Meiosis

• Sex and cell division - the production of gametes
• DNA in a cell replicates only once, but cell divides twice
  • First division
    • Random segregation of homologous chromosomes
    • This produces 2 haploid cells
    • Chromosomes are still duplicated (made up of 2 chromatids)
  • Second division
    • Random segregation of chromatids
    • Centromeres divide
    • Chromatids are pulled to opposite sides
    • This produces 4 haploid gametes → sperm/ovum
• Diploid cell (2n = 46 chromosomes) produces 4 haploid daughter cells (n = 23 chromosomes)
• Fertilisation of sperm (n = 23) and ovum (n = 23) produces one zygote (2n = 46)

Down's Syndrome

• Caused by non-disjunction → mistake in meiosis produces an extra chromosome 21
• Pair of chromosomes of one parent fails to separate during second division of meiosis
  • Chromatids fail to segregate in one haploid cells
    • One gamete has NO chromosome 21
    • Other gamete has 2 chromosomes 21
  • Other haploid cells produce 2 normal daughter cells/gametes
    • They each contain one chromatid
• Fertilization
  • Normal gamete (from partner) fuses with abnormal gamete that contains 2 chromosome 21s
  • Zygote ends up with 3 copies of chromosome 21!
  • Total chromosome count is 47 rather than 46!
• [Background Knowledge]
  • Risk increases with age
  • Triple test during pregnancy screens babies for Down’s syndrome
  • Only shows the risk of developing Down’s syndrome - not a diagnosis!
  • Diagnosis by amniocentesis - allows early abortion
• Clinical features
  • Learning difficulties
  • Short stature
  • Different physical appearance
  • Severity of symptoms varies from person to person