Cell Cycle, Cell Division & Cell Structure

• Cells divide for various reasons:

  • To reproduce
  • To grow bigger
  • To repair injuries
  • To become more efficient
• In multicellular organisms, the distance for substances to enter cells is larger due to a higher surface area to volume ratio
• Multicellular organisms require specialised exchange surfaces for efficient gas exchange of carbon dioxide and oxygen due to their higher surface area to volume ratio
• Limits of Cell Growth:

  1. Volume grows faster than surface area

  2. Larger cells have difficulty getting oxygen, nutrients, and removing wastes

  3. Solution: Cell division
• Rates of Cell Growth:

  1. Vary depending on cell type

  2. E. Coli divides every 30 minutes

  3. Some cells take longer to divide or "stop" dividing for a while
• Prokaryotic Cell Division:

  • Binary Fission (asexual)
• Chromosomes:

  1. Chromosomes are "colored bodies" seen only when the cell divides and contain genetic info in the form of DNA

  2. Composition of Chromatin:

  • Composed of DNA and histones
  • DNA in a chromosome is 10,000x longer than the chromosome itself
  • Nucleosomes coil into supercoils, then into chromosomes

  3. Chromosome Structure:

  • Each chromosome replicates and is paired as sister chromatids joined at a centromere
  • Human cell has 46 chromosomes (2n)
• Cell Division: Mitosis & Cytokinesis:

  • Define mitosis and cytokinesis
  • Describe the cell cycle and changes during interphase
  • Discuss events and significance of mitosis
• The Cell Cycle:

  • Interphase (G1, S, G2)
  • Mitosis (PMAT)
  • Cytokinesis
• Mitosis - Prophase:

  • Nuclear membrane disintegrates
  • Chromosomes condense and centrioles move to opposite poles
• Mitosis - Metaphase:

  • Chromosomes attach to spindle fibers and line up in the middle
• Mitosis - Anaphase:

  • Centromeres split and spindles pull sister chromatids apart towards opposite poles
• Mitosis - Telophase:

  • Chromosomes cluster at poles, uncoil, nuclear envelope reforms, and cell membrane begins to pinch in the middle
• Cytokinesis:

  • Cell membrane moves inward and pinches in the middle forming two identical cells
  • In plant cells, a cell plate and cell membrane appear separating the new cells
• Controls on Cell Growth:

  1. Some cells rarely/never divide (nervous and heart cells)

  2. Some cells divide daily (skin and digestive tract cells)

  3. Cells stop growing when they bump into each other

  4. Cut or break in tissue causes cells to grow rapidly
• Regulating the Cell Cycle:

  • Cyclins regulate cell cycle timing
  • Internal and external regulators control cell cycle progression
  • Apoptosis is programmed cell death
• Uncontrolled Cell Growth:

  • Results in cancer
  • Cells lose ability to control growth rate and continue dividing uncontrollably
• Cancer:

  • Tumor mass of cells not responding to "stop" signals
  • Benign tumors do not spread, malignant tumors spread and destroy healthy tissue
  • Metastasis is the spreading of cancer
• Cell Differentiation:

  • Process by which cells become specialized into different types
  • Stem cells are totipotent or pluripotent
• Meiosis is a type of cell division where one germ cell produces 4 gametes with half the number of chromosomes
• Meiosis occurs only in germ cells in gonads, such as testes and ovaries
• Meiosis also occurs in flowers, specifically in the ovary and anther
• During Prophase I of meiosis, chromosomes condense and synapsis occurs, where homologous chromosomes pair up to form a tetrad
• Crossing over in meiosis may occur in the tetrad, between nonsister chromatids, leading to genetic variation
• In Metaphase I of meiosis, paired homologues align, and independent assortment occurs, adding variation
• During Anaphase I of meiosis, homologous chromosomes separate towards the poles, while sister chromatids remain attached
• In Telophase I of meiosis, each pole has a haploid set of chromosomes, and cytokinesis occurs to form two haploid daughter cells
• Meiosis II does not have an interphase II and is similar to mitosis in its phases: Prophase II, Metaphase II, Anaphase II, and Telophase II
• Gamete formation in animals differs between male (spermatogenesis) and female (oogenesis) gametes
• In spermatogenesis, all 4 meiotic products develop into sperm cells
• In oogenesis, cytokinesis in meiosis is uneven, leading to the formation of one large egg cell and three small polar bodies that break down
• Mitosis involves somatic cells, producing 2 identical daughter cells with the same number and kind of chromosomes as the parent cell
• Meiosis, on the other hand, involves germ cells of gonads, producing 4 different gamete cells with half the chromosome number as the parent cell
• Cell Theory:

  • All living things are made up of cells
  • Cells are the smallest working units of all living things
  • All cells come from preexisting cells through cell division
• Definition of Cell: A cell is the smallest unit capable of performing life functions
• Examples of Cells:

  • Amoeba Proteus
  • Plant Stem
  • Red Blood Cell
  • Nerve Cell
  • Bacteria
• Two Types of Cells:

  • Prokaryotic
  • Eukaryotic
• Prokaryotic Cells:

  • Do not have structures surrounded by membranes
  • Few internal structures
  • One-celled organisms, Bacteria
• Eukaryotic Cells:

  • Contain organelles surrounded by membranes
  • Most living organisms: Plant, Animal
• Cell Parts:

  • Organelles
  • Surrounding the Cell