Cell Cycle and Mitosis Notes

Cell Division

• Adult human bodies contain between 10-50 trillion cells.
• Cell cycle is a series of events that leads to cell division

Eukaryotic Cell Division

• Mitosis: two daughter cells receive the same amount of genetic information as the mother cell which produced them
• Meiosis (sexual reproduction): cells which have two sets of chromosomes produce daughter cells with a single set of chromosomes
• Functions of Cell Division:
  • In unicellular organisms, division of one cell reproduces the entire organism
  • Enables multicellular eukaryotes to develop from a single cell and, once fully grown, to renew, repair, or replace cells as needed

Eukaryotic Chromosomes

• Cytogenetics – field of genetics involving microscopic examination of chromosomes and cell division
• Karyotype reveals number, size, and form of chromosomes in an actively dividing cell
• Humans have 23 pairs of chromosomes (46 total chromosomes)
  • Autosomes – 22 pairs in humans
  • Sex chromosomes – 1 pair in humans – XX or XY
• Ploidy
  • Diploid or 2n – humans have 23 pairs of chromosomes
  • Haploid or n – gametes (egg cell or sperm) have 1 member of each pair of chromosomes or 23 total chromosomes
• In diploid species (2n), members of a pair of chromosomes are called homologs (homologous chromosomes)

Replication and compaction of chromosomes

• Each chromosome replicates prior to mitosis.
• At the start of mitosis, the chromosomes become compact.

Cell Cycle

• Interphase: cell grows and copies its chromosomes (G1, S, G2)
  • G1 (first gap)
  • S (synthesis, chromosomes duplicated)
  • G2 (second gap)
• M phase (mitosis and cytokinesis)
• S phase = synthesis of DNA
• G1 phase = cell growth, protein synthesis, organelle production
• G2 phase = cell growth prepares for cell division
• Checkpoints of the cell cycle control system
  • Critical regulatory points
  • Checkpoint proteins act as sensors to determine if the cell is in proper condition to divide
  • Three checkpoints in eukaryotes:
    • G1 checkpoint (restriction point)
    • G2 checkpoint
    • Metaphase checkpoint
• Cyclins and cyclin dependent kinases (cdks) are responsible for advancing a cell through phases of the cell cycle
  • Cyclins are proteins
  • Concentration of cyclins vary in the cell cycle
  • To be active, cyclins must be bound to cdks
  • Cyclin dependent kinases (cdks)
  • Enzymes (kinases)
  • Phosphorylates proteins needed to advance the cell to the next phase

Mitotic Cell Division

• Cell divides to produce two new cells (daughter cells) that are genetically identical to the original cell (mother cell)
• Eukaryotic cell division consists of
  • Mitosis: the division of the genetic material in the nucleus
  • Cytokinesis: the division of the cytoplasm
• Chromosomal replication produces a pair of sister chromatids
• Centromere is where the two chromatids are most closely attached

Mitotic spindle

• Organizes and sorts chromosomes during cell division
• Composed of microtubules
• Microtubules growth and organization starts at two centrosomes
• The centrosome contains a pair of centrioles
• The centrosome duplicates during interphase (forming two centrosomes)
• Mitosis is conventionally divided into five phases
  • Prophase
  • Prometaphase
  • Metaphase
  • Anaphase
  • Telophase
• Cytokinesis overlaps the latter stages of mitosis
• During cell division, the two sister chromatids of each duplicated chromosome separate and move into two nuclei
• Once separate, the chromatids are considered to be individual chromosomes

Phases of Mitosis:

• Interphase: Prior to mitosis, cells are in interphase. Consists of G1, S, and G2 phases. Chromosomes have replicated in S phase and are decondensed. Nucleolus is visible
• Prophase: Chromosomes have already replicated and are joined as pairs of sister chromatids. Chromatids condense into highly compacted structures that are readily visible by light microscopy. Nuclear membrane dissociates into small vesicles. The nucleolus is no longer visible
• Prometaphase: Nuclear envelope completely fragments. Mitotic spindle is completely formed during this phase. Centrosomes move apart and demarcate the two poles. Spindle fibers interact with sister chromatids. Two kinetochores on each pair of sister chromatids are attached to kinetochore microtubules from opposite poles. Kinetochore microtubules begin to move the chromosomes
• Metaphase: At metaphase, the centromeres of all the chromosomes are at the metaphase plate
• Anaphase: sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell. Each individual chromatid is linked to only one pole by kinetochore microtubules. Kinetochore microtubules shorten, pulling chromosomes toward the pole to which they are attached
• Telophase: Chromosomes have reached their respective poles and decondense. Nuclear membranes now re-form to produce two separate nuclei. Cytokinesis begins during anaphase or telophase, and the spindle eventually disassembles

Cytokinesis

• Two nuclei are segregated into separate daughter cells
• Animals - cleavage furrow constricts like a drawstring to separate the cells
• Plants - cell plate forms a cell wall between the two daughter cells

Binary Fission in Bacteria

• Chromosome replicates, beginning at the origin of replication
• Chromosomes anchored to membrane
• The two daughter chromosomes actively move apart while the cell elongates
• FtsZ proteins form ring at separation site
• The plasma membrane pinches inward, dividing the cell into two
• Mitosis in Eukaryotes evolved from binary fission

Loss of Cell Cycle Controls in Cancer Cells

• Cancer cells do not respond to signals that normally regulate the cell cycle
• Cancer cells do not need growth factors to grow and divide