The Cell Cycle

1
The Cell Cycle

• Chapter 12

2
Cell Division

• Described by Rudolf Virchow in 1855.
• involves the distribution of genetic material
  (chromosomes) from parent to daughter cell
• Functions
• reproduction of an entire organism
  (single-celled)asexual reproduction
• Allows multicellular organisms to develop from a
  single cell
• renewal and repair (replacing dead cells and
  healing wounds)

3
Cell Cycle

• life of a cell from time it was split from parent
  cell until it divides into two daughter cells
• Different in cell types
• Prokaryotes
• Binary fission
• Eukarytotes--somatic cells (body cells)
• mitosis (identical cells)
• Eukaryotes--gametes (sperm and egg)
• meiosis (non-identical cells)

4
Chromosome Anatomy

• DNA exists as chromatin in cells not undergoing
  division (mitosis)
• During mitosis chromatin condenses into
  chromosomes
• Duplicated chromosome sister chromatid
• attached at centromere

5

• Interphase
• 90 of the cell cycle
• divided into
• G1 growth/gap 1 (grow)
• S synthesis (chromosomes copied)
• G2 growth/gap 2 (grow and prepare for division)

6

• Mitotic (M) Phase (mitosis)--division of genetic
  material
• prophase- chromosomes condense (sister
  chromatids), mitotic spindle forms, centrosomes
  (centrioles) migrate to opposite ends
• prometaphase- nuclear membrane disappears,
  microtubules (spindle fibers) begin to attach to
  chromatids (centromere/kinetochore)
• metaphase- chromosomes line up on metaphase plate
  (middle of cell), all microtubules attached to
  chromatids
• anaphase- chromatid pairs are pulled apart and
  move toward opposite ends of cell
• telophase- two nuclei begin to form, chromosomes
  unwind, cytoplasm begins to pinch inward
  (cleavage)
• Cytokinesis--complete division of cytoplasm

7
Regulation of Cell Cycle

• Frequency of cell division varies with cell type,
  which is crucial to normal growth, development
  and maintenance
• Cell cycle regulated at certain checkpoints by
  internal and external signals.
• a stop and go signal which regulates the cycle
• Allows the cell to determine if there is enough
  nutrients and raw materials to move to the next
  phase of the cycle
• located in G1, G2, and M phases

8

• In Eukaryotes, cellular reproduction must be
  controlled to maintain the form and function of
  different parts of the body
• Prokaryotes reproduce constantly when
  environmental conditions are optimal
• Progression through the phases of the cell cycle
  is tightly regulated
• Some cells never reproduce and enter G0 phase, a
  non-dividing phase
• Cells may be here for a designated period of time
  or forever
• Nerve and muscle cells

9

• G1 checkpoint triggered by DNA damage
• S checkpoint triggered by incomplete DNA
  replication
• G2 checkpoint triggered by DNA damage
• M checkpoint triggered by chromosome not
  attaching to spindle

10

• Regulatory Molecules
• protein kinases
• enzymes that activate or inactivate other
  proteins by phosphorylation help in cell
  signaling
• go signals in G1 and G2
• cyclins
• Proteins that activate the kinases by attaching
  (cyclin-dependent kinases--Cdks)
• Allosteric regulation
• levels rise and fall in response to concentration
  of kinase

11

• MPF (maturation/mitosis promoting factor)
  triggers passage into M-phase. Concentration
  declines as mitosis proceeds, eventually stopping
  mitosis
• PDGF (platelet derived growth factor) produced
  in blood (platelets), helps stimulate connective
  tissue to heal after injury

12
Loss of Control Cancer

• Regulatory mechanisms are missing in cells (due
  to mutations)
• Cells divide excessively and become invasive
  faulty cell cycle control
• benign vs. malignant tumors
• chemotherapy alters cell cycle (prevents it from
  occurring), but targets cancerous as well as
  normal cells
• can stop dividing at random points in cell cycle,
  or can be immortal and divide indefinitely
• HeLa Cells

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