Cells Growth and Division

• Cell Cycle

  Life cycle of the cell which includes the stages of initiation, growth and death.

  Initiation: division of mother cell

  Growth: grows for a period

  Death: ceases to exist by dividing and produces 2 daughter cells.

  ·  About 25M cell division per second occurs in the human body In plants, - cell cycle arrest as plants reach their final form, cells stop dividing (e.g. leaf)
• 2 PHASES

   

  Growth Phase

  (G1 Phase, S phase, G2 Phase)

   

  Division Phase

  ·   Karyokinesis - division of the nucleus

  Mitosis (4stages PMAT)

  Meiosis (2x undergoes PMAT)

  Cytokinesis – Division of the Protoplasm
• CELL CYCLE – GROWTH PHASE

  ·       No cell division, only cell growth

  ·       Also called Interphase or Resting phase

  ·       3 Distinct Phases within interphase

  o   G1 Phase

  o   S1 Phase

  o   G2 Phase
• G1 PHASE

  ·       Known as Gap 1 phase

  ·       It is the first stage after division.

  ·      Cell is recovering from division and is conducting most of its normal metabolism.

  ·       Generally, it is the longest part of the cell cycle

  ü  Single-celled organisms (algae) – several hours

  ü  Plants that grow slowly – 2 to 3 days or weeks or months
• After replication, duplicate DNA molecules are still attached to the centromere.
• The Synthesis Phase, also known as Synthesis, is the phase where genes in the nucleus are replicated.
• A Gene is a polymer of nucleotides; it has a unique sequence of nucleotides.
• Many higher plants and animals need about 100,000 types of genes to store all information required to make enzymes, proteins and hormones necessary for the organism’s life.
• During S phase, the entire chromosome, not just the DNA is replicated.
• Histones are a special class of proteins which complexes with DNA and gives it both protection and structure.
• Centromere is a structural feature of the chromosome which is usually located at the center; it holds the two chromatids together.
• Chromatid is half of the double chromosome.
• Chromosomes contain thousands of genes that are attached together in a linear sequence.
• The Genome is the whole set of genes.
• G2 PHASE

  ·       Known as Gap 2 Phase

  ·       Cells prepare for division.

  ·       Usually lasts for 3 to 5 hours.

  ·       Produce factors in the cytoplasm necessary to start nuclear division.

  ·       Synthesis of alpha and beta tubulin for spindle microtubules

  ·       Proteins necessary for processing chromosomes  

  ·       Breaking down of nuclear envelope.
• CELL CYCLE – DIVISION PHASE

  2 Processes:

  ·       Karyokinesis - Division of the nucleus

  o   Mitosis

  o   Meiosis

  ·       Cytokinesis - Division of the protoplasm
• MITOSIS

  ·       M phase – shortest of the stage

  ·       Duplication division - nuclear genes are first copied; then one set of genes is separated from the other and are packed into its own nucleus.

  ·       Nuclei that are produced are more or less exact copies of the original nucleus, except for occasional errors.

  ·       More common type of karyokinesis

  ·       Used by eukaryotic unicellular organisms that are not undergoing sexual reproduction.

  ·       4 Phases of Mitosis

  o   Prophase

  o   Metaphase

  o   Anaphase

  o   Telophase
• Prophase - At the onset of mitosis, the nuclear membrane of the cell is intact and as this stage progresses, the nuclear membrane disappears.
• Prophase - In the nucleus, chromatin material becomes tightly coiled as it condenses into discrete chromosomes.
• Prophase - The condensed chromosomes appear as two identical sister chromatids united along the centromere or kinetochore.
• Prophase - The mitotic spindle in the cytoplasm starts to form, with the spindle fibers composed of microtubules.
• Prophase - Centrioles move away from each other and towards the opposite poles, propelled by the lengthening bundles of microtubules found between them.
• Metaphase is the stage where the centrioles have finally reached their respective poles.
• Metaphase - Chromosomes start to assemble at the metaphase plate, an imaginary plane midway between the spindle’s two poles.
• Metaphase - The centromeres of all chromosomes are aligned with one another, and sister chromatids of each chromosome are assembled at the equator of the cell.
• During metaphase, the centromeres of the sister chromatids face the opposite poles of the cell.
• Metaphase - The identical chromatid of each chromosome are attached to the microtubules radiating from the opposite poles of the cell.
• The spindle fibers are very abundant during metaphase.
• TELOPHASE

  The daughter nuclei start to form at the two poles of the cell, where the chromosomes have assembled
• Telophase -

  Nuclear membrane starts to reappear, and are reconstructed from the fragments of the cell’s former nuclear envelope.
• Telophase The nucleoli reappear, and the chromatin fiber of each chromosome uncoils.
• telophase - The division of one nucleus into two genetically identical nuclei is now complete (karyokinesis). This is followed by cytokinesis, the division of cytoplasm into two, eventually forming two new daughter cells.
• Summary of Early Prophase

  mitotic spindle starts to form

  chromosomes starts to condense

  nucleolus is gone

  Summary of Late Prophase (Prometaphase)

  nuclear envelope breakdown

  chromosomes fully condensed
• Summary of Metaphase

  chromosomes line up at metaphase plate
• Summary of anaphase

  microtubules push poles apart

  kinetochore microtubules pull chromosomes towards pole
• Summary of Telophase

  chromosomes starts to decondense

  spindle disappears

  nuclear membrane reappears

  nucleolus reapprears
• CYTOKINESIS

  ·       Division of the protoplast

  ·       Simpler than mitosis

  ·       There is random distribution of organelles.
• ·       In plants, it involves formation of phragmoplast

  o   Phragmoplast – short microtubules aligned parallel to spindle microtubules; formed at the center of the cell where the metaphase plate had been.

  o   It traps dictyosome vesicles that then fuse into a large, flat, platelike vesicle in which two new primary walls and middle lamella begin to form
• o   Cell Plate – composed of the phragmoplast, vesicle and walls. 
• Mitosis - daughter nuclei replicates the original mother nuclues

  Meiosis - two sex cells (gametes) fuse together, forming a zygote which grows into new adult.