CELL DIVISION

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CELL DIVISION
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Cells divide. This makes cells small. Why do
cells divide? Why must cells be small?
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PURPOSES OF CELL DIVISION

• 1. Growth- increase in size of the organism (by
  dividing cells, not by increasing the size of the
  cells)
• 2. Repair needed because of worn out or
  injured cells (your skin cells are replaced every
  28 days your stomach every 7)
• 3. Reproduction (2 types)
• Asexual one parent. Offspring identical to
  parent - mitosis
• Sexual combination of genetic material from two
  parents - meiosis

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CELLS ARE SMALL

• Surface area to volume ratio must remain high
  for the cell
• To take in sufficient nutrients and oxygen to
  maintain life
• To remove waste
• To move molecules across the cell efficiently
• When cells in an organism divide, growth and
  repair result.

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High Surface Area to Volume Ratio?????
Surface area 6 x height x width 6 Volume l
x w x h 1 Ratio 61 6
Surface area 6 x height x width 150 Volume
l x w x h 125 Ratio 150125 or 1.2
6 is higher than 1.2!
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• Asexual reproduction passes on chromosomes
  through mitosis to make a clone (like binary
  fission in bacteriamore later)
• Sexual reproduction chromosomes are separated
  in meiosis (more later) and then combine to make
  a new organism (like egg sperm fusing to make a
  zygotemore later)

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Vocabulary

• Chromosome structure found in the nucleus of
  eukaryotic cells that contains the genetic
  material made of chromatin
• Chromatin Strands of DNA found in the nucleus
  makes up chromosomes when condensed around
  proteins

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Vocabulary

• Chromatidsone of the two strands of a chromosome
  that become visible during mitosis or meiosis
• Centromerethe
• region of the
• chromosome that
• holds the two
• sister chromatids
• together

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Vocabulary

• Mitosis Process of nuclear division
  karyokinesis
• Cytokinesis - Process of division of the cytoplasm

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The Cell Cycle

• The cell cycle is a continuous process that
  occurs in SOMATIC CELLS (body cells ex. Skin).
  It is an ordered set of events of cell growth and
  division resulting in two daughter cells, which
  then start the process again.
• 2 main parts
• Growth preparation
• Interphase (90 of the cells life)
• G1
• S
• G2
• II. Cell division
• Mitosis division of the nucleus
• Cytokinesis division of the cytoplasm usually
  follows mitosis, but sometimes doesnt occur
• What would this result in?

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STAGES OF CELL CYCLE
I. Growth Preparation (Must occur before
mitosis) INTERPHASE- getting ready stage (happens
before mitosis, can often see nucleolus, DNA
threadlike chromatin)
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INTERPHASE

• 90 of the time, the cell is in this phase
• Grows
• Performs operations unique to the type of cell

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INTERPHASE

• GROWTH 1 STAGE G1
• Decides whether or not the cell will divide
• Makes its structural proteins and enzymes to
  perform its functions
• A pancreas cell will produce and secrete insulin
• Salivary gland will produce and secrete enzymes
  in the mouth to aid in digestion
• Each chromosome is a single molecule of DNA and
  associated proteins

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INTERPHASE

• S Synthesis (DNA Replication)
• Each of the chromosomes is copied (in humans this
  makes 92 chromatids held by 46 centromeres)

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INTERPHASE

• GROWTH 2 PHASE G2
• DNA replication is checked by DNA repair enzymes
• Cell prepares for mitosis
• Proteins organize themselves to form a series of
  fibers called the spindles
• Involved in chromosome movement during mitosis
• Spindle fibers composed of microtubules

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INTERPHASE IN AN ANIMAL CELL
INTERPHASE IN A PLANT CELL
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II. Cell Division - MITOSIS

• Continuous process
• 4 main parts
• prophase, metaphase, anaphase, telophase
• P-MAT

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PROPHASE
ANAPHASE
METAPHASE
TELOPHASE
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MITOSIS

• PROPHASE
• Condensing of 2 chromatids to form chromosome
  hinged by a centromere
• Coil up
• Become visible
• Centrioles begin to migrate to opposite sides of
  the cell
• Nuclear envelope dis-assembles

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MITOSIS

• METAPHASE
• Spindle fibers align the chromosomes along the
  middle of the cell nucleus.
• This line is referred to as the metaphase plate.
• This organization helps to ensure that in the
  next phase, when the chromosomes are separated,
  each new nucleus will receive one copy of each
  chromosome

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MITOSIS

• ANAPHASE
• Chromatids move apart from one another
• Each chromosome is attached to a spindle which
  moves it toward one pole
• Results in equal separation and distribution of
  chromosomes

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MITOSIS

• TELOPHASE
• Chromatids arrive at opposite poles of cell
• New membranes form around the daughter nuclei.
• The chromosomes disperse (uncoil) and are no
  longer visible under the light microscope.
• The spindle fibers continue to dis-assemble
• Cytokinesis may also begin during this stage.
• This phase reverses many of the processes of
  prophase

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Tissue sample showing cells in multiple phases of
mitosis
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CYTOKINESIS

• Process in which the cytoplasm divides and two
  separate cells form.
• In animals, it begins with the formation of a
  cleavage furrow
• Microfilaments (actin fibers) contract during
  cleavage and assist the division of the cell into
  two daughter cells
• Think of a string being pulled tight around a
  cube of jello (gelatin)

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CYTOKINESIS

• In plant cells, cleavage does not occur
• New cell wall is formed in the center of the cell
  by vesicles from the Golgi
• As the vesicles join, they form a double membrane
  called the cell plate
• Forms in middle and moves outward
• Separates the daughter cells

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Plant cell plate
Animal cell cleavage
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• MITOSISSummary
• Animation http//www.hybridmedicalanimation.com/w
  ork/animation/the-stages-of-mitosis/
• PROPHASE- chromosomes evident, nuclear membrane
  disappearing (P for Phat (fat), chromosomes
  condense/fatten and become visible)
• METAPHASE- sister chromatids lined up in the
  middle/equator (M for middle, chromosomes lined
  up in the middle of cell)

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• ANAPHASE- sister chromatids pulled apart (A for
  Apart or Away because the chromatids pull apart
  and move away from center)
• TELOPHASE- chromosomes are at ends of cell, cells
  prepare to separate (T for Two new nuclear
  envelopes are forming)

Cleavage furrow Cell plate
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Not all cells reproduce

• Some leave the cell cycle here and do not undergo
  cell division
• Red Blood Cells which kick out their nucleus
  to make room for the hemoglobin and therefore
  cant divide
• Brain and spinal cord cells rarely if ever
  divide called G0 (pronounced G naught)

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Other cells cant stop dividing
Uncontrolled cell growth is known as cancer.
Read how this can occur on pp. 126-7 138-9 in
your text.
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Websites

• Cell Mitosis Lab Practice http//www.biology.arizo
  na.edu/cell_bio/activities/cell_cycle/activity_des
  cription.html
• Mitosis pictures Video
• http//www.iknow.net/CDROMs/cell_cdrom/cell3.html
  mitosis
• Cell Cycle Interactive Game
• http//nobelprize.org/medicine/educational/2001/ce
  llcycle.html