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. 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
• 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