Cell Growth and Division (Mitosis and Meiosis) Prentice Hall pg. 240-255, 275-278 Chapters 10, 11.4 Biology 1 2004-2005

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Cell Growth and Division(Mitosis and Meiosis)
Prentice Hall pg. 240-255, 275-278Chapters 10,
11.4Biology 12004-2005

• 101 Cell Growth
• A. Limits to Cell Growth
• 1. DNA Overload
• 2. Exchanging Materials
• 3. Ratio of Surface Area to Volume
• 4. Cell Division
• 102 Cell Division
• A. Chromosomes
• B. The Cell Cycle
• C. Events of the Cell Cycle
• D. Mitosis
• 1. Prophase
• 2. Metaphase
• 3. Anaphase
• 4. Telophase
• Cytokinesis
• 103 Regulating the Cell Cycle
• Controls on Cell Division
• Asexual Reproduction

• 114 Meiosis
• A. Chromosome Number
• B. Phases of Meiosis
• 1. Meiosis I
• 2. Meiosis II
• C. Gamete Formation
• D. Comparing Mitosis and Meiosis

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10-1 Cell GrowthWhy do cells divide rather than
continuing to grow indefinitely?

• On average, the cells of an adult animal are no
  larger than those of a young animalthere are
  just more of them.
• The larger a cell becomes, the more demands the
  cell places on its DNA and the more trouble the
  cell has moving enough nutrients and wastes
  across the cell membrane.

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Limits to Cell Growth

• DNA Overload
• information that controls a cells function is in
  DNA (eukaryotic cells, DNA is found in the
  nucleus of the cell)
• when a cell is small, the information stored in
  that DNA meets the cells needs
• cell must have enough DNA to support the protein
  needs of the cell
• in many large cells, more than one nucleus is
  present
• large amounts of DNA in many nuclei ensure that
  cell activities are carried out quickly and
  efficiently

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• Exchanging Materials
• food, oxygen, and water enter a cell through its
  cell membrane. Waste products leave in the same
  way.
• the rate at which this exchange takes place
  depends on the surface area of the cell, which is
  the total area of its cell membrane
• the rate at which food and oxygen are used up and
  waste products are produced depends on the cells
  volume (the space inside the cell)
• understanding the relationship between a cells
  volume and its surface area is the key to
  understanding why cells must divide as they grow

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Ratio of Surface Area to Volume in Cells

• as the cell gets bigger, the volume increases
  much more rapidly than the surface area, causing
  the ratio of surface area to volume to decrease
• this decrease creates serious problems for the
  cell

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Surface area-to-volume ratio

• If cell size doubled, the cell would require
  eight times more nutrients and would have eight
  times more waste to excrete.
• The surface area, however, would increase by a
  factor of only four.
• The cell would either starve to death or be
  poisoned from the buildup of waste products.

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10-2 Cell Division

• Cell division is the process by which two
  daughter cells are produced from one cell.
• Cell division results in two cells that are
  identical to the original, parent cell.

Cytokinesis
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The discovery of chromosomes

• Chromosomes are the carriers of the genetic
  material that is copied and passed from
  generation to generation of cells
• Accurate transmission of chromosomes during cell
  division is critical
• The chromosomes condense into compact, visible
  structures at the beginning of cell division
• The cells of every organism have a specific
  number of chromosomes
• fruit flies-8 human cells-46 carrot
  cells-18

• A Human Chromosome. This is a human chromosome
  shown as it appears through an electron
  microscope. Each chromosome has two sister
  chromatids attached at the centromere.

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The Cell Cycle

• During the cell cycle, a cell grows, replicates
  its DNA, prepares for division, and divides to
  form two daughter cells, each of which then
  begins the cycle again.  

Interphase

• The majority of a cells life (75) is spent in
  the growth period known as interphase.
• Following interphase, a cell enters its period of
  nuclear division called mitosis.
• Following mitosis, the cytoplasm divides,
  separating the two daughter cells.

Mitosis
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Interphase

• Interphase, the busiest phase of the cell cycle,
  is divided into three parts (G1, S, G2) .
• G1 cells increase in size and synthesize new
  proteins, organelles and do most of their
  growing.

• S chromosomes are replicated and the synthesis
  of DNA molecules takes place. Key proteins
  associated with the chromosomes are made.
• G2 After the chromosomes have been duplicated,
  the cell enters another shorter growth period in
  which mitochondria and other organelles are
  manufactured and cell parts needed for cell
  division are assembled.

Interphase
S
DNA synthesis and replication
G1
Rapid growth and metabolic activity
G2
Centrioles replicate cell prepares for division
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Biologists divide the events of mitosis into four
phases prophase, metaphase, anaphase, and
telophase.

• Mitosis is the division of the cell nucleus

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Prophase

• centrioles, two tiny structures located in the
  cytoplasm near the nuclear envelope, separate and
  take up positions on opposite sides of the
  nucleus
• condensed chromosomes become attached to spindle
  fibers at a point near the centromere
• chromosomes coil more tightly (the chromosomes
  become visible)
• the nucleolus disappears
• the nuclear envelope breaks down

Centromere
Sister chromatids
Spindle fibers
Disappearing nuclear envelope
Doubled chromosome
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Metaphase

• the chromosomes line up across the center of the
  cell, called the equator (MIDDLE)
• microtubules connect the centromere of each
  chromosome to the poles of the spindle

Centromere
Sister chromatids
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Anaphase

• the centromeres that join the sister chromatids
  separate, allowing the sister chromatids to
  separate and become individual chromosomes
• chromosomes continue to move until they have
  separated into two groups near the poles of the
  spindle
• Anaphase ends when the chromosomes stop moving.

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Telophase

• the chromosomes, which were distinct and
  condensed, begin to disperse into a tangle of
  dense material (unravel)
• nuclear envelope re-forms around each cluster of
  chromosomes
• the spindle begins to break apart, and a
  nucleolus becomes visible in each daughter
  nucleus.
• Mitosis is complete. However, the process of cell
  division is not complete.

Nuclear envelope reforming
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Cytokinesis

• As a result of mitosis, two nucleieach with a
  duplicate set of chromosomesare formed, usually
  within the cytoplasm of a single cell.
• Cytokinesis usually occurs at the same time as
  telophase.
• cytokinesis is the division of the cytoplasm
  itself.
• Cytokinesis can take place in a number of ways.

Nuclear envelope reappears
Two daughter cells are formed
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Cytokinesis in plant and animal cells

• Animal cells
• the cell membrane is drawn inward until the
  cytoplasm is pinched into two nearly equal parts.
  Each part contains its own nucleus and
  cytoplasmic organelles.

• Plants Cells
• a structure known as the cell plate forms midway
  between the divided nuclei, as shown below.
• The cell plate gradually develops into a
  separating membrane. A cell wall then begins to
  appear in the cell plate.

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103 Regulating the Cell Cycle 

• cell growth and cell division are highly
  controlled
• different cells move through the cell cycle at
  various rate
• muscle cells and nerve cells do not divide at all
  once they have developed
• skin cells,cells of the digestive tract, and
  cells in the bone marrow that make blood cells,
  grow and divide rapidly throughout life. Such
  cells may pass through a complete cycle every few
  hours.
• This process provides new cells to replace those
  that wear out or break down.
• Scientists spend a great deal of time studying
  how cell division is regulated

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Control of Cell Division

• 1. place some cells in a petri dish containing
  nutrient broth (broth provides food)
• -most cells will grow until they form a
  thin layer covering the bottom of the dish
• 2. when cells come into contact with other
  cells, they respond by not growing
• if cells are removed from the center of the dish,
  
• the cells bordering the open space will begin
  dividing until they have filled the empty space
  (5)
• -These experiments show that the controls
  on cell growth and cell division can be turned on
  and off.

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Control of Cell Division in the body

• Something similar to the previous experiment
  happens within the body.
• When an injury such as a cut in the skin or a
  break in a bone occurs, cells at the edges of the
  injury are stimulated to divide rapidly.
• This action produces new cells, starting the
  process of healing.
• When the healing process nears completion, the
  rate of cell division slows down, controls on
  growth are restored, and everything returns to
  normal.

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Normal Growth and Repair

• Control of Mitosis - normal growth and repair
  occur as cells produce new cells by mitosis and
  as new cells grow larger
• 1) differentiation - changes that take place in
  cells as they develop determined in part by the
  surrounding cells
• - cells may differentiate into muscle, blood,
  bone, skin, etc.
• 2) growth factors - chemicals that stimulate the
  division and differentiation of new cells during
  growth highly specific cause mitotic growth of
  only one type of cell

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• 3) regeneration - process of growing back lost
  body parts cells at wound site become less
  specialized and divide new cell differentiate
  into needed tissues
• - lizard - may regenerate lost tail which
  includes many types of tissues
• - birds mammals - may generate some tissues
  but not entire limbs

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Asexual reproduction - one parent produces
offspring by cell division

• DNA identical to parent
• unicellular organisms - can reproduce through
  mitotic division
• ex bacteria, fungi, plants, some animals

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Multicellular organisms - 3 forms of mitotic
division

• 1) budding - organisms grown from a piece of
  another organism
• - bud forms on some part of parent identical
  to parent
• - separates from parent to continue growth on
  its own
• - yeasts, hydra

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• 2) fragmentation - separated pieces of parent
  organism develop into new individuals
• - flatworm - cut in half each half forms new
  organism

- sea star - any piece containing part of central
disk can grow into a new organism
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• 3) vegetative reproduction - new plants grow from
  the stems, roots, or leaves of an existing plant
• - runners - spider plants
• - bulbs of tulips
• - potato tubers, etc.

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Cell Cycle Regulators

• cells in mitosis contain a protein that when
  injected into a non-dividing cell, would cause a
  mitotic spindle to form
• cyclins regulate the timing of the cell cycle in
  eukaryotic cells.

The sample is injected into a second cell in G2
of interphase.
A sample of cytoplasm is removed from a cell in
mitosis.
As a result, the second cell enters mitosis.

• Internal regulators- regulatory proteins inside
  the cell
• External regulators- growth factors (usually work
  most during embryonic development and wound
  healing)

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Uncontrolled Cell Growth

• cancer cells do not respond to the signals that
  regulate the growth of most cells
• they form masses of cells called tumors that can
  damage the surrounding tissues
• cancer cells may break loose from tumors and
  spread throughout the body, disrupting normal
  activities and causing serious medical problems
  or even death

• What causes the loss of growth control that
  results in cancer?
• smoking tobacco
• radiation exposure
• viral infection

Normal cells
Cancer Cells
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Meiosis and sexual reproduction

• Sexual reproduction - when chromosomes of two
  parents combine to produce offspring chromosomes
  are contained in reproductive cells called
  gametes (eggs or sperm)
• Meiosis - forms gametes only occurs in
  eukaryotic cells
• - results in the production of daughter cells
  that have half the number of chromosomes of the
  parent
• - daughter cells are not alike because their
  chromosomes may be different

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• Mitosis 2 daughter cells with each containing 2
  sets of chromosomes
• Meiosis 4 daughter cells with each containing 1
  set of chromosomes
• Chromosome Number
• diploid cells - (2Ndiploid number) contain two
  complete sets of chromosomes almost all body
  cells are diploid
• - humans have 23 chromosomes in 1 set body
  cells have 46 chromosomes (2 sets)

• different organisms have different numbers of
  chromosomes
• fruit fly (Drosophila)chromosomes. Each of the
  fruit flys body cells has 8 chromosomes.

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• Haploid cells - (Nhaploid number) contain one
  set of chromosomes found only in gametes
  produced through meiosis
• - human gametes have 23 chromosomes (1 set)
• fertilization - when an egg cell and a sperm cell
  of the same type of organism join to produce a
  new individual
• - zygote - cell that results from fertilization
  first cell of new individual contains two
  complete sets of chromosomes (one from each
  gamete)

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• - homologous pairs - matching pairs of
  chromosomes in a diploid cell (every chromosome
  in the egg has a matching chromosome from the
  sperm) pairs code for the same traits
• - chromosome pairs are known as tetrads

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Crossing Over

• Crossing over is the exchange of genes between
  pairs of homologous chromosomes
• produces new combinations of genes

• only occurs during prophase I of meiosis because
  homologous pairs are still tangled together
• chromosomes break where they meet and exchange
  genes
• Crossing over increases genetic variability

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Meiosis I
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Meiosis II
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Phases of Meiosis - includes meiosis I and
meiosis II

• Meiosis I - divides homologous chromosome pairs
  (tetrads) produces 2 cells from 1 parent cell

1) Prophase I - chromosomes become thick and
visible nucleoli and nuclear envelope disappear
spindle fibers form
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• 2) metaphase I - homologous chromosomes are
  together and line up in the middle of the cell
• 3) anaphase I - homologous pairs of chromosomes
  separate from each other spindle fibers pull
  whole sister chromatids to opposite ends of the
  cell

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• 4) telophase I - nuclear envelope reforms (in
  most organisms)
• - cytokinesis takes place - resulting cells have
  sister chromatids (contain one chromosome from
  each parent) (cytokinesis is NOT part of meiosis)

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Meiosis II - divides sister chromatids similar
to mitosis

• 1) prophase II - nuclear envelope disappears
  spindle fibers form

2) metaphase II - sister chromatids line up in
center of cell
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• 3) anaphase II - centromere divides sister
  chromatids of each chromosome separate when
  centromere is pulled by spindle fibers

4) telophase II - nuclear envelope reforms
cytokinesis takes place 4 single chromosome,
haploid daughter cells formed
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Gamete Formation

• This female gamete is called an egg in animals
  and plants
• only one of the cells produced by meiosis is
  involved in reproduction
• This egg cell receives most of the cytoplasm, the
  other three cells produced are known as polar
  bodies, which are reabsorbed by the body

• In male animals, the haploid gametes produced by
  meiosis are called sperm.
• In some plants, pollen grains contain haploid
  sperm cells.

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Meiosis and evolution

• Evolution - process of change in populations over
  time meiosis always brings change
• variation - differences among members of a
  population may allow for survival of a
  population
• - results from meiosis, fertilization, may
  result from crossing over
• environmental changes can be disastrous for
  organisms variety of traits within the
  population increases the chance that some
  individuals will survive the change

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Comparison of Mitosis and Meiosis

• Mitosis results in the production of two
  genetically identical diploid cells, whereas
  meiosis produces four genetically different
  haploid cells.

Mitosis Mitosis Meiosis
of divisions 1 2
of daughter cells produced 2 4
Type of reproduction associated with Asexual Sexual
Diploid or haploid cell produced? Diploid Haploid
Is daughter cell identical to parent cell Yes No
Gamete or body cell produced? Body (somatic) Gamete (egg or sperm)
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