Why do cells divide?
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To maintain a workable volume to surface area ratio
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Volume is the amount of space inside of a cell. This would include the cytoplasm
and all of the organelles.
Surface area is the total amount of exterior which is exposed to the environment
around the cell. This would include the outside of the cell membrane.
Volume increases faster than surface area. The surface area must be large
enough for a sufficient amount of materials to enter the cell. The materials must
enter quickly enough that all of the cell will get what it needs. Thus, a larger
surface area: volume ratio is preferable.
Examples:
Larger surface area: volume ratio
Smaller surface area: volume ratio
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Smaller cube (1 cm X 1 cm X 1 cm)
The surface area of one side = 1 cm X 1 cm = 1 square cm (or 1 cm2).
There are 6 sides, so the total surface area = 6 X cm2 = 6 cm2.
Volume = 1 cm X 1 cm X 1 cm = 1 cubic cm (or 1 cm3)
Surface:Volume = 6 cm2/1 cm3 = 6 cm2/cm3 (or 6 square cm of surface area for each cubic cm of
volume)
Larger cube (10 cm X 10 cm X 10 cm)
The surface area of one side = 10 cm X 10 cm = 100 square cm (or 100 cm2).
There are 6 sides, so the total surface area = 600 X cm2 = 600 cm2.
Volume = 10 cm X 10 cm X 10 cm = 1000 cubic cm (or 1000 cm3)
Surface:Volume = 600 cm2/1000 cm3 = 0.6 cm2/cm3 (or 0.6 square cm of surface area for each
cubic cm of volume).
Notice that the larger cube has more surface area and more volume but less surface
area for each cubic centimeter of volume.
Every cell is surrounded by a plasma membrane (discussed below and in the next
chapter). Most cells are very small and therefore have a high ratio of plasma
membrane surface to cell volume.
Why
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do cells divide?
To make more cells
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Growth of a multicellular organism requires the addition of cells.
Larger organisms do not necessarily have bigger cells, but they will
have more cells than smaller organisms.
Repair and replacement of cells lost due to injury or cell death
requires cell division. A healthy cell will divide to replace the lost
cell(s).
Cell division occurs at different rates depending on the organism
and the type of cell.
 Plant root cells would divide more rapidly because this is an
area of active growth.
 Some nerve cells enter a phase of
no division.
 Some bacteria cells divide very rapidly.
For example, E. coli can divide
every 20 minutes in ideal conditions.
How do cells divide?
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Prokaryotic vs. Eukaryotic Division
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The result of all cell division is the production of daughter cells. In
order for the daughter cells to contain enough DNA, the genetic
material (DNA) of the parent cell must be copied.
Prokaryotic division differs from eukaryotic division because
prokaryotic cells do not contain a nucleus or membrane-bound
organelles.
Eukaryotic division requires the replication of the nucleus and
genetic material (DNA) as well as the allocation of the organelles
into each daughter cell.
How do cells divide?
 Eukaryotic Cell Cycle and Mitosis
 Interphase is the “normal” part of the cell cycle. The cells
spend most of the cycle (life of the cell) in interphase.
During interphase the cell carries out life functions
(STERNGR) and then prepares for division:
 During most of interphase DNA exists in a “relaxed”,
stringy form called chromatin. After being copied, there
are two complete copies of the DNA in the cell. These
copies are attached to each other.
 Each copy of DNA is called a chromatid. The two
chromatids are attached in a region called the centromere.
At this point in interphase, the chromatids are coiled /
condensed. The entire structure is called a doubled
chromosome..
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< Chromatid
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Centriole>
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Stages of Mitosis (Eukaryotic cell division)
 Prophase
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 Coiled chromosomes become visible
 Nuclear membrane is broken down
 Spindles (protein fibers that will attach
to chromosomes and aid in
chromosome movement) start to form
Stages of Mitosis (Eukaryotic cell division)
 Metaphase
 Spindle fibers attach to the
centromere regions
 Chromosomes are moved to the
middle of the cell
Stages of Mitosis (Eukaryotic cell division)
 Anaphase
 Spindle fibers shorten
 Doubled chromosomes are separated
into chromatids
 Chromatids begin to move to opposite
poles/ends of the cell
Stages of Mitosis (Eukaryotic cell division)
Telophase
 Spindle fibers are broken down
 Nuclear membrane begins to form
around the clusters of chromatids at
each pole of the cell
Stages of Mitosis (Eukaryotic cell division)
 Cytokinesis is the separation of the
cytoplasm (including all organelles) into two
identical daughter cells.
 Animal cells constrict in the middle to pinch
apart forming two different cells.
 Plant cells also produce a cell plate dividing
the two cells. This cell plate will become
the cell wall.
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http://www.johnkyrk.com/mitosis.html
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Mitosis and Cancer
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Typically, cell division is controlled by genes in the cell, contact with
other cells, and available nutrients in the environment.
Cancer is a class of diseases characterized by uncontrolled cell
division and the ability of these cells to invade other tissues.
Cancer occurs when the genes that control cell division do not
function properly. This may occur because of an inherited genetic
defect, a spontaneous genetic mutation, or a mutation caused by
environmental factors.
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Examples: Radiation, some viruses
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Mutagens are environmental factors that increase genetic mutations.
Carcinogens are environmental factors that increase the likelihood of
cancer. Carcinogens are often mutagens, and any mutagenic agent
can cause cancer if it mutates a gene that controls cell division.
Examples: Cigarette smoke, DDT
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1.
Review Questions
Why is a large surface area: volume ratio
preferable?
Volume increases faster than surface area and the cell can more efficiently move material in and out of the cell.
2. Name three reasons cells must divide.
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Replace injured or worn out cells
Growth
Maintain a workable surface area to volume ratio
3. Why is eukaryotic cell division more complex than
prokaryotic cell division?
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prokaryotic cells do not contain a nucleus or membrane-bound organelles.
4. What are the two basic stages of the cell cycle?
Interphase, phases of mitosis (prophase, metaphase, anaphase, telophase)
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5. How is the genetic material “prepared” for cell
division during interphase?
exists in a “relaxed”, stringy form called chromatin. Copied, called chromatid, attached to a
centromere, then coiled into a double chromosome
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6. How many daughter cells are produced as a result
of mitosis, and how do these cells compare to the
parent cell?
2, Identical
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7. How are mitosis and cancer connected?
Cells that are cancerous undergo uncontrolled cell division. The cell division includes
mitosis.
Why does reproduction require two types
of cell division?
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Asexual reproduction and cell division
Asexual reproduction is reproduction involving only one source of
genetic material (i.e. one parent). This means that offspring will be
genetically identical (clones) to the parent.
There are several types of asexual reproduction:
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Binary fission occurs when a unicellular organism (such as
bacteria or an amoeba) divides into two equally sized cells for the
purpose of reproduction.
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Budding occurs when simple organisms (such as yeast or hydra)
produce much smaller cells than those of the parent organism.
Binary
Fission
Budding
–Asexual reproduction and cell division
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Sporulation occurs when an organism (such as
mushrooms) produce spores – tiny packets of DNA – for
the purpose of reproduction.
Vegetative propagation occurs when a portion of a plant
goes through cell division in order to produce another
plant.
Regeneration occurs when a part of an animal (such as
an earthworm) undergoes cell division to produce a new
organism
Advantages and Disadvantages of Asexual
Reproduction:
 Advantages of asexual reproduction include speed of
reproduction and producing multiple copies of
successful genetic combinations.
 Disadvantages of asexual reproduction include
increased risk of a single factor affecting an entire
population due to lack of genetic variation
Asexual reproduction and cell division
4. Mitosis is usually the mechanism that
allows asexual reproduction to occur.
For example, when an earthworm is cut
in half, cells must use mitosis to divide in
order to produce cells which will
reconstruct the mission portion.
Why does reproduction require two types of cell
division?
B.
Sexual reproduction and cell division
1. Sexual reproduction is reproduction involving
two sources of genetic material (i.e. two
parents). This means that offspring will be
genetic combinations of the two parents.
a. Gametes are sex cells, such as sperm and egg.
The purpose of a gamete is to fuse with another
gamete to combine genetic material (fertilization).
b. A zygote is the cell which is produced by
fertilization. The zygote will develop into an
embryo.
Mitosis
Why does reproduction require two types
of cell division?
2. Advantages and Disadvantages of Sexual
Reproduction:
a.
b.
The main advantage of sexual reproduction is genetic
variation. With many genetic possibilities, the likelihood
of a successful combination of traits for a particular
environment is high.
The disadvantages of sexual reproduction include the
necessity of mating for fertilization (which requires more
time) and the risks of unfavorable genetic combinations.
Why does reproduction require two types of cell division?
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Although sexual reproduction begins differently than asexual
reproduction (with two parents as opposed to one parent),
sexual reproduction also relies on cell division.
 Meiosis is a type of cell division which produces gametes.
Meiosis is a unique type of cell division because it divides
the genetic material in half, allowing for fertilization.
Example: Human body cells contain 46 chromosomes.
In order for a zygote (fertilized egg) to contain 46
chromosomes, the sperm and egg must each contain only 23
chromosomes.
 Mitosis is the process which allows the zygote to begin
dividing to produce a multicellular organism. The cells
also must differentiate (become different types of cells to
carry out different functions).
The Process
Meiosis (to produce the gametes)
Fertilization (union of the
gametes)
Mitosis (necessary
for growth
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Review Questions
1. How many sources of genetic material are involved
in asexual reproduction? In sexual reproduction?
1, 2
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2. Name five types of asexual reproduction.
Binary Fission, Budding, Sporulation, Vegetative Propagation, Regeneration
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3. Why is mitosis necessary for asexual reproduction?
To increase the number of cells in the organism
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4. Name one advantage and one disadvantage of
asexual reproduction.
Speed of reproduction, lack of genetic variation
5. What are gametes? How is a zygote produced
from gametes?
Product of Meiosis, where the chromosome number is 50%, The gametes
unite in fertilization creating the zygote
6. Name one advantage and one disadvantage of
sexual reproduction.
Offspring are a genetic combination of both parents. Takes longer to
produce offspring.
7. How are both mitosis and meiosis necessary
for sexual reproduction?
Meiosis is needed to produce gametes and mitosis is necessary for growth
of the organism.
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How does meiosis produce gametes?
A. Recall that a gamete is very different from a
somatic (body) cell.
1. A somatic cell is diploid, which means it contains two of
each type of chromosome. These chromosome pairs
are called homologous chromosomes. For example, a
human has 23 types of chromosomes. A human
somatic cell has 46 total chromosomes, consisting of
23 homologous pairs.
2. A gamete is haploid, which means it contains only one
of each type of chromosome (one from each
homologous pair). For example, a human egg cell
(ovum) contains 23 total chromosomes.
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http://www.johnkyrk.com/meiosis.html
B. Eukaryotic Cell Cycle and Meiosis
1. At the end of interphase (the longest part of the cell
cycle in which the cell completes normal life
functions), the cell duplicates the DNA. This
creates doubled chromosomes. The cell is now
ready to divide.
2. Meiosis requires two cell divisions:
a. Meiosis I is called the reduction division. In this
division, the homologous pairs of doubled
chromosomes are separated. The end result of
Meiosis I is two daughter cells, each of which is
haploid but contains doubled genetic material.
b. Meiosis II uses the same basic steps as mitosis.
In this division, the doubled chromosomes are
separated. The end result of Meiosis II is four
daughter cells, each of which is haploid and
contains no duplicated DNA.
Graphical representation of
Meiosis:
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46
Doubled
Meiosis I
23
Doubled
23
Doubled
Meiosis II
23
Single
d
23
Singled
23
Singled
23
Single
d
3. Meiosis provides several sources of genetic variation:
a. Gene mutations can occur during the duplication of DNA at the
end of interphase before meiosis (or mitosis) begins.
b. Crossing over occurs when the homologous pairs come
together during Meiosis I. In crossing-over a part of one
chromosome can switch places with the same part of the
homologous chromosome. Then, when the homologous pairs
are separated, each chromosome will be different than the
original.
c. Random assortment of chromosomes into daughter cells during
Meiosis I allows for a mix of the chromosomes inherited from
each parent.
Independent Assortment Animation
d. Nondisjunction, when homologous chromosomes do not
properly separate, may occur during Meiosis I, creating a
gamete with one too many or one too few chromosomes.
e. The random fertilization of any one egg by any one sperm
allows for numerous genetic combinations in offspring.
Nondisjunction/Fertilization Aniatmion
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Review Questions
1. How is the chromosome number of a gamete
different from the chromosome number of a
somatic cell?
Gamete has one copy of the chromosome (haploid) and a somatic cell
has two copies (diploid)
2. What types of cells are diploid? Haploid?
Somatic is a diploid and a Gamete is a haploid
3. What is the purpose of meiosis?
It provides several sources of general variation and produces the
gametes needed for fertilization
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4. What is another name for Meiosis I?
Reduction Division
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5. How is the purpose of Meiosis I different from
the purpose of Meiosis II?
Meiosis 1 divides the chromosome number by half, while Meiosis II
separates the sister chromatids of the doubled chromosomes.
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6. Name five sources of variation meiosis provides.
Gene mutation
Crossing Over
Random assortment of chromosome
Nondisjunction
Random Fertilization
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7. Describe crossing-over.
Occurs when the homologous pairs come together during Meiosis I and portions of the
homologous chromosomes switch