Cell Division
T
WO major types of cell division take place in flow-
ering plants. They are mitosis and meiosis. Mitosis, along with cytokinesis, involves the division of
cells for growth and development. Cells produced
through mitosis and cytokinesis make up the plant
body, which consists of the roots, stem, leaves, and flower
parts. Meiosis is associated with the production of sex cells and
sexual reproduction.
Objective:
þ
Describe the processes of mitosis and
meiosis.
Key Terms:
Ñ
anaphase
chromatid
clone
crossing over
cytokinesis
diploid
gamete
haploid
homologous chromosomes
interphase
meiosis
metaphase
mitosis
mitotic spindle
polyploid
prophase
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synapsis
telophase
zygote
Mitosis
Plant cells either stop growing or divide once they reach a
certain size. Growth and division of cells is how a plant
increases in size. Cell division takes place through two main
processes, mitosis and cytokinesis. Mitosis is a complex process in which the nucleus of a cell divides. The resulting two
nuclei have the same number and types of chromosomes as
the original nucleus. Cytokinesis is the process by which the
cytoplasm of a cell divides, providing each new cell with the
essential organelles.
The life of a cell follows a cycle of growth and division. The
cell cycle can be broken down into stages, or phases. Most of
the cell’s life cycle is spent in interphase, a time between
periods of cell division. During interphase the cell grows and
synthesizes needed materials, such as proteins. Interphase is
further divided into three phases. The first is the G1 phase,
during which the cell increases enzyme activity for DNA synthesis. This stage is followed by the S phase, a period when
chromosomes duplicate. Each half of duplicated chromosomes
is called a chromatid. The third phase, the G2 phase, sees an
increase in protein synthesis.
Toward the end of interphase, mitosis begins. Mitosis is
divided into four distinct stages. The first stage is prophase.
During prophase, the chromatin condenses, or masses, in
the form of chromosomes. Sister chromatids, identical units
created during S phase, are drawn together at a region called
the centromere. Note that a dividing cell is typically described
as being similar to a globe, in that it has an equator and polar
regions. While the chromatins are massing, fibers are forming
at the poles of the cell. These fibers are referred to as mitotic
spindles. Toward the end of prophase, the nuclear membrane breaks down, and spindles begin to attach to chromatids.
Prophase is followed by metaphase. During metaphase,
the mitotic spindles attach to the centromeres of the chromosomes. The chromosomes also line up along the equator.
E-unit: Cell Division
MITOSIS
PROPHASE
No synapsis of homologous
chromosomes
METAPHASE
Individual chromosomes line
up on metaphase plate
ANAPHASE
Sister centromeres move to
opposite poles
TELOPHASE
Two 2n cells identical to the
parent cell
FIGURE 1. Mitosis is a complex process
in which the nucleus of a cell divides.
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During the third stage, called anaphase, the chromatids part at the centromere. One set of
chromosomes moves toward a pole, and an identical set of chromosomes moves toward the
other pole.
The final stage of mitosis sees the reformation of two nuclei. This phase is known as
telophase.
Cell division is completed during cytokinesis. The result is two cells identical to the parent
cell in every way except size. These cells start the cycle anew. All the cells of plants propagated
by asexual reproduction are produced through mitosis. The cells of the new plants are identical. These genetically identical offspring are said to be clones of the parent.
Meiosis
Meiosis is the mechanism behind sexual reproduction. In meiosis, four gametes are produced from a single cell. Gametes are specialized sex cells, the microspore (sperm) and
megaspore (egg). An egg fertilized by a sperm is a zygote. The significance of meiosis is that
genetic material is recombined during the process. All offspring differ genetically from all others.
Chromosomes normally exist in pairs. Each set of two chromosomes is called homologous
chromosomes. Homologous chromosomes share a similar size, shape, and position of the
centromeres. When a cell contains the normal two sets of two chromosomes, it is said to have a
diploid (2n) number of chromosomes. A haploid (n) cell has a single set of chromosomes.
Gametes contain a haploid number of chromosomes. Many grasses and flowering plants have
three or more sets of chromosomes. They are called polyploid. Plants with extra sets of chromosomes are typically larger and more vigorous than diploid members of the same species.
Bread wheat is a good example of a polyploid plant used in agriculture. It is hexaploid.
Meiosis differs from mitosis in
some important ways. Meiosis
involves two cell divisions that
result in four cells. Each of the
four cells produced has a haploid
number of chromosomes. During
meiosis, the genetic information
from each parent is rearranged so
that the resulting gametes have a
unique combination of genes.
The two nuclear and cytoplasmic divisions of meiosis are
named meiosis I and meiosis II.
Each division includes a prophase,
FIGURE 2. The polyploid Stella de Oro daylily on the left is larger than the
metaphase, anaphase, and
diploid Stella de Oro daylily on the right. (Courtesy, Klehm Nursery)
telophase.
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During the S phase of interphase,
the chromosomes duplicate just
before meiosis I. The chromosome
PROPHASE I
PROPHASE II
pairs join at their centromeres. Since
a diploid number is duplicated, there
are four chromatids, or two homologous pairs of chromosomes. The four
Synapsis of homologous
Chromosomes are still duplicated
chromatids are referred to as a tetrad.
chromosomes to form tetrads
In prophase I, these chromosomes lie
side by side, a process called
METAPHASE II
METAPHASE I
synapsis. At this time, genetic
material may be exchanged between
the homologous chromatids in a process called crossing over. Enzymes
cut each chromatid, sections are
Tetrads line up on metaphase plate
Chromosomes line up on the
metaphase plate
switched, and then the sections are
fused together. In this way, genetic
ANAPHASE II
ANAPHASE I
material is altered, leading to greater
genetic variation among the offspring. Also, the nuclear envelope
vanishes.
Metaphase I begins with the tetHomologous centromeres move
Sister centromeres move to
rads aligned along the equatorial
to opposite poles
opposite poles
plane. Spindle fibers attach to homologous chromosomes. In anaphase I,
TELOPHASE II
TELOPHASE I
the homologous chromosomes of
each pair separate and are pulled
toward the opposite poles. In
telophase I, the nuclear envelope
Each chromosome is still duplicated
Four n cells
reappears, and cytokinesis takes place.
FIGURE 3. In meiosis, four gametes are produced from a single cell.
Meiosis II begins with a diploid
number of chromosomes in each of
the two cells. No further duplication
of genetic material takes place prior to meiosis II. Prophase II is brief, since the chromatids are
already condensed. Spindle fibers form, and the nuclear envelope breaks down. In
metaphase II, the chromosomes line up along the equator. Anaphase II sees the chromatids
pulled to opposite poles by spindle fibers. The nuclear envelope reforms, and the cytoplasm
divides.
The two divisions of meiosis result at telophase II in four haploid cells, each with a different
genetic makeup. When fertilization takes place during sexual reproduction, the haploid
gametes fuse to form a diploid cell called the zygote. The zygote develops into the embryo. It
inherits a single set of chromosomes from the female parent and a single set from the male
parent.
MEIOSIS
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Summary:
2
Cells undergo division. The two major types of cell division that take place in flowering plants are mitosis and meiosis. Mitosis, along with cytokinesis, involves the
division of cells for growth and development. Roots, stems, leaves, and flower parts
are made of cells produced through mitosis and cytokinesis. Meiosis is cell division
that results in sex cells, or gametes. The two divisions of meiosis result in four haploid cells, each with a different genetic makeup. When fertilization takes place during sexual reproduction, the haploid gametes fuse to form a diploid cell called the
zygote.
Checking Your Knowledge:
´
1. Differentiate between mitosis and cytokinesis.
2. What are the steps involved in mitosis?
3. During what stage of mitosis is most of a cell’s life cycle spent?
4. What are the steps involved in meiosis?
Web Links:
:
The Biology Project—Mitosis Tutorial
http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html
Computer Animations by John Kyrk—Meiosis
http://www.johnkyrk.com/meiosis.html
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