Mitosis and Meiosis
H
OW DO bacterial cells multiply and become colonies of
millions of bacteria? How do complex multicellular
organisms, such as trees and animals, grow? How can offspring look different from their parents? The answers to
these questions lie in two processes, mitosis and meiosis.
Objective:
þ
Explain the processes of mitosis and meiosis.
Key Terms:
Ñ
anaphase
chromatid
clones
crossing over
cytokinesis
diploid
haploid
homologous
chromosomes
interphase
meiosis
metaphase
mitosis
mitotic spindles
polyploid
prophase
synapsis
telophase
Cell Division
Two major types of cell division take place in organisms. 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 an organism’s body. Meiosis is associated with the production of sex cells and sexual reproduction.
MITOSIS
Cells either stop growing or divide once they reach a certain size. Growth and division of
cells are how organisms increase 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
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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 a cell’s life cycle is spent in a period between cell
division known as interphase. 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. It is followed by the S phase, a period
when chromosomes duplicate. Each half of duplicated chromosomes is called a chromatid. The third phase, called 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, as
chromosomes. Sister chromatids 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.
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.
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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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 the
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. Some varieties are
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
FIGURE 2. The polyploid Stella De Oro daylily on the left is larger than the
from each parent is rearranged so diploid Stella De Oro daylily on the right. (Courtesy, Klehm Nursery)
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, metaphase, anaphase, and telophase.
During the S phase of interphase, the chromosomes duplicate just before meiosis I. The
chromosome pairs join at their centromeres. Since a diploid number is duplicated, there are
four chromatids, or two homologous pairs of chromosomes. The four chromatids are referred
to as a tetrad. In prophase I, these chromosomes lay side by side, a process called 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 switched, and then the sections are fused together. In this way, genetic material is altered, leading to greater genetic variation among the offspring. Also, at this time the nuclear envelope, or membrane, vanishes.
Metaphase I begins with the tetrads aligned along the equatorial plane. Spindle fibers attach
to homologous chromosomes. In anaphase I, the homologous chromosomes of each pair sepa-
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rate and are pulled toward the
opposite poles. In telophase I, the
nuclear envelope reappears and
cytokinesis takes place.
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 telephase II in four haploid cells, each with a different
genetic make up. 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
PROPHASE I
PROPHASE II
Synapsis of homologous
chromosomes to form tetrads
Chromosomes are still duplicated
METAPHASE I
METAPHASE II
Tetrads line up on the
metaphase plate
Chromosomes line up on the
metaphase plate
ANAPHASE I
Homologous centromeres move
to opposite poles
ANAPHASE II
Sister centromeres move to
opposite poles
TELOPHASE I
TELOPHASE II
Each chromosome is still duplicated
Four n cells
FIGURE 3. In meiosis, four gametes are produced from a single cell.
Summary:
2
Cells undergo division. The two major types of cell division that take place in
organisms are mitosis and meiosis. Mitosis, along with cytokinesis, involves the
division of cells for growth and development. The body of an organism is made of
cells produced through mitosis and cytokinesis. Meiosis is a 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.
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Checking Your Knowledge:
´
1. What are the major phases of mitosis?
2. What is cytokinesis?
3. How does meiosis differ from mitosis?
Expanding Your Knowledge:
L
Obtain a set of prepared slides that show cells in the stages of mitosis and meiosis.
Study the slides under a microscope and relate what you see to the discussion in
this E-unit.
Web Links:
:
Splitting Up
http://www.biology4kids.com/files/cell2_mitosis.html
It’s About Duplicating an Organism
http://www.biology4kids.com/files/cell2_meiosis.html
Animal Cell Mitosis
http://www.cellsalive.com/mitosis.htm
Animal Cell Meiosis
http://www.cellsalive.com/meiosis.htm
Agricultural Career Profiles
http://www.mycaert.com/career-profiles
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