10. CELL DIVISION
READING
• OpenStax College. (2018). Biology (2nd ed.) Houston, TX: OpenStax.
o Chapter 10: Introduction & All Sections
o Chapter 11: Section 1
o Chapter 43: Sections 3 - 5
REQUIRED MATERIALS
• Bead Set from Lab Kit
• Deck of Cards
• Coin
WEB SITES
• https://www.khanacademy.org/science/biology/cellular-molecular-biology
• http://www.bozemanscience.com/cell-division
• http://www.bozemanscience.com/mitosis
• http://www.bozemanscience.com/phases-of-mitosis
• http://www.bozemanscience.com/meiosis
• http://www.bozemanscience.com/phases-of-meiosis
• http://www.bozemanscience.com/028-cell-cycle-mitosis-and-meiosis
• http://www.bozemanscience.com/the-sodaria-cross
• https://www.khanacademy.org/partner-content/crash-course1/partner-topic-crashcourse-bio-ecology/crash-course-biology/v/crash-course-biology-112a
• https://www.khanacademy.org/partner-content/crash-course1/partner-topic-crashcourse-bio-ecology/crash-course-biology/v/crash-course-biology-112b
OBJECTIVES
• Describe the process of binary fission.
• Distinguish between asexual and sexual reproduction
• Name the stages of the cell cycle and describe their characteristics.
• Name the phases of mitosis and meiosis and describe their characteristics.
• Identify the phases of mitosis when dividing cells are viewed with a microscope.
• Compare and contrast the processes and end products of mitotic and meiotic cell division.
• Describe the significance of mitotic and meiotic cell divisions.
• Explain how independent assortment, crossing over, and random fertilization contribute to
genetic variation in sexually reproducing organisms.
INTRODUCTION
All new cells are formed by the division of preexisting cells. A key part of cell division is the
replication of DNA in the parent cell and the distribution of DNA to the daughter cells. Recall that
DNA is the molecule that controls cellular functions, and it contains the hereditary information
passed from parent cell to daughter cells. Large sections of DNA are condensed into chromosomes.
Genes, the determiners of inheritance, are relatively short sequences of nucleotides in DNA
molecules and many genes are found on a single chromosome.
In prokaryotic cells, cell division is relatively
simple. The process is known as binary fission.
Binary fission involves (1) replication of the
single circular chromosome (a DNA molecule)
and attachment of the two chromosomes to
separate points on the plasma membrane, (2)
separation of the chromosomes by linear
growth of the cell, and (3) division of the cell
by membrane invagination and formation of a
septum, which ultimately constricts "pinching"
the two daughter cells apart. Figure 1 depicts
the process of binary fission. The prokaryotic
cells in which this process occurs are too small
for you to observe directly with a compound
light microscope in the lab, however.
In eukaryotic cells, cell division is more
complex. Usually there are many
chromosomes in the cell nucleus, and each
eukaryotic chromosome consists of a DNA
molecule plus associated protein molecules.
Two different types of cell division occur in
eukaryotic cells: mitotic cell division and
meiotic cell division. Daughter cells formed by
mitotic cell division contain the same number
and composition of chromosomes as the parent cell. In contrast, cells formed by meiotic cell
division have only one-half the number of chromosomes as the parent cell. Thus, these two types of
cell division differ in the way the chromosomes are dispersed to the new cells that are formed. The
terms mitosis and meiosis refer to the orderly Figure 1. Binary fission in a prokaryotic cell. Source:
OpenStax Biology
process of separating and distributing the
replicated chromosomes to the new cells.
Cytokinesis (i.e., division of the cytoplasm) is the process of actually forming the daughter cells.
In animals and most plants, body cells contain two sets of chromosomes, and the chromosomes
occur in pairs. Both members of a chromosome pair contain similar hereditary information.
Therefore, members of a chromosome pair are homologous chromosomes. A cell with two sets of
chromosomes is said to be diploid (2n), whereas a cell with only one set of chromosomes is haploid
(n). Mitotic cell division may occur in either diploid or haploid cells, but meiotic division occurs
only in diploid cells.
Each organism has a characteristic number of chromosomes in its body cells. For example, fruit
flies have 8 chromosome (4 pairs), onions have 16 (8 pairs), and humans have 46 (23 pairs).
Gametes of these organisms are always haploid and contain 4, 8, and 23 chromosomes,
respectively.
EXERCISE 1: INTRODUCTION
1. Complete item 1 on your Laboratory Report.
MITOTIC CELL DIVISION
In unicellular and a few multi-cellular eukaryotic organisms, mitotic cell division serves as a means
of reproduction. In all multi-cellular organisms, it serves as a means of growth and repair. As
worn-out or damaged cells die, they are replaced by new cells formed by mitotic division in the
normal maintenance and healing processes. Millions of new cells are formed in the human body in
this manner.
Mitotic cell division is an orderly, controlled process, but it sometimes breaks out of control to form
massive numbers of nonfunctional, rapidly dividing cells that constitute either a benign tumor or a
cancer. Seeking the causes of uncontrolled mitotic cell division is one of that major efforts of
current biomedical research.
The Cell Cycle
A cell passes through several recognizable stages during its life span. These stages constitute the
cell cycle. There are two major stages. Mitosis, the M stage, accounts for only 5% to 10% of the cell
cycle. The interphase forms the remainder. See Figure 2.
Interphase has three subdivisions. A
growth period, the G1 stage, occurs
immediately after mitosis. Cells that will
not divide again will enter a stage where
they will carry out normal functions,
termed G0. Cells preparing to divide enter
the next stage, the synthesis (S) stage,
during which chromosome replication
occurs. Each replicated chromosome
consists of two sister chromatids joined at
the centromere. See Figure 3. A second
growth stage, the G2 stage, follows and
prepares the cell for the next mitotic
division. The centrioles replicate in the G2
Figure 2. The Cell Cycle. Source: OpenStax Biology.
stage in animal cells.
Mitotic Phases in Animal Cells
The process of mitosis is arbitrarily divided into recognizable stages or phases to facilitate
understand, although the process is a continuous one. These phases are: prophase, prometaphase,
metaphase, anaphase, and telophase. The characteristics of each phase as observed in animal cells
are noted here to aid your study. Interphase is also included for comparative purposes. Compare
these descriptions to Figure 4.
Interphase
Cells in interphase have a distinct nucleus and, in the G2 stage, two pairs of centrioles. The
chromosomes are uncoiled and are visible only as chromatin granules.
Prophase
During prophase, (1) the nuclear
membrane and nucleolus
disappear, (2) the replicated
chromosomes coil tightly to
appear as rod-shaped structures,
(3) each pair of centrioles begin to
migrate to opposite ends of the
cells, and (4) spindle microtubules
begin to form, extending between
the pairs of centrioles.
Prometaphase
During prometaphase, (1) the
chromosomes continue to
Figure 3. Chromosome Structure. Source: A. Kuczynski.
condense, (2) kinetochores form
on the replicated chromosomes
and (3) kinetochore microtubules from the centrioles attach to the kinetochores.
Metaphase
This brief phase is characterized by the chromosomes lining up at the metaphase plate of the cell. It
is the lengthening and shortening of the kinetochore microtubules that move the chromosomes to
the metaphase plate. Spindle microtubules that are not attached to the kinetochores are arranged
so they span the length of the cell. These are termed polar microtubules.
Anaphase
Anaphase begins with the separation of the centromeres of the sister chromatids. Then, the
shortening of the kinetochore microtubules pulls the sister chromatids toward opposite poles.
Once the sister chromatids separate and begin moving toward opposite poles of the spindle, they
are called daughter chromosomes. Thus, a cell in anaphase contains two complete sets of
chromosomes.
Telophase
In telophase, (1) a new nuclear membrane forms around each set of chromosomes to form two new
nuclei, (2) the nucleolus reappears, (3) the chromosomes start to uncoil, and (4) the kinetochore
and polar microtubules disassemble. Usually cytokinesis, cytoplasmic division, begins prior two the
end of telophase. A cleavage furrow forms, which continues to constrict until the parent cell divides
to produce two daughter cells.
Figure 4. Mitotic cell division in an animal cell. Source: OpenStax Biology.
Mitotic Division in Plant Cells
Mitotic division in plant follows the same basic pattern that occurs in animals, with some notable
exceptions. Most plants do not possess centrioles, although a spindle of fibers is present in dividing
cells. The rigid cell wall prevents the formation of a cleavage furrow during cytokinesis; instead, a
cell plate forms to separate the parent cell into two daughter cells, and new cell wall forms along
the cell plate. Cytokinesis usually, but not always, occurs during telophase.
EXERCISE 2: MITOTIC DIVISION
Procedure:
1. Complete item 2 on the Laboratory Report.
MEIOTIC CELL DIVISION
In contrast to mitotic cell division, meiotic cell division consists of two successive divisions but only
one chromosomes replication. This results in the formation of four cells that have only half the
number of chromosomes of the diploid (2n) parent cell. Thus, the daughter cells have a haploid (n)
number of chromosomes since they each contain only one member of each chromosome pair (i.e.,
homologous pair). In addition to reducing the chromosome number in the daughter cells, meiosis
also reshuffles the genes, hereditary units formed of small segments of DNA, and this greatly
increases the genetic variability among the daughter cells.
In humans and most animals, cells formed by meiotic division become gametes, either sperm or
eggs. In plants, meiotic cell division results in the formation of meiospores that grow into haploid
gametes that, in turn, produce gametes by mitotic division. In either case, the basic result of
meiosis is the same: haploid cells increased genetic variation.
Meiotic Phases in Animal Cells
Study Figure 5 as you read the following description of meiotic cell division in an animal cell.
Chromosome replication occurs in the S stage of interphase prior to the start of meiosis.
Meiosis I
Meiosis I is sometimes called “reductional division” as the number of sets of chromosomes, or
ploidy is reduced between parent and daughter cells. Prophase I exhibits the following
characteristics. Each chromosome is composed of two sister chromatids joined together at the
centromere. The replicated members of each chromosome pair join together in a side-by-side
pairing called synapsis. Chromosomes in synapsis are often called tetrads because they consist of
four chromatids. An exchange of chromosome segments (cross-over) frequently occurs between
members of the tetrad and increases the genetic variability of the cells produced by meiotic
division. The chromosomes coil tightly to appear as rod-shaped structures, the nuclear membrane
and nucleolus disappear, and spindle microtubules begin to form. The centrioles begin to migrate
towards the poles.
As the cell enters prometaphase I, the kinetochore microtubules from the centrioles attach to the
kinetochores on the chromosomes and the centrioles continue to migrate towards the poles.
Metaphase I is characterized by the synapsed chromosomes, being pushed and pulled by the
kinetochore microtubules, lining up in pairs at the metaphase plate. The polar microtubules span
from the two poles, overlapping each other at the cell's equator.
Anaphase I begins with the separation of the homologous pairs, pulled towards the poles by
kinetochore microtubules. The centromeres do not separate so each chromosome still consists of
two chromatids joined at the centromere. The polar microtubules "walk" past each other causing
the cell to elongate.
During telophase I a nuclear membrane may form around each set of chromosomes and the
chromosomes may decondense. Cytokinesis separates the parent cell into two daughter cells
contain only one member of each chromosome pair in a replicated state. Thus, each daughter is
haploid (n).
Meiosis II
Meiosis II is sometimes called “equational division” as the ploidy of the parent cells is equal to the
ploidy of the daughter cells produced. Both cells formed by meiosis I divide again in meiosis II, but
for discussion purposes we will follow only one of these cells in the second division. In interphase
between meiosis I and II, the centrioles replicate, but chromosomes do not replicate again. Recall
that they are already replicated.
Prophase II exhibits the following characteristics. If the chromosomes have decondense after
meiosis I, they recondense. If the nuclear envelope reformed, it will begin to disappear and the
spindle microtubules begin to form. The centrioles begin to migrate towards the poles.
As the cell enters prometaphase II, the kinetochore microtubules from the centrioles attach to the
kinetochores on the chromosomes and the centrioles continue to migrate towards the poles.
Metaphase II is characterized by the replicated chromosomes, being pushed and pulled by the
kinetochore microtubules, lining up single file at the metaphase plate. The polar microtubules span
from the two poles, overlapping each other at the cell's equator.
Anaphase II begins with the separation of the replicated chromosomes at the centromere, pulled
towards the poles by kinetochore microtubules. The sister chromatids, no called daughter
chromosomes, move toward opposite poles of the spindle. The polar microtubules "walk" past each
other causing the cell to elongate.
Telophase II proceeds as usual to form the new nuclei, and cytokinesis divides the cell to form two
haploid (n) daughter cells.
Because each cell entering meiosis II forms two daughter cells, a total of four haploid (n) cells are
produced from the original diploid (2n) parent cell entering meiosis I.
EXERCISE 3: MEIOTIC DIVISION
Procedure:
1. Complete item 3 on your Laboratory Report.
Figure 5. Meiotic division in an animal cell. Source: OpenStax Biology.
FOLLOWING CHROMOSOMAL MOVEMENT WITH BEADS
The majority of cells in the human body have two copies of every chromosome, one from their
father and one from their mother. During the formation of gametes these diploid cells undergo
meiotic cell division to produce haploid gametes (i.e., eggs and sperm), containing just one copy of
each chromosome. In this exercise, you will work on your own and follow the movement of the
chromosomes through meiosis I and II using a set of chromosomes made from interlocking beads.
*** Special Note: You will only be following chromosomal movement in this exercise. There
are other significant events that occur in the phases
of meiosis that you are still responsible for knowing
(e.g. nuclear envelope disappearing, spindle
microtubule formation, etc.). ***
EXERCISE 4: CHROMOSOMAL MOVEMENT
You will need:
• A set of beads
Procedure:
1. Obtain a set of beads. Before beginning the
exercise, ensure that your bead chromosomes
are constructed exactly as shown in Figure 6,
adding or removing beads as necessary. The
yellow beads will represent the chromosomes
inherited from this organism’s mother and the
blue beads the chromosomes inherited from the
organism’s father. The centromere on each
chromosome is represented by a white bead.
2. Most of the cells in a multicellular eukaryotic
organism do not perform meiotic cell division.
Only those cells which are responsible for the
production of gametes will undergo this process.
As these cells prepare for meiosis during G1 of
interphase, the chromosomes are uncondensed
and un-replicated in the nucleus. Figure 7 shows
the cell’s nucleus during G1 (we will temporarily
overlook the fact that if the chromosomes are
uncondensed, then they would not be able to be
represented by our bead chromosomes). The
cell is diploid at this stage because there are 2
sets of each chromosome, one from the father
and one from the mother.
Figure 6. Fully constructed bead
chromosomes. Source: A. Kuczynski.
Un-replicated homologous chromosomes
Figure 7. Chromosomes during G1 of
interphase. Source: A. Kuczynski.
Replicated homologous chromosomes
Sister chromatids
Figure 8. Chromosomes after S phase of
interphase. Note the differences between
sister chromatids and homologous
chromosomes. Source: A. Kuczynski
3. As the cell enters S phase of interphase, the
chromosomes are replicated (See Figure 8). This
means that each chromosome is composed of two
sister chromatids connected at the centromere.
Attach each sister chromatid by connected the
two white centrosome beads together. Complete
items 4a – 4c on the Laboratory report.
4. Although the chromosomes are not fully
condensed as the start of prophase I, the
homologous chromosomes form a
tetrad and crossing over between the
homologs will occur (See Figure 9).
During crossing over, the homologous
pairs align so that their gene loci are
perfectly lined up. Enzymes then
exchange portions between the
maternal and paternal homologous pair. We will
simulate this process by removing some beads
from the paternal chromosome and placing them
on the maternal chromosome and vice versa.
5. As the cell enters metaphase I, the homologous
chromosomes still in synapsis (i.e., connected)
line up along the cell’s equator (i.e., the metaphase
plate). Which homolog faces which pole is
determined randomly and is independent to how
another pair of homologous chromosomes orient
themselves at the cell’s equator (See Figure 10).
Complete item 4d on the Laboratory Report.
Figure 9. Homologous pairs arranged in
tetrads with crossing over occurring. Source:
A. Kuczynski.
Figure 10. Cell enters metaphase I and
homologous pairs line up along the cell's
equator (i.e., the metaphase plate). Source:
A. Kuczynski.
Figure 11. Homologous chromosomes pulled
towards opposite poles of cell. Source: A.
Kuczynski.
6. In anaphase I the homologous chromosomes
untangle as they are pulled towards the poles by
the kinetochore microtubules. The chromosomes
are still replicated at this point as the sister
chromatids remain attached at the centromere
(See Figure 11).
7. During telophase I the chromosomes gather near
the pull and cytokinesis begins. After the cleavage
furrow has “pinched” the cell into two, the result
is two new haploid daughter cells (See Figure 12).
Complete items 4e – 4g on the Laboratory Report.
Two separate cells
Figure 12. Two haploid, genetically different
daughter cells result after meiosis I and
cytokinesis. Source: A. Kuczynski.
8. The process of meiosis II occurs in both of the
daughter cells produced by meiosis I and looks
very similar in appearance to the process of
mitotic cell division. In prophase II the
chromosomes may recondense and the nuclear
envelope may disappear if necessary.
9. As the cell enters metaphase II, the kinetochore
microtubules have pushed and pulled the still
replicated chromosomes so they aligned single
file across the cell’s equator (See Figure 13).
Two separate cells
Figure 13. Two cells during metaphase II in
meiosis II. Source: A. Kuczynski.
10. During anaphase II the sister chromatids are
separated as they are pulled towards the poles
by the kinetochore microtubules (See Figure 14).
11. During telophase II the now un-replicated
chromosomes gather near the poles of the cell
and the nuclear envelope begins to remove.
After the process of cytokinesis there are now
four genetically different haploid cells (See
Figure 15).
Two separate cells
Figure 14. Both cells produced from
meiosis I undergoing anaphase II.
12. Complete item 4 on your Laboratory Report.
4 haploid daughter cells
Figure 15. Four haploid, genetically
different daughter cells result after meiosis
and cytokinesis. Source: A. Kuczynski.
MEIOSIS WITH CARDS
This exercise is designed to show how chromosomes can be passed from grandparent, to parent, to
offspring. As you complete the activity, examine Figure 16 and compare which portions of the
diagram are simulated by which steps of the activity. You should identify the different processes
that contribute to genetic variation during meiosis and when each occurs.
EXERCISE 5: MEIOSIS WITH CARDS
You will need:
• Standard deck of playing cards
• Coin
Procedure:
1. Divide the deck of cards into 4 separate piles by suit (i.e., hearts, diamonds, spades, and
clubs).
2. Arrange each suit (i.e., pile) from high to low (count the ace as higher than the king).
3. The two red suits (i.e., hearts and diamonds) represent the chromosomes of the mother.
She received one set from her father (i.e., diamonds), who is the maternal grandfather of our
offspring and one set from her mother (i.e., hearts), who is the maternal grandmother of our
offspring.
4. The two black suits (i.e., spades and clubs) represent the chromosomes of the father. He
received one set from his father (i.e., clubs), who is the paternal grandfather of our offspring
and one set from his mother (i.e., spades), who is the paternal grandmother of our offspring.
5. Two cards of the same color and same face value (e.g., three of diamonds and three of
hearts) represent a pair of homologous chromosomes.
6. Complete item 5a on the Laboratory Report.
Egg formation:
7. Now you are ready to demonstrate what happens to the chromosomes during egg
formation. Placed the diamond stack and the heart stack of cards side-by-side to represent
the mother’s homologous pairs of chromosomes in cells specialized for egg formation.
8. Assume that the mother received the heart suit complement of chromosomes from her
maternal parent (i.e., her mother) and the diamond suit complement from her paternal
parent (i.e., her father). Since the chromosomes assort randomly during meiosis (i.e.,
independent assortment), flip a coin to decide which member of a homologous pair enters
the egg (which, of course, will be eventually fertilized to form the zygote in this example). If
the coin is heads, select the top heart card from the heart stack. If the coin is tails, select the
diamond card. Remember to discard the unused card from the opposite stack each time a
selection is made.
9. Flip the coin separately for each homologous pair, placing one member of the pair into the
egg stack and the other into the discard stack, until all the cards are selected. The egg stack
should contain a card of every face value (i.e., ace to two). Set the unused stack of cards
aside to avoid confusion later.
Sperm formation:
10. After you form the egg, you are read to form the sperm from the father. Assume the father
received the spades from his maternal parent (i.e., his mother) and the clubs from his
paternal parent (i.e., his father).
11. Now, repeat the above procedure to form a sperm stack. If the coin is heads, select the
spade member of the father’s homologous pair. If the coin is tails, select the club member.
Again, do not forget to discard the unused complement into a discard stack each time a
selection is made.
Zygote (i.e., Offspring) Formation:
12. Use the egg stack and the sperm stack to form a zygote (i.e., offspring) by having the sperm
fertilize the egg (i.e., place the stacks together).
13. Complete item 5 on the Laboratory Report.
Figure 16. Diagram of chromosomes being passed from grandparent, to parent to offspring of current generation.
Source: genetics.thetech.org