Lab Exercise 9 - Cell Division
Mitosis and Meiosis
Introduction:
Mitosis (Figure 9.1):
Cell division, or cell reproduction, is the phenomenon whereby new cells are produced by older
cells. Mitosis is the type of division that is a constant process throughout the human body that is
essential if the body is to grow from a microscopic, unicellular zygote (fertilized egg) to an
incredibly complex, multi-cellular adult. It is also required for the constant renewal of “worn
out” cells, for the replacement of many body tissues. Somatic (body) cells produce additional
cells by this process of division known as mitosis.
The division of one cell into two involves: (1) division of the nucleus (mitosis), and (2) division
of the cell cytoplasm (cytokinesis). As a result of mitosis, two new daughter nuclei are formed,
each of which contains the same number of chromosomes contained in the original nucleus. In
order to understand how it is possible to convert one set of chromosomes into two sets, we must
examine this process of mitosis. It is a continuous process, proceeding smoothly from start to
finish. The period between cell divisions is known as interphase. For convenience and ease of
description we usually divide mitosis into four artificial phases or “stages’: (1) prophase, (2)
metaphase, (3) anaphase, and (4) telophase. Telophase leads directly to the formation of
daughter cells by the process called cytokinesis.
Figure 9.1 - Mitosis Overview
Gametogenesis by Meiosis:
Gametes, or sex cells, produce daughter cells by a somewhat different process, called meiosis.
Mitosis is also involved in gametogenesis, as the first step in production of sperm and ova by
allowing the initial proliferation of the precursor cells. The meiotic steps which follow this
initial division differ from mitosis in that there is a second division and the number of
chromosomes in each resulting cell is half the normal number in somatic cells. In the language
of genetics, the normal human diploid number of chromosomes (46) is reduced to the haploid
number (23); characteristic of sex cells.
Binary fission:
Most unicellular organisms multiply by a process termed binary fission. By this process, after
the chromosomes and other organelles have duplicated, such cells divide the amount of
chromosomes and organelles and the membrane of the cell pinches in and divides the original
cell into two identical cells.
Objectives:
Upon completion of this exercise, you should be able to:
1. Describe the differences between the somatic (body) cells and sex cells.
2. Describe the steps involved in cell division by mitosis.
3. Describe the steps involved in cell division by meiosis.
4. Describe binary fission and the life forms that use it.
5. Describe the effects of crossing over and independent assortment.
6. Define all the terms that are bolded and underlined.
Interphase (cell growth preparation for mitosis) (Figures 9.2a and 9.2b):
No nuclear division occurs during this “stage”, thus it is sometimes called the resting stage.
But, the cell is far from resting, since active chromosome duplication is taking place during
interphase. Although the process is not readily seen with the microscope, each chromosome
splits longitudinally (down its long axis) and forms two exact replicas of itself. These are known
as chromatids. Cells also continue to carry-out their designed functions during interphaseInterphase constitutes 90% of the cell cycle.
Figure 9.2a - Animal Cell Interphase
Figure 9.2b - Plant Cell Interphase
Mitosis:
Prophase (Figures 9.3a and 9.3b):
Several events occur during this “stage”. Each centriole forms a daughter centriole. The four
centrioles then separate with a pair moving toward opposite ends (poles) of the cell. A series of
microtubules called spindle fibers extend between the centrioles, forming a structure called the
spindle. As prophase progresses the nuclear envelope and nucleolus are disrupted, the
chromatin shortens and thickens becoming chromosomes. This shortening and thickening is
the result of tight, spring-like coiling by the formerly strand-like chromatin. The chromosomes
are quite visible and can be seen with the microscope. The chromosomes exist in pairs that are
attached to each other at a single point called the centromere and are now referred to as sister
chromatids. In late prophase sister chromatids begin to migrate toward the equatorial plate of
the cell.
Figure 9.3a - Animal Cell Metaphase Figure 9.3b - Plant Cell Metaphase
Metaphase (Figures 9.4a and 9.4b):
This easily identified “stage” is marked by the alignment of the sister chromatids, chromosomes
in the center of the cell at the so-called equatorial plate. Each pair of chromatids has migrated
along a spindle fiber, and each chromatid is attached at the kinetochore in the centromere region
to a fiber.
Figure 9.4a - Animal Cell Metaphase
Figure 9.4b - Plant Cell Metaphase
Anaphase (Figures 9.5a and 9.5b):
The centromeres split in early anaphase, and each chromatid becomes a separate, single
chromosome. As anaphase continues, the chromosomes are pulled to opposite ends of the cell
by the spindle fibers. The centromere seems to “lead the way” with the arm-like chromosomes
dangling along behind.
Figure 9.5a - Animal Cell Anaphase
Figure 9.5b - Plant Cell Anaphase
Telophase (Figures 9.6a and 9.6b):
In this phase the chromosomes are at the opposite ends of the cell and the duplication and
division processes of the nucleus are essentially completed. The nuclear envelope and nucleolus
of each daughter cell is formed, and the spindle has retracted to centrioles. The chromosomes
uncoil and begin to assume the thread-like chromatin form characteristic of interphase (which
follows shortly). Late in telophase, the cell membrane begins to constrict, furrow, between the
two nuclei.
Figure 9.6a - Animal Cell Telophase
Figure 9.6b – Plant Cell Telophase
Cytokinesis:
Now that telophase is completed, the furrowing of the cellular membrane continues dividing the.
cytoplasm. In plant cells a cell plate forms by the coalescence of molecules of cellulose. The
single cell is divided into two independent daughter cells and the cell cycle is repeated for each
of them beginning with interphase.
Meiosis:
Spermatogenesis:
This is the process of the production of spermatozoa (sperm cells). This process occurs in the
seminiferous tubules of the male’s testes, starting at the onset of puberty or sexual maturity and
continuing throughout life. Early sex cells, called spermatogonia, divide several times by
mitosis, and then enlarge at puberty to form primary spermatocytes. These undergo meiosis
and form secondary spermatocytes, each of which goes through a second meiotic division. The
result is four haploid cells called spermatids. In about two weeks, without any further dividing,
each of these matures into a spermatozoan. Thus, four mature sperm cells come from each
primary spermatocyte. The mature spermatozoa are released into the lumen of the vas deferens.
Oogenesis:
This is the process of the production of an ovum or egg cell, and occurs in the oviduct of the
female. The early ova or oogonia of the unborn female child divide mitotically and form diploid
primary oocytes. All of the ova produced by a woman during her lifetime, from puberty to
menopause, derive from these primary oocytes which are present before birth. The first meiotic
division produces a regular secondary oocyte with full cytoplasm, plus a smaller secondary
oocyte known as the first polar body which contains little cytoplasm and is non-functional.
During the second meiotic division the secondary oocyte divides again, to produce a mature,
functional ovum plus another non-functional polar body. The first polar body might also divide
to give two polar bodies. The end result of all this meiotic division is a single-haploid ovum
with a lot of cytoplasm, and two or three haploid, non-functional polar bodies, with very little
cytoplasm.
Lab Activity 9 - Cell Division
Mitosis
Required Materials:





Compound Microscope
Prepared Whitefish Blastula Slides showing stages of Mitosis
Prepared onion root tip slides showing stages of Mitosis
Plant cell Model showing the stages of Mitosis
Animal cell Model showing the stages of Mitosis
Assignment 1
Your biology instructor has previously set-up several models of plant and animal cells showing stages of
mitosis. You are responsible for viewing the models and identifying the stage of mitosis, stating whether it is a
plant or animal cell and giving a brief description of the phase. Record your answers in the table given under
the assignment -1 of the lab report.
Assignment 2
Identify the indicated phase and whether the specimen is plant or animal and record your results under
assignment – 2 of the lab report.
Assignment 3
Fill in the type of “cell division” that matches each descriptive phrase given under assignment -3 of the lab
report.
Assignment 4
Fill in the blanks with suitable terms for the assignment - 4 given under the lab report.
Assignment 5
Choose and give the correct answer for the questions under assignment – 5 of the lab report.
Lab Report 9 - Cell Division - Mitosis
Name: ________________________________
Date:
____________________________
Class Index: ___________________________
Instructor: ____________________________
Before you begin filling out this lab report you must read Exercise 9 - Cell Division in your lab manual.
Complete Assignments given below. You can use your Lab Manual results and Textbook to complete the
information below. Sometimes the magnifications in your lab manual and textbook will vary from what you
viewed under the microscope.
Fill in the Lab Report below.
Assignment 1: Your Instructor will arrange the Animal and Plant cell models showing the stages of cell
division. After looking at the models answer the following question.
Model No.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Animal
Plant Mitotic Phase
Description of Phase
Assignment 2: Identify the phase and whether the specimen is plant or animal cell.
Specimen
Phase/ plant or animal cell
i. _______________
ii. _______________
A.
i. ______________
B.
ii. ______________
i. ______________
ii. ______________
C.
i. ______________
D.
ii. ______________
i.
E.
______________
ii. ______________
i. ______________
ii. ______________
F.
i. ______________
ii. ______________
G.
i. ______________
ii. ______________
H.
i.
I.
______________
ii. ______________
i. ______________
J.
ii. ______________
Assignment 3: Fill in the type of “cell division” that matches each descriptive phrase
Statement
Type of cell division
A. This type of cell division results in the formation of either gametes
(in animals) or spores (in plants).
B. This type division occurs in somatic (non-sex) cells
Assignment 4: Fill in the blanks:
a. Division of the cytoplasm is called ____________________________.
b. During _____________________ phase of interphase, DNA replication occurs.
c. During ______________________phase of mitosis, each pair of chromatids separate and are moved by
the spindle fibers toward opposite poles.
d. Nuclear envelope and nucleoli reappears during __________________ phase of mitosis.
e. How many daughter cells are formed in meiosis? _____________________________.
Assignment 5 Choose the correct answer:
1. Before undergoing mitotic cell division, a parent cell had 44 chromosomes. After cell division, each
of the two daughter cells should have ________
a. 11 chromosomes
b. 22 chromosomes
c. 44 chromosomes
d. 88 chromosomes
2. The process of cytokinesis is exactly the same in plant cells and animal cells. ____
a.
True
b. False
3. Which of the following does mitosis normally accomplish?
a. production of a cancer cell
b. production of four haploid daughter cells.
c. production of two daughter cells with identical chromosomes like the parent
d. unequal division of the cytoplasm.