Cellular Reproduction*
two sister chromatids held together by a
centromere.
Objectives
1. Define the terms chromatin, (sister)
chromatid, chromosome, diploid, and
haploid, homologous chromosome
2. Describe the purpose of mitosis and how
it is used in living things
3. Describe the phases of mitosis and
identify animal and plant cells in these
phases
4. Describe the purpose of meiosis and how
it is used in living things
5. Describe the phases of meiosis
6. State which phases of meiosis create
variation in the gametes
7. Compare mitosis and meiosis with
regards to starting cells, ending cells,
ploidy, and genetic similarity between
cells.

Label the drawings of the
chromosomes below using the words:
 centromere
 replicated chromosome
 chromatid
 chromosome
Chromosome Structure
Chromosomes are highly coiled bodies
of protein and DNA found inside the
nucleus of a eukaryotic cell. These
structures are only visible during cellular
division when they become very tightly
coiled and compact. Between cell divisions,
these structures relax and “unwind”. This
relaxed structure is called chromatin.
The life of a cell can be divided into a
resting phase (interphase) and a dividing
phase (mitotic phase). Normally there is
only one copy of every chromosome in a
cell. However, when a cell is about to
divide, it every chromosome is duplicated in
a process called replication. Replication
occurs during interphase before the start of
the mitotic phase. Each chromosome
becomes a replicated chromosome, with
Chromosome Number
The number of chromosomes in a cell
depends upon what role that particular cell
plays in the life of an organism. Most cells
in living things have a “full set” of
chromosomes and are said to be diploid. As
seen in a karyotype chart, there are 46
*Adapted from Kathleen Duncan, Foothill College, California
Bios 140 Biology Lab Manual
pg. 1
chromosomes in a human body cell. The
diploid number (or “2n” number) is 46.
Chromosome chart (karyotype)
of a human body cell
thus two kinds of cellular division with
different purposes:
Mitosis: cell reproduction in which the
chromosome number does not change. This
form of reproduction is used for growth and
repair, although some organisms use mitosis
to make more individuals (asexual
reproduction).
Meiosis: cell reproduction in which the
chromosome number is reduced by half.
This form of reproduction is used to make
sexual cells, or gametes, for mating
sexually.
Notice in the chart above that each
chromosome is paired with another
chromosome of the same length and banding
pattern. These pairs, called homologous
chromosome pairs, are always present in a
diploid cell. There are 23 pairs of
homologous chromosomes in a human cell.
Some cells, particularly sex cells called
gametes, have half of a “full set” of
chromosomes. These cells are called
haploid (or “1n”). Haploid cells only have
one member of every homologous
chromosome pair present in a cell. There
are therefore only 23 chromosomes in a
human gamete cell, and no homologous
pairs.
Both mitosis and meiosis are cell
divisional events that may occur during the
lifetime of a cell. The life of a cell can be
described as the Cell Cycle, in which cells
spend most (or all) of their time in a stage
called interphase. During interphase, cells
perform metabolism and repair functions.
When a cell “decides” to divide by mitosis
or meiosis, the chromosomes are replicated
during interphase.
Subsequently, the cell moves into the
dividing stage of the Cell Cycle, called M
phase. After M phase is
complete, the new cells are once again in
interphase.
The Cell Cycle
Cellular Division
The Cell Theory states that all cells
come from pre-existing cells. This
means that cells must have some way of
reproducing themselves. In sexually
reproducing organisms, cells must
undergo division in order to reduce the
number of chromosomes. There are
Bios 140 Biology Lab Manual
pg. 2
Mitosis
Mitosis may occur in cells that are
diploid or haploid. Cells that are
diploid will produce two identical diploid
cells, called daughter cells. Cells that are
haploid will produce two identical haploid
daughter cells.
Mitosis is used by living things when it is
important that offspring cells have exactly
the same genes as the parent cell.
The process of mitosis involves four
stages or phases. These phases are called
prophase, metaphase, anaphase, and
telophase.
 Mitosis in an Onion Root Tip
1. Obtain a photomicrograph (picture taken
through a microscope) of an onion root
tip. Root tips are areas of growth in
plants, so many cells are performing
mitosis here. Look for cells that seem to
have worms in them (chromosomes)..
2. Identify five separate onion root tip
cells, one each in interphase, prophase,
metaphase, anaphase, and telophase.
Use the displays in the lab, your
textbook, and the chart below, to
determine which cells are in these
stages. Then draw a sketch of these five
separate cells in Table I on the next
page.
 Mitosis Using Pop-Beads
1. Build eight chromosome strands,
each with 5 pop beads connected
through a magentic white
"centromere" to another 5 pop-beads
below. Make 2 identical strands
(chromatids) of yellow, 2 strands of
blue, 2 strands of green, and 2
strands of red.
2. Make a large circle on your desktop
with string to indicate the plasma
membrane. Place one strand of each
color into the circle (one red, blue,
green, and yellow) to represent a
diploid cell with a 2n number of 4.
3. First, run the cell through S phase of
the cell cycle and replicate each
chromosome by adding a sister
chromatid of the same color to each
one in the circle (use the magenetic
centromeres to hold each replicated
chromosome together).
4. With your lab partner, slowly talk
your way through the events of
prophase, metaphase, anaphase, and
telophase, describing to each other
what is happening to the
chromsomes and what else is
happening in the cell that you are not
visually representing.
Bios 140 Biology Lab Manual
pg. 3
Table I. Mitosis in Onion Root Tip Cells
Name
Phase
Interphase
Appearance and Events
No chromosomes visible
Nuclear membrane visible
Nucleolus (dark spot) visible
DNA replication, metabolism occurs
Prophase
Nuclear membrane dissolves
Centrioles move to opposite sides of the cell
Chromosomes condense, become visible
Nucelolus absent
Spindle fibers grow towards chromosomes
Metaphase
Chromosomes line up in middle of cell
Centrioles at opposite sides of cell
Spindle fibers reach to chromosomes
Nucleolus and nuclear membrane absent
Anaphase
Sister chromatids begin to separate
Spindle fibers begin to shorten
Telophase
Sister chromatids at opposite poles of cell
Nuclear membrane beginning to reform
Chromosomes begin to unwind
Centrioles and spindle fibers disappearing
Furrow forms as cytoplasm separates
Cell plate (new cell wall) forms in plants
Sketch
Bios 140 Biology Lab Manual
pg. 4
Meiosis
because it provides insurance against
catastrophic changes in the environment.
Purpose of Meiosis
Variation is specifically created in
processes called crossing over (Prophase I)
and the segregation of alleles (Anaphase I).
Crossing over involves the exchange of
chromosomal segments between
homologous chromosomes. The segregation
of alleles occurs when each pair of
homologous chromosomes separates in one
of two possible ways.
Meiosis is the process where cells reduce
the number of chromosomes by half. There
are two cell divisions in meiosis, compared
with only one in mitosis. Meiosis occurs
only in specialized cells of organisms
capable of sexual reproduction. These
specialized cells are called germ cells or
germ-line cells. In humans, these cells are
found in the testicles of males and the
ovaries of females.
Meiosis always begins in a diploid cell
and involves two cell divisions. The
products of the first cell division are haploid.
These cells then undergo a second cell
division and differentiate into gametes. In
some species, one of more cycles of mitosis
occurs before differentiation. Mature male
gametes are called sperm. Mature female
gametes are called eggs or ova. Gametes are
always haploid.
Parts of Meiosis
Meiosis is divided into two parts,
Meiosis I and Meiosis II. The phases of
these two parts include prophase,
metaphase, anaphase, and telophase.
Variation in Meiosis
During meiosis, a shuffling of gene
combinations occurs. When some genes are
recombined with other genes, new
combinations result. This produces gametes
that are all genetically different from each
other.
These differences produce the
variation in features we see among sexually
reproducing organisms. Variation is an
advantage in a population of living things
 Steps of Meiosis
1. In the Table II on the next page draw a
picture of a cell with 6 chromosomes (2n=6)
going through the eight phases of meiosis.
Refer to the lab displays and your textbook
for help in drawing these pictures.
Genes on Chromosomes
Chromosomes carry genes on them that
dictate what a cell and a whole organism
will look like. Every characteristic or
feature in a living thing is dictated by two
genes. These two genes interact to produce
the external appearance of a living thing,
called the phenotype. All cells carry two
genes for every characteristic, and these
genes are carried on a pair of homologous
chromosomes. One homologous
chromosome carries one gene, and the other
chromosome carries the other.
According to the laws of genetics set
down by Gregor Mendel, every
characteristic or trait has two forms, or two
alleles. These alleles are usually
symbolized by small and capital letters (e.g.
G and g). Furthermore, every cell carries
two of these alleles at a time. Thus, a cell
can carry genes for a characteristics in three
possible combinations of two: GG, Gg, and
gg. These specific combinations of genes
Bios 140 Biology Lab Manual
pg. 5
for a single trait are called genotypes. The
physical expression of these gene
combinations is called the phenotype.
Between the two alleles for a
characteristic, usually one allele is dominant
over the other. Dominant genes mask the
expression of non-dominant or recessive
genes. The Thus, if G were the symbol for
green flowers and g were the symbol for
white flowers, then the genotype Gg would
produce a phenotype of green.
In the next exercise, you will work with
four different characteristics, having the
gene symbols F or f, B or b, A or a, and Y or
y.
Meiosis and Fertilization of the
Doodlebug
 Building the Doodlebug Chromosomes
chromatid. At the bottom of each red
chromatid, place a piece of tape with the
letter b.
4. Build a homologous replicated
chromosome out of yellow beads. Again
connect a chain of 12 through a
connector to a chain of 8. Do this twice
to make two sister chromatids, and push
them together at their centromeres.
5. Place a piece of tape with the letter f at
the top of each of the yellow sister
chromatids. Then place a piece of tape
with the letter B at the bottom of each of
the chromatids.
6. Build another set of replicated
homologous chromosomes in the
following way. Connect a 7-unit green
chain through a connector to another 7unit chain. (Continue with Step 15)
1. Work in groups of two for this exercise.
Locate the source of pop beads and
connectors in your lab room. Get 40
yellow, 40 red, 28 green, and 28 blue
pop beads as well as eight magnetic
rubber-tube connectors and take them to
your seat.
2. Link together two 12-unit chains and
two 8-unit chains of red pop beads. Join
each of the 12-unit chains to separate
rubber tube connectors. Then join the
two 8 unit chains to the bottom of each
of the two 12-unit chains. You should
now have two red chromosomes with a
magnetic centromere in the middle.
Push the two centromeres together to
form a replicated chromosome with two
sister chromatids. See the picture of
finished chromosomes
3. Place a piece of tape with the letter F
at the top of each of the red sister
chromatids. Be sure the letter F is at the
same relative location on each
yellow
red
green
A finished set of replicated Doodlebug chromosomes ready
for meiosis.
Bios 140 Biology Lab Manual
pg. 6
blue
Table II. Steps of Meiosis for a cell with 6 chromosomes
Phase
Interphase
Appearance and Events
No chromosomes visible
Nucleolus visible
Events: DNA replication, transcription, translation
Prophase I
Chromosomes condense, become visible
Homologous chromosomes are paired up
Nuclear membrane disappears
Centrioles move to opposite sides of cell
Spindle fibers grow toward chromosomes
Events: Chromosomes exchange segments (crossing over)
Homologous chromosome pairs line up in the middle of the cell
Spindle fibers attach to chromosomes
Events: Chromosome pairs can line up in two different ways
Homologous pairs begin to separate
Spindle fibers begin to shorten
Events: Segregation of alleles for each trait begins, genes are
combined in unique ways
Metaphase I
Anaphase I
Telophase I
Prophase II
Sketch
Homologous pairs at opposite sides of cell
Nuclear membrane beginning to form
Centrioles and spindle fibers disappearing
Furrow forming as cytoplasm separates
Events: Segregation completed, cells are haploid
Chromosomes condense, become visible
Nuclear membrane disappears
Centrioles move to opposites sides of cell
Metaphase II
Chromosomes line up in middle of cell
Spindle fibers reach out to chromosomes
Anaphase II
Sister chromatids begin to separate
Spindle fibers begin to shorten
Events: The number of chromatids become zero
Telophase II
Chromosomes reach opposite sides of cell
Chromosomes begin to unwind
Nuclear membrane begins to form
Centrioles and spindle fibers disappearing
Furrow forms as cytoplasm separates
Bios 140 Biology Lab Manual
pg. 7
7. Make another green chromatid in the
same way, and push the 14 unit green
chromosomes together. Then build
another two more chromatids (again, 7
units + 7 units) out of blue beads, and
push them together. The green and blue
replicated chromosomes are homologous
to each other.
8. Place a piece of tape with the letter A on
the top of both blue chromatids. Place a
piece of tape with the letter Y on the
bottom of both blue chromatids.
9. Place a piece of tape with the letter a on
the top of both green chromatids. Place a
piece of tape with the letter Y on the
bottom of both green chromatids.
In doodlebugs, there are genes for tail shape,
antennae length, numbers of toes, and body
pattern.
c. What is the whole genotype of the cell?
F = curly tail
f = straight tail
y = spotted body pattern
Y = striped body pattern
A = 8 toes per foot
a = 4 toes per foot
b = short antennae
B = long antennae
The capital letters indicate the dominant
genes. Lower case letters are recessive
genes.
d. What does this parental Doodlebug look
like? (Describe the phenotype)
e. In order for meiosis to begin, what
important event must occur during
interphase?
Performing Meiosis
1. Begin by separating each of the sister
chromatids from each other and laying
just one chain of each color in a pile on
the lab tabletop. Put the unused
chromosomes off to the side. Place a
piece of string around all the beads to
represent a nuclear membrane. Then put
a larger string circule around the nucleus
to represent the cell membrane. This
now represents a germ cell in the ovary
or testes of the Doodlebug during
interphase.
2. Now replicate all of the chromosomes in
the germ cell. Magnetically attach the
chromatid of the same color that you set
aside earlier to each chromosome inside the
cell.
a. How many chromosomes are in this
parent germ cell before meiosis?
2. Now move the cell into Prophase I of
Meiosis.
a. Make the nuclear membrane dissolve
by erasing the inner chalk circle
b. Is the cell diploid or haploid? How can
you tell?
a. How many total chromatids are there?
b. How many replicated chromosomes are
there?
Bios 140 Biology Lab Manual
pg. 8
b. Draw the centrioles at the sides of
the cell, starting to form spindle
fibers (use chalk)
c. Put the red replicated chromosome
on top of the yellow chromosome,
and the green on top of the blue.
Make sure they pair up exactly. This
represents the pairing up or synapsis
that occurs in Meiosis Prophase I.
3. During Prophase I, segments of
chromatids from one replicated
chromosome exchange with another.
This is called crossing over. Switch a
segment containing the letter F on one
red chromatid with an equal segment
containing the letter f on the
corresponding paired yellow chromatid.
Then switch a segment containing the
letter a on a green chromatid with an
equal segment containing the letter A on
the corresponding homologous blue
chromatid. You may switch the B’s or
the Y’s if you like, or leave them alone.
4. Using a diagram of meiosis from a book
or your notes, determine what happens
next in Metaphase I. Hint: the replicated
homologous chromosomes line up as
pairs down the middle of the cell. Say
aloud to your partner what happens in
this phase. Ask your instructor if you
are unsure of how to set this up.
5. Continue meiosis through anaphase I
where the homologous replicated
chromosomes separate. Be sure not to
separate sister chromatids in this phase!
How many different ways can the paired
homologous chromosomes separate to
produce genetically different cells in
Telophase I? Ask your instructor if you
don't understand this.
membranes and nuclear membranes for
both cells. Be sure to discuss with your
partner what is happening in each phase
of Meiosis I.
7. Continue into Meiosis II with your two
cells.
a. What structures separate in
Anaphase II that did not separate in
Anaphase I?
8. Complete Meiosis and examine your
gametes.
 How many chromosomes are in each
gamete?
 How many gametes result from the
process of meiosis?
 How many chromatids are present in
each gamete?
 Pick ONE of your gametes and write
down its genotype.
 Are your gametes genetically identical?
Explain how you know the answer to
this from what you see before you.
 In what phase of meiosis did the
homologous chromosomes separate
from each other?
 In what phase of meiosis did the sister
chromatids separate from each other?
 In which phases of meiosis is variation
(gene scrambling) introduced?
and
6. At Telophase I, simulate cytokinesis by
modifying your chalk circles. Draw cell
Bios 140 Biology Lab Manual
pg. 9
9. Select one of your gametes at random
and find another team to have sex with
right on the desktop! Make a zygote by
fusing the two cells together
(fertilization). Image this zygote
dividing by mitosis to form a baby
doodlebug.
 Draw a picture of your doodlebug in
the space below, showing all the
phenotypic characteristics.
 What is the genotype of your baby
Doodlebug?
 Describe the phenotype of your
doodlebug.
Bios 140 Biology Lab Manual
pg. 10
Bios 140 Biology Lab Manual
pg. 11
Lab Report
Questions
Meiosis and Mitosis
Name
*** Turn in the whole lab, not just this worksheet
1. Name an example of asexual reproduction in plants.
2. Name an example of asexual reproduction in animals.
3. What are the advantages of asexual reproduction?
4. Name the phase for each of the onion root tip cells indicated by the pointers .
From Biology by N. Campbell, 4th Ed. 1996 Benjamin Cummings
Bios 140 Biology Lab Manual
pg. 12
5. Complete the following chart involving mitosis:
Organism
Number of
chromosom
es in cell
before
mitosis
Amoeba
50
Yeast
32
Corn
40
Chimpanzee
48
Human
46
Number of
replicated
chromosomes
in mitosis
prophase
Number of
chromatids
in mitosis
prophase
Number of
chromosomes
in daughter
cells after
mitosis
6. Test your understanding of meiosis by completing the table for the following organisms.
Write down the number of structures found in a single cell for each column.
The round worm parasite Ascaris has 4 chromosomes in each of its diploid cells.
Phases
Chromosomes
Chromatids
Number of
homologous
pairs present
Full or half set
of
chromosomes?
Haploid or
Diploid?
Prophase I
Telophase I
Prophase II
Telophase II
The garden pea (Pisum sativum) has 7 chromosomes in each of its haploid cells
Phases
Chromosomes
Chromatids
Number of
homologous
pairs present
Full or half set
of
chromosomes?
Haploid or
Diploid?
Prophase I
Telophase I
Prophase II
Telophase II
Bios 140 Biology Lab Manual
pg. 13
7. Complete the following comparison table.
CHARACTERISTIC
MITOSIS
MEIOSIS
Purpose
Haploid or Diploid at
beginning of cell division?
Change in chromosome
number
Type of cells that undergo
this process (somatic or
germ-line cells)
Number of cell divisions
Number of cells produced
Descriptions of the genes
of the new cells compared
to the parent cell (identical
or different)
Description of the genes of
the new cells compared to
each other (identical or
different)
Bios 140 Biology Lab Manual
pg. 14