EXERCISE: Mitosis and Meiosis
Objectives
After completing this exercise, you should be able to:
1. Describe the activities of chromosomes, centrioles, and microtubules in the cell cycle,
including all phases of mitosis and meiosis.
2. Describe differences in mitosis and meiosis.
Introduction
The nuclei in cells of eukaryotic organisms contain chromosomes with clusters of genes, discrete
units of hereditary information consisting of double-stranded deoxyribonucleic acid (DNA),
Structural proteins in the chromosomes organize the DNA and participate in DNA folding and
condensation. When cells divide, chromosomes and genes are duplicated and passed on to
daughter cells. Single-celled organisms divide for reproduction. Multicellular organisms have
reproductive cells (eggs or sperm), but they also have somatic (body) cells that divide for growth
or replacement.
In somatic cells and single-celled organisms, the nucleus divides by mitosis into two daughter
nuclei, which have the same number of chromosomes and the same genes as the parent cell. In
multicellular organisms, in preparation for sexual reproduction, a type of nuclear division called
meiosis takes place. In meiosis, nuclei of certain cells in ovaries or testes (or sporangia in plants)
divide twice, but the chromosomes replicate only once. This process results in four daughter
nuclei with differing alleles on the chromosomes. Eggs or sperm (or spores in plants) are
eventually formed. Generally, in both mitosis and meiosis, after nuclear division the cytoplasm
divides, a process called cytokinesis.
Events from the beginning of one cell division to the beginning of the next are collectively called
the cell cycle.
The cell cycle is divided into two major phases; interphase and mitotic phase (M). The M phase
represents the division of the nucleus and cytoplasm.
Exercise 1. Modeling the Cell Cycle and Mitosis
Materials
Approximately 60 pop beads of one color
Approximately 60 pop beads of another color
4 magnetic centromeres
Introduction
Scientists use models to represent natural structures and processes that are too small, too large, or
too complex to investigate directly. Scientists develop their models from observations and
experimental data, usually accumulated from a variety of sources. Building a model can
represent the culmination of a body of scientific work, but most models represent a welldeveloped hypothesis that can then be tested against the natural system and modified.
Linus Pauling’s novel and successful technique of building a physical model of hemoglobin was
based on available chemical data. This technique was later adopted by Francis Crick and James
Watson to elucidate the nature of the hereditary material, DNA. Watson and Crick built a wire
model utilizing evidence collected by many scientists. They presented their conclusions about
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the structure of the DNA helix in the journal Nature in April 1953 and were awarded the Nobel
Prize for their discovery in 1962.
Today in lab you will work with a partner to build models of cell division: mitosis and meiosis.
Using these models will enhance your understanding of the behavior of chromosomes,
membranes, and microtubules during the cell cycle. After completing your model, you will
consider ways in which it is and is not an appropriate model for the cell cycle. You and your
partner should discuss activities in each stage of the cell cycle as you build your model. After
going through the exercise once together you will demonstrate the model to each other to
reinforce your understanding.
In the model of mitosis that you will build, your cell will be a diploid cell (2n) with four
chromosomes. This means that you will have two homologous pairs of chromosomes. One pair
will be long chromosomes, the other pair, short chromosomes. (Haploid cells have only one of
each homologous pair of chromosomes, denoted n.)
Procedure
1. Build a homologous pair of single-stranded chromosomes using 10 beads of one color for one
member of the long pair and 10 beads of the other color for the other member of the pair. Place
the centromere at any position in the chromosome, but note that it must be in the same position
on homologous chromosomes. Build the short pair in the same manner, but use fewer beads. You
should have enough beads left over to duplicate each chromosome.
2. Using the chromosomes you have built, model replication during the S phase of Interphase
and Prophase, Prometaphase, Metaphase, Anaphase and Telophase of Mitosis. Be sure to draw
out each step you have modeled.
EXERCISE 2. Modeling Meiosis
Materials
60 pop beads of one color
8 magnetic centromeres
60 pop beads of another color
Introduction
Meiosis takes place in all organisms that reproduce sexually. In animals, meiosis occurs in
special cells of the gonads; in plants, in special cells of the sporangia. Meiosis consists of two
nuclear divisions, meiosis I and II, with an atypical interphase between the divisions during
which cells do not grow and synthesis of DNA does not take place. This means that meiosis I
and II result in four cells from each parent cell, each containing half the number of
chromosomes, one from each homologous pair. Recall that cells with only one of each
homologous pair of chromosomes, are haploid (n) cells. The parent cells, with pairs of
homologous chromosomes, are diploid (2n). The haploid cells become sperm (in males), eggs (in
females), or spores (in plants). One advantage of meiosis in sexually reproducing organisms is
that it prevents the chromosome number from doubling with every generation when fertilization
occurs.
Working with another student, you will build a model of the nucleus of a cell in interphase and
meiosis. You and your partner should discuss activities in the nucleus and chromosomes in each
stage. Go through the exercise once together, and then demonstrate the model to each other to
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reinforce your understanding. Compare activities in meiosis with those in mitosis as you build
your model.
Procedure
1. Build the premeiotic interphase nucleus much as you did the mitotic interphase nucleus. Have
two pairs of chromosomes (2n = 4) of distinctly different sizes and different centromere
positions. Have one member of each pair of homologues be one color, the other, a different
color.
2. Using the chromosomes you have built, model replication during the S phase of Interphase
and ALL phases of Meiosis. Be sure to draw out each step you have modeled.
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