Cell Cycle and Cell Division

I. Topic: Meiosis
1.2 Differentiate mitosis from meiosis

Objectives:
1. define meiosis
2. identify the stages of meiosis
3. describe the stages of Meiosis
4. contrast Meiosis I and Meiosis II

III. Science Concepts:

1. Meiosis is a process of nuclear division that reduces the number of chromosomes by half and produces
haploid cells from diploid cells.

2. Through meiosis the chromosome number of a species can remain the same from one generation to the next.

3. In meiosis, there are two successive divisions: Meiosis I and Meiosis II.

a. Meiosis I - separates the homologous chromosomes.

 Special cells in the reproductive organs with diploid number of chromosomes undergo
nuclear division resulting into two cells with haploid number but double stranded
chromosome. It is composed of Prophase I, Metaphase I, Anaphase I and Telophase I. A brief
interphase follows after telophase I but no duplication of chromosomes occurs.

 Prophase I: (Gr. pro = before) DNA condensates and crossing-over (exchange of genetic
material) occurs between twisted homolog chromosomes. Prophase I is divided into
Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis

 Metaphase I (Gr meta= middle): The homolog chromosomes (bivalents) are paired in the
equatorial plane. The chromatids are strongly condensed. Bivalents are positioned in such a
way that homolog centromeres lay at either side of the equatorial plane. Only chiasmata
(crossings) hold the bivalents of homolog chromosomes together.

 Anaphase (Ana = apart) begins when the chromosomes are pulled to the opposite poles in the
cell. The units of a homolog pair move apart (separation of bivalents) in opposite direction;
however, the chromatids of each chromosomes stay joined.

 Telophase (Telos = end): The chromosomes have reached the poles and decondense, while a
new nuclear envelop is formed.

b. Meiosis II. In this phase, the two cells from meiosis I undergo a nuclear division identical to
what happened in mitosis producing four un-identical cells with haploid number of
chromosomes which are single stranded. It is composed of Prophase II, Metaphase II, Anaphase
II and Telophase II.

 Prophase II : typical for this stage is the presence of a haploid number of chromosomes that
condense again. The two sister chromatids of a chromosome are still coupled together at the
centromere.
 Metaphase II: The chromosomes move again to the equatorial plane at the equatorial plate.
However, this plane is oriented perpendicularly with respect to the previous one of meiosis I.
 Anaphase II: The centromeres separate and the sister chromatids are pulled apart to opposite
poles by the spindle.
 Telophase II: The former sister chromatids have reached the poles. A nuclear envelope is
formed around each nucleus, while chromosomes disintegrate again.

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IV. Activities:

A. Defining Meiosis. Form the definition of meiosis using the different words inside the box.

reproductive organs haploid organisms

nuclear division number one-half
four reduced

Meiosis is a process of ____________________ occurring in the ________________ of ____________ in

which the ____________ of chromosomes in cells is ___________ by _________ thus producing ________
________ cells.

B. Identifying the stages of meiosis I by writing the specific stage below each drawing. (Prophase I,
Metaphase I, Anaphase I and Telophase I). Write also the sequence number (1, 2, 3, 4) of each stage
on the box.

C. Identifying the stages of meiosis II by writing the specific stage below each drawing (Prophase II,
Metaphase II, Anaphase II, Telophase II). Write also the sequence number (1, 2, 3, 4) of each stage
on the box.

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 C. Describing the stages of Meiosis
1. Meiosis I. Match the correct description of the stages of Meiosis I. Write the letter/s that correctly
match with the correct description.
Phases of Meiosis I Answers Description
1. Prophase I a. homologous chromosomes (centromeres intact)
separate and move toward the opposite poles
b. spindle disappears, nuclear envelopes reform, daughter
2. Metaphase I nuclei become haploid, and each chromosome is still
duplicated.
c. Already duplicated chromosomes appear, homologous
3. Anaphase I pairs synapse, crossing over occurs, the nuclear
envelope is broken
d. spindle formation is complete, homologous
4. Telophase I chromosomes or tetrad are aligned at the center of the
pole.
e. Cytokinesis begins
f. Cytokinesis results in two daughter cells

2. Meiosis II. Match the correct description of the stages of Meiosis II. Use a line to connect the name of
the stage with its correct description.
Phases of Meiosis II Answer Description of the Different Phases of Meiosis II
a. Chromosomes are still duplicated, spindle formation
1. Prophase II begins, nuclear envelope fragments
b. Centromeres divide, haploid sets of daughter
chromosomes move toward opposite poles.

c. Cytokinesis begins
2. Metaphase II
d. Spindle disappears; nuclear envelope reform;
daughter nuclei are haploid and genetically
dissimilar from parent cell and each other.
3. Anaphase II
e. Cytokinesis results in four haploid daughter cells

f. Spindle formation is complete; duplicated

chromosomes are aligned at the center of the pole;
kinetochore fibers of sister chromatids point to
4. Telophase II opposite poles.

V. Generalization:
1. Why do you think meiosis is an important process of cell cycle in sexually reproducing organisms?

2. How does meiosis maintain the specific chromosome number of every species? Give one example.

VI. Evaluation.

1. Which of the following describes the new cells that result from the meiotic division?
a. with half the number of chromosomes of the original cells.
b. same size as the original cell
c. with the same number of chromosomes as the original cell.
d. double the size in the original cell

2. How many daughter cells are produce at the end of meiosis II?
a. 1 b. 2 c. 4 d.
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3. Which event is NOT common in meiosis I and meiosis II?
a. chromosomes move separately and move toward opposite poles.
b. synapses and crossing over occur.
c. nuclear membrane disintegration.
d. reappearance of nucleoli

4. Mitosis occurs in different types of cells. In what type of cells listed below does mitosis NOT occur?
a. blood cells c. muscle cells
b. sex cells d. nerve cells

5. Which event normally occurs in both mitosis and meiosis.

a. crossing over of homologous chromosomes
b. Nuclear membrane disintegration
c. Haploid cells are produced.
d. chromosome replication