Biology Lab Shahimermaid

UMUC Biology 102/103

Lab 5: Meiosis

INSTRUCTIONS:

· On your own and without assistance, complete this Lab 5 Answer Form electronically and submit it via the Assignments Folder by the date listed on your Course Schedule (under Syllabus).

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· Save your Lab5AnswerForm in the following format: LastName_Lab5 (e.g., Smith_Lab5).

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Experiment 1: Following chromosomal DNA movement

Procedure

Meiosis I

Prophase I—

Metaphase I—

Anaphase I—

Telophase I—

Meiosis II

Prophase II—

Metaphase II—

Anaphase II—

Telophase II—

Questions

1. What is the state of the DNA at the end of meiosis I? What about at the end of meiosis II?

2. Why are chromosomes important?

3. How are Meiosis I and Meiosis II different?

4. Name two ways meiosis contributes to genetic recombination.

5. Why do you use non-sister chromatids to demonstrate crossing over?

6. How many chromosomes were present when Meiosis I started?

7. Why is it necessary to reduce the chromosome number of gametes, but not other cells of an organism?

8. If humans have 46 chromosomes in each of their body cells, determine how many chromosomes you would expect to find in the following:

Sperm _________________

Egg _________________

Daughter cell from mitosis _______________

Daughter cell from Meiosis II _________________

9. Investigate a disease that is caused by chromosomal mutations. When does the mutation occur? What chromosome is affected? What are the consequences?

TYPE YOUR FULL NAME:

63

The Cell

Lab 5
Meiosis

64

Lab 5: Meiosis

65

Introduc on

Meiosis only occurs in organisms that reproduce sexually. The process generates haploid (1n) cells
called gametes (sperm cells in males and egg cells in fe
males), or spores in some plants, fungi, and pro sts, that
contain one complete set of chromosomes. Haploid cells
fuse together during fer liza on to form a diploid cell with
two copies of each chromosome (2n).

Genes are the units of heredity that have speci c loci
(loca ons) on the DNA strand and code for inheritable
traits (such as hair color). Alleles are alterna ve forms of the same gene (brown vs. blue eyes). Homol
ogous chromosomes contain the same genes as each other but o en di erent alleles. Non sex cells
(e.g. bone, heart, skin, liver) contain two alleles (2n), one from the sperm and the other from the egg.

Mitosis and meiosis are similar in many ways. Meiosis, however, has two rounds of division—meiosis I
and meiosis II. There is no replica on of the DNA between meiosis I and II. Thus in meiosis, the parent
cell produces four daughter cells, each with just a single set of chromosomes (1n).

Meiosis I is the reduc on division– the homologous pairs of chromosomes are separated so that each
daughter cell will receive just one set of chromosomes. During meiosis II, sister chroma ds are sepa
rated (as in mitosis).

Concepts to explore:

Meiosis
Diploid cells
Haploid cells
Chromosomal crossover

Concepts to explore:

There are over two meters of DNA pack
aged into a cell’s nucleus. It is coiled and
folded into superhelices that form chro
mosomes, which must be duplicated be

fore a cell divides.

Each of the 23 human chromosomes
has two copies. For each chromosome,
there is a 50:50 chance as to which copy

each gamete receives.

That translates to over 8 million possi
ble combina ons!

Lab 5: Meiosis

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Meiosis I:

Prophase I: The sister chroma ds condense and a ach to their homologous counterparts
(chromosomes with the same genes but poten ally di erent alleles). This is the stage
where crossing over occurs (homologous chromosomes exchange regions of DNA). The
centrioles, which will serve as intracellular anchors during division, appear.

Metaphase I: The chromosomes line up in the middle of the cell. The orienta on of each
pair of homologous chromosomes is independent from all other chromosomes. This
means they can “ ip op” as they line up, e ec vely shu ing their gene c informa on
into new combina ons. Microtubules (long strands) grow from each centriole and link
them together while also a aching to each pair of homologous chromosomes.

Anaphase I: The microtubules pull the homologous chromosomes apart (the sister chro
ma ds remain paired).

Telophase I

: One set of paired chromosomes arrives at each centriole, at which me a nu
cleus forms around each set.

Cytokinesis: The plasma membrane of the cell folds in and encloses each nucleus into two
new daughter cells.

Meiosis II:

Prophase II: Before any replica on of the chromosomes can take place, the daughter cells
immediately enter into Prophase II. New spindle bers form as the nucleus breaks down.

Metaphase II: The sister chroma ds align in the center of the cell, while the microtubules
join the centrioles and a ach to the chromosomes. Unlike Metaphase I, since each pair of
sister chroma ds is iden cal, their orienta on as they align does not ma er.

Anaphase II: The sister chroma ds are separated as the microtubules pull them apart.

Telophase II: The chroma ds arrive at each pole, at which me a nucleus forms around
each.

Cytokinesis: The plasma membrane of the cell folds in and engulfs each nucleus into two
new haploid daughter cells.

We brie y discussed “crossing over” in Prophase I. Since the chromosomes of each parent undergoes
gene c recombina on, each gamete (and thus each zygote) acquires a unique gene c ngerprint.

The closeness of the chroma ds during Prophase I, creates the opportunity to exchange gene c mate
rial (chromosomal crossover) at a site called the chiasma. The chroma ds trade alleles for all genes
located on the arm that has crossed.

The process of meiosis is complex and highly regulated. There are a series of checkpoints that a cell

Lab 5: Meiosis

67

must pass before the next phase of meiosis will begin. This ensures any mutated cells are iden ed
and repaired before the cell division process can con nue.

One of the muta ons that is of par cular concern is a
varia on in the amount of gene c material in a cell. It
is cri cal that the gamete contain only half of the chro
mosomes of the parent cell. Otherwise the amount of
DNA would double with each new genera on. This is
the key feature of meiosis.

Figure 1: The stages of meiosis

Muta ons that are not caught by the cell’s
self check system can result in chromosomal

abnormali es like Down’s syndrome, in
which there are 3 copies of chromosome 21.

Interphase I:
Cellular growth
and DNA repli
ca on occur to
prepare cell for
meiosis.

Metaphase I: Paired
chromosomes align
at metaphase plate.

Prophase I: Sister
chroma ds pair
up. Crossing over
may occur.

Prior to meiosis.

Anaphase I: Microtubules pull
chromosomes to opposite poles.
Sister chromosomes remain paired.

Telophase I: Paired
chromosomes arrive
at polar ends of cell.
Cytokinesis occurs
to bisect cell.

Prophase II: Daughter
cells immediately enter
Prophase II. Nuclei are
broken down

Metaphase II:
Sister chroma
ds align at

center of cell.

Anaphase II:
Sister chro
ma ds are
pulled apart.

Telophase II: Sister chroma ds
arrive at polar ends of cell. Nu
clei are created and cytokinesis
completes bisec on.

Lab 5: Meiosis

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Experiment 1: Following chromosomal DNA movement

Every cell in the human body has two alleles that condense into single chromosomes held together by
a centromere. These “sister” chroma ds replicate and pair with the newly made homologous chromo
somes. In this exercise we will follow the movement of the chromosomes through meiosis I and II to
create haploid (gamete) cells.

Procedure

Meiosis I

A. As prophase I begins, chromosomes coil and condense in prepara on for replica on.

1. Using one single color of bead, build a homologous pair of duplicated chromosomes.
Each chromosome will have 10 beads with a di erent colored centromere in it.

For example, if there are 20 red beads, 10 beads would be snapped together to
make two di erent strands. In the middle of each of the 10 bead strands, snap
a di erent colored bead in to act as the centromere.

Now, repeat these steps using the other color of bead.

2. Assemble another homologous pair of chromosomes using only 12 (that’s 6 per
strand) of the rst color bead. Place another, di erent colored bead in the middle of
each to act is its centromere. Repeat this step (2 strands of 6 beads plus a centro

Figure 2: Bead Set up

Materials

2 sets of di erent colored snap
beads (32 of each)
8 centromeres (snap beads)
Blue and red markers*
*You must provide

Lab 5: Meiosis

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mere) with the other color of beads.

B. Bring the centromeres of two units of the same color and length together so they can be held
together to appear as a duplicated chromosome.

1. Simulate crossing over. Bring the two homologues pairs together (that’d be the two
pairs that both have 10 bead strands) and exchange an equal number of beads be
tween the two.

C. Con gure the chromosomes as they would appear in each of the stages of meiosis I.

Meiosis II

A. Con gure the chromosomes as they would appear in each stage of meiosis II.

B. Return your beads to their original star ng posi on and simulate crossing over. Track how this
changes the ul mate outcome as you then go through the stages of meiosis I and II.

C. Using the space below, and using blue and red markers, draw a diagram of your beads in each
stage. Beside your picture, write the number of chromosomes present in each cell.

Meiosis I

Prophase I

Metaphase I

Anaphase I

Telophase I

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Meiosis II

Prophase II

Metaphase II

Anaphase II

Telophase II

Ques ons

1. What is the state of the DNA at the end of meiosis I? What about at the end of meiosis II?

2. Why are chromosomes important?

3. How are Meiosis I and Meiosis II di erent?

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4. Name two ways meiosis contributes to gene c recombina on.

5. Why do you use non sister chroma ds to demonstrate crossing over?

6. How many chromosomes were present when meiosis I started?

7. Why is it necessary to reduce the chromosome number of gametes, but not other cells of an
organism?

8. If humans have 46 chromosomes in each of their body cells, determine how many chromo
somes you would expect to nd in the following:

Sperm ___________________
Egg ___________________
Daughter cell from mitosis ___________________
Daughter cell from Meiosis II ___________________

9. Inves gate a disease that is caused by chromosomal muta ons. When does the muta on
occur? What chromosome is a ected? What are the consequences?

We have discussed and worked with cells and their functions at some length. Now we should have a look at how cells reproduce themselves by cellular division. There are two kinds of cell divisions: Mitosis, which happens in somatic cells, and meiosis, which takes place in gametes, the sex cells. There are number of differences between these two types of cell division, because we need to have different outcomes.

Meiosis requires two divisions to happen; after the first division, the two daughter cells are haploid. I have posted a special descriptive comparison between these two processes into the Summaries of Special Topics section under Conferences – PLEASE read it carefully!

Lab 6 – Meiosis

Before you begin your work, please read very carefully the Introduction in the Lab Manual and study Figure 1 carefully. You will then simulate four chromosomes by using snap beads (they are in your kit). You also need red and blue markers (not in your kit).

Then read carefully Meiosis I and build your four chromosomes. Take it step by step, and please review the importance of crossing-over during the tetrad formation. Then do Meiosis II , and then do the drawings of chromosomes during the four phases of Meiosis I and Meiosis II. Please be sure to write down the number of chromosomes and chromatids you are working with, and also provide a brief description at each step. Following that, you will answer nine questions about meiosis. Please read them very carefully, and do not make any assumptions! You should remember that each and every species has different numbers of chromosomes. For example, humans have 23 pairs, that is, 46 total number of chromosomes. Mice have 40 chromosomes, dogs have 76, giraffes have 30, and some plants have over 200 chromosomes. The number of chromosomes found in the cells of individual species, are the result of their individual evolutionary history. In our lab exercise, you are working with an imaginary species that has four chromosomes.

This is a fun exercise, and you may want to involve your children or other interested family members or friends.

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