BIO Lab 07: Meiosis - catholictexts.com · 5. Meiosis has two consecutive cell divisions termed Meiosis I and Meiosis II. Meiosis I begins with a parent cell with a ploidy that is

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BIO Lab 07: Meiosis

Meiosis Jesus answering said….”But from the beginning of the Creation, God made them male and female. For this

cause a man shall leave his father and mother; and shall cleave to his wife. And they two shall be in one

flesh….What therefore God hath joined together, let not man put asunder.” Mark 10:6-9

Introduction

In a previous lab, you examined the process by which a diploid (2n) parent cell divides to

produce two daughter cells with the same number and the same kind of chromosomes. The

process, mitosis, results in diploid (2n) cells and is typical of ordinary tissue growth.

Meiosis is cell division in which a diploid

(2n) parent cell divides to produce four daughter

cells with half the number of chromosomes

called the haploid (n) number. The daughter

cells are specialized gametes designed to restore

the diploid number at fertilization as shown.

Not only is chromosomal number reduced in

meiosis, but any given chromosome in the

parent cell may differ from the same chromosome in the daughter cell due to one or more

crossover events. In this lab, you will follow the movement of chromosomes through meiosis in

order to create four haploid daughter cells.

Definitions Definitions involved in meiosis: ❖ Allele – Alternate forms of the same gene found at the same location on a homologous chromosome

❖ Chiasma – The area on homologous chromosomes where crossover occurs

❖ Chromatid – One of two identical copies of chromosomes held together by a centromere, formed

when a chromosome replicates itself during cell division. Also called “sister chromatids”

❖ Chromosome – Highly condensed structure of DNA surrounded by proteins found when cells are

undergoing division to ensure that genetic information is divided equally between the two resulting

daughter cells

❖ Crossover – A process of genetic exchange

between homologous chromosomes occurring in

the tetrad structure during meiosis I, resulting in

recombinant chromosomes

❖ Diploid – Cells that have two copies of each

chromosome, one from each parent (called 2n)

❖ Gamete – A cell containing a haploid number of

chromosomes

❖ Gene – A DNA sequence on a chromosome

coding for a specific protein

❖ Haploid – Cells that have one copy of each

chromosome (called n)

❖ Homologous Chromosomes – One chromosome from each homologous pair comes from mother

(maternal chromosome) and one comes from father (paternal chromosome). They are similar but not

identical – each have genes in same order but alleles for each trait may differ.

❖ Ploidy – The number of copies of chromosomes in a cell. In animals, usually diploid (2n) or haploid

(n).


BIO Lab 07: Meiosis

❖ Recombinant Chromatid – Chromatid produced when segments of DNA are exchanged at a cross

over event during Prophase I or Metaphase I

❖ Somatic Cells – Diploid cells not involved in gamete formation

❖ Tetrad – Four homologous chromosomes made up of two sister chromatids and formed during the

prophase of meiosis

Learning Objectives: • Model the steps of meiosis in order to gain a better understanding of the process

• Compare and contrast meiosis and mitosis events and processes

• Examine crossover and the effect it has on chromosomes in daughter cells

Materials Required: From Biology Kit Student Supplied

Chenille Stems (pipe cleaners) (4 lengths)

Beads, Color 1 (10)

Beads, Color 2 (10)

Safety

• Beads are a choking hazard. Keep away from small children. Do not put in mouth.

Preparation:

1. Build Paternal Chromosomes:

• You will need a total of 10 beads of

one color

• Use the diagram to the right to guide

your construction

• Chromosome 1:

▪ Place 2 beads on a pipe cleaner

▪ Tie a knot in the pipe cleaner to

model the centromere

▪ Place 1 remaining bead on the

end

▪ Secure both ends so the beads do

not fall off

▪ Duplicate this so that you have

two Paternal copies of

Chromosome 1

▪ See figure to the right

• Chromosome 2:

▪ Place 1 bead on a pipe cleaner

▪ Tie a knot in the pipe cleaner to model the centromere

▪ Place 1 remaining bead on the end


BIO Lab 07: Meiosis

▪ Secure both ends so the beads do not fall off

▪ Duplicate this chromatid so that you have two Paternal copies of Chromosome 2

▪ See figure to the right

2. Build Maternal Chromosomes:

• You will need a total of 10 beads of a different color

• Use the diagram above to guide your construction

• Chromosome 1:

▪ Using 3 beads of a different color, construct Maternal Chromosome 1

▪ Duplicate this so that you have 2 Maternal copies of Chromosome 1

• Chromosome 2:

▪ Using 2 beads of the maternal color, construct Maternal Chromosome 2

▪ Duplicate this so that you have 2 Maternal copies of Chromosome 2

3. Let us review what you have:

• Sometime during Interphase, S-phase will occur, in which each chromosome will

duplicate

• The cell will have copies of each chromosome

• Using our organism today as an example, we have:

Duplicate copies (or sister chromatids) of Chromosome 1 derived from this

organism’s mother

Sister chromatids of Chromosome 1 derived from this organism’s father

These are called Homologous Pairs

Genes at equivalent points on the maternal gene and the paternal gene are called

homologous genes or alleles

• The figures below show two sets of homologous pairs of chromosomes


BIO Lab 07: Meiosis

Experiment 1 – No Crossover Use your text and notes to review the important processes at each stage of Meiosis.

1. Assemble a tetrad

• Pair the homologous chromosome you have constructed into a tetrad

• Locate the cell diagrams following Experiment 2 below

▪ Use the cell outlines to place your models in, or as a model to draw your own cell

outline

• Locate Table 1 in Lab Report and use it to describe your models

2. Using the bead “chromosomes”, model each stage of meiosis and cytokinesis:

• Prophase I and II

• Metaphase I and II

• Anaphase I and II

• Telophase I and II

• Cytokinesis I and II

3. As you model each stage, use Table 1 to note your findings, especially:

• Major activities that occur in each step

• The correct number of chromosomes present in the cell for each phase

• The notable appearance or disappearance of structures such as:

▪ Diploid or haploid state

▪ Number of chromosomes in cell

▪ Centromeres

▪ Chromatin

▪ Sister chromatids

▪ Spindles and spindle fibers

▪ Centrioles

▪ Metaphase plate

▪ Nuclear envelope

▪ Cleavage furrow

▪ Other structures that appear or disappear

3. Perform Data Analyses and Conclusions


BIO Lab 07: Meiosis

Experiment 2 – Crossover Use your text and notes to review Crossover events during Meiosis.

1. Assemble chromosomes

• Reassemble the two homologous chromosomal pairs as you did before

• Locate Table 2 in Lab Report and use it for discussing your models

2. Model a Crossover Event at Prophase I

• Bring Homologous Pair 1 close together

• Create a chiasma

Cross ONE (and only one) sister chromatid of one pair with the

homologous sister chromatid of the second pair

• Exchange an equal number of beads between the two pairs

• Ensure that the resulting recombinant chromosomes are equal in

length but now have different combination of colors

Describe in detail the resulting homologous pair in Table II

3. Model the remaining stages of Mitosis and Cytokinesis:

• Prophase II

• Metaphase I

and II

• Anaphase I

and II

• Telophase I

and II

• Cytokinesis

4. As you model each stage

• Pay attention to the following:

Crossover event:

• Follow the new recombinant chromatids

• Note which cells the new recombinant chromatids end up in

• There are 4 cells at the end of meiosis, describe how they differ.

Make sure you indicate the correct number of chromosomes present in each cell

for each phase

• Describe the notable appearance or disappearance of:

Recombinant chromatid, haploid, diploid, spindles and spindle fibers, centrioles,

nuclear envelope, centromeres, sister chromatids, chromatin,

5. Perform Data Analyses and Conclusions


BIO Lab 07: Meiosis

Following are cell outlines for your use. You may wish to use these to lay your

chromosome models in. Or, if you wish, you can use these to as models to

draw your own.

INTERPHASE I INTERPHASE II

PROPHASE I PROPHASE II


BIO Lab 07: Meiosis

METAPHASE I METAPHASE II

ANAPHASE I ANAPHASE II


BIO Lab 07: Meiosis

TELOPHASE I TELOPHASE II

CYTOKINESIS I CYTOKINESIS II


BIO Lab 07: Meiosis

Lab Report for: _______________________________

Table 1 – Meiosis (Without Crossover Event)

Stage # Chromosomes

Per Cell Describe in Detail

Prophase I 4 Metaphase I Anaphase I Telophase I Prophase II

Metaphase II Anaphase II Telophase II Cytokinesis

Table 2 – Meiosis (With Crossover)

Stage # Chromosomes

Per Cell Describe in Detail

Prophase I 4

Metaphase I

Anaphase I

Telophase I

Prophase II

Metaphase II


BIO Lab 07: Meiosis

Anaphase II

Telophase II

Cytokinesis

Data Analyses and Conclusions 1. What is the purpose of meiosis?

2. In the cell you modeled, what was the diploid number (2n) for the parent cell? What was the

haploid number?

3. Metaphase is drastically different between Mitosis and Meiosis I:

• Describe what is observed at the metaphase plate

• How is Metaphase similar between Mitosis and Meiosis I?

• How is Metaphase different between Mitosis and Meiosis I?

• What does this difference lead to during the next step?

COMPARISON OF METAPHASE BETWEEN MITOSIS AND MEIOSIS

MITOSIS MEIOSIS

Describe what is observed

at the Metaphase Plate

How are they the same? How are they different?

What is the result of

difference?

4. Review the figure (right) which portrays a cell

in Metaphase I. Which scenario, if any, is most

likely? Which Law or Principle informs your

answer and explain?


BIO Lab 07: Meiosis

5. Meiosis has two consecutive cell divisions termed Meiosis I and Meiosis II. Meiosis I begins

with a parent cell with a ploidy that is diploid. In this model, the diploid number was 2n=4.

At what point, was the ploidy reduced to haploid (n=2)….at the end of Meiosis I or at the

end of Meiosis II? Explain.

6. End Result – No Crossover:

• Describe, in detail, the end result of Mitosis and Meiosis.

• How are they similar?

• How are they different? What have similarities and differences. Compare and

contrast mitosis and meiosis in the table below.

COMPARISON OF END RESULT BETWEEN MITOSIS AND MEIOSIS

Assuming No Crossover MITOSIS MEIOSIS

Describe End Result

How are they the similar?

How are they different?

(# of Cells; Ploidy, etc.)

7. Consider each bead to represent a gene. In the crossover event explain what took place

between the homologous pairs.

8. Compare the resulting 4 cells of meiosis – With and without a Crossover Event:

• Describe the chromosomes in each of the 4 cells WITH and WITHOUT a Crossover

Event

• How are the gametes similar?

• How are the gametes different?

COMPARISON OF END RESULT

WITH AND WITHOUT CROSSOVER No Crossover CROSSOVER Describe the chromosomes

in gametes WITH

Crossover.


BIO Lab 07: Meiosis

Describe the chromosomes

in gametes WITHOUT

Crossover.

How are the gametes the

same? How are the gametes

different?

9. Why is it necessary to reduce the number of chromosomes in gametes but not in skin cells?

10. Review the figure below. Assume these are 4 female gametes produced from one parent cell

undergoing meiosis, resulting in 4 egg cells.

What would be the result if each egg could be fertilized with the SAME male gamete, the

sperm? Would the four individuals be identical? Similar? Explain.

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BIO Lab 07: Meiosis - catholictexts.com · 5. Meiosis has two consecutive cell divisions termed Meiosis I and Meiosis II. Meiosis I begins with a parent cell with a ploidy that is