The Cell Lab 6 Meiosis 66 Lab 6: Meiosis 67 Introduction

26 Jun The Cell Lab 6 Meiosis 66 Lab 6: Meiosis 67 Introduction

Question
The Cell

Lab 6

Meiosis

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Lab 6: Meiosis

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Introduction

Meiosis only occurs in organisms that reproduce sexually. The process generates haploid (1n) cells

called gametes (sperm cells in males and egg cells in females),

or spores in some plants, fungi, and protists, that

contain one complete set of chromosomes. Haploid cells

fuse together during fertilization to form a diploid cell with

two copies of each chromosome (2n).

Genes are the units of heredity that have specific loci

(locations) on the DNA strand and code for inheritable

traits (such as hair color). Alleles are alternative forms of the same gene (brown vs. blue eyes). Homologous

chromosomes contain the same genes as each other but often different 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 replication 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 reduction division– the homologous pairs of chromosomes are separated so that each

daughter cell will receive just one set of chromosomes. During meiosis II, sister chromatids are separated

(as in mitosis).

Concepts to explore:

Meiosis

Diploid cells

Haploid cells

Chromosomal crossover

Concepts to explore:

There are overtwo metersof DNA packaged

into a cell’s nucleus. It is coiled and

folded into superhelices that form chromosomes,

which must be duplicated before

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 possible

combinations!

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Meiosis:

Prophase I: The sister chromatids attach to their homologous counterparts (same chromosome

– different version). This is the stage where crossing over occurs (homologous chromosomes

exchange regions of DNA). Structures which will serve as anchors in the cell

(centrioles) during the division process appear.

Metaphase I: The chromosomes line up in the middle of the cell. The orientation of each

pair of homologous chromosomes is independent from all other chromosomes. This

means they can “flip flop” as they line up, effectively shuffling their genetic information

into new combinations. Microtubules (long strands) grow from each centriole and link

them together while also attaching to each pair of homologous chromosomes.

Anaphase I: The microtubules pull the homologous chromosomes apart (the sister chromatids

remain paired).

Telophase I: One set of paired chromosomes arrives at each centriole, at which time a nucleus

forms around each set.

Cytokinesis: The plasma membrane of the cell folds in and encloses each nucleus into two

new daughter cells.

Prophase II: Before any replication of the chromosomes can take place, the daughter cells

immediately enter into prophase II. New spindle fibers form as the nucleus breaks down.

Metaphase II: The sister chromatids align in the center of the cell, while the microtubules

join the centrioles and attach to the chromosomes. Unlike metaphase I, since each pair of

sister chromatids is identical, their orientation as they align does not matter.

Anaphase II: The sister chromatids are separated as the microtubules pull them apart.

Telophase II: The chromatids arrive at each pole, at which time 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 briefly discussed “crossing over” in Prophase I. Since the chromosomes of each parent undergoes

genetic recombination, each gamete (and thus each zygote) acquires a unique genetic fingerprint.

The closeness of the chromatids during prophase I, creates the opportunity to exchange genetic material

(chromosomal crossover)at a site called the chiasma. The chromatids 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

must pass before the next phase of meiosis will begin. This ensures any mutated cells are identified

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and repaired before the cell division process can continue.

One of the mutations that is of particular concern is a

variation in the amount of genetic material in a cell. It is

critical that the gamete contain only half of the chromosomes

of the parent cell. Otherwise the amount of DNA

would double with each new generation. This is the key

feature of meiosis.

Figure 1: The stages of meiosis

Mutations that are not caught by the cell’s

self‐check system can result in chromosomal

abnormalities like Down’s syndrome, in

which there are 3 copies of chromosome

21.

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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” chromatids replicate and pair with the newly made homologous chromosomes.

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 preparation for replication.

1. Using one single color of bead, build a homologous pair of duplicated chromosomes.

Each chromosome will have 10 beads with a different colored centromere in it.

For example, if there are 20 red beads, 10 beads would be snapped together to

make two different strands. In the middle of each of the 10 bead strands, snap

a different 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 first color bead. Place another, different 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 different colored snap

beads (32 of each)

8 centromeres (snap beads)

Blue and red markers*

*You must provide

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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 between

the two.

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

Meiosis II

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

B. Return your beads to their original starting position and simulate crossing over. Track how this

changes the ultimate 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

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?

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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?

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