Meiosis Lab

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Meiosis

Lab 4

 

 

 

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Lab 4: Meiosis

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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 speciÞc 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 4: Meiosis

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Meiosis:

Prophase I: The sister chroma ds a ach to their homologous counterparts (same chromo-

some – di erent version). This is the stage where crossing over occurs (homologous chro-

mosomes 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 orienta on of each

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

means they can “ßip ßop” 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.

Prophase II: Before any replica on of the chromosomes can take place, the daughter cells

immediately enter into prophase II. New spindle Þbers 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 brießy 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 Þngerprint.

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

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

 

 

Lab 4: Meiosis

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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 chromo-

somes 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 chromoso-

mal abnormali es like Down’s syndrome, in

which there are 3 copies of chromosome

21.

 

 

Lab 4: 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. a) Using one single color of bead, build a homologous pair of duplicated chromo-

somes. 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.

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

 

 

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

2. a) Assemble another homologous pair of chromosomes using only 12 (that’s 6 per

strand) of the Þrst color bead. Place another, di erent colored bead in the middle

of each to act is its centromere.

b) Repeat this step (2 strands of 6 beads plus a centromere) 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. Your beads should appear as they do in Fig-

ure 2.

C. Simulate crossing over between the blue and red chromosomes. Bring the two homologous

pairs together (that’d be the two pairs that both have 10 bead strands) and exchange an equal

number of beads between the two. This simulates what occurs during prophase I.

D. ConÞgure the chromosomes as they would appear in each of the remaining stages of meiosis I.

Use Figure 1 to guide you.

 

Meiosis II

E. ConÞgure the chromosomes as they would appear in each stage of meiosis II. Use Figure 1 to

guide you.

F. 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. This work

will help you answer the ques ons in the lab, but does not need to be submi ed for grading.

 

Meiosis I

Prophase I

 

 

Metaphase I

 

 

 

 

 

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Anaphase I

 

 

 

Telophase I

 

 

 

Meiosis II

Prophase II

 

 

 

Metaphase II

 

 

 

Anaphase II

 

 

 

Telophase II

 

 

F. Return your beads to their original star ng posi on and simulate crossing over again. Track

how this changes the ul mate outcome as you then go through the stages of meiosis I and II.

 

 

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Ques ons

1. Why is crossing over important in heredity?

2. Provide two ways that meiosis I and meiosis II are di erent.

3. a) In this lab, how many chromosomes were present in each cell when meiosis I started?

b) How many chromosomes were present in each daughter cell at the end of meiosis II?

4. If humans have 46 chromosomes in each of their body cells, determine how many chromo-

somes you would expect to Þnd in the following:

Sperm ___________________

Egg ___________________

Daughter cell from mitosis ___________________

Daughter cell from meiosis II ___________________

5. Why is it necessary to reduce the chromosome number of gametes, but not of other cells of

an organism?