Biology Lab Shahimermaid

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The Cell

Lab 5 Meiosis

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Lab 5: 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 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!

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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

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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.

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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

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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?