Name:

TA:

Lab Section:

 

MITOSIS AND MEIOSIS

 

Textbook reading:

Cell cycle- Fig. 12.4

Mitosis- Fig. 12.5

Meiosis- Fig. 13.6

 

Introduction:

There are many different cell types in your body or in that of a plant and they all have many things in common. One of theses things is that at some time in their development they had to replicate. Cellular replication (the cell cycle) results in the production of new cells which are either identical to that of the original cell or genetically different from the original cell. The growth phase of the cell cycle is called interphase. DNA replicates itself during interphase and thus is critical to the formation of two daughter nuclei. The process of replication which results in identical offspring is called mitosis, while variation in the offspring results from meiosis. The basic features of mitosis and meiosis are discussed in your text book. Read through the processes before you start this lab and make sure you are familiar with the different phases of mitosis and meiosis (prophase, metaphase, anaphase, and telophase) before you begin the lab. In this lab you will use diagrams and models to become familiar with each stage in the cell cycle and view slides of plant and animal cells in each phase of the cell cycle.

 

Procedure:

 

MITOSIS

 

Somatic cells divide by a process called mitosis. Mitosis has four stages, prophase, metaphase, anaphase and telophase. Using the chromosome kits provided, centrioles (pennies), and nuclear membranes (thread), go through each phase as described below.

 

Prophase : Chromosomes are visible within the nuclear membrane. Each contains two sister chromatids joined at the centromere. Near the nucleus are two bundles called centrioles, which at the beginning of the division phase of the cell, start to separate and move to opposite poles of the cell. As the centrioles move apart, spindle fibers (composed of microtubules) form between them. The nuclear membrane begins to break down (remove the thread).

 

Metaphase: Spindle fibers attach to the chromosomes at their centromeres and move the chromosomes to the equator of the cell.

 

Anaphase: The two sister chromatids separate (separate the chromatids) one being dragged by the spindle fibers in the direction of one centriole, the other in the opposite direction to the other centriole. Now each sister chromatid represents a single chromosome.

 

Telophase: When the chromosomes reach the centrioles, the spindle fibers break down and the chromosomes return to their thread-like form. A nuclear membrane forms around each set of chromosomes (replace the string) leaving the centriole outside. Now each nucleus contains a set of chromosomes which are identical in number and composition to that of the parent cell. The cytoplasm is divided between the two nuclei resulting in two identical cells, each containing identical chromosomes and enough cytoplasm to start the growth and reproduction cycle again.

 

Without looking at the procedure notes, work in pairs and explain to each other the process of mitotic division. Write down as many things as you can remember about each stage of mitosis.

 

Inerphase

 

Prophase________________________________________________________________

 

Metaphase_______________________________________________________________

 

Anaphase _______________________________________________________________

 

Telophase_______________________________________________________________

 

 

Mitosis results in two genetically identical nuclei. What does this mean in terms of the amount and type of DNA in each cell? ___The amount and type of DNA in each cell is identical._____________________________________________________________________

 

________________________________________________________________________

 

 

The word clone means geneticaly identical. Are cells that result from mitosis clones?

 

__Yes______________________________________________________________________

 

 

Identical twins occur because within the first few divisions of the zygote, the new cells separate - are the twins clones? _____yes___________________________________________________________________

 

 

On the bench are prepared slides of whitefish blastula cells (blastula is an early stage of embryo development). See if you can identify cells in interphase as well as the different stages of mitosis. How many of each stage can you find in one view at 400x magnification? Count the number of cells in each phase and write that number in the table. Repeat this two more times, be sure to shift the field of view each time, and find the average number of cells in each phase.

 

 

interphase

prophase

metaphase

anaphase

telophase

total # counted

view 1

           

view 2

           

view 3

           

average #

           

Percentage

           

 

If the process of replication occurred in a 24 hour period, what percentage of time would each phase take up of that 24 hour period? Draw a pie chart to illustrate your data.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1 ________________________________________________________

 

 

 

 

Now take a root tip slide and view the stages of mitosis in these slides. Repeat the identification of each stage and place your counts in the table below.

 

 

 

 

interphase

prophase

metaphase

anaphase

telophase

total # counted

view 1

           

view 2

           

view 3

           

average #

           

Percentage

           

 

Table 2 ________________________________________________________

 

 

 

 

 

 

 

 

 

 

 

If the process of replication occurred in a 24 hour period, what percentage of time would each phase take up of that 24 hour period? Draw a pie chart to illustrate your data.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2 ________________________________________________________

 

Are there any differences in the time for each stage between the animal and plant cells? List some of the major differences between the process of mitosis in plant and animal cells.

 

________animal cells lack a cell wall; plant cells divide by a cell plate; most plant cells lack centrioles.

______________________________________________________________

 

______________________________________________________________________

 

______________________________________________________________________

 

 

 

MEIOSIS

 

Meiotic division occurs to produce sex cells or gametes (at least in animals), which contain only one set of chromosomes, half the amount of DNA, found in diploid somatic cells. Gametes are thus considered to be haploid (one set of chromosomes). Meiosis is the first step in sexual reproduction. Sexual reproduction also results in variation in the genotype of the offspring because during the formation of gametic cells a process called crossing over occurs. At interphase each cell contains pairs of homologous chromosomes, one from each parent. During interphase each chromosome replicates in a manner similar to that of mitosis.

Meiosis has two distinct stages meiosis; I and meiosis II.

 

 

 

Meiosis I

 

Prophase I: For this simulation you need two pairs of chromosomes, each with two sister chromatids connected at the centromeres. One set of sister chromatids will be red and the other yellow. To demonstrate crossing over, put the yellow set of sister chromatids close to the red set. Remove four beads from one of each of the yellow and red chromatids and exchange them. You now have one yellow pair of sister chromatids with a red section on the end of one chromatid and a red pair of chromatids with a yellow part attached.

Each centriole should start to move to the poles and the nuclear membrane breaks down.

 

Metaphase I: The homologous chromosomes move to the equator of the cell.

 

Anaphase I: Separate the homologous chromosomes (each contains sister chromatids) toward the centrioles. Important- do not separate the sister chromatids, only the pairs of homologous chromosomes.

 

Telophase I: Pile each chromosome near its centriole and place a second centriole near the first. Nuclear membrane forms and cytoplasmic division occurs to produce two cells. By the end of telophase I, the daughter nuclei are haploid and genetically different from the original nucleus.

 

 

Meiosis II

 

Each student in the pair should take one of the new somatic cells generated in the last part of the exercise and proceed. The phases of meiosis II are virtually identical to the process of mitosis.

 

Interphase II/Interkinesis: Similar to normal interphase, except DNA synthesis does not take place.

 

Prophase II: Centrioles migrate to the poles. Nuclear membrane breaks down.

 

Metaphase II: Bring the chromosomes to the equator of the cell.

 

Anaphase II: Sister chromatids separate and move to poles.

 

Telophase II: Pile each of the chromosomes close to the centriole and replace nuclear membrane. Cytoplasmic division occurs resulting in a total of four nuclei.

 

Repeat this process until you are completely familiar with it.

 

Without looking at the procedure notes, repeat meiotic division and write down two things about each stage of meiosis.

 

Interphase I _____________________________________________________________

 

Prophase I ______________________________________________________________

 

Metaphase I _____________________________________________________________

 

Anaphase I _____________________________________________________________

 

Telophase I _____________________________________________________________

 

Interphase II (Interkinesis) ___________________________________________________________

 

Prophase II _____________________________________________________________

 

Metaphase II ____________________________________________________________

 

Anaphase II _____________________________________________________________

 

Telophase II _____________________________________________________________

 

 

Which part of meiosis is identical to mitosis? ___________________________________

 

 

In animals, meiosis occurs in ovarys and testis and directly produces gametes (sperm and egg cells). In plants, however, meiosis occurs in flowers. Microspores are formed by meiosis within anthers. Microspores eventually develop into pollen which will eventually form sperm cells. Megaspores are formed by meiosis within ovules. Megaspores eventually develop into embryo sacs which eventually form an egg cell. You will learn more about plant reproduction in future labs.

 

Observe a prepared slide of a cross section through an ovary. Sketch what you see in the space below.

 

 

 

 

 

 

 

Can you identify the large nucleus within the egg cell? The various stages of meiosis are very difficult to distinguish (especially when you only observe one section). If the slide was of a human ovary, how many chromosomes would be present in the egg cell? How about the surrounding cells?

 

__23 chromosomes in the egg cell and 46 chromosomes in the surrounding cells.___________________________________________________________________ __

 

Observe a prepared slide of testis. Again, it is very difficult to distinguish the various stages of meiosis. Sketch what you see in the space below. Using Fig. 46.12in your textbook as a guide, label a cell that is diploid, one that has completed meiosis I, and one that has completed meiosis II (give it your best shot).

 

 

 

 

 

 

 

 

 

 

Observe the prepared slides which show various stages of meiosis in plants. These slides are sections through anthers (Fig. 38.4, left panel). Pollen grains, which are formed in anthers, are the "male" structures formed by meiosis in plants. Embryo sacs (which contain egg cells) are the "female" structures formed by meiosis in plants. We’ll just focus on meiosis in anthers as our demonstration.

 

Observe and sketch the following stages:

microsporocytes- these cells are diploid and capable of undergoing meiosis.

dyads- two cells; the end result of meiosis I.

tetrads- four cells; the end result of meiosis II.

mature pollen- haploid structures that are shed from plants.

 

 

 

 

 

 

 

 

 

 

 

 

At which of the above stages are haploid nuclei first formed?

 

____dyads____________________________________________________________________

 

 

At which of the above stages is genetic variation first evident?

 

_____dyads (after meiosis I)

______________________________________________________________

 

 

Within a tetrad of pollen, are the cells genetically identical? How about among the tetrads within the anther?

 

______no, no

__________________________________________________________________

 

 

Another very important phenomenon of meiosis is independent assortment of chromosomes. This topic will be covered in lecture, but you should realize that it, along with crossing over, provides tremendous genetic variation in gametes.

 

 

 

 

MEIOSIS CD-ROM ASSIGNMENT

 

Use the computer lab in room 121 to complete this assignment.

 

Do NOT put headphones on until the CD has started. Volume can be controlled by the two small buttons on the right of the computer.

 

Double click the "Cell Biology I" icon.

 

Double click the "Power CD" icon.

 

The program will begin to run by itself. Once started, click and hold down on "Contents", choose "Feature Presentations", then doube click #12 (Meiosis: 09:10 - 13:42).

 

This meiosis part of the CD should lasts approximately 15 minutes.

 

Answer the questions provided as the program progresses and turn in to your TA as part of your assignment this week.

 

NOTE: If the program goes a little too fast, you can click on the middle button on the lower left of the screen (the one with two vertical lines). This will stop the program until you click the button again.

 

 

MEIOSIS CD-ROM QUESTIONS

 

1. At what phase of meiosis does exchange of genetic material between homologous chromosomes take place?

 

Meiosis I

 

2. Do crossovers occur at a specific region or can they occur anywhere along the length of a chromosome?

 

Anywhere

 

3. What is the first visible sign that a crossover event has taken place?

 

Chiasmata- site of crossing over.

 

4. Do sister chromatids separate during anaphase I of meiosis?

 

No

 

5. Are the two daughter nuclei that result from meiosis I haploid or diploid?

 

Haploid

 

6. Are chromosomes replicated during the interphase between meiosis I and meiosis II?

 

No

 

7. If crossovers did not occur, how many genotypes (per chromosome) would result from meiosis?

 

2

 

8. In what two ways does meiosis introduce genetic variation into a population?

 

Independent assortment of chromosomes

 

Crossing over

 

9 & 10. List two main differences between mitosis & meiosis.

Any 2 is fine.