To understand the differences between Mitosis and Meiosis To understand Oogenesis

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Gwendolyn Lloyd
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1 Lab 5: Cell Division LABORATORY OBJECTIVES To learn and understand the cell cycle To understand the process of nuclear division called Mitosis To understand the process of nuclear division called Meiosis To understand the differences between Mitosis and Meiosis To understand Oogenesis INTRODUCTION: All living things are made of cells and all cells arise only from pre-existing cells; these statements are the basis of cell theory. So, how do cells arise from pre-existing cells? What is the process that allows a cell to make an exact copy of itself in order for us to grow and repair? And how is it that all the cells end up with exactly the same amount of chromosomes and organelles as its parent cell? And how do we pass on a cell to the next generation so it too can arise from an already pre-existing cell? How do we ensure that there is variation from one generation to the next meaning, how do we ensure that we are not just a clone of the previous generation? Mitosis and meiosis are only a small part of the process of making cells from pre-existing cells. Cells must grow and develop before they can make copies of themselves. The cell cycle describes the life cycle of a typical cell. Right after a cell divides, each of the daughter cells must go through a growth and development phase called interphase. Interphase is described as having 3 phases. The first phase is referred to as G1 in which the cell itself will grow in size and then late in G1, organelles within the cell will start to duplicate in preparation for another cell division. The next phase of interphase is referred to as S Phase; during this phase each strand of chromatin (which is DNA) will create a replica of itself this is the only time in the cell cycle where DNA replicates. During this stage, each DNA double helix will unzip and make a copy of each strand, thus making 2 chromosomes from 1 these 2 sisters stick together until anaphase in mitosis. The cell will continue into the third and final phase of Interphase, called G2. During this phase the cell will produce proteins to aid in cell division and do any final tweeking necessary before entering the cell division stage of the cell cycle. Once Interphase is complete, the cell will enter the Cell Division phase of the cell cycle. The cell division phase is Mitosis and cytokinesis together (the primary oocyte or primary spermatocyte would have also completed Interphase before entering meiosis cells made via meiosis, however, end up as products egg and sperm so unless there is fertilization, they will not go through another cell cycle. In spermatogenesis, once meiosis creates the daughter cells (sperm) most will never go into another cycle, and in the case of a primary oocyte, the daughter will only start another cycle if it becomes fertilized by one of the sperm a story to be continued later) 1

During mitosis and cytokinesis, the cell will divide and become 2 daughter cells: The parent cell starts out as a 2n (diploid) cell with 46 chromosomes and, by the end of mitosis and cytokinesis,

2 Mitosis is the part of the cell life cycle in which the nucleus divides. Mitosis occurs during growth and repair of our cells. Humans have 46 chromosomes (= diploid (2n) -23 pairs of chromosomes 2(23)=46 total chromosomes) in each one of our cells. These 46 chromosomes (our DNA) are unique to each individual. During mitosis and cytokinesis, the cell will divide and become 2 daughter cells: The parent cell starts out as a 2n (diploid) cell with 46 chromosomes and, by the end of mitosis and cytokinesis, will produce 2, 2n daughters (it splits itself in half) each with 46 chromosomes as well as duplicate organelles the same as the original parent (remember all the duplicating and replicating happened during interphase so that enough parts were made for 2 daughter cells with the same amount of parts as the original parent). Meiosis on the other hand, occurs only in the reproductive organs to make gametes in the case of humans, this is the production of sperm for males and eggs for females. In order for the next generation to start with the same number of chromosomes (23 pairs, 46 total) it is essential that, at one stage, the number of chromosomes is reduced by half so that eggs and sperm end up with total of 23 chromosomes each, instead of the original 46. This way, when the egg becomes fertilized by the sperm (when they join together), each bring 23 chromosomes, so now the total is back up to 46 chromosomes to start the next generation. Although the above paragraphs only begin to describe the mitosis and meiosis stories, it is possible to now get a glimpse of the essential roles these processes play in the production of cells from pre-existing cells; both within our bodies and between generations. 1 1 Meiosis from: phenomena.nationalgeographic.com mitosis from Science Life - The University of Chicago Medicine 2

3 ACTIVITY 1: The cell cycle MATERIALS Paper 1. This activity is short and is only intended to remind you that mitosis and meiosis are only a small part of the cell cycle. For this activity simply draw the cell cycle and label and explain what happens during each part of the cycle. Put this figure under your data and observations section. There are no discussion questions for this section. ACTIVITY 2: Observing mitosis in a living cell MATERIALS Computer film from: 1. Observe the short film Mitosis (Somatic Cell Division) pay attention to what is happening in the various phases of mitosis. This is observation only; you do not need to write anything down. 2. After you have finished all the activities in the lab, come back and watch the film again and see if you can identify the phases as they happen without prompting from the narrator. Phases Prophase Metaphase Mitosis cell division stage of the cell cycle Anaphase Telophase Cytokinesis 3

4 1. On the film, which are the phases that appear to last the longest? Which phases appear to happen the quickest? ACTIVITY 3: Observing Mitosis under the microscope MATERIALS Compound light microscope Onion root tip slides Oil for oil immersion lens Lens cleaning liquid and lens paper 2 1. Place an onion root tip slide on a compound light microscope and put a drop of oil on the slide. Following proper procedure (bring it into focus with the scanning objective lens 4x, then move up to 10x, FINE focus, move to 40x, fine focus, then to 100x carefully using FINE FOCUS ONLY!) go up to your 100x oil immersion lens. 2. Using the xy translational control knobs (note figure below to see which knobs these are) to move the stage and slide around, scan the slide and find cells in every stage of mitosis Prophase, Metaphase, Anaphase and Telophase and record the number in each on a table under Data and Observations. You will also see lots of cells that may be in Interphase. It is best to be methodical in your search so as not to recount a section start at one end and slowly work your way across and down to the root tip or vice versa. 2 Slide of microscope stage from 4

DATA AND OBSERVATIONS 3 Table 1: Cells in various stages of mitosis in an onion root tip at 1000x magnification. Prophase Metaphase Anaphase Telophase : 1.

5 DATA AND OBSERVATIONS 3 Table 1: Cells in various stages of mitosis in an onion root tip at 1000x magnification. Prophase Metaphase Anaphase Telophase : 1. Describe what is happening in each phase that allowed you to recognize it as that particular phase. ACTIVITY 4: Observing meiosis in a living cell MATERIALS Computer film called La méiose 1. Observe the short film La méiose pay attention to what is happening in the various phases of meiosis. This is observation only; you do not need to write anything down. You will be unable to see when the homologous pairs undergo crossing over, but remember, it is happening! 3 Photo of mitosis in plant cell from Slideshare.net 5

6 2. After you have finished all the activities in the lab, come back and watch the film again and see if you can identify the phases as they happen without prompting from the narrator. Reminder: Interphase happens before the cell enters meiosis (Reminder Interphase is the first part of the cell cycle and is NOT part of the cell division stage of the cell cycle) ALSO remember that Interphase only happens once when a cell undergoes meiosis because at the end of meiosis II and cytokinesis you produce a product a gamete (in humans this is an egg or sperm) and that gamete must become fertilized before it too will start a new cell cycle. Phases of Meiosis Prophase I Metaphase I Meiosis I Anaphase I Telophase I Interkinesis Cell division stage of the cell cycle Prophase II Metaphase II Meiosis II Anaphase II Telophase II Cytokinesis 1. Were you able to see when the homologous pairs joined together? And at which phase did the homologous chromosomes separate from each other? 6

7 ACTIVITY 5: Making models to compare meiosis and mitosis MATERIALS: 11 x17 paper 2 colors of clay (Play-Doh) for the chromosomes Different color clay for the centrosomes Different color clay (or string) for spindle fibers 1. On one 11 x 17 sheet of paper, draw circles representing each phase of mitosis (PMAT). 2. Imagine you are observing mitosis in a creature who has a total of 4 chromosomes in their diploid cell - 2 from the mother and 2 from the father (hence, the 2 different colors of clay) (reminder: humans have 46 in our diploid cell 23 from the mother s side and 23 from the father s side). 3. Using 2 different colors of clay to represent the maternal and paternal chromosomes, show what happens in each phase. 4. Label each phase, describe in words what is going on in that phase. Also, state whether that phase is haploid or diploid. 5. Now do the same for meiosis (both meiosis I and meiosis II). Make sure to represent synapsis and crossing over. DATA AND OBSERVATIONS 1. Take a picture of both your models and attach a copy to your lab report labeling them as figures. 1. Describe the differences between Prophase in Mitosis and Prophase in Meiosis I. 2. The parent cell going into meiosis I is diploid. By the end of meiosis I what is it? Haploid or diploid? 3. What phase is Meiosis I referred to as; duplication or reduction? 4. Which is the same as mitosis as far as what is happening -Meiosis I or Meiosis II? What is, essentially, the only difference between this stage of meiosis and mitosis? 7

8 ACTIVITY 6: Making models to demonstrate what happens during Oogenesis MATERIALS: 11 x17 paper 2 colors of clay (Play-Doh) for the chromosomes Different color clay for the centrosomes Different color clay (or string) for spindle fibers 1. On one 11 x 17 sheet of paper, draw pencil or pen outlines to represent the cell in each phase of oogenesis. 2. Imagine you are observing oogenesis in a creature who has a total of 4 chromosomes in their diploid cell - 2 from the mother and 2 from the father (hence, the 2 different colors of clay) (reminder: humans have 46 in our diploid cell 23 from the mother s side and 23 from the father s side). 3. Using 2 different colors of clay to represent the maternal and paternal chromosomes, show what happens in each phase of oogenesis (both meiosis I and meiosis II). Make sure to represent synapsis and crossing over as well as when a sperm comes along to complete oogenesis. 4. Label each phase, describe in words beside the clay cell, what is going on in that phase. Also, state whether that phase is haploid or diploid. DATA AND OBSERVATIONS 1. Take a picture of your model and attach a copy to your lab report labeling it as a figure. 1. Write a description in paragraph form (write the story), of what is happening at the various stages of oogenesis up to, and including, fertilization and the production of a zygote (fertilized egg) from the 2 gametes (egg + sperm). 8

Sexual Reproduction. The two parent cells needed for sexual reproduction are called gametes. They are formed during a process known as meiosis.

Sexual Reproduction. The two parent cells needed for sexual reproduction are called gametes. They are formed during a process known as meiosis. Sexual Reproduction Recall that asexual reproduction involves only one parent cell. This parent cell divides to produce two daughter cells that are genetically identical to the parent. Sexual reproduction,