1.6: Mitosis and Meiosis II
Introduction to Mitosis
In eukaryotic cells, the time and phases from the beginning of one cell division until the beginning of the next cell division is called the cell cycle (Figure 1) . The first phase of the cell cycle is interphase. Interphase is the time during which the cell performs its normal functions and prepares for cell division. Cells spend most of their time in this phase. During interphase, chromosomes are not visible because they are decondensed (present only as a tangled mass of thin threads of DNA with associated proteins, called chromatin). The nuclear membrane is present, and visible, as is the nucleolus .
Figure 1. The cell cycle.
Interphase includes two gap phases, G1 and G2, where the cell increases in size and synthesizes new organelles, enzymes, and other proteins that are needed for cell division. In between the two gap phases, the DNA replicates in preparation for cell division. This stage is called the S phase . At the beginning of S phase, chromosomes are single and unreplicated. By the end of S phase, each chromosome has made an exact copy of itself and consists of two sister chromatids . At this point in the cell cycle the sister chromatids are held together tightly at the centromere . While the two sister chromatids are physically joined together they are still considered one replicated chromosome (Figure 2). When the sister chromatids physically separate, later during the cell cycle, they are then considered to be individual chromosomes.
In animal cells, interphase is also when the centrosome (consisting of two centrioles ) is replicated. Spindle fibers form from and radiate outward from the centrosomes to attach to and move chromosomes during cell division.
Figure 2. Chromosomes and sister chromatids.
Interphase is followed by mitosis (in the somatic cells) or meiosis (in reproductive cells), which is when replicated chromosomes and cytoplasm separate, during the process of karyokinesis and cytokinesis respectively.
Chromosomes that are the same length, have the same centromere location and the same gene sequences and positions are called homologous chromosomes. Human somatic cells contain pairs of homologous chromosomes. Each homologous pair consists of one maternal chromosome and one paternal chromosome. Cells that contain two copies of each chromosome are called diploid (2n, where n is the number of different chromosomes in a single set). Human sex cells (eggs and sperm) contain only one copy of each chromosome. Cells with only one copy of each chromosome are haploid (n). When the haploid sperm (n) and egg (n) combine during fertilization this forms a diploid zygote (2n).
MITOSIS
Mitosis is nuclear division that results in two cells containing the same number of chromosomes as the parent cell. Most human cells (skin, muscle, bone, etc.) divide by mitosis. This process is necessary for the normal growth and development of a multicellular eukaryotic organism from a zygote (fertilized egg), as well as growth and the repair and replacement of cells and tissues. At the end of mitosis, two daughter cells are formed that are identical to the original (parent) cell. Mitosis is also a form of asexual reproduction in unicellular eukaryotes.
Mitosis is a complex and highly regulated process. It occurs in the following 4 separate phases: prophase, metaphase, anaphase, and telophase. Telophase is quickly followed by cytokinesis.
Prophase: Cells prepare for division by coiling and condensing their chromatin into chromosomes. By late prophase, individual chromosomes can be seen, each consisting of two sister chromatids joined at a centromere. Spindle fibers begin to form from the centrosomes, which have begun to migrate to opposite “poles” of the cell. The nucleoli and the nuclear membrane degrade. (Figure 3)
Figure 3. Cell in prophase
Metaphase: Spindle fibers (called kinetochore microtubules or kinetochore spindle fibers) that emanate from the centromeres attach to the kinetochore (a proteinaceous area) on the sister chromatids. The fibers pull and otherwise manipulate the chromosomes to align them on the plane that passes through the center of the cell (metaphase plate) (Figure 4). This “plate” is not an actual structure; it merely signifies the location of replicated chromosomes prior to their impending separation.
Other non-kinetochore spindle fibers or tubules (aka polar microtubules), emanating from the two centrosomes, elongate and eventually overlap with each other near the metaphase plate.
Figure 4. Spindle fibers attaching to kinetochores in metaphase.
Anaphase: The centromeres divide, with the help of separase enzymes, and separate the sister chromatids (Figure 5). This happens simultaneously in all the chromosomes. The kinetochore spindles shorten and pull each chromatid to which they are attached toward the pole (and centrosome) from which they originate. This equally distributes exactly half the chromosomal material to each side of the cell. In late anaphase, the non-kinetochore spindles begin to elongate, lengthening the cell.
Figure 5. Cell in anaphase.
Telophase: The non-kinetochore microtubules continue to elongate, further elongating the cell in preparation for cytokinesis (splitting of the cytoplasm). The chromosomes reach their respective poles. The kinetochores disappear. The two nuclear membranes (one in each half of the cell) begin to form around the chromosomes. Nucleoli reappear and the chromosomes in each soon-to-be new cell begin to decondense back into chromatin. Mitosis is complete at the end of this stage.
Cytokinesis (splitting of the cytoplasm):
In animal cells and all other eukaryotes without a cell wall, cytokinesis is achieved by means of a constricting “belt” of protein fibers that slide past each other near the equator of the cell. As this occurs, the diameter of the belt decreases, pinching the cell to form a cleavage furrow around the cell’s circumference. As constriction proceeds, the furrow deepens until it eventually slices its way into the center of the cell. At this point, the cell is divided into two.
Plant cell walls are far too rigid to be split apart by contracting proteins. Instead, these cells assemble membrane proteins (in vesicles that bud off the Golgi apparatus) in their interior at right angles to the spindle apparatus. This expanding membrane partition, called a cell plate, continues to grow outward until it reaches the interior surface of the plasma membrane and fuses with it. This divides the cell in two.
The phases of Mitosis
Exercise 1: Modeling the Phases of Mitosis with Pop Beads
Materials:
- Chalk (can be used directly on lab bench to draw cellular structures and then washed off)
-
Chromosome modeling kits
- Commercially available pop bead kits (e.g Carolina Biological Supply Company, Item #171100)
- 40 pop beads of one color (red)
- 40 pop beads of another color (yellow)
- 8 magnetic centromeres
- 4 laminated pictures of centrosomes (each consisting of a pair of centrioles)
- Paper towels / Kimwipes
Procedure:
Using models is a great way to represent natural structures and processes that are too small, or too large, or too complex to observe directly. By building chromosomes from the pop beads and manipulating them to model cell division (mitosis and meiosis) you will enhance your understanding of the nature of chromosomes and the cellular structures needed to perform cell division. In this simulation, your cell will be a diploid (2n) cell with 4 chromosomes. It will contain 2 homologous pairs of chromosomes. In order to differentiate between the pairs of homologs, one pair of homologous chromosomes will be longer than the other. One chromosome of each pair is red and represents maternal DNA (genetic material contributed by a female’s egg). The other chromosome of each pair is yellow and represents paternal DNA (genetic material contributed by a father’s sperm). Thus, for each pair of homologous chromosomes, one should be red and one should be yellow. The strands of pop beads represent the DNA in the form of chromatin during the G1, S, and G2 phases. They then represent chromosomes as they enter the phases of mitosis.
This diploid cell with 2 homologous pairs of chromosomes will be modeled as it moves through the following phases of mitosis:
- Interphase (uncondensed DNA) Before Synthesis of DNA (G1)
- Interphase (uncondensed DNA) After Synthesis of DNA (G2)
- Prophase
- Metaphase
- Anaphase
- Telophase
- Cytokinesis
- You will build 2 pairs of homologous chromosomes (2n = 4). The first homologous pair of single chromosomes should be constructed using 12 red beads for one member of the long pair and 12 yellow beads for the other member of the pair. You may place the magnetic centromere at any position along the chromosome, but it must be at the same position on both members of the homologous pair.
- The second homologous pair of single chromosomes should be constructed using 6 red beads for one member of the short pair and 6 yellow beads for the other member of the pair. You may place the magnetic centromere at any position along the chromosome, but it must be at the same position on both members of the homologous pair.
- Use the pop beads to model the G1 phase of interphase (before synthesis of the DNA). On your lab bench, use the chalk to draw the cell membrane, nucleus, and nucleolus. Place a laminated centrosome in the cytoplasm of your cell. Place the 4 assembled chromosomes in the nucleus of your cell drawing. Remember that these represent a mass of chromatin.
- DNA replication takes place during the S phase of interphase. Model the S phase of interphase by assembling a second strand that is identical to each of the 4 single chromosomes. The identical strands are considered sister chromatids, which are held together by their magnetic centromeres. (Note: In a living cell, the centromere is a single unit until it separates in anaphase. Therefore, consider the pair of magnets to be a single centromere.)
- Model the G2 phase of interphase by letting your 4 assembled replicated chromosomes rest in the nucleus of your cell. Duplicate the centrosomes by placing another laminated centrosome in the cytoplasm of your cell drawing. Begin to move them towards opposite poles of the cell drawing.
- To model the prophase stage of mitosis, leave the chromosomes where they are in your cell drawing. In this stage, the chromatin coils and condenses into chromosomes. Using the paper towel (or Kimwipe) start erasing some of the nuclear membrane on your cell drawing. This will simulate the breakdown of the nuclear membrane. Also erase the nucleolus, as the nucleolus disappears in this stage. Continue to move your centrosomes to opposite sides of the cell. Use the chalk to draw spindle fibers beginning to form and radiating outward toward the chromosomes.
- At prometaphase the centrosomes are at opposite poles of the cell. During this phase the chromosomes continue to condense, the nuclear membrane completely breaks down, and the spindle fibers start to attach to the kinetochores of the centromeres. These are called kinetochore microtubules . Other spindle fibers radiate outward from the centrosomes, but these do not attach to the kinetochores. These spindle fibers are called nonkinetochore microtubules.
- To model metaphase , move the centromeres of your chromosomes to lie on an imaginary plane midway between the centrosomes that are now positioned at opposite poles of the cell. Line up the individual chromosomes in the middle (equator) of the cell. Sister chromatids remain attached at the centromere during metaphase.
- To model anaphase , pull the magnetic centromeres apart and slide them towards opposite poles of the cell. Keep them attached to your drawn kinetochore microtubules, but use the paper towel to erase them, making them shorter. With the chalk, lengthen the nonkinetochore microtubules and have them overlap in the middle of your cell. After separation at the centromere, each sister chromatid is now referred to as an individual chromosome. Anaphase ends when the chromosomes reach the opposite ends of the cell.
- Model telophase by piling up your chromosomes at each pole. Erase your spindle fibers, as they disappear during this stage. Even though we can’t simulate it, the chromosomes will start to decondense and uncoil back into chromatin. Redraw a nuclear envelope around each pile of chromosomes. Also add nucleoli to your drawing. The formation of separate nuclear envelopes divide the nuclei and marks the end of telophase.
- The division of the cytoplasm, or cytokinesis, results in the formation of two separate cells. To model cytokinesis in animal cells, fungi, and slime molds, leave the piles of chromosomes at their separate poles. Draw indentations of the cell membrane inwards, towards the cytoplasm, on the sides of the cell where there are no centrosomes. This represents a cleavage furrow ,which eventually “pinches” the cell’s cytoplasm into two separate cells. The proteins actin and myosin contribute to the formation of the cleavage furrow. In plant cells, membrane-bound vesicles migrate to the center of the cell (the equatorial plane) and fuse together to form a cell plate . The cell plate eventually divides the cytoplasm into two separate cells. Materials needed to build the cell wall are released from the vesicles at the cell plate, forming a new cell wall.
- How do the daughter cells you formed compare to the original parent cell? _________________________________________________________________
Observe the phases of Mitosis in Plant Cells
Exercise 2: Observing the Phases of Mitosis in the Onion Root Tip
Materials:
- Prepared slide of the onion root tip
- Compound light microscope
Procedure:
- Examine a slide of a longitudinal section of an onion root tip. Adjust the slide to view the region just above the root cap, where there are likely to be dividing cells.
- Focus on the dividing cells using the 4x scanning objective lens, then switch to the 10x objective and then the 40x objective.
- Survey the slide to find a cell in each phase of mitosis. Draw a cell for each phase in the boxes provided.
Interphase
DNA is uncondensed and in the form of chromatin. Individual chromosomes are not visible. The nuclear membrane is intact. The nucleolus is visible.
Prophase
Chromatin begins to condense into distinguishable chromosomes. These “puffy” structures are seen throughout the nucleus. Nucleoli begin to disappear. In late prophase (often called prometaphase) the nuclear membrane is no longer visible.
Metaphase
The chromosomes line up in the middle of the cell. Spindle fibers attach to kinetochores at the centromere and extend to the poles of the cell.
Anaphase
Centromeres split, separating each former chromatid into two individual chromosomes. The chromosomes move toward opposite poles.
Telophase and Cytokinesis
Chromosomes reach the poles. The nuclear envelopes begin to reform. The formation of a cell plate forms between the two cells to carry out cytokinesis.
Observe the phases of Mitosis in Animal Cells
Exercise 3: Observing the Phases of Mitosis in the Whitefish Blastula
Materials:
- Prepared slide of whitefish blastula
- Compound light microscope
Procedure:
The blastula is an early embryonic stage where many of the cells are dividing at any one time.
- Focus on the dividing cells using the 4x scanning objective lens, then switch to the 10x objective and then the 40x objective.
- Survey the slide to find a cell in each phase of mitosis. Draw a cell for each phase in the boxes provided.
Interphase
The DNA is uncondensed and in the form of chromatin. Individual chromosomes are not visible. The nuclear membrane is intact. The nucleolus is visible.
Prophase
Chromatin begins to condense and chromosomes are distinguishable. These “puffy” structures are seen throughout the nucleus. The nucleoli begin to disappear. In late prophase (often called prometaphase) the nuclear membrane is no longer visible.
Metaphase
The chromosomes line up in the middle of the cell. Spindle fibers attach to kinetochores at the centromere and extend to the poles of the cell.
Anaphase
Centromeres split, separating each former chromatid into two individual chromosomes. The chromosomes move toward opposite poles.
Telophase and Cytokinesis
Chromosomes reach the poles. The nuclear envelopes begin to reform. A cleavage furrow forms between the two cells to carry out cytokinesis.
- Why would the method of cytokinesis in animal cells not work in plant cells? Explain. __________________________________________________________________
How Long Does a Cell Spend in Each Phase of the Cell Cycle?
Exercise 4: Determining Time Spent in Different Phases of the Cell Cycle (Optional)
Materials:
- Laptop
- Calculator
Procedure:
- Obtain a laptop.
- Open a web browser and go to the following site: http://www.biology.arizona.edu
- This site will provide an interactive test of your ability to identify the stages of mitosis. It will also allow you to calculate the duration of the stages identified in the laboratory exercise you just completed, but the website will give standard results for the entire class.
- Go to the Cell Biology section, and find the activity “ Online Onion Root Tips ”.
- Keep clicking on “Next” at the bottom of the page until you get to the screen: Determining Time Spent in Different Phases of the Cell Cycle
- Click on “Next” at the bottom of the page.
- Identify each stage shown to you by the program. When a picture of a cell pops up in a stage of mitosis, simply click on the phase in which the cell belongs. If you make a mistake, read the explanation for why you were mistaken before making a new selection.
- When you are finished, use the formula given below and record your results in the table.
- The duration of each stage of mitosis can be determined by using the following formula. Compute the length of time for each stage and place your calculations in the table below.
(Number of cells in a stage ÷ Total number of cells) x 1440 (min in a day) = minutes a cell spends
in each stage in one day
|
Number of cells in each stage |
Interphase ______ |
Prophase ______ |
Metaphase ______ |
Anaphase _______ |
Telophase _______ |
Total 36 |
|---|---|---|---|---|---|---|
|
Percent of cells (# of cells / Total) |
100% |
|||||
|
Time (in minutes) spent in Stage – use calculation above |
Practical Challenge Questions
- In the circle below, sketch a 2n=6 diploid cell in metaphase of mitosis. Be sure to label the centromere, centrioles, and spindle fibers.
References
Belwood, Jacqueline; Rogers, Brandy; and Christian, Jason, Foundations of Biology Lab Manual (Georgia Highlands College). “Lab 10: Mitosis & Meiosis,” (2019). Biological Sciences Open Textbooks . 18. CC-BY
https://oer.galileo.usg.edu/biology-textbooks/18
Meiosis
Introduction to Meiosis (aka “Reduction Division”)
Meiosis is a special type of cell division in which the daughter cells produced have half the number of chromosomes (n) as their parent cell. In animals, this division occurs in the reproductive organs (gonads -- testes of males or ovaries of females) of species that reproduce sexually, and results in the formation of gametes (eggs or sperm) that contain half the number of chromosomes as the original cell. Sexual reproduction involves the joining of gametes (fertilization) to form a zygote, which then has two copies of each chromosome (2n). Meiosis is a critical process, as it increases genetic diversity within a species.
Cells that divide by meiosis prepare for cellular division (during interphase) much like every other cell. Meiosis progresses through the same phases as mitosis (prophase, anaphase, metaphase, telophase, and cytokinesis). However, unlike mitosis, meiosis involves two rounds of cellular division (meiosis I and meiosis II). Meiosis involves only one round of DNA replication where each chromosome replicates to form sister chromatids. Therefore, when meiosis is completed, each daughter cell contains only half the number (n) of chromosomes as the original cell.
The stages of meiosis:
- Meiosis I
- Prophase l
- Metaphase l
- Anaphase l
- Telophase l
- Cytokinesis l
- Meiosis II
- Prophase ll
- Metaphase ll
- Anaphase ll
- Telophase ll
- Cytokinesis ll
Prophase I: During prophase of meiosis I, the chromosomes join in homologous pairs. Homologous chromosomes (aka homologs) are the same length, have their centromere in the same position, and carry genetic information (genes) for the same traits, but not necessarily the same versions (alleles) of the gene.
For example, human chromosome #19 contains a gene for eye color. In one person, one allele might code for blue eyes and the other allele codes for green eyes. Since every human inherits two copies of chromosome 19 (one from the mother’s egg and one from the father’s sperm) a person could have 2 blue alleles, 2 green alleles, or one of each.
Paired homologous chromosomes are called tetrads and are said to be in synapsis. During synapsis, equivalent pieces of homologous chromatids are exchanged between the chromosomes. This is called crossing-over and can occur several times along the length of the chromosomes. As occurs in the mitotic division, prophase of meiosis I also involves the degradation of the nuclear membrane, disassembly of the nucleolus, and formation of spindle fibers.
Metaphase I: Metaphase of meiosis I occurs when the joined homologous chromosome pairs are moved to the center of the cell by spindle fibers (Figure 6). The fibers arrange the pairs so that homologs are on opposite sides of the metaphase plate (aka equatorial plane).
Figure 6. Homologous pairs line up at the equatorial plate in Metaphase l.
Anaphase I follows, as homologs are pulled apart, toward opposite poles of the cell (Figure 7). [*Note: this is significantly different from the separation of sister chromatids that occurs during mitosis].
Figure 7. In Anaphase l mitotic spindles pull homologs to opposite poles of the cell.
Telophase I marks the end of meiosis I, as new nuclei form and cytokinesis separates the cytoplasm forming two daughter cells.
At the end of meiosis I, the two daughter cells have half the number of chromosomes as did their parent cell. Thus, the cells have been reduced from diploid (2n) to haploid (n) (Figure 8). [n refers to the number of chromosomes in a set that are characteristic for a species. Humans have one set (n) of 23 unique chromosomes (n = 23). A diploid human cell has 2 sets (2n) of 23 unique chromosomes (2n = 46).
Figure 8. Meiosis l results in two haploid cells.
*Meiosis II follows meiosis I, which proceeds very much like mitosis.
During Prophase II , chromosomes containing two sister chromatids are lined up on the equator of each daughter cell by the spindle fibers. This is completed by the end of Metaphase II (Figure 9).
Figure 9. Metaphase ll of Meiosis
The centromeres separate and sister chromatids are pulled to each pole of the cell during Anaphase ll (Figure 10) . Cytokinesis II occurs after Telophase II to complete cell division and ultimately the production of four (4) daughter cells (Figure 11). The cells produced (egg or sperm, in humans) are haploid (n rather than 2n) and will either unite (via fertilization) or die. They do not divide further on their own as meiosis is not a cycle.
Figure 10. Anaphase ll of Meiosis
Figure 11. Meiosis results in four haploid cells.
The Phases of Meiosis
Exercise 1: Modeling the Phases of Meiosis
Materials:
- Chalk (can be used directly on lab bench to draw cellular structures and then washed off)
-
Chromosome modeling kits
- Commercially available pop bead kits (e.g Carolina Biological Supply Company, Item #171100)
- 40 pop beads of one color (red)
- 40 pop beads of another color (yellow)
- 8 magnetic centromeres
- 8 laminated pictures of centrosomes (each consisting of a pair of centrioles)
- Paper towels / Kimwipes
- Fine point sharpie
- Small round stickers
Procedure:
A diploid cell with 2 homologous pairs of chromosomes (as in the previous modeling exercise) will be modeled as it moves through the meiosis. First, you will model meiosis l. Then, you will model meiosis ll as described below.
Model Meiosis l (1 diploid cell → 2 haploid cells)
-
tetrads form, crossing over occurs, homologues separate
- Interphase Before Synthesis of DNA (G1)
- Interphase After Synthesis of DNA (G2)
- Prophase l
- Metaphase l
- Anaphase l
- Telophase l
- Cytokinesis l
Model Meiosis ll (2 haploid cells → 4 haploid cells)
- sister chromatids separate
- Prophase ll
- Metaphase ll
- Anaphase ll
- Telophase ll
- Cytokinesis ll
Modeling Meiosis l
- You will build 2 pairs of homologous chromosomes (2n = 4). The first homologous pair of single chromosomes should be constructed using 12 red beads for one member of the long pair and 12 yellow beads for the other member of the long pair. You may place the magnetic centromere at any position along the chromosome, but it must be at the same position on both members of the homologous pair.
- The second pair of single chromosomes will represent the X and Y sex chromosomes in a male. The X chromosome should be constructed using 8 red beads. To make manipulation of the chromosomes easier, place the magnetic centromere in the middle of the X chromosome so that you have 4 red beads on one side of the centromere and 4 red beads on the other side. The Y chromosome should be constructed using 2 yellow beads with a magnetic centromere between them.
- Alternative forms of genes are called alleles. We will use the letter “B” or “b” to represent the gene for eye color. Labeled stickers will be used to represent alleles. With the Sharpie, label one small sticker with a “B” and another small sticker with a “b”. “B” will represent the dominant allele for brown eyes and “b” will represent the recessive allele for blue eyes. You may place the “B” sticker on any of the red beads of the long red chromosome. The “b” sticker must be placed at the same bead position of the “B” sticker, but on the long yellow chromosome.
- To model the G1 phase of interphase (before synthesis of the DNA). On your lab bench, use the chalk to draw the cell membrane, nucleus, and nucleolus. Place a laminated centrosome in the cytoplasm of your cell. Place the 4 assembled chromosomes in the nucleus of your cell drawing. Remember that these represent a mass of chromatin.
- DNA replication takes place during the S phase of interphase. Model the S phase of interphase by assembling a second strand that is identical to each of the 4 single chromosomes. The identical strands are considered sister chromatids, which are held together by their magnetic centromeres. (Note: In a living cell, the centromere is a single unit until it separates in anaphase. Therefore, consider the pair of magnets to be a single centromere.)
- Model the G2 phase of interphase by letting your 4 assembled replicated chromosomes rest in the nucleus of your cell. Duplicate the centrosomes by placing another laminated centrosome in the cytoplasm of your cell drawing. Begin to move them towards opposite poles of the cell drawing.
- To model the prophase I stage of meiosis, leave the chromosomes where they are in your cell drawing. In this stage, the chromatin coils and condenses into chromosomes. Using the paper towel (or Kimwipe) start erasing some of the nuclear membrane on your cell drawing. This will simulate the breakdown of the nuclear membrane. Also erase the nucleolus, as the nucleolus disappears in this stage. Continue to move your centrosomes to opposite sides of the cell. Use the chalk to draw spindle fibers beginning to form and radiating outward toward the chromosomes.
- Prophase l of meiosis has some notable differences compared to prophase of mitosis. To further model prophase l of meiosis, arrange the condensed chromosomes so that homologous chromosomes are paired. Put homologous chromosomes next to each other in your cell drawing to simulate this process of synapsis (homologs pairing). Homologous pairs of chromosomes paired in this way are called tetrads . When tetrads form, the inner non-sister chromatids of the tetrad pair can exchange DNA by a process known as crossing over . Pieces of equivalent segments of non-sister chromatids can be exchanged from one non-sister chromatid to the other. Crossing over can occur several times along the length of the chromosomes.
- How many tetrad complexes do you have in your cell, which is 2n = 4? ____________________________________________________________
- Exchange segments of the inner non-sister chromatids of the red and yellow beads that contain the two different alleles, “B” and “b”. Remember that the red beads represent DNA from the maternal chromatid and the yellow beads represent DNA from the paternal chromatid. You now have one crossover event for this tetrad.
- Metaphase l of meiosis is also notably different than metaphase of mitosis. To model metaphase l of meiosis, move your tetrads to the equator, midway between the two centrosomes, which should now be positioned at the two opposite poles of the cell. Draw spindle fibers radiating out from the centrosomes to the homologous pairs (tetrads) which are lined up along the equatorial plane of the cell. Homologs are paired together but the maternal and paternal chromosomes are on opposite sides of the equatorial plate along the middle of the cell. Every pair of chromosomes is arranged independent of another. The side of the equatorial plate where each chromosome is arranged is completely random and independent of the side of the equatorial plate on which other chromosomes are located. Therefore, there are several different arrangements that can occur (Figure 12).
Figure 12. Independent Assortment in a cell with 2 homologous pairs.
- In anaphase l of meiosis, homologous chromosomes separate toward opposite poles of the cell. Centromeres do not split as they do in mitosis. For each homologous pair, move the duplicated maternal chromosome and duplicated paternal chromosome to opposite poles of the cell. Remember that in our model, the two magnets (one of each of the two sister chromatids) represent one centromere. Erase the spindle fibers and redraw them shorter and shorter as each homologous pair moves away from one another towards opposite poles of the cell.
- How does the structure of chromosomes in anaphase I of meiosis differ from that in anaphase of mitosis? ____________________________________________________________
- Anaphase ends and telophase l begins when chromosomes reach opposite poles of the cell. Arrange the chromosomes in groups at opposite ends of the cell. You should have one long chromosome of 12 beads and either the X chromosome or the Y chromosome at each pole. Nuclear division happens in telophase. The formation of separate nuclear envelopes divide the nuclei and mark the end of telophase.
- Model cytokinesis l by drawing the formation of a cleavage furrow to divide the cytoplasm into two and form two separate cells. Redraw the nuclear membrane around the chromosomes and draw a nucleolus inside of each nucleus. These two cells will now enter meiosis ll. Note that each daughter cell has half the number of chromosomes as the parental cell. Thus, the cells have been reduced from diploid (2n) to haploid (n) . [n refers to the number of pairs of chromosomes that are characteristic for a species . Humans have a “n” of 23, so a diploid human cell has 2(23), or 46 chromosomes].
Meiosis ll
- There is no DNA replication before the second cell division stage of meiosis. The stages of meiosis ll proceed very much like mitosis. The two cells created in meiosis l will enter into prophase ll . The chromosomes in each cell contain two sister chromatids, which are condensed and distributed throughout the nucleus. Add another laminated centrosome. There should be two centrosomes in each new cell. In your drawing be sure to note that the nuclear envelope begins to break down, the nucleolus disappears, the centrosomes move towards opposite poles, and spindle fibers begin to form and radiate toward the chromosomes. Spindle fibers attach to kinetochore proteins at the centromeres of the chromosomes.
- To model metaphase ll , line up the individual chromosomes on the equator (middle) of each cell. Sister chromatids remain attached at the centromere during metaphase ll.
- Model anaphase ll by pulling the two magnetic centromeres of each duplicated chromosome apart. The sister chromatids should be separated and moved toward opposite poles of each cell. After separation at the centromere, the chromatids are now called chromosomes. In anaphase II single chromosomes move towards opposite poles.
- Anaphase ll ends and telophase ll begins when the chromosomes reach opposite poles of the cells. Nuclear division happens in telophase. The spindle fibers disassemble. Nuclear envelopes and nucleoli reappear. Condensed chromosomes begin to decondense and uncoil. The formation of separate nuclear envelopes divide the nuclei and marks the end of telophase.
- Model cytokinesis ll by drawing the formation of a cleavage furrow to divide the cytoplasm of each cell into two separate cells.
- What is the total number of nuclei and cells now present? ___________________
- How many cells were present when meiosis began? _______________________
- How many chromosomes were present in the original parental cell? ___________
- How many chromosomes are in each new daughter cell? ___________________
- Are the chromosomes in the new daughter cells identical to the chromosomes in the original parental cell? Explain your results in terms of independent assortment and crossing over. __________________________________________________
Questions for Review
- What is the meaning of diploid? What abbreviation do we use to represent diploid? Name 2 diploid cells in humans.
- What is the meaning of haploid? What abbreviation do we use to represent haploid? Name 2 haploid cells in humans.
- In what stage of the cell cycle does S phase occur? Explain why the DNA must be duplicated during the S phase of the cell cycle, prior to mitosis taking place.
- What specific feature of cytokinesis in animal cells can you use to distinguish this process from cytokinesis in plant cells?
Practical Challenge Questions
- In the circle below, sketch a 2n=6 haploid cell in metaphase l of meiosis.
- A monogenic gene gives rise to a trait from a single set of alleles. A polygenic gene gives rise to a trait from several sets of alleles. Give an example of a monogenic and polygenic trait.
References
Belwood, Jacqueline; Rogers, Brandy; and Christian, Jason, Foundations of Biology Lab Manual (Georgia Highlands College). “Lab 10: Mitosis & Meiosis,” (2019). Biological Sciences Open Textbooks . 18. CC-BY