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CD CO B The Cell O The Cell Cycle O Interphase A F B O Cytokinesis G O Mitosis O H Phase (Cell Division) Cycle O G? Phase .. . .C Phases of H Mitosis O Gj Phase D O Prophase H, O S Phase E O Metaphase H? -45- O Anaphase H3 O Telophase H4 O Mother Cell a O Daughter Cells b CHAPTER TWO Chanter 2-16: MitOSl'S During the cell division phase of the cell cycle, the cell under goes mitosis and then cytokinesis. Mitosis is the process in which the duplicated chromosomal pairs separate and, in cytokinesis, the cell splits to form two new cells. This plate will explore the As mitosis continues, the chromatids separate into chromosomes. Continue your reading and color the structures as you encounter them in the plate. process of mitosis. The next phase of mitosis is metaphase. Here the chromatid pairs align themselves along the equator of the cell, at an area called the metaphase, or equatorial plate. The chromatids (G,) are linked near the middle of the chromosome at a development called the kinetochore (I). There is one kinetochore located on each sister chromatid, and their compositions are unknown. At this stage, the spindle fibers (H) are distinct, and they extend out from the centrioles. The remainder of the cytoplasm (A) should be This plate shows six diagrams that portray the phas es of a cell undergoing mitosis. This process is a continuous one, and is generally described in terms of four phases; interphase, prophase, metaphase, and telophase. Use the same colors throughout the six diagrams. These diagrams tend to be fairly detailed, so light colors are recommended. colored in a light color. In anaphase, the DNA at the kinetochore (I) has duplicated, As you may remember, the DNA in the nucleus of the cell repli cated during the S phase of the cell cycle, but is not distinguished and the chromatids have separated. Each chromatid is now a phase. The nucleus (B) contains the DNA in a diffuse mass called chromatin. The nucleolus (C) is seen clearly in the interphase cell, and the nuclear membrane |D) encloses the nucleus. Color the them by their centrioles. An equal number of chromosomes move as distinct chromosomes during the first phase of mitosis, inter cytoplasm (A) a light color. Two submicroscopic bodies (also duplicated prior to mitosis) that participate in mitosis are the centrosomes. Each of the centrosomes contains two cylindrical structures that are arranged at right angles to each other, called centrioles (E), which are involved in the organization of microtubules during cell division. We now begin the process of mitosis; our cell is in early prophase. The same colors that were used for the interphase cell should be used here. Prophase is the longest phase of mitosis. It begins when the chromatin of the cell nucleus condenses to form distinct chromo somes. Because DNA replication has taken place during interphase, each chromosome is composed of two identical strands, known as chromatids (G,). Notice that, in early prophase, the centrioles (E) are surrounded by a series of micro tubules that radiate outward; these are called asters |F). In late prophase, the centrioles (E) have moved to opposite poles of the cell and the asters (F) are still visible. Spindle fibers (H) can be seen extending between the centrioles and should be traced with a light color such as yellow. Spindle fibers are com posed of microtubules and associated proteins. Notice that the chromatids (G,) have continued to compact, becoming shorter and thicker. The nuclear membrane begins to break apart and disappear as the cell proceeds through late prophase. chromosome (G2). Four chromosomes are seen moving to the bottom of the diagram, and four to the top of the diagram. The chromosomes resemble "V's" because the spindle fibers lead to the opposite poles of the cell. In a human cell, for example, forty-six chromosomes move to one pole and forty-six chromo somes move to the opposite pole. We will now examine a cell in telophase; the phase that signals the end of mitosis and immediately pre cedes cytokinesis. As the dividing cell enters telophase, you can see that the chro mosomes (G2) arrive at opposite ends of the cell, where they become thinner and less distinct. The spindle fibers (H) begin to break down in this phase, the nuclear membrane |D) begins to form around the chromosomal material, and the nucleolus (C) reappears. As telophase comes to an end, the cytoplasm (A) is divided between the two new daughter cells. At the center of the cell in animal cells a cleavage furrow (J) begins to form as the mem brane pinches in from both sides. The appearance of the cleavage furrow signals the end of telophase and the beginning of cytokinesis. The furrow pushes inward from opposite sides of the cell until two cells are created. These cells are referred to as the daughter cells. -46- H Interphase Prophase (Early) Prophase (Late) D Metaphase Anaphase Telophase Mitosis O Cytoplasm A O Centrioles O Nucleus (Chromatin) B O Asters O Nucleolus C O Chromatids E F G, O Nuclear Membrane D O Chromosomes G, -47- O Spindle Fibers O Kinetochore O Cleavage Furrow H I J CH.A PTER TUO Chapter 2-17: The process of mitosis, discussed in the previous plate, occurs in cells that are reproducing during growth and wound healing, and the replacement of dead cells. The two cells that arise from mitosis are genetically identical to their parent cell. Certain cells undergo another form of cell division known as meiosis. In this process, a single parent cell produces four cells, each of which has half the number of parental chromosomes. The parent has two sets of chromosomes and is said to be diploid (2N), while the cells that result from meiosis each have a single set of chromo somes and are said to be haploid (N). Meiosis takes place in the reproductive organs and results in cells that are used during reproduction. These cells, which are sperm and egg cells, are called gametes. At fertilization, the fusion of two haploid gametes forms a single cell, called the zygote, which is diploid. Mei'OSl'S At the end of meiosis I, the chromosome pair has sep arated and a chromosome that consists of two sister chromatids has moved to each daughter cell. The sister chromatids are held together at the centromere. Each of the two daughter cells will now enter meiosis II. The two daughter cells now enter meiosis II, shown at the top of the second column. View 2a shows prophase. Again, we see the centrioles (F) and the cytoplasm (E), which should be colored in a pale color. Sister chromatids 1 (H,) are in the left cell, and sister chromatids 2 (H2) are in the right cell. In view 2b, the sister chromatids line up along the equator of each cell. Then the kinetochores separate the sister chromatids. Now, in view 2c, anaphase is in process, and the sister chro matids are considered chromosomes. In the left cell, chromosome In this plate, we trace the two main phases of meio sis. Many of these processes are similar to those of mitosis, and you should refer to the previous plate whenever necessary. We will follow a single pair of chromosomes through the process of meiosis and will note how they are distributed to four cells. The process of meiosis involves two rounds of cell division, known as meiosis I (A) and meiosis II (B). The bars that indicate these two rounds should be colored. The first round results in daughter cells that have reduced numbers of chromatids. In the second round, these chromatids are distributed to the gametes. Each round of meiosis contains a prophase, metaphase, anaphase, and telophase, as is the case in mitosis. We will begin with meiosis I. Here we see a parent cell with a distinctive nucleus (C) and nucleolus (D). The cytoplasm (E) should be colored in a pale color. The centriole (F) functions in meiosis as it does in mitosis. The phase designated 1 a represents prophase. Prophase continues in view 1 b. Here a single pair of chromo somes is considered (remember that humans have twenty-three pairs of chromosomes per cell). We see homologous chromosome 1 (G,) and homologous chromosome 2 (G2). The DNA in each chromosome has replicated. Here, the chromosomes have come together, and crossing over (chapter 3) may take place. View 1 c represents metaphase. The homologous chromosomes line up along the equator of the cell, and we see that each consists 1 (I,) moves to one side of the cell, while chromosome 2 (l2) moves to the other. Chromosome 3 (l3) and chromosome 4 (l4) separate in the second cell. As telophase commences, in view 2d, the chromosomes are situated at the poles, and the nuclei are tak ing shape once again. Cell division (cytokinesis) begins. In the final view, 2e, we see the four cells that result from cytokinesis. Each cell is haploid, meaning that it contains a single chromosome from the original chromosome pair. Recall that we began with two chromosomes. Now in the final view, each cell has one chromosome from that original pair. In the human male, these cells will undergo further development to become sperm cells, and in the human female, one of these cells will become an egg cell. Meiosis is linked to sexual reproduction in plants and animals because haploid cells join to form a fertilized diploid cell. In ani mals, the haploid stage is very brief, but in simple plants, the haploid stage predominates over the diploid stage, as you will see in the plates on plant biology. of sister chromatids. Chromosome 1 has sister chromatids 1 (H,), and chromosome 2 consists of sister chromatids 2 (H2). Anaphase is shown in view 1 d. Sister chromatids 1 move to the left pole of the cell while sister chromatids 2 move to the right. In telophase, sister chromatids 1 are contained in the left daugh ter cell, and sister chromatids 2 are in the right daughter cell. This marks the end of meiosis I. -48- Meiosis O Mei osi s I A O Meiosis II B O Nucleus C O Nucleolus D O Cytoplasm E O Centrioles O Homologous F 1 G, O Homologous Chromosome 2 Chromosome G, O Sister Chromatids 1 H, O Sister Chromatids 2 H, CD O Go 00 B to to 8 8 O Chromosome 1 O Chromosome 3 I3 O Chromosome 2 O Chromosome 4 I4 -49-