Mitosis and meiosis - B. Biernacka - Campbell Biology in Focus PDF

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B. Biernacka...

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Mitosis and Meiosis 1.1 Cell Division Cell division is an important process that needs to occur in order for an organism to grow and survive. Old or damaged cells need to be replaced in organisms by new cells. Cells that get used up, such as blood cells, also need to get replaced. Cell division is an important part of the cell cycle in which the genetic information of a cell gets duplicated and then equally distributed into two cells which eventually separate and generate new cells. The cell cycle: The cell cycle consists of an interphase as well as mitosis and cytokinesis. Interphase can further be broken down into three phases, G1, S, and G2. G1 (first gap phase) is a growth phase in which the cell grows and produces cell components such as RNA, ribosomes, and enzymes. The G1 phase is followed by the S (synthesis) phase in which DNA replication occurs. DNA replication is extremely important in the production of new cells because each cell produced during cell division needs to have the same DNA content as the original (except gametes which will be discussed with meiosis). The S phase is then followed by the G2 (second gap) phase. During the G2 phase, spindle fiber proteins are made which leads into the start of mitosis. There is also a G0 phase, which will not be discussed in this course. Figure 1 shows a diagram of the cell cycle.

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Figure 1: The cell cycle Chromatin is the mass of DNA and proteins in the nucleus that make up chromosomes, the gene carrying structures. Chromosomes are single stranded during the G1 phase and become double stranded after DNA replication occurs in the S phase (Figure 2). Each duplicated chromosome has two identical sister chromatids joined by a centromere and is shaped like an “X”. The double stranded chromosomes separate during mitosis and one set of genes goes into each of the two daughter cells.

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Figure 2: a) The DNA of a single stranded chromosome replicates to become b) a double stranded chromosome made up of two genetically identical sister chromatids.

1.2 Mitosis Mitosis follows the G2 phase of interphase and is the process in which the cell divides and equally distributes the replicated DNA to each of two daughter cells and therefore the chromosome number for each cell remains the same. This is especially useful for organisms because cells that get used up or damaged can be replenished and replaced with cells that are genetically identical. Mitosis can be broken down into four stages; prophase, metaphase, anaphase, and telophase. However, each stage is continuous with the next and there is no distinct cut off point from one stage to the other. Prophase: Prophase, the first stage of mitosis, happens when chromosomes begin to coil up and become visible. During interphase the individual strands of the chromosome are not visible because they are not coiled up as much. During prophase however, each individual chromosome becomes visible as the familiar “X” shape (Figure 3). At the same time, the nuclear membrane starts to break apart and the centrioles (in animals only) start to move to opposite sides of the cells. Recall the chapter on cell structure and function. Spindle fibers continue to be produced and move to opposite sides of the cells as well (Figure 4).

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Figure 3: During mitosis each chromosome has two identical strands of DNA known as chromatids that are joined together via a centromere to form a chromosome.

Figure 4: During prophase the chromosomes become visible, the nuclear membrane breaks apart, and spindle fibers form and extend across the cell. Q. How many chromosomes are there in Figure 4? Are the chromosomes single or double stranded? Metaphase: During metaphase, the second stage of mitosis, the chromosomes line up along the center (equator) of the cell. The center of the cell (equator) is often referred to as metaphase plate. When chromosomes lined up, spindle fibers attach to them at the centromere (Figure 5).

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Note that there is no nuclear envelope around the chromosomes in metaphase, because it disappeared in prophase.

Figure 5: During metaphase the chromosomes line up at the equator of the cell. Q. How many sister chromatids are present? Anaphase: Anaphase occurs as the chromatids separate and are pulled to opposite ends of the cell by the spindle fibers. The chromatids are then referred to as daughter chromosomes. Each daughter chromosome has the same genetic information as the other half it got separated from (Figure 6).

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Figure 6: During anaphase the chromosomes get separated into daughter chromosomes and get pulled to opposite ends of the cell. Q. How many sister chromatids are present? How many daughter chromosomes are present? Telophase: The final stage of mitosis is called telophase. During telophase, the nuclear membranes reform and the spindle fibers disassemble. Two genetically identical daughter nuclei form in which the chromosomes are single stranded again (Figure 7). The chromosomes also unwind and become no longer visible. Hence, a lot of events happening in telophase are opposite to prophase. Throughout telophase, the process of cytokinesis occurs in which the cytoplasm splits and forms two separate cells. These two daughter cells are genetically identical to the parent cell. The chromosome number is exactly the same in the daughter cells as was in the parent cell, the only difference is that the chromosomes are now single stranded instead of double stranded. This is an important aspect of mitosis since the newly formed daughter cells are used for repairing and replenishing damaged, or worn out cells and they need to be identical to the original parent cell.

Figure 7: During telophase the daughter nuclei form and the spindle fibers disassemble. Cytokinesis occurs at the same time to separate the cytoplasm into two different cells. Q. If a parent cell started out with 16 double stranded chromosomes, how many chromosomes would each daughter cell have and would they be single or double stranded? Cytokinesis: Cytokinesis occurs slightly differently in animals than in plants. In animal cells a cleavage furrow forms by the contraction of microfilaments in order to split the cytoplasm and pinch off the ends. In plant cells a cell plate forms instead because plants have a cell wall (Figure 8).

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Figure 8: Cytokinesis in a) an animal cell and b) a plant cell.

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Table 1: An overview of the cell cycle and mitosis.

1.3 Meiosis Meiosis is a process of cell division that leads to the production of gametes. Gametes are the egg and sperm cells of human beings and other animals. They have only half of the original chromosome number but an entire set of chromosomes (n) and are referred to as haploid. When the haploid gametes of two individuals join together a diploid organism is formed (the original

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number of chromosomes restored) with genetic material from both parents. Diploid organisms have two sets of chromosomes (2n). Meiosis occurs in two divisions, meiosis I and meiosis II and each division is further divided into four stages similar to that of mitosis. Meiosis I reduces the number of chromosomes from diploid (2n) to haploid (n) and meiosis II produces four haploid daughter cells (gametes). Prophase I: During prophase I the DNA, which has already been replicated in S phase, coils up and becomes visible as chromosomes, each made up of two chromatids (Figure 9). Chromosomes that code for the same genes pair up next to each other and are known as homologous chromosomes. In a homologous pair, one chromosome originally came from the mother and one from the father so they are never totally identical, they simply code for the same genes. The pairing of homologous chromosomes is known as synapsis. Once the homologous chromosomes are paired up, crossing-over can occur. Crossing-over is the exchange of the equivalent genetic material between the non-sister chromatids of the homologous pairs and allows for genetic diversity in offspring to occur by producing recombinant chromosomes (Figure 10). Recombinant chromosomes are chromosomes that have genes from both parents. In prophase I spindle fibers also continue to form as the centrioles (in animals only) move to opposite poles.

Figure 9: Early prophase I.

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Figure 10: The process of crossing-over produces genetic variation. Metaphase I: During metaphase I the homologous chromosomes line up along the equator of the cells together and the spindle fibers elongate and connect to only one of each pair of chromosomes. Each pair of maternal and paternal homologues is arranged randomly along the equator so there is a random amount of maternal and paternal homologues being pulled to each side of the cell (Figure 11). This is known as independent assortment, and along with crossing-over, allows for genetic diversity to occur in organisms (Figure 12).

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Figure 11: Metaphase I. Q. How many homologous chromosomes are present in Figure 11? How many chromatids are present in Figure 11?

Figure 12: Independent assortment allows for genetic diversity to occur. Anaphase I: The homologous chromosomes are pulled apart during anaphase I and are pulled to opposite sides of the cell. Each chromosome that is being pulled still has two chromatids present (Figure 13). Note that in anaphase of mitosis, sister chromatids are pulled apart.

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Figure 13: Anaphase I. Q. How many chromatids are present in Figure 13? Telophase I: During telophase I, two haploid daughter cells are formed but the chromosomes remain double stranded (Figure 14). The two daughter cells are still considered haploid because they each have only one set of chromosomes (n), regardless of being single or double stranded. Cytokinesis also occurs during this time to separate the cytoplasm into two different cells. This leads into the next division of meiosis, meiosis II which is very similar to mitosis.

Figure 14: Telophase I. Q. How many chromosomes are present in each of the daughter cells? Prophase II: During prophase II the spindle fibers form again and move to opposite sides of each of the two cells. In animal cells, the centrioles also move to opposite sides as well (Figure 15).

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Figure 15: Prophase II. Q. How many sister chromatids are present in each new daughter cell? Metaphase II: In metaphase II the chromosomes in each of the two cells line up at their equators and the spindle fibers lengthen and attach to each chromosome (Figure 16).

Figure 16: Metaphase II. Q. Are the daughter cells in Figure 16 haploid or diploid? Anaphase II: During anaphase II the chromosomes get separated into daughter chromosomes and are pulled to opposite sides of the cells (Figure 17). Note that at the start of anaphase, the chromosomes are double stranded, each consisting of two sister chromatids. Thus, in this stage sister chromatids are pulled apart and as soon as it happens, each of them becomes a daughter chromosome.

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Figure 17: Anaphase II. Q. How many daughter chromosomes are present in Figure 17? Telophase II: Both of the cells that were formed in meiosis I split into two more cells during telophase II, each with a single stranded chromosomes. The overall process of meiosis produces four haploid cells that are all genetically different (Figure 18). In humans, these form the gametes (eggs or sperm cells) which each have 23 chromosomes. The cells of the ovaries and testes, where gametes are formed, have 46 chromosomes. Prior to meiosis I, the primary spermatocytes of the testes, and oocytes of the ovaries are diploid (46 chromosomes: 2 sets of 23). After meiosis I occurs, both become haploid (23 chromosomes: 1 set of 23) and are known as secondary spermatocytes and oocytes. After meiosis II haploid sperm and egg cells are formed still containing 23 chromosomes. Figure 19 shows the process of spermatogenesis and Figure 20 shows the process of oogenesis.

Figure 18: Telophase II. Q. How many chromosomes would each of the four gametes have if the original parent cell had 46 chromosomes? Would they be single or double stranded?

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Table 2: An overview of meiosis I.

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Table 3: An overview of meiosis II.

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