Mitosis and Meiosis 1. A. Describe the Process of Mitosis and Meiosis Mitosis Mitosis is a process of cell duplication, or reproduction, during which one cell gives rise to two genetically identical daughter cells. Strictly applied, the term mitosis is used to describe the duplication and distribution of chromosomes, the structures that carry the genetic information. In the context of the cell cycle, mitosis is the part of the cell division process in which the DNA of the cell's nucleus is split into two equal sets of chromosomes. Mitosis is absolutely essential to life because it provides new cells for growth and for replacement of worn-out cells. Mitosis may take minutes or hours, depending upon the kind of cells and species of organisms. It is influenced by time of day, temperature, and chemicals. The process: Phases of mitosis. This refers to how a cell divides to make two genetically identical cells. Mitosis has four stages: prophase, metaphase, anaphase, and telophase. Prophase is subdivided into an early phase (called prophase) and a late phase (called prometaphase). These phases occur in strict sequential order. Cytokinesis - the process of dividing the cell contents to make two new cells starts in anaphase or telophase. Assignment I: Mitosis and Meiosis 1 Just before mitosis, the cell is in interphase late G2 phase and has already copied its DNA, so the chromosomes in the nucleus each consist of two connected copies, called sister chromatids. The cell has also made a copy of its centrosome, an organelle that will play a key role in orchestrating mitosis, so there are two centrosomes. (Plant cells generally don’t have centrosomes). 1. Early prophase (Prophase). In early prophase, the cell starts to break down some structures and build others up, setting the stage for division of the chromosomes. → The chromosomes start to condense (making them easier to pull apart later on). → The mitotic spindle begins to form. The spindle is a structure made of microtubules, strong fibers that are part of the cell’s skeleton. Its job is to organize the chromosomes and move them around during mitosis. The spindle grows between the centrosomes as they move apart. → The nucleolus (or nucleoli, plural), a part of the nucleus where ribosomes are made, disappears. This is a sign that the nucleus is getting ready to break down. Assignment I: Mitosis and Meiosis 2 2. Late prophase (prometaphase). In late prophase (prometaphase), the mitotic spindle begins to capture and organize the chromosomes → The chromosomes become even more condensed, so they are very compact. → The nuclear envelope breaks down, releasing the chromosomes. → The mitotic spindle grows more, and some of the microtubules start to capture chromosomes. 3. Metaphase. In metaphase, the spindle has captured all the chromosomes and lined them up at the middle of the cell, ready to divide. → All the chromosomes align at the metaphase plate (not a physical structure, just a term for the plane where the chromosomes line up). → At this stage, the two kinetochores of each chromosome should be attached to microtubules from opposite spindle poles. Assignment I: Mitosis and Meiosis 3 Before proceeding to anaphase, the cell will check to make sure that all the chromosomes are at the metaphase plate with their kinetochores correctly attached to microtubules. This is called the spindle checkpoint and helps ensure that the sister chromatids will split evenly between the two daughter cells when they separate in the next step. If a chromosome is not properly aligned or attached, the cell will halt division until the problem is fixed. 4. Anaphase. In anaphase, the sister chromatids separate from each other and are pulled towards opposite ends of the cell. → The protein “glue” that holds the sister chromatids together is broken down, allowing them to separate. Each is now its own chromosome. The chromosomes of each pair are pulled towards opposite ends of the cell. → Microtubules not a�ached to chromosomes elongate and push apart, separating the poles and making the cell longer. All of these processes are driven by motor proteins, molecular machines that can move along microtubule racks and carry a cargo. In mitosis, motor proteins carry chromosomes or other microtubules as they move. Assignment I: Mitosis and Meiosis 4 5. Telophase. In telophase, the cell is nearly done dividing, and it starts to re-establish its normal structures as cytokinesis (division of the cell contents) takes place. → The mitotic spindle is broken down into its building blocks. → Two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli reappear. → The chromosomes begin to de-condense and return to their stringy form. Cytokinesis Assignment I: Mitosis and Meiosis 5 The division of the cytoplasm to form two new cells, overlaps with the final stages of mitosis. It may start in either anaphase or telophase, depending on the cell, and finishes shortly after telophase. In animal cells, cytokinesis is contractile, pinching the cell in two like a coin purse with a drawstring. The “drawstring” is a band of filaments made of a protein called actin, and the pinch crease is known as the cleavage furrow. Plant cells can’t be divided like this because they have a cell wall and are too stiff. Instead, a structure called the cell plate forms down the middle of the cell, splitting it into two daughter cells separated by a new wall. When cytokinesis finishes, we end up with two new cells, each with a complete set of chromosomes identical to those of the mother cell. The daughter cells can now begin their own cellular “lives,” and –depending on what they decide to be when they grow up – may undergo mitosis themselves, repeating the cycle. Assignment I: Mitosis and Meiosis 6 Mitosis is used for almost all of your body’s cell division needs. It adds new cells during development and replaces old and worn-out cells throughout your life. The goal of mitosis is to produce daughter cells that are genetically identical to their mothers, with not a single chromosome more or less. Meiosis Meiosis is used for just one purpose in the human body: the production of gametes, sex cells, or sperm and eggs. Its goal is to make daughter cells with exactly half as many chromosomes as the starting or original cell Meiosis in humans is a division process that takes us from a diploid cell—one with two sets of chromosomes—to haploid cells—ones with a single set of chromosomes. In humans, the haploid cells made in meiosis are sperm and eggs. When a sperm and an egg join in fertilization, the two haploid sets of chromosomes form a complete diploid set: a new genome. The process of meiosis. In many ways, meiosis is a lot like mitosis. The cell goes through similar stages and uses similar strategies to organize and separate chromosomes. In meiosis, however, the cell has a more complex task. It still needs to separate sister chromatids (the two halves of a duplicated chromosome), as in mitosis. But it must also separate homologous chromosomes, the similar but non-identical chromosome pairs an organism receives from its two parents. Assignment I: Mitosis and Meiosis 7 These goals are accomplished in meiosis using a two-step division process. Homologue pairs separate during a first round of cell division, called meiosis I. Sister chromatids separate during a second round, called meiosis II. Since cell division occurs twice during meiosis, one starting cell can produce four gametes (eggs or sperm). In each round of division, cells go through four stages: prophase, metaphase, anaphase, and telophase. Phases of meiosis. Meiosis I. Before entering meiosis I, a cell must first go through interphase. As in mitosis, the cell grows during G1 phase, copies all of its chromosomes during S phase, and prepares for division during G2 phase. During prophase I, differences from mitosis begin to appear. As in mitosis, the chromosomes begin to condense, but in meiosis I, they also pair up. Each chromosome carefully aligns with its homologue partner so that the two match up at corresponding positions along their full length. For instance, in the image below, the letters A, B, and C represent genes found at particular spots on the chromosome, with capital and lowercase letters for different forms, or alleles, of each gene. The DNA is broken at the same spot on each homologue—here, between genes B and C— and reconnected in a crisscross pattern so that the homologues exchange part of their DNA. Assignment I: Mitosis and Meiosis This process, in which homologous chromosomes trade parts, is called crossing over. It's helped along by a protein structure called the synaptonemal complex that holds the homologues together. The chromosomes would actually be positioned one on top of the other—as in the image below—throughout crossing over; they're only shown side-by-side in the image above so that it's easier to see the exchange of genetic material. You can see crossovers under a microscope as chiasmata, cross-shaped structures where homologues are linked together. Chiasmata keep the homologues connected to each other alter the synaptonemal complex breaks down, so each homologous pair needs at least one. It's common for multiple crossovers (up to 25) to take place for each homologue pair. The spots where crossovers happen are more or less random, leading to the formation of new, remixed chromosomes with unique combinations of alleles. 8 Assignment I: Mitosis and Meiosis 9 Alter crossing over, the spindle begins to capture chromosomes and move them towards the center of the cell (metaphase plate). This may seem familiar from mitosis, but there is a twist. Each chromosome attaches to microtubules from just one pole of the spindle, and the two homologues of a pair bind to microtubules from opposite poles. During metaphase I, homologue pairs—not individual chromosomes—line up at the metaphase plate for separation. When the homologous pairs line up at the metaphase plate, the orientation of each pair is random. For instance, in the diagram below, the pink version of the big chromosome and the purple version of the little chromosome happen to be positioned towards the same pole and go into the same cell. But the orientation could have equally well been flipped, so that both purple chromosomes went into the cell together. This allows for the formation of gametes with different sets of homologues. In anaphase I, the homologues are pulled apart and move apart to opposite ends of the cell. The sister chromatids of each chromosome, however, remain attached to one another and don't come apart. Finally, in telophase I, the chromosomes arrive at opposite poles of the cell. In some organisms, the nuclear membrane re-forms and the chromosomes de-condense, although in others, this step is skipped—since cells will soon go through another round of division, meiosis II. Cytokinesis usually occurs at the same time as telophase I, forming two haploid daughter cells. Assignment I: Mitosis and Meiosis Meiosis II. Cells move from meiosis I to meiosis II without copying their DNA. Meiosis II is a shorter and simpler process than meiosis I, and you may find it helpful to think of meiosis II as “mitosis for haploid cells." The cells that enter meiosis II are the ones made in meiosis I. These cells are haploid— have just one chromosome from each homologue pair—but their chromosomes still consist of two sister chromatids. In meiosis II, the sister chromatids separate, making haploid cells with non-duplicated chromosomes. 10 Assignment I: Mitosis and Meiosis 11 During prophase II, chromosomes condense and the nuclear envelope breaks down, if needed. The centrosomes move apart, the spindle forms between them, and the spindle microtubules begin to capture chromosomes. The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles. In metaphase II, the chromosomes line up individually along the metaphase plate. In anaphase II, the sister chromatids separate and are pulled towards opposite poles of the cell. In telophase II, nuclear membranes form around each set of chromosomes, and the chromosomes de-condense. Cytokinesis splits the chromosome sets into new cells, forming the final products of meiosis: four haploid cells in which each chromosome has just one chromatid. In humans, the products of meiosis are sperm or egg cells. How meiosis mixes and matches genes. The gametes produced in meiosis are all haploid, but they're not genetically identical. For example, take a look the meiosis II diagram above, which shows the products of meiosis for a cell with 2n = 4 chromosomes. Each gamete has a unique "sample" of the genetic material present in the starting cell. As it turns out, there are many more potential gamete types than just the four shown in the diagram, even for a cell with only four chromosomes. The two main reasons we can get many genetically different gametes are: Crossing over. The points where homologues cross over and exchange genetic material are chosen more or less at random, and they will be different in each cell that goes through meiosis. If meiosis happens many times, as in humans, crossovers will happen at many different points. Assignment I: Mitosis and Meiosis 12 Random orientation of homologue pairs. The random orientation of homologue pairs in metaphase I allows for the production of gametes with many different assortments of homologous chromosomes. In a human cell, the random orientation of homologue pairs alone allows for over 8 million different types of possible gametes B. State the Differences Between Mitosis and Meiosis i. Cell Division Mitosis: A somatic cell divides once. Cytokinesis (the division of the cytoplasm) occurs at the end of telophase. Meiosis: A reproductive cell divides twice. Cytokinesis happens at the end of telophase I and telophase II. ii. Daughter Cell Number Mitosis: Two daughter cells are produced. Each cell is diploid containing the same number of chromosomes. Meiosis: Four daughter cells are produced. Each cell is haploid containing one-half the number of chromosomes as the original cell. iii. Genetic Composition Mitosis: The resulting daughter cells in mitosis are genetic clones (they are genetically identical). No recombination or crossing over occur. Meiosis: The resulting daughter cells contain different combinations of genes. Genetic recombination occurs as a result of the random segregation of homologous chromosomes Assignment I: Mitosis and Meiosis 13 into different cells and by the process of crossing over (transfer of genes between homologous chromosomes). iv. Length of Prophase Mitosis: During the first mitotic stage, known as prophase, chromatin condenses into discrete chromosomes, the nuclear envelope breaks down, and spindle fibers form at opposite poles of the cell. A cell spends less time in prophase of mitosis than a cell in prophase I of meiosis. Meiosis: Prophase I consists of five stages and lasts longer than prophase of mitosis. The five stages of meiotic prophase I are leptotene, zygotene, pachytene, diplotene, and diakinesis. These five stages do not occur in mitosis. Genetic recombination and crossing over take place during prophase I. v. Tetrad Formation Mitosis: Tetrad formation does not occur. Meiosis: In prophase I, pairs of homologous chromosomes line up closely together forming what is called a tetrad. A tetrad consists of four chromatids (two sets of sister chromatids). vi. Chromosome Alignment in Metaphase Mitosis: Sister Chromatids align at the metaphase plate (a plane that is equally distant from the two cell poles). Meiosis: Tetrads (homologous chromosome pairs) align at the metaphase plate in metaphase I. vii. Chromosome Separation Assignment I: Mitosis and Meiosis 14 Mitosis: During anaphase, sister chromatids separate and begin migrating centromere first toward opposite poles of the cell. A separated sister chromatid becomes known as daughter chromosome and is considered a full chromosome. Meiosis: Homologous chromosomes migrate toward opposite poles of the cell during anaphase I. Sister chromatids do not separate in anaphase I. viii. Meiosis takes place only in the gametes, or sex cells (sperm in men and eggs in women) whereas mitosis occurs almost in all body cells ix. In Mitosis, during metaphase, individual chromosomes line up on cell’s equator whereas in meiosis, pairs of chromosomes line up on cell’s equator x. Mitosis occurs in all organisms except in viruses whereas meiosis occurs only in animals, plants and fungi Assignment I: Mitosis and Meiosis 15 C. Mention any Five Enzymes that are Involved in Cell Cycle Regulation and Their Sites of Action No. Enzyme Site of Action 1. Cyclin C- Cdk3 G1 phase 2. Cyclin D- Cdk4 G1 phase 3. Cyclin E- Cdk2 G1/S transition 4. Cyclin A- Cdk2 S phase, G2 phase 5. Cyclin B- Cdk1 M phase 6. Anaphase-Promoting M phase on M cyclins Complex/Cyclosome (APC/C) Assignment I: Mitosis and Meiosis 16 2. State the Differences Between Eukaryotic and Prokaryotic Cells Feature Size Cell type Cell Wall Presence of Nucleus Shape of DNA Mitochondria Ribosome Golgi Apparatus Endoplasmic Reticulum Mode of Reproduction Cell Divison Prokaryotic cells 0.5-3um Single-cell Cell wall present, comprise of peptidoglycan or muco-peptide (polysaccharide). Well-defined nucleus is absent, rather nucleoid is present which is an open region containing DNA. Circular, double-stranded DNA. Absent Eukaryotic cells 2-100um Multicellular Usually cell wall absent, if present (plant cells and fungus), comprises of cellulose (polysaccharide). A well-defined nucleus is present enclosed within nuclear membrane. 70S Absent 80S Present Absent Present Asexual Most commonly sexual Binary Fission, (conjugation, transformation, transduction) Lysosomes Absent and Peroxisomes Chloroplast (Absent) scattered in the cytoplasm. Transcription Occurs together. And Translation Organelles Organelles are not membrane bound, if present any. Replication Single origin of replication. Number of Only one (not true called as a plasmid). Chromosomes Linear, double stranded DNA. Present Mitosis Present Present in plants, algae. Transcription occurs in nucleus and translation in cytosol. Organelles are membrane bound and are specific in function. Multiple origins of replication. More than one. Assignment I: Mitosis and Meiosis References The Editors of Encyclopaedia Britannica. (2020). Mitosis. In Encyclopædia Britannica. https://www.britannica.com/science/mitosis Khan Academy. (2020). Cell cycle. https://www.khanacademy.org/science/ap-biology/cellcommunication-and-cell-cycle/cell-cycle/v/interphase Scitable. (2014). CDK. Nature Education. https://www.nature.com/scitable/topicpage/cdk14046166/ Open Educational Resources. (n.d.). Cell division and cell cycle. In Lumen Learning. https://courses.lumenlearning.com/suny-biology1/chapter/control-of-the-cell-cycle/ Creative commons. (n.d.). Cyclin-dependent kinase. In Wikipedia. https://en.m.wikipedia.org/wiki/Cyclin-dependent_kinase Ohkura, H. (2015). Meiosis: an overview of key differences from mitosis. Cold Spring Harbor perspectives in biology, 7(5). https://doi.org/10.1101/cshperspect.a015859 17
