1. 5. Purpose of meiosis
1.1. Meiosis is specialized cell division which occurs in all sexually reproducing organisms. The sex cells are called gametes and have half the chromosomes of the parent cell. Male gametes are called sperm and female gametes are called eggs. The process that produces gametes is meiosis. The purpose of meiosis is to reduce chromosome number by half (diploid to haploid) which occurs thanks to two consecutive cell divisions (meiosis I and meiosis II) so they can fuse with a different gender gamete and produce offspring with the same amount of chromosomes as each parent.
2. 6. How genetic diversity is related to meiosis
2.1. Meiosis allows there to be genetic diversity because it allows offspring to be created that in not an exact genetic copy of the parents. It creates gametes that join with another individuals gametes to form Homologous chromosomes. Homologous chromosomes are corresponding (carry the code for the same characteristic) chromosomes from each parent. Having these homologous chromosomes produces the variation among individual members of a spices because the offspring are different then any other individual.
2.1.1. Even though each homologous chromosome has a corresponding gene, these genes may be slightly different from one another. The different forms of the same gene are called alleles. There are dominant and recessive alleles. A dominant allele will express its trait if it is present. A recessive allele will only be expressed if both chromosomes contain recessive allele. This will cause more genetic diversity as well.
2.2. Meiosis also causes genetic diversity because during Prophase 1, the replicated homologous pair of chromosomes comes together and crossover happens which is where sections of the chromosomes are exchanged. After crossing over, the resultant chromosomes are neither entirely maternal nor entirely paternal, but contain genes from both parents.
3. 7. Diploid cells vs. Haploid cells
3.1. Cells that have a complete set of chromosomes are called diploid cells and cells with half the chromosome number of the parent are called haploid cells.
4. 8. How the offspring of sexual reproduction is not exact copies of either parent
4.1. They are not exact copies because of crossover and homologous chromosomes; half from each parent, as said in #6.
5. 9. Phases of meiosis and how it is different than mitosis
5.1. In many ways, meiosis is a lot like mitosis. The cell goes through similar stages and uses similar tactics to organize and separate chromosomes. In meiosis, however, the cell has a more complex task. It still needs to separate sister chromatids, as in mitosis, but it must also separate homologous chromosomes. Meiosis is composed of 2 phases, Meiosis 1 and meiosis 2 to accomplish this. Cell division occurs twice during meiosis so one starting cell can produce four gametes.
5.1.1. Interphase and prophase 1 in Meiosis 1 are similar to Mitosis. During interphase, the chromosomes replicate to form joined sister chromatids. During prophase I, differences from mitosis begin to appear. As in mitosis, the chromosomes begin to condense, but in meiosis 1, they also pair up. Each chromosome carefully aligns with its homologous partner so that the two match up at corresponding positions along their full length. Then crossover happens where the DNA is broken at the same spot on each homologous side and reconnected in a criss-cross pattern so that the homologous exchange part of their DNA. The spots where crossovers happen are more or less random, leading to the formation of new remixed chromosomes with unique combinations of alleles.
5.1.1.1. During Metaphase 1, the spindle begins to capture chromosomes and move them towards the center of the cell. This may seem familiar from mitosis, but there is a twist. Each chromosome attaches to microtubules from just one pole of the spindle. So, during metaphase 1, homologous pairs, not individual chromosomes, line up at the metaphase plate for separation.
5.1.1.1.1. In anaphase 1, the homologous are pulled apart and move apart to opposite ends of the cell. The sister chromatids of each chromosome remain attached to one another and don't come apart unlike in mitosis. Finally, in telophase 1, the chromosomes arrive at opposite poles of the cell. Since cells will soon go through another round of division, Cytokinesis usually occurs at the same time as telophase 1, forming two haploid daughter cells.
5.2. Phases of meiosis
6. 1. The purpose of mitosis
6.1. Mitosis is the type of cell division used for growth, repair, asexual reproduction and to replace worn out cells. Mitosis occurs wherever new cells are needed. A parent produces two daughter cells that are identical to the parent cell and each other. Constant cell division ensures that cells never become too large. It is also the only way that organisms which reproduce asexually can reproduce.
6.1.1. The point of mitosis is to make sure that each daughter cell gets a perfect, full set of chromosomes. Cells with too few or too many chromosomes usually don’t function well: they may not survive, or they may even cause cancer. So, when cells undergo mitosis, they don’t just divide their DNA at random. Instead, they split up their duplicated chromosomes in a carefully organized series of steps.
7. 2. Phases of mitosis
7.1. Interphase: the phase of the cell cycle in which a typical cell spends most of its life. During this phase, the cell copies its DNA into sister chromatids in preparation for mitosis. The centrioles are also replicated.
7.1.1. Prophase: the first stage in mitosis. The sister chromatids become joined at or near the center and look like an X. The nucleolus is no longer visible. The nuclear membrane breaks down and the chromosomes spread out in the cytoplasm. The centrioles move to opposite poles of the cell and start to form spindle fibers (microtubules that grow toward the center of the cell). These spindle fibers form the spindle.
7.1.1.1. Metaphase: the second stage in mitosis. The spindle is now completely formed and the sister chromatids attach to the spindle fibers. The sister chromatids also line up along the middle of the cell, halfway between the poles of the cell.
7.1.1.1.1. Anaphase: the third stage in mitosis. The sister chromatids get pulled apart by the spindle fibers and move toward the opposite poles of the cell. They are now chromosomes again.
8. 3. The chromosomes in each phase
8.1. Chromatin is condense DNA to fit within the nucleus of the cell. Throughout most of the cell cycle, DNA is packaged in the form of chromatin. However, during mitosis chromatin exists in an additional level of organization known as a chromosome. Chromosomes are an even denser packaging of chromatin. Chromatids are chromosomes in their duplicated state which happens during interphase. What happens to them at each stage is explained in #2.
9. 4. Spore formation, binary fission and cloning
9.1. Theses are all different types of asexual reproduction ( only one organism needed to reproduce). Asexual reproduction causes all the offspring to be genetically identical to each other and the parent. It also allows a single organism to produce large numbers of offspring.
9.1.1. Many fungi, algae and non-flowering plant reproduce by forming spores. Spores are cells with thick cell walls and are produced by cell division.
9.1.2. Binary fission is where a parent undergoes cell division and divides in half to produce two smaller identical daughter cells or offspring. Only single-celled organisms (like bacteria) and some algae use binary fission. Binary Fission allows for rapid population under good conditions.
9.1.3. Cloning is where a single cell or part of an organism in used to grow a new, genetically identical organism. This new organism is a clone of the parent. Humans have made cloning possible for animals that cannot naturally form clones themselves.