1. A duplicated, or “replicated,” chromosome is two sister chromatids joined at a centromere by cohesion proteins.2. The cell cycle refers to two components: cell growth and mitotic cell division. Events, orstages, involved are interphase (which includes G1, S (DNA synthesis) and G2 phases) and the mitotic phase (includes mitosis and cytokinesis).3. Cells and organisms need to regulate or control the cell cycle so that damaged cells cannot be allowed to replicate and cause tumors or cancer and so cell growth is not over stimulated, also causing cancer.4. Cells regulate or control the cell cycle through molecular signals (signal molecules and regulatory proteins) in the cytoplasm. Checkpoints are meant to regulate the cell at various times in its life cycle. They pause the cycle and receive signals from the environment on what to do. In the G2 checkpoint, signals assess preparations and if it’s “ok,” they direct the cell to enter the mitotic phase. The MPF signal puts the cell throughthe G2 checkpoint and initiates mitosis. MPF is a cyclin-Cdk complex. A particular cyclin is synthesized during S and G2, and when this cyclin level becomes high, it gets bound to and activated by a particular Cdk (this complex is now called MPF).5. The general purposes of mitotic cell division are development and tissue growth and repair. Mitotic cell division begins with prophase where centrosomes move to opposite poles, spindles start to form, the nuclear envelope starts to dissolve, the nucleolus disintegrates, and chromatin begins to condense into distinct chromosomes. Secondly is pro-metaphase where the nuclear envelope is now disassembled, chromosomes are clearly distinct, a kinetochore appears at each centromere, and the spindle apparatus is complete. Next is metaphase, where the spindle microtubules move chromosomes and the chromosomes like up single file at the metaphase plate (midline, cell equator). Next is anaphase where the centromere splits, spindle microtubules pull apart sister chromatidstoward opposite poles, and each separated chromatid is now a distinct chromosome. The last step is telophase where the spindle disassembles, the nuclear membrane reforms, nucleolus reappears, and mitosis is now complete. Cytokinesis happens last and the cell membrane pinches in at the cleavage furrow and two daughter cells result. The number of chromosomes in the beginning of mitosis is the same as the number of chromosomes in each daughter cell at the end. 6. Cancer and the cell cycle have a lot to do with each other. Cancer results when there are errors in the cell cycle. It is caused by mutations in genes. Gene regulation also has a lot to do with cancer. If genes are not regulated properly, cancer can occur. Normal tumor-suppressing genes restrict cell division, but if there is an error it can result in over stimulation of cell division. And normal proto-oncogenes stimulate cell division, but if there is an error they will over stimulate a cell to divide when it shouldn’t. 7. Somatic cells are diploid and have two sets of chromosomes (maternal and paternal sets). They are body cells in places such as skin, muscle, and nerves. Their function is development and the growth and repair of the body. They are formed through mitosis and in humans, the chromosome number is 46. Germ cells are sex cell or gametes. They are haploid and contain only one set of chromosomes. In humans, they have 23 chromosomes. They are found in oocytes, or egg cells, and sperm cells and are formed through meiosis. 8. A karyotype is an orderly way of displaying the number and types of chromosomes in a cell. It displays visible (condensed) already replicated chromosomes (ones undergoingmitosis). A karyotype reveals how many chromosomes are in that organism and detect the gender. They are also used to detect abnormalities in chromosome number, size, etc.9. When we say that in eukaryotes “chromosomes exist in pairs” it means that there is a chromosome from the father that matches a chromosome from the mother. The two chromosomes that make up a pair are called homologous chromosomes. They are similarto each other in that they have the same number and kinds of genes. They are possibly different from each other in their alleles, which are different versions of the same kind of gene.10. A diploid has twice the number of chromosomes as a haploid cell. Diploid cells are somatic cells and haploid cells are germ cells or gametes.11. The general purpose of meiotic cell division is to produce four haploid sex cells. There are 2 cell divisions in meiosis, meiosis 1 and meiosis 2. The first division is a “reduction” division (you go from 2n to n). Chromosomes duplicate in a diploid cell, homologous chromosomes pair up and form a tetrad and nonsister chromatids exchange segments (crossing-over), tetrads line up at the metaphase plate, and homologous pairs separate. Two haploid cells are formed, but the sister chromatids are still attached at centromeres. Meiosis two then occurs, which is alike a regular mitosis but in a haploid cell. We get 4 haploid cells with just one set of chromosomes. Compared to the beginning of meiosis, after meiosis 1, there are half the number of chromosomes. And after meiosis two, there is still half the number of chromosomes. 12. (a) In meiosis, tetrads are formed by the synapsis of homologous chromosomes. Chromosomes line up at the metaphase plate in mitosis, but tetrads line up at the plate in meiosis. In mitosis, sister chromatids separate during anaphase, but homologous chromosomes are separate in anaphase 1 in meiosis. Sister chromatids don’t separate until anaphase II in meiosis. Mitosis produces two diploid daughter cells identical to each other and the parent, but meiosis produces four haploid daughter cells that are genetically different. (b) (c)13. Unicellular organisms reproduce asexually through mitosis. Meiosis can contribute to genetic variation among diploid organisms by independent assortment of chromosome pairs and crossing over. Independent assortment is when the two chromosomes in a pair separate independently of how other chromosome pairs separate. So a gamete can receive a mix of both maternal and paternal genes. Crossing-over is when non-sister chromatids in a tetrad break and exchange sections at the chiasma. This results in new allele combinations. This is important so for evolutionary reasons.14. Mendel’s
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