The Cell Cycle, Cellular Growth, and Cancer Questions

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Bring on the tough stuff

1. A scientist tells you that he has found a cool single celled organism that is about to divide. He tells you that the homologous chromosomes are not paired, and that it doesn't look like any recombination has occurred. Is the organism dividing by mitosis or meiosis? Explain.
     
2. Explain how mitosis and meiosis are different, and how are they the same?
     
3. Draw out and annotate the steps of mitosis. Make sure to clearly label the chromosomes, sister chromatids, mitotic spindle, and each of the cell cycle steps.
     
4. Explain what key events would be red flags that a cell is going through meiosis and not mitosis?
     
5. How is our life cycle as humans (birth, growth, reproduction, and so on) connected to the cell cycle of the cells that make up our bodies?
     
6. How is chromosome structure important for chromosome segregation?
     
7. A scientist tells you that he has a cell with 46 chromatids. He asks you if the cell is diploid or haploid. What is the answer? Does your answer change if he tells you that the original cell had 23 chromatids? What has happened to the cell?
     
8. Describe what events are special to prophase I of meiosis.
     
9. Describe the three phases that make up interphase. How are they alike? Different?

Answers

1. The organism is dividing by mitosis. Homologous chromosomes only pair during meiosis, so if there is no pairing (and therefore, no recombination), the cell can't be undergoing meiosis
      
2. Different: Mitosis gives 2 genetically identical daughter cells, meiosis gives 4 genetically different daughter cells; mitosis – cells produced are diploid, meiosis – cells produced are haploid; there are two cell divisions in meiosis, but only one in mitosis; pairing of homologous chromosomes and recombination occurs in meiosis (prophase I), but not mitosis; homologous pairs of chromosomes line up together in metaphase I, but not in mitosis; whole chromosomes move in anaphase I, only single chromatids move in anaphase
   
   Same: DNA replication of chromosomes, chromosome condensation, nuclear envelope breakdown, centrosomes produce spindle apparatus, chromosomes align along the cell's equator, spindle apparatus pulls chromosomes into position, single chromatids move in anaphase and anaphase II, chromosome decondensation, nuclear envelope reformation, cytokinesis
     
3. Refer to mitosis diagrams
     
4. Pairing of homologous chromosomes (prophase I); chiasmata formation/recombination (prophase I); pairs of chromosomes aligned on the cell's equator (metaphase I); whole chromosomes moving (anaphase I)
     
5. We are formed from a fertilized egg; the egg and the sperm that fertilized it were both produced by meiosis in our parents' gonads. After fertilization, the single-celled embryo divides again and again by mitosis to grow and develop into a fetus. After birth, mitosis continues to provide new cells for us as we grow. It also provides new cells to replace those that are damaged or worn out. When we reach puberty, meiosis begins (or begins again, if you are female) to produce our own sex cells, ready for reproduction. As we grow older, cells become senescent and cannot divide as well, or may lose the control of their cell cycle, leading to diseases such as cancer
     
6. Chromosomes in a resting cell contain a single strand of DNA. When the cell needs to divide, that DNA is replicated, giving two identical sister chromatids. If the chromosomes did not copy their genetic material, then chromosome segregation could not occur correctly. There would be random segregation of chromosomes between daughter cells, rather than both cells getting a complete complement of chromosomes. The centromere is also important for chromosome segregation, as it provides an attachment site for the spindle apparatus. Pairing of homologous chromosomes allows recombination and the reshuffling of genetic material, leading to greater genetic diversity, an important survival feature.
     
7. The cell could be diploid OR haploid. Without knowing whether the chromatids are paired or separate, you can't tell. Yes, your answer does change: the cell with 46 chromatids is diploid. If it had 23 chromatids to start with, but it now has 46, two haploid cells must have fused to form a diploid cell, i.e. the original cell has been fertilized.
     
8. Pairing of homologous chromosomes and the formation of chiasmata, which enables the reshuffling of genetic material between the homologs via the process of recombination. The number of recombination events (cross-overs) will depend on the length of the homologs involved; the longer the chromosome, the more chance there is for recombination to occur
     
9. G₁: Growth, replication of materials, normal cell functions
   S: replication of DNA
   G₂: growth, replication of materials, normal cell functions
   
   All 3 phases involve the replication of materials necessary for cell division (organelles, proteins, DNA, and so on). DNA replication only takes place during S phase. Cell growth takes place in phases G₁ and G₂