Mitosis Replication of Eukaryotic Cells

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Exercise 13

1. a. Mitosis and cytokinesis are often referred to collectively as "cellular division." Why are they more accurately called cellular replication? The result of mitosis is production of two cells (replicates) identical to the parent cell. The genetic material is replicated rather than divided. b. Does the cell cycle have a beginning and an end? The organization of our study of cellular events indicates a beginning and end, but the events continue to repeat. 2. a. If a nucleus has eight chromosomes during interphase, how many chromosomes does it have during metaphase? During metaphase there are 8 double-stranded chromosomes. b. How many does it have after mitosis is complete? After mitosis (and separation of chromatids) there are 16 chromosomes distributed between two nuclei. 3. a. Why would we choose an embryonic mass of cells for procedure 13.3 in which to study the stages of mitosis? embryonic cells are replicating and growing rapidly b. Which stage of mitosis most often is associated with the beginning of cytokinesis? usually late anaphase 4. a. What region of a root has the most mitotic activity? meristem, just above cap b. Why is pinching of the cytoplasm inadequate for cytokinesis in plant cells? Pinching of cytoplasm is not appropriate for rigid cell walls. c. Locate a plant cell in late telophase. What is the volume of the new cells relative to a mature cell? Each cell is smaller (approximately 1/2) than the parent cell. 5. a. Why are the combined data from all the class members more meaningful than your results alone? Combined results reduce random error. b. How accurate were your predictions for length of each stage of mitosis? probably not close c. What sources of error can you list for this technique to determine the time elapsed during each stage of mitosis? variation in onion roots; student's ability to distinguish stages; microscopic cross sections of roots may or may not pass through the nuclear material

Exercise 14

1. a. Why would shuffling genetic material and producing new combinations of characteristics be advantageous to a species? New genetic combinations allow adaptation to changing environments. b. When would it be deleterious? New combinations always produce a percentage of disadvantageous characteristics; the risk of bad gene combinations may be inappropriate in a stable, beneficial environment. 2. a. Synapsis occurs after chromosomal DNA has replicated. How many chromatids are involved in crossing-over of a homologous pair of chromosomes? 4 b. Suppose synapsis occurred between two homologous chromosomes, and one had alleles for blue eyes and brown hair and the other had alleles for green eyes and blonde hair. How many different combinations of these alleles would be possible? 4 Remember that crossing-over does not necessarily occur between the loci, so the original combinations may remain intact. 3. a. If a nucleus has eight chromosomes when it begins meiosis, how many chromosomes does it have after telophase I? Telophase II? 4 with two chromatids each; 4 with one chromatid each b. What are the major differences between the events of meiosis and mitosis? (see table 14.2) c. What are some minor differences, and why do you consider them minor? minor differences would be those unrelated to chromosomal events 4. a. During gametogenesis a sperm cell undergoes considerable structural change. What are the basics of sperm structure and how does it relate to function? The basics include small head with nucleus; an acrosomal process with enzymes to digest the outer membranes of the egg; a head and neck rich in mitochondria; and tail used to propel the cell to the egg. b. What is the advantage of producing sperm in a system of tubes rather than in solid tissue? because sperm cells can be expelled easily from the body through a system of tubes c. What is...
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