11 Mitosis: Cell Division Yowr instrvesor may colleer these endoferereise querno

Question

11 Mitosis: Cell Division Yowr instrvesor may colleer these endoferereise quernonr so please fill your name and lab section, End-of- Exercise Questions During which part of the cell cycle does DNA replication occur? Pease chromosomes first become visible in mitosis? What is the difference between the terms chromosome and chromatid 4. What is a spindle? What is its function? 5. During what stages of mitosis are chromosomes composed of two chromatids? 6. During what stages of milosis are chromosomes single structures composed of one chromatid? 7. How does cytokinesis differ in plant and animal cells? 8. Most cells spend the longest amount of time in What evidence do you have to support this statement? 9. Why was it difficult to find interphase cells in the whitefish blastula slide? 10. Why are you more likely to see a polar view in animal cells than in plant cells?

Explanation / Answer

Ans. PAGE 1.

#1. S phase

#2. Prophase- as “beads on strings” or thin threads

#3. Note that each chromosome consists of a single dsDNA molecule ins supercoiled state.

Chromosome- It is a single dsDNA molecule in supercoiled state.

Chromatids- Two chromosomes (One original copy + its duplicated copy made during S-phase) attached to each other are called chromatids. When the chromatids separate to from two independent chromatids at the end of anaphase, they again called “chromosomes” because each chromatid is now a single DNA molecule or chromosome.   

#4. Spindle is microtubules. It mediates chromosome segregation and cell shape change during mitosis.

#5. Prophase, Metaphase, at the beginning of Anaphase

#6. At the end of anaphase, Telophase

#7. Cytokineses in animal cells is brought about by formation of contractile rings that pinches off the mother cell into two daughter cells. In plant cell, the cell plate laid at center finally extends to cell periphery and forms cell wall between the two daughter cell, thus separating them.

#8. G1 phase.

Most cells spend most their time in G1 phase, during which cell prepares itself for cell division, if necessary and if it is capable of division. Preparation for cell division, like strong nutrients and biomolecules, increase in cell size etc. is a time-consuming process. G1 constitute around 90% time of most cell cycles. Many cells, say neurons, cardiomyocyte, or any other somatic cells divide almost never after they’re formed in human body. Some cells that divide for tissue repair, do so less frequently- only when needed or once in few days or so on. Most somatic cells do not require to divide, so they spend almost all their life in G1-phase.

#10. The “pinching off” mother cell into two daughter cells makes the animal cell cytokinesis view more polar.   

PAGE 2:

#2. Once all the sister chromatid pairs align at the equatorial plate, the cell is said to be in metaphase. The hypothetical plane of alignment of all pairs of sister chromatids during metaphase is called metaphase plate. The movements of chromosomes during and after alignment at metaphase plate is a result of simultaneously coordinated forces namely polar winds (polar ejection force), poleward flux (microtubule flux) and kinetochore- generated poleward force. Polar ejection force arises due to attachment of plus end of interpolar microtubules to chromosome arms through motor proteins (kinesin-4 and kinesin-10). Due to enhanced rate of polymerization at plus end, microtubules push the chromosome away from the spindle pole. This force helps in positioning the chromosomes towards the equatorial plate during prometaphase and metaphase.

#5. Recombination or Crossing over

#6. It reduces the chromosome number too half in the daughter cells- the gametes. The haploid gametes (n) when fuse together during fertilization, restore the diploidy (2n) nature of the zygote- new organism.

#7. Segregation during Anaphase I.

#8. A. Synapsis (= pairing) , Zygotene, Prophase I

B. Pachytene, Prophase I

C. Anaphase I

D. Cytokinesis I

E. Anaphase II

F. Anaphase I

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