Question

1. Why are chromosomes in their chromatin form prior to mitosis (during interphase)?

2. Why would DNA wind tighter around histones prior to the start of mitosis?

3. Is cancer caused by errors during mitosis or is it caused by something else? Hint: why is it so difficult to treat cancer?

4. How do bacterial cells reproduce? How does it compare to eukaryotes (what’s the same /different)?

Answer 1

1. Chromosomes exist as chromatin threads DNA wrapped by histone proteins, and during mitosis it condeses to chromosomes. The chromosome formation helps them to move smoothly without getting entangled and breaking. Once the mitosis is over chromosomes decondense and exist as chromatin threads or nucleosomes during interphase. In nucleosomes, the binding of proteins to DNA in chromatin protects regions of the DNA from nuclease digestion, so that the enzyme can attack DNA only at sites separated by approximately 200 base pairs.

Chromatin structure is thus intimately linked to the control of gene expression in eukaryotes. During the period of cell cycle, called interphase, the genes are transcribed and the DNA is replicated in preparation for cell division. A more decondensed state allows transcription.

2.  Packaging of DNA into chromatin fiber shortens its length approximately sixfold. The chromatin can then be further condensed by coiling into 30-nm fibers. Interactions between histone H1 molecules appear to play an important role in this stage of chromatin condensation.

3. Cancer is due to uncontrolled growth of cells. The cell division process is controlled at G1, M and G2 checkpoints. Each checkpoint verifies whether the cell is ready for the next phase, either to pause or not. But in cancer, the mutations does not obey the checkpoint rules, the cell division is proceeded without control and the premature death of the cell occurs.

Once the cancer cell is formed, due to clonal heterogeneity, it can form many sub clones. The cancer cells can communicate with healthy cells nearby. They can induce the normal cells for the formation of blood vessels to feed the tumor and to remove the blood waste products. These cancer cells interact with the immune system to suppress its function. Even a single cancer stem cell can seed the growth of a new tumor. Hence treating cancer has become a real task.

4. Bacteria divides by binary fission by just doubling the content. In every 20 minutes, one bacterial cell divides into two.

Superficially, the eukaryotic cell also divides into two by mitosis. But the number of processes in bacterial cell division and eukaryotic cell division are different. Eukaryotic cell division is very complex.

In prokaryotes, the process includes DNA replication, chromosome segregation, and finally the separation into two daughter cells. In eukaryotes, at first the nucleus divides to make sure that the offsprings have diploid number of chromosomes. This is followed by cytokinesis- division of cytoplasm along with the organelles.