Our body cells are able to make copies of themselves so that we can continue to grow, and also in order to replace damaged tissues. The development of a cell occurs when existing cells divide to produce new cells during what is known as the "cell cycle". The stage of the cell cycle during which the cell actually divides is known as mitosis.


The stages of the cell cycle you should already be familiar with, but for ease of reference there will be shown here now:




  • In the cell that is not currently being divided, the DNA is all spread out in long strings.


  • Before the cell divides the number of subcellular structures such as mitochondria and the ribosomes needs to be increased, this means that the cell has to grow before the division can take place.


  • The DNA is then duplicated so that there will be one copy of each chromosome for each new cell. The DNA is copied and forms X shaped chromosomes, each arm of the chromosome is an exact duplicate of the other.


  • The chromosomes line-up at the centre of the cell and the cell fibres then start to pull them apart. The two arms of each chromosome go to opposite ends of the cell.


  • Membranes then form around each the new sets of chromosomes which become the nuclei of the two new cells, in other words the nucleus of the initial cell has divided.


  • Lastly the cytoplasm and cell membrane divide, this forms the two new "daughter" cells which will contain exactly the same DNA as each other, as well as the originating cell (the parent cell)



Q. A section of growing tissue is observed under a microscope, 240 cells are visible in a particular field of view. It can be seen that 64 of these cells are in the replication stage of the cell cycle and one complete cycle takes 36 hours with this particular type of tissue. How long do the cells spend in the replication stage of the cell cycle, give your answer in minutes.


A. We are told that 64/240 cells are currently in the replication stage, and the total cell cycle time is thirty-six hours which is 36×60=2160 minutes. Therefore the cells spend 64/240ths of 2160 minutes in the replication stage:





Q. In a tissue sample of 2000 cells, 74 observed to be in the last stage in mitosis. For this particular tissue type the cell cycle takes approximately 800 minutes. How long does this last stage in mitosis take for an average cell in this tissue, state your answer in hours.


A.We are given all of the necessary information, that 74/2000 cells are in the last stage of mitosis, during a cell cycle of approximately 800 minutes. We therefore calculate our answer in the same way as we did for the previous question: