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A Beginners' Guide to Molecular Biology

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Division into two identical cells


Mitosis is the name for the way that a cell duplicatesask Dr Chromo!itself so that each daughter cell receives an identical copy of its genetic material. At the end of mitosis, there will be two cells instead of one. They will be identical to each other.

The events that occur differ in prokaryotesask Dr Chromo! and in eukaryotesask Dr Chromo!.

Before we start describing the different phases of this cell division, we need to remember what a cell looks like in its normal state, which is known as the interphaseask Dr Chromo!. A normal eukaryotic cell has pairs of chromosomes (2n chromosomes). During this phase, DNA is being replicated. At the end of interphase, the amount of DNA will be twice the normal amount. For a transient moment the cell will have 4n chromosomes.

Cell division

The act of cell division is normally a single process, but it can be broken down into a number of "phases". It is convenient to give a name to each phase:

At each phase an identifiable process is taking place.


Prophase is the first stage of cell division. Here, the cell prepares itself for division. The nucleus swells, and chromosomesask Dr Chromo! become visible. Each chromosome has two chromatidsask Dr Chromo! as a result of duplication of the DNA which took place during interphase. The two chromatids are linked together at a centromereask Dr Chromo!. The centrosome (2 centriolesask Dr Chromo!) duplicates into 2 diplosomes, and each diplosome, or asterask Dr Chromo! moves toward opposite poles of the nucleusask Dr Chromo!.



Microtubulesask Dr Chromo! assemble, and form a network (the spindle fibres). The chromosomes move towards the equator of the cell, where they are visible. This is the phase in which morphological studies of chromosomes are carried out, often for clinical purposes.




The two sister chromatids separate.Each one migrates to opposite ends of the cell. So each daughter cell has an identical complement of chromosomes . The nuclear membrane has disappeared at this stage. The cell membrane expands as the cell itself elongates. The diameter of the cell decreases at the equator.



A new membrane forms around the new nuclei and two cells are quickly formed. The chromatid, now called a chromosome, uncoils, and the nucleolus becomes visible again.

Each cell contains a pair of chromosomes (2n chromosomes)


Some remarks!

What triggers mitosis?

External signals, hormonesask Dr Chromo!, internal factors, growth factors (proteins able to trigger mitosis): these proteins have a receptor on the cell membrane. When the protein is attached to the receptor, it triggers several events in the cell that end with the division of the cell.

What is Cancer?

A cell becomes cancerous when:

  • it loses the ability to stop dividing when encountering another cell (hence the formation of a tumour),
  • it becomes immortal.


    1. One cell with 2n chromosomes divides to create two cells with 2n chromosomes : the number of chromosomes per cell is conserved.
    2. The replication of DNA preceding the division of the cell has prepared chromosomes with two chromatids. The material has been multiplied by two before the division takes place: the quantity of material per cell is conserved.
    3. The replication of DNA results in two identical chromatids: the quality is conserved.
    4. In the chromosome cycle, from interphase to mitosis, the same material, goes from being in a diffuse form (during interphase) to being in a condensed form (as chromatids, during some phases of the mitosis). Only the condensed material can be separated.

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