The most important part of the cell is the nucleus, for this is where all the instructions for the cell’s activities are kept. These instructions are in the chromosomes. This nuclear material is kept safe, away from the rest of the cell, by the nuclear membrane.

When cells are not dividing, you cannot see the chromosomes, and the nucleus looks like this photograph of a cell seen under the microscope where you can see the nuclear membrane surrounding a salmon pink area. At this stage, the chromosomes are not visible – just this mass of nuclear material called chromatin. This is where DNA (deoxyribose nucleic acid) is found.

Scientists who study the nucleus and how cells divide are called cytogeneticists. Just before cells start to divide, the chromosomes become visible. Cytogeneticists stain the dividing nucleus and look at them under high-powered microscopes to examine these visible chromosomes. They line them up and sort these chromosomes into different types. All the chromosomes in a human cell is called a karyotpe.

Nature of Science

The development of the cell theory (that cells divide into new cells) shows that many people can contribute to the development of a theory to explain science.

Chromosomes can be identified by size and shape. There are 22 pairs of chromosomes that carry most of the genetic messages in the human cell.

The 23 rd pair of chromosomes are called the sex chromosomes. Because of their shape they are called X chromosomes or Y chromosomes. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). Here is an arrangement of chromosomes from a human female. Can you see the two X chromosomes? How many chromosomes can you count? There should be 46 or 23 pairs.

When chromosomes are visible, they look like this. Each chromosome is made up of two identical strands called chromatids (1). Where they join is called the centromere (2).

Along these chromatids are arranged the genetic messages called genes. The genes are made up of DNA. Genes carry the information that is passed on when cells divide.


Cells grow and divide by a process called mitosis. During mitosis, the DNA that makes up the chromosomes copies itself and unzips.

At the end of this process, there are two new cells that have the same number and type of chromosomes as the original cell. This whole process is called the cell cycle.

However, sometimes things go wrong with the replication process and mutations occur.

Useful links

See what cells and chromosomes look like when they are dividing.

The cell cycle.

    Published 29 July 2008