Cell division
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.
Structure of an animal cell
Outline drawing of an animal cell showing the nucleus, cell membrane and some other organelles.
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.
Human immune B cell
The nucleus is very clear in this cell. This cell is used in experimental work because it has been changed so that it can divide indefinitely.
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.
Karyogram from a human female lymphocyte
Karyogram showing 23 pairs of chromosomes from a human female.
Source: Bolzer et al., (2005) Three-Dimensional Maps of All Chromosomes in Human Male Fibroblast Nuclei and Prometaphase Rosettes. PLoS Biol 3(5): e157 DOI: 10.1371/journal.pbio.0030157, Figure 7a
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.
Chromosome
Chromosome with two chromatids linked with a centromere.
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.
Mitosis
Cells grow and divide by a process called mitosis. During mitosis, the DNA that makes up the chromosomes copies itself and unzips.
DNA unzipping and copying itself
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: www.cellsalive.com/mitosis.htm
The cell cycle: www.cellsalive.com/cell_cycle.htm
