The cell nucleus is the most important organelle, and it’s here that we find our DNA (deoxyribonucleic acid) packaged up tightly into structures called chromosomes. Chromosomes are long thread-like structures made of a DNA molecule and protein. During cell division, chromosomes coil up tightly into X shapes and are more easily visible under a microscope. Human cells have 23 pairs of chromosomes.
A DNA molecule is made up of a series of nucleotides arranged in 2 strands that resemble a ladder and twist to form a double helix.
Nucleotides are made up of a base, a sugar and a phosphate. The 4 bases – adenine (A), guanine (G), cytosine (C) and thymine (T) – pair with each other (A with T and G with C). It is the order or sequence of these base pairs that provides the information needed for the growth and development of our bodies. It can help to visualise the bases as letters in the alphabet that form words when they are in a certain order. These ‘words’ are calledgenes and act like a set of instructions for our cells.http://www.sciencelearn.org.nz/Contexts/Uniquely-Me/Science-Ideas-and-Concepts/DNA-chromosomes-and-gene-expression
Simply explained
http://learn.genetics.utah.edu/content/begin/tour/
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