DNA (deoxyribonucleic acid) is a long molecule that is contained within almost all of our cells in a compartment called the nucleus. The information in DNA is stored as a code made up of four chemical bases which are adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of approximately 3 billion bases, and more than 99 percent of these bases are the same in all people. It is the order of these bases that determines the available information for building and maintaining an organism.
DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule and together, this is called a nucleotide. Nucleotides are arranged in two long strands that form a double helix spiral.
The double helix is similar in form to a ladder; the base pairs form the rungs and the sugar and phosphate molecules form the ladders vertical sidepieces. A piece of DNA consists of millions of base pairs long, in conjunction with some proteins is known as a chromosome. One of the wonders of DNA is that it can replicate itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell.
What is a Chromosome?
Chromosomes are long DNA strands packaged into thread-like structures. They are located in the nucleus (core) of each cell. Each chromosome is made up of DNA tightly coiled around proteins called histones.
Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or "arms." The short arm of the chromosome is labeled the "p arm." The long arm of the chromosome is labeled the "q arm." The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.
What are Genes?
A gene is a very small piece of a DNA strand. The sequence of DNA bases in a gene contains instructions for making molecules called proteins. Proteins are the building blocks of all life forms and each protein serves a particular function in the body.
Human genes vary in size from a few hundred DNA bases to more than 2 million bases. It is estimated that humans have between 20,000 and 25,000 genes.
Every person has two copies of each gene, one inherited from each parent. Most genes are the same in all people, but a small number of genes are slightly different between people. These small differences contribute to each person's unique physical features and tendencies. That is, your genes contain instructions for building everything about you: your eye, hair, and skin color, your tendency to gain weight, your athletic tendencies, your likelihood to develop addictions or diseases, and your other unique characteristics.
Humans inherit 23 chromosomes from each of their parents for a total of 46 chromosomes. Forty-four of these chromosomes are identical in men and women and these chromosomes are called autosomes. The remaining two are called sex chromosomes, and are designated X and Y.
Women inherit two X chromosomes and men inherit one X chromosome from their mother and one Y chromosome from their father.
It is the combination of the genes we inherit that define certain traits that resemble our mother and others that resemble our father.
As mentioned earlier, each gene is made up of a series of bases, and those bases provide instructions for making a single protein. Any change in the sequence of bases and therefore in the instructions is known as a mutation. Some mutations have little or no effect on the protein, while others cause the protein not to function at all.
A gene mutation is a permanent change in the DNA sequence that makes up a gene. Mutations range in size from a single DNA base to a large segment of a chromosome. Gene mutations can be inherited from a parent or acquired during a person's lifetime. Mutations that are passed from parent to child are hereditary mutations, and they are present throughout a person's life in virtually every cell in the body.
Mutations that occur only in an egg or sperm cell, or just after fertilization, are called new (de novo) mutations and may explain genetic disorders in which an affected child has a mutation in every cell, but has no family history of the disorder.
Acquired mutations occur in the DNA of individual cells at some time during a person's life. These changes can be caused by environmental factors such as ultraviolet radiation from the sun. Acquired mutations in cells that are not eggs or sperm cannot be passed on to the next generation.
Some genetic changes are very rare while others are common. Genetic changes that occur in more than 1 percent of the population are called polymorphisms. They are common enough to be considered a normal variation in the DNA. Polymorphisms are responsible for many normal differences between people such as eye color, hair color, and blood type.