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DNA, RNA, Genes, Chromosomes - Basic Explanation

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Questions and Answers about Gene Therapy

Genes are the biological units of heredity. They determine obvious traits, such as hair and eye color, as well as more subtle characteristics, such as the oxygen-carrying ability of the blood. Complex traits, such as IQ and physical strength, may be shaped by the interaction of a number of different genes along with environmental influences.

It is estimated that humans have 100,000 genes. A flaw in virtually any one of them can result in disease. Each gene acts as a blueprint for making a specific enzyme or other protein. However, only certain genes in a cell are active at any given moment and, as cells mature, many of their genes become permanently inactive. It is the pattern of active and inactive genes in a cell and its resulting protein composition that determines what kind of cell it is and what it can and cannot do.

9. What is DNA, and how is it related to genes?

In chemical terms, genes are composed of segments of deoxyribonucleic acid, or DNA. DNA is a very long molecule, composed of individual units called nucleotides. Each nucleotide contains phosphate, the sugar deoxyribose, and one of four nucleic acid bases: adenine, thymine, guanine, or cytosine. It is these bases that carry the information content, or "code," of the DNA molecule.

There are about 3 billion pairs of nucleotides in the DNA of a typical human cell. Each individual's genetic material has a unique nucleotide sequence (except in the case of identical twins), although a large percentage of the genome (total set of genes) of every human is identical to that of every other.

DNA is organized in two chains that form a double helix. Wherever adenine appears on one chain, it is matched (and physically linked) with thymine on the other chain. Similarly, guanine is matched with cytosine. This consistent pairing of complementary bases allows DNA to duplicate itself accurately when cells divide.

10. What is RNA?

Ribonucleic acid, or RNA, is a molecule that is chemically similar to DNA and carries the same code, using different bases. When DNA is biochemically "read," or transcribed, the transcription product is RNA. This RNA is read in turn, and converted into a corresponding protein.

In certain viruses, RNA, rather than DNA, serves as the genetic material.

11. How are genes related to chromosomes and cells?

The genes are arranged on chromosomes -- rod-like structures composed of DNA and protein. In humans, each cell -- the basic unit of living organisms -- contains 46 chromosomes (23 pairs), located within a central structure known as the nucleus.

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