BACKGROUND:

The blueprint for the structure and functioning of our bodies is contained in the genetic material found in the nucleus. The genetic material (chromatin) is composed of DNA (Deoxyribonucleic acid) and protein. During certain times in a cell's life the chromatin will condense and form x-shaped structures called chromosomes. Chromosomes are found in the nucleus of cells at some time in their life spans. Human beings have 46 chromosomes, arranged in 23 pairs. Heredity is encoded in DNA within the chromosomes. A gene is a very small cluster of chemical units which group up to form the DNA molecule. RNA (ribonucleic acid) is the messenger of DNA within the cell. Forms of RNA direct the cell to manufacture specific enzymes and other proteins. Modern biological research is developing a much more complicated picture than what is described above. Students in the fifth grade should learn the general outline and parts, but to really understand what is going on is not really completely known.

DNA functions by carrying the template or "map" of chemical compounds, amino acids, that are used to build proteins. DNA directs the production of proteins by providing the sequence of amino acids necessary to produce specific proteins. Amazingly, there are only 20 amino acids that are utilized to produce every protein in the human body. Of course, certain "modified" amino acids exist, but these still require the starting 20 amino acids. In order to gain an understanding of DNA, we must start at the heart of the matter, the cell.

This summary mainly is concerned with human cells but the basic theory holds for most organisms. Most human cells have a distinguishable central structure called the nucleus. The nucleus is the "storage area" for the cell's genetic material. The nucleus houses DNA and its corresponding helper molecule, RNA (ribonucleic acid). The nucleus is often referred to as being the "control center" of the cell and, the nucleus does in fact regulate cell activity. This regulation is brought about under the influence of the genetic information that is housed in the cell. This genetic information informs the cell of what activities it is to undertake and what it is to accomplish. The cell carries out these "orders" by performing specific biochemical reactions, often times under the influence of enzymes (proteins that are catalysts) that are produced in the cell. The nucleus is the depository for nucleic acids (DNA and RNA). It is in the nucleus where DNA replication occurs.

The sugar-phosphate backbone consists of deoxyribose sugar groups connected together by phosphate groups. The sugar groups are, in turn, connected to the four bases: adenine, guanine, cytosine, and thymine. Adenine and guanine are called purines while cytosine and thymine are called pyrimidines. The bases are often abbreviated as A, G, C, and T. A purine base can only bond to a pyrimidine base, and a pyrimidine only to a purine. Adenine will only bond with thymine and guanine will only bond with cytosine. The nucleotide base pairs are held together by hydrogen bonds. It follows then that the number of adenine bases will equal the number of thymine bases and the number of guanine bases will equal the number of cytosine bases. This presumption led the way for the formulation of Chargaff's rules which was the basis of DNA investigation prior to the discovery of the double helical nature of DNA.

DNA carries a template that determines amino acid sequences which are then used to produce proteins at the ribosomes. Since proteins cannot produce other proteins DNA serves as a "storage" center for the amino acid sequences of all the proteins produced by an organism. A particular amino acid sequence that codes for a specific protein is called a gene. The genes of an organism are stored in structures called chromosomes, which are the familiar x-shaped structures often seen in biology books. An organism's DNA is not always organized into chromosomes, rather these structures appear at particular times during the life cycle of a cell. A chromosome consists of DNA associated with a group of proteins known as the histone proteins. DNA is a long, linear, polymer (poly=many, mer=unit) that if stretched out could be up to several or even thousands of meters long. One chromosome consists of a single molecule of DNA.

Humans reproduce sexually through the successful union of a sperm and egg cell. The sperm and egg, referred to as gametes, contain one half the number of chromosomes found in other human cells. When a cell contains one half the number of chromosomes it is referred to as being haploid. Conversely, when a cell contains the full number of chromosomes it is referred to as being diploid. Human sperm and egg cells are haploid and contain 23 chromosomes. The diploid number for human cells is 46 chromosomes. When an egg and sperm unite each cell contributes 23 chromosomes and the resulting fertilized egg has 46 chromosomes. Through this mechanism genetic variability and heredity is expressed. Therefore a child will have received one half of his DNA from his mother and one half from his father. The traits that are then expressed in the child are a function of which DNA (ie, the father's or mother's) was expressed. The study of how traits are inherited and passed on through generations is referred to as genetics.

DNA has an analogous helper molecule called RNA (ribonucleic acid.) RNA's structure is similar to DNA's except in the following manners:

1. RNA contains the sugar ribose, whereas DNA contains the sugar deoxyribose. Deoxyribose has one less oxygen than ribose, hence the name deoxy-.
     
2. RNA contains the base uracil instead of thymine.
     
3. RNA is usually single stranded.

PROCEDURE:

1. Discuss with students DNA and RNA. Use the information above to help you discuss this important concept.
     
2. Color the backbone which hold the pyrimidines (thymine and cytosine) and purines (adenine and guanine). The small pentagon and circle.
     
3. Color each of the pyrimidines and purines. The purine base adenine always bonds with the pyrimidine base thymine, and guanine always bonds with cytosine.