Objectives Students will have opportunities to
1. Collect and relate information concerning the work of Mendel, Watson, Crick, Wilkins, McClintock and Rosalind Franklin to the rest of the class.
2. Describe the structure and nature of DNA.
Approximate time one week.
One of the greatest scientific wonders is the human body and how it reproduces. What element is responsible for this rapid growth? The secret of how life is reproduced lies in the genes and their chemicals, DNA and RNA. DNA or deoxyribonucleic acid is the code of heredity. For example, it controls the plan of a person’s appearance, intelligence or body structure. This hereditary information is passed on to the cells by chemical messengers called RNA or ribonucleic acid. DNA therefore directs their production and growth of every cell in the human body. Sometimes the message in the code tells some cells to switch off some genes and at another time to start some genes. In this way different cells are produced for varied purposes.
An English physicist, Francis Crick and an American biochemist, James Watson spent a great deal of time studying the DNA molecule, and in 1953, announced that they had worked out a model of how the Whole DNA molecule is constructed. They likened this DNA molecule to a spiral staircase, winding round and round— built like a double helix. The edges of the staircase are made up of bits of sugar deoxyribose, joined together by phosphate groups. The steps of the staircase are made up of pairs of the four organic bases. THese bases are paired up in very definite ways. Adenine on one strand of the helix is always paired with thymine on the other, and guanine is always paired with cytosine.
One may ask, Why can’t there be another combination, say for example an adenine pairing with cytosine or two cytosines join together? Would this combination permit proper functioning of the molecule? A little deviation from this structural sequence will result in malfunctioning of the molecule which wilL ultimately affect the structures as a whole. This lies in the fact that these bases are joined together by a hydrogen bond; one must have a hydrogen atom and the other must have a nitrogen or oxygen of a special kind. These atoms must be in the right places to form hydrogen bonds. Thus the wrong pair of bases could not hold the two chains of DNA together.
As mentioned earlier, DNA sends its orders to the cell by making RNA messengers. When a DNA molecule is ready to make RNA, part of it unzips. The hydrogen bonds that hold the two chains together are not as strong as other kinds of chemical bonds. Therefore these bonds can break and the chains can separate. Along one of the chains, an RNA base moves into place beside each DNA base that is uncovered. The RNA base guanine will line up next to a cytosine in the DNA, and a cytosine next to a guanine. An adenine will form a hydrogen bond with the thymine in DNA, and since RNA never contains thymine, a uracil will be joined into place next to an adenine in
DNA. (See diagram below).
After the RNA chain is made it breaks away, and the two DNA chains coil up together again into a double helix. This new RNA now has DNA’s message to be translated in the cell.
Genes are composed of some of the most common elements such as carbon, hydrogen, oxygen, nitrogen and phosphorous. They are made of deoxyribonucleic acid (DNA). Genes are capable of exerting their influence through the proteins which they cause to be produced in the cells. These proteins are manufactured on the ribosomes in the cytoplasm of each cell. Genes in the nucleus produce messenger-RNA which passes into the cytoplasm and directs the information of polypeptide chains of amino-acids at the site of the ribosomes.
It is in this DNA molecule, the code of life that stores the secret of all our differences and similarities among all forms of life.
1. Have students visit the library and use the various resources, say books, magazines, journals, periodicals, microfiLms or the computer to gather information from Watson and Crick discovery to modern genetics. Students will organize themselves into small groups of four and gather related facts concernINg the work of Mendel, Wilkins, McClintock and Rosalind Franklin. Each group will be given the opportunity to relate their work to the rest of the class.
2. Have students construct models of the double helix using colored construction paper and also utilize the DNA made-easy board to reinforce which bases pair up with other bases.
3. Teacher demonstration—use of colored paper clips, colored clothes pins, fruit loop cereal or gumdrops, could be used to show the linkage of DNA molecule.