A. Relate the structure of DNA and RNA to chromosomes and genes.
B. Make models of the structure of DNA and RNA.
Approximate time: one week.
Before proceeding further with any discussion of how development is affected by the genes on chromosomes, it might be appropriate to pause and examine the structure of the chromosome.
Chromosomes are made up of a substance called Deoxyribonucleic acid (DNA). The function of DNA is to encode genetic information that will be transcribed to Ribonucleic acid (RNA) and then on to proteins from amino acids. This DNA is a large molecule of sugar, phosphates, 2 purine bases, adenine (A) ana guanine (G), and 2 pyrimidine bases, thymine (T) and cytosine (C). In RNA uracil (U) replaces thymine and the sugar is ribose instead of deoxyribose. The structure of DNA as proposed by Watson and Crick in 1953 suggests a ladderlike or double helix configuration. Figure 3 details a procedure for making models of DNA and RNA that can be used most effectively in the classroom. These instructions by this author were published in the
Connecticut Journal of Science Teaching,
March 1974. The base A always pairs with T and G with C. The bases are attached to the sugar molecule like the rungs of a ladder. The sugar is attached alternately with the phosphate on the sides of the “ladder”. The structures of DNA and RNA can be checked in any biology textbook. Three bases constitute a codon, a unit of genetic code which determines what amino acid will be added to a nascent protein chain. For example the codon GUG (and its complementary CAC) is for valine which can be part of a protein molecule. The genes are therefore composed of codons. If the genetic material is abnormal, incorrect coding of proteins will occur; e.g. sickle cell anemia which will be discussed later.