I. THE GENETIC MATERIAL

Several classic experiments were done to determine the nature of the genetic material: click here for required supplemental reading

1. F. Griffith (1928) studied two strains of the bacterium Streptococcus pneumoniae, rough (R) and smooth (S) (see handout). Mice injected with the smooth strain died while the rough strain was avirulent. However, heat-killed smooth bacteria when mixed with live rough bacteria were virulent. Further, the dead mice were found to contain live smooth bacteria indicating that genetic transformation had occurred. Griffith did not identify the agent of this "transforming activity."
2. B.Oswald Avery (1944) identified the transforming agent as DNA by partially purifying it. He showed that the transforming material was not sensitive to proteases (enzymes that destroy protein) suggesting that protein was not responsible for the transformation. However, at the time, DNA was viewed as "chemically boring" and was not seriously being considered as the genetic material.
3. Martha Chase and Alfred Hershey (1952) provided further evidence that the genetic material is nucleic acid. They infected Escherichia coli bacteria with labeled T-even phages. Nucleic acids were labeled with 32P while proteins were labeled with 35S. After injection of the genetic material, a blender was used to remove attached viruses. It was found that only the 32P-labeled material entered the bacteria, although complete virions were the ultimate outcome of the infection, thus suggesting that the genetic material was nucleic acid.

II. NUCLEIC ACIDS (DNA and RNA)

Nucleotide components:

1. Sugar - DNA contains the sugar deoxyribose while RNA contains the sugar ribose
2. Base - the bases are flat, aromatic and nitrogenous. They can absorb UV light and are capable of hydrogen bonding
3. Phosphate - up to three phosphate groups are linked as an ester to the sugar. The phosphates link nucleotides in the structure of nucleic acids. They confer a negative charge.
4. N-glycosidic linkage - refers to the type of bond between the sugar and base

Oligonucleotides

1. Oligonucleotide growth occurs in the 5' to 3' direction via an attack by 3' hydroxyl groups upon the 5' alpha phosphate of nucleotide triphosphates (Figure 5.3)
2. The resulting linkage is a phosphodiester

The structure of DNA was solved by the efforts of several investigators:

1. Rosalind Franklin (1951) studied DNA crystals by X-ray diffraction and concluded that: "...the results suggest a helical structure (which must be very closely packed) containing probably two, three or four co-axial nucleic acid chains per helical unit and having the phosphate groups near the outside."
2. Erwin Chargaff (1950) looked at the biochemical composition of DNA and noted that the quantity of A = T and C = G (Chargaff's Rule).
3. J. Watson and F. Crick (1953) built a model consistent with the available data (Figure 5.5). Their DNA model was that of a double helix consisting of two antiparallel strands of nucleic acid held together by hydrogen bonding between bases (Figure 2.12). Adenine pairs with thymine via two hydrogen bonds while three hydrogen bonds occur between cytosine and guanine. Ten base pairs exist per helical turn spanning 34 Angstroms. Base pairs, of equal width, occur in the center of the helix while the phosphates occur on the outside surface of the structure. The backbone of the helix is punctuated by major and minor grooves. The major grooves (and occasionally the minor grooves) may serve as sites for interaction with proteins (Figure 5.4). In their classic paper in Nature, Watson and Crick state that:
• "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."
4. i.e. since the information in the two strands of the double helix is redundant, a duplex could be replicated by using each strand as a template for building two new DNA molecules.

III. GENETIC ELEMENTS

are structures that contain genetic information

1. Chromosomes carry the information required for life under all conditions.
2. Four kinds of non-chromosomal elements are: mitochondria and chloroplast DNA, plasmids, viruses and transposable elements.
1. Mitochondria and chloroplasts are organelles believed to have arisen by endosymbiosis. They contain DNA, but cannot exist independently.
2. Plasmids are usually circular and composed of double-stranded DNA. They have their own orgin of replication (ori) and do not exist extracellularly. They may confer a selective advantage (e.g. antibiotic resistance).
3. Viruses are non-cellular genetic elements that enlist specific cells for their own replication. They consist of protein and nucleic acid (double- or single-stranded DNA or RNA).
4. Transposable elements were first identified in maize by Barbara McClintock (1945). Although they replicate as part of another genetic element (e.g. a chromosome or plasmid), they are capable of moving from site to site. There are two types - insertion sequences and transposons. Insertion sequences contain only the genetic information conferring mobility and no other genes. Transposons contain additional genes. Transposable elements provide evidence that genetic material is not stable, but instead fluid in nature.