Chapter 16

Inheritance has a molecular component. This first was demonstrated by the research of Griffith on transformation in 1928. Avery, MacLeod and McCarty identified the transforming agent as DNA. Not only did this demonstrate that DNA is the molecule of inheritance, but it also indicated that the transmission of DNA could also result in the production of other categories of organic molecules.

Chargaff noted that in DNA the amount of A=T and C=G, and that the relative amounts of these pairs differed from species to species.

Watson and Crick proposed a model of the DNA molecule that, built on X-ray crystallography work by Wilkins and Franklin and Chargaff's work, suggested that DNA was a double helix. The helix contained two sugar phosphate backbones oriented in an antiparallel configuration. The nitrogen bases were oriented toward the inside of the helix and bases on one backbone were complementary to the ones on the opposite backbone. Hydrogen bonds formed between these complementary bases A with T and C with G. One complete turn of the helix occurred every 10 bases. The double helix has alternating major and minor grooves. (Fig. 16.5, 16.7, 16.8)

Replication refers to DNA being created from DNA. DNA replication is semiconservative. That means when two double stranded DNA molecules arise from one initial DNA molecule that each of the new molecules consists of one old strand and one new strand. Watson and Crick predicted this, and it was experimentally demonstrated by Meselson and Stahl. (Fig. 16.9, 16.10, 16.11)

When DNA is replicated the two strands must be unwound. This unwinding begins at sites referred to as points of origin. Prokaryotes have one origin and eukaryotes have hundreds to thousands. The enzyme that unwinds the strands is called helicase. When the strands separate there is a tendency for them to come back together. To avoid this the strands have to be stabilized. Single stranded binding proteins (SSBPs) attach to each strand conferring stability on them. The unwound region of DNA forms what is called a replication bubble. Each replication bubble has two replication forks. We generally discuss what is going on at each replication fork, but recognize that similar events are occurring at all forks. The enzyme that actually joins the nucleotides together to form the DNA polymer is called DNA polymerase. Each of the original DNA strands acts as a template for the synthesis of its complement. The original DNA strands are read in a 3' to 5' direction, meaning that the new DNA strands are synthesized in a 5' to 3' direction. (Fig. 16.12, 16.13)

Because of the antiparallel nature of the DNA molecule, the replication fork is near the 5' end of one strand and the 3' end of the other. As the fork continues to unzip, the blueprint strand having its 5' end near the replication fork can be read in a continuous fashion, but the other strand must be read in a discontinuous fashion. The former strand is called the leading strand the latter is called the lagging strand. DNA polymerase is unable to begin synthesizing a complementary strand to the blueprint unless it is extending the 3' end of a nucleic acid. Because of this, replication must be primed before it can begin. An enzyme, primase, first synthesizes an RNA sequence complementary to a region of DNA and this RNA is then extended by DNA polymerase. This effectively occurs once for the leading strand, but multiple times for the lagging strand. The discontinuous lagging strands are composed of short stretches of double stranded DNA that are referred to as Okazaki fragments. Afterwards the RNA is digested by a form of DNA polymerase and replaced by DNA. Then adjacent DNA stretches are joined together using enzymes called ligases. (Fig. 16.13, 16.14, 16.15, 16.16, 16.17)

When DNA is replicated it occurs with a very high fidelity. However, mistakes do occur. If an incorrect match is made between the blueprint DNA strand and the DNA strand being synthesized a mutation results. There are a number of mechanisms available to correct such errors. DNA polymerase has proofreading capabilities, and checks that the base incorporated are indeed complementary to the template. If they are not they are digested out and replaced by the correct nucleotide. Even after this occurs there are other repair systems (mismatch repair, excision repair) that work to reduce the mutation rate that occurs in cells.