DNA Replication and Chromosome Reproduction

Abstract

In each DNA synthetic (S) phase, the two strands of the DNA double helix separate by unwinding. Each strand serves as a template for synthesis of a completely new complementary strand from the deoxyribonucleotides dA, dG, dC, and dT, hereafter called A, G, C, and T. Since the new DNA double helix consists of one conserved strand and one newly synthesized strand, replication is called semi-conservative. This was first demonstrated at the chromosomal level by autoradiography, growing cells in the presence of [³H]thymidine during one cell cycle and in the absence of this radioactive DNA precursor during the next cycle. If replication is semi-conservative and each chromatid contains a single DNA molecule, label will be incorporated into the newly replicated strand of DNA in each chromatid at the first cycle but into neither new strand of DNA at the second cycle. The result will be a radiolabeled strand in only one of the two sister chromatids. This was confirmed autoradiographically by placing a photographic emulsion on metaphase chromosome preparations. Electrons produced by radioactive decay of the tritium (³H) produced silver grains in the emulsion, mostly within 1 micrometer (µm) of the source, and these were concentrated over one of the two chromatids (Fig. 3.1). Occasionally, however, a region of the second chromatid would be labeled while the corresponding region of the other chromatid lacked label. This reciprocal labeling pattern is the result of breakage and rejoining of sister chromatids, resulting in an exchange between them (Taylor, 1963).