5.8 DNA Replication

. DNA Replication Replication of DNA takes place during the S phase of cell cycle. During replication, each DNA molecule gives rise to two DNA strands, identical to each other as well as to the parent strand. Three hypotheses of DNA replication have been proposed. They are conservative replication, dispersive replication, and semi- conservative replication. In conservative replication, the original double helix serves as a template. The original molecule is preserved intact and an entirely new double stranded molecule is synthesized. In dispersive replication, the original molecule is broken into fragments and each fragment double helical DNA = total number of base pairs × distance between two consecutive base pairs). If the length of E. coli DNA is . mm, the number of base pairs in E. coli is × bp ( . × m/ . × - ). The length of the DNA double helix is far greater than the dimension of a typical mammalian nucleus (approximately - m). How is such a long DNA polymer packaged in a cell? Chromosomes are carriers of genes which are responsible for various characters from generation to generation. Du Praw ( ) proposed a single stranded model (unineme), as a long coiled molecule which is associated with histone proteins in eukaryotes. Plants and animals have more DNA than bacteria and must fold this DNA to fit into the cell nucleus. In prokaryotes such as E. coli though they do not have defined nucleus, the DNA is not scattered throughout the cell. DNA (being negatively charged) is held with some proteins (that have positive charges) in a region called the nucleoid. The DNA as a nucleoid is organized into large loops held by protein. DNA of prokaryotes is almost circular and lacks chromatin organization, hence termed genophore . In eukaryotes, this organization is much more complex. Chromatin is formed by a series of repeating units called nucleosomes . Kornberg proposed a model for the nucleosome, in which molecules of the four histone proteins H2A, H2B, H3 and H4 are organized to form a unit of eight molecules called histone octamere