5.8 DNA Replication · Part 2

organized to form a unit of eight molecules called histone octamere . The negatively charged DNA is wrapped around the positively charged histone octamere to form a structure called nucleosome . A typical nucleosome contains bp of DNA helix. The histone octameres are in close contact and DNA is coiled on the outside of nucleosome. Neighbouring nucleosomes are connected by linker DNA (H1) that is exposed to enzymes. The DNA makes two complete turns around the histone octameres and the two turns are sealed XII Std Zoology Chapter XII Std Zoology Chapter Molecular Genetics ' ' ' ' ' Daughter strands Parental strand Parent Parental strand ' ' ' A C TA T C T A T A T A T A T A T A T A T A T A T A T A A T A T A T A T A T C G C G C G C G C G C G C G C G C G C G C G G C G C G C Fig. . Semiconservative DNA replication complementary fragments. Finally two new molecules are formed which consist of both old and new fragments. Semi-conservative replication was proposed by Watson and Crick in . This mechanism of replication is based on the DNA model. They suggested that the two polynucleotide strands of DNA molecule unwind and start separating at one end. During this process, covalent hydrogen bonds are broken. The separated single strand then acts as template for the synthesis of a new strand. Subsequently, each daughter double helix carries one polynucleotide strand from the parent molecule that acts as a template and the other strand is newly synthesised and complementary to the parent strand (Fig. . ) .