Hình 1,16. Bao bì của DNA vào nhiễm sắc thể có nhân điển hình. DNA 2 nm chuỗi xoắn kép được gói thành nucleosome như minh họa trong hình 1,15. Tại một số điểm histone H1 vào phức tạp, có thể tại những điểm vào / xuất cảnh DNA trên nucleosome | 28 DNA STRUCTURE AND FUNCTION 1 Double-stranded DNA Beads on a string 30 nm Protein scaffold 2 nmI Folded and coiled loops 10 nm Mitotic chromosome Cross-section of 30 nm fibre Loops of 30 nm fibre Figure . Packaging of DNA into the eukaryotic chromosome. The 2 nm DNA double helix is wrapped into nucleosomes as illustrated in Figure . At some point histone H1 enters the complex possibly at the DNA entry exit point on the nucleosome. The nucleosomes can form extended 10 nm fibres which are long arrays of ordered nucleosomes. The nucleosomes can further condense to form a 30 nm fibre. This may be a zig-zag array of nucleosomes. The 30 nm fibre is then wrapped onto a protein scaffold which can be additionally folded and coiled to form the mitotic chromosome that is observed under the electron microscope The Replication of DNA Watson and Crick finished their famous 1953 paper with the statement It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material . It was apparent to them from the arrangement and nature of the bases that each strand THE REPLICATION OF DNA 29 of the DNA double helix could serve as a template for the synthesis of a complementary strand Figure . They proposed that if the double helix were unwound each nucleotide on the parental strands would have an affinity for its complementary nucleotide Watson and Crick 1953b . Thus each of the parental DNA strands could act as a template for the synthesis of a new DNA strand that would result in the production of two identical double-stranded DNA duplexes. In this model each of the newly synthesized DNA molecules consists of one parental and one newly synthesized DNA strand hence the process is known as semi-conservative replication. Two other modes of replication that also rely on the parental strands serving as templates for new DNA synthesis were also considered. In conservative New Old Old New .
