Final answer:
The linking number (Lk) of a closed-circular, double-stranded DNA molecule can be changed via epigenetic modifications, including the addition or removal of histone proteins and nucleosomes, as well as by mutations in the DNA sequence introduced by intercalating agents, all of which affect gene accessibility and DNA topology.
Step-by-step explanation:
Changes in Linking Number (Lk) and DNA Structure
The linking number (Lk) of a closed-circular, double-stranded DNA molecule can be changed by various mechanisms. Specifically, Lk can be affected by epigenetic modifications such as the addition of reversible changes to histone proteins and DNA, the removal of nucleosomes from the DNA, or the addition of more nucleosomes to the DNA.
Moreover, mutations in the DNA sequence, especially those introduced by intercalating agents, can also alter the Lk. Such changes impact the DNA's overall topology and can affect the compactness and accessibility of genetic material.
DNA molecules consist of two strands that form a double helix, with each strand having a deoxyribose and phosphate backbone and nitrogen bases that form hydrogen bonds with complementary bases on the opposite strand. During processes such as replication and repair, enzymes like DNA polymerase and DNA ligase play crucial roles in synthesizing new strands and sealing together the sugar-phosphate backbones, respectively.
Mutations or changes brought about by exposure to factors like radiation or intercalating agents can result in errors when DNA polymerase synthesizes a new DNA strand. Such changes have the potential to alter the DNA's structure, which can directly or indirectly influence the Lk.