(a) : In genome all the genes are contained in a single set of chromosomes. The instructions in our genome are present in the form of $DNA$. $DNA$ has a complicated structure in the form of a double helix. Single strands of $DNA$ are coiled up into structures called chromosomes. Within the chromosomes, segments of $DNA$ are “read” together to form genes. Thus, a gene is a segment of $DNA$ or chromosome situated at a specific locus (gene locus) which carries coded information associated with a specific function and can undergo crossing over as well as mutation. A nucleotide is the basic unit of $DNA$ made up of a pentose sugar, phosphoric acid and a nitrogenous base.