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A chromosome is a long DNA molecule that carries many genes arranged along its length. This description fits a thread of DNA made up of a string of genes because the genes are lined up along the same DNA molecule and form a single unit of heredity. The genome, by contrast, is the entire set of genetic material in an organism; a nucleotide is one building block of DNA; an allele is a version of a gene. So the term that best matches a single continuous thread of DNA containing multiple genes is chromosome.