DNA

DNA is the hereditary material in nearly all living organisms. It is a double-stranded helix of nucleotides (with the bases A, T, C, and G) that stores genetic instructions for the development and functioning of an organism.

• DNA is mostly located in the cell nucleus (nuclear DNA), with a small amount in mitochondria (mtDNA). Each human cell's nucleus contains <u>46 DNA molecules (chromosomes)</u>.
• The two strands of DNA are complementary: A always pairs with T, and C pairs with G. This allows DNA to be copied (semi-conservative replication) reliably when cells divide.
• Nearly all living organisms use DNA as genetic material, but a few viruses have RNA genomes instead. DNA's sugar is deoxyribose (no oxygen on the 2' carbon), distinguishing it from RNA's ribose.

• If a question mentions a "double helix" or nucleotides with <u>thymine (T)</u> bases, it's referring to DNA (as opposed to RNA, which is single-stranded and has uracil).
• Questions about the storage of genetic information (in chromosomes, genes, etc.) are targeting DNA. For example, "genetic blueprint in the nucleus" = DNA.
• Be prepared for comparisons between DNA and RNA: e.g., identifying that DNA stays in the nucleus (except mitochondrial DNA) while RNA can travel to the cytoplasm.