The nucleic acids, which are macromolecules made up of nucleotide units, exist in two types: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) (RNA). DNA is the genetic substance that can be found in all living things, from single-celled bacteria to multicellular animals like you and me. Although some viruses employ RNA rather than DNA as their genetic material, they aren’t really living (since they cannot reproduce without help from a host). DNA is found in the nucleus, a specialized, membrane-bound vault in the cell, as well as in certain other types of organelles in eukaryotes, such as plants and animals (such as mitochondria and the chloroplasts of plants). Although DNA is contained in a specific cell area termed the nucleoid in prokaryotes, such as bacteria, it is not covered in a membrane envelope. DNA is often divided up into a number of very long, linear segments called chromosomes in eukaryotes, but chromosomes in prokaryotes, such as bacteria, are much smaller and often circular (ring-shaped). A chromosome can include tens of thousands of genes, each of which gives instructions on how to manufacture a specific product that the cell requires. Many genes code for protein products, which means they define the amino acid sequence utilized to make a certain protein. However, before this information can be utilized to make proteins, an RNA copy (transcript) of the gene must be created. Messenger RNA (mRNA) is a form of RNA that acts as a link between DNA and ribosomes, which are molecular engines that read mRNA sequences and use them to make proteins. The “core dogma” of molecular biology is the pathway from DNA to RNA to protein. Not all genes produce protein products, which is important to note. Some genes, for example, code for ribosomal RNAs (rRNAs), which are structural components of ribosomes, or transfer RNAs (tRNAs), which are cloverleaf-shaped RNA molecules that transport amino acids to the ribosome for protein synthesis. Other RNA molecules, such as small microRNAs (miRNAs), operate as gene regulators, and new non-protein-coding RNAs are being found all the time. DNA and RNA are polymers (sometimes very long polymers in the case of DNA) made up of monomers called nucleotides. A polynucleotide is formed when these monomers unite to form a chain. A nitrogen-containing ring structure termed a nitrogenous base, a five-carbon sugar, and at least one phosphate group make up each nucleotide. With the base connected to one of its carbons and the phosphate
