In The Gno

DNA is the inherited genetic material that controls gene expression through protein synthesis.  This is a fancy way of saying that DNA is a chemical form of a genetic blueprint.  The genetic instructions contained in DNA are housed in genes.  Genes are smaller segments that, when strung together, comprise a DNA molecule.  Genes are actually unique linear sequences of nucleotide monomers named for the nitrogenous base they contain.

Wait – nucleotide monomers and nitrogenous bases?  This blog is supposed to demystify genetics by explaining concepts and ideas in everyday language, but sometimes that is difficult to do without a deeper understanding of chemistry.

Yes, there is much chemistry intertwined in biology; in fact, they go hand in hand.  Without going into too much unnecessary detail, a nucleotide monomer is a chemical unit that is made of a sugar with five carbon atoms (aptly named a “five-carbon sugar” or “pentose sugar”), a phosphate atom bonded to four oxygen atoms (called a phosphate group), and one of four possible nitrogen-containing bases called adenine (A), cytosine (C), thymine (T), or guanine (G).  They are called bases because these chemical compounds, collectively made mostly of carbon, nitrogen, and hydrogen atoms, have basic properties as opposed to acidic properties.

Acids and bases are a deeper lesson in chemistry; right now it is more important to focus on the fact that the four bases (A, C, T, or G) are categorized into two types:  Purines and pyrimidines.  Purines are chemically a double-ringed structure, whereas pyrimidines form a single ring.  Adenine and guanine are classified as purines, while cytosine and thymine are pyrimidines.

DNA double helix

One strand of DNA might look like a straight ladder cut down the middle, making the left side of the ladder and the right side of the ladder.  One strand complements the other in a way that is also related to chemical structure.  The component nucleotide bases (the rungs of the ladder) of a DNA strand (one half of the ladder) pair with each other by chemically bonding to the nucleotide bases of the second DNA strand (the other half of the ladder).  Together, the two DNA strands unite to form what looks like one single ladder.

Watch a video on the unique structure of DNA

For genes to properly function, the nucleotides must pair up correctly.  They do this by adhering to a strict set of base-pairing rules that were discovered by pioneers in the science of genetics, James Watson and Francis Crick.  Base-pairing rules dictate that a purine always pairs with a pyrimidine.  In other words, adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G).  When nucleotide bases pair correctly, that is, when A pairs with T, or C pairs with G, the rungs on two halves of the ladder (single DNA strands) come together to form a single ladder.  The ladder appears twisted due to complex chemical bonding, such as weak hydrogen bonds or ionic and covalent bonding, between the various elements in the DNA molecule itself.  The “twisted ladder” shape of DNA is what gives the molecule its unique name – the double helix.