Under-stranding DNA

By Hannah Jeoung, Genetic Biology

DNA.

Students hear this term used throughout their lives - ever since they step foot into their first science class - but do they really know what it means?

One complicated way to describe it would be as "the molecular basis of inheritance". A simpler way would be to call it the molecule that makes up our genes, a special biological code that makes us who we are. Think of it as comparable to computer code, where a series of instructions creates a program and result that is essential to whatever task the programmer seeks to complete. Only in this case, the code is made up of various sequences of nucleotides.

The stereotypical image of a DNA strand consists of double helices in antiparallel arrangement. Antiparallel means that subunits run in opposite directions. Imagine getting a flexible ladder and twisting it at even intervals. The end result would be an accurate depiction of a DNA strand, where the ladder rungs would be paired up nucleotide bases. Its essential structure was discovered by scientists Watson and Crick once they examined X-ray diffraction images of DNA produced by the brilliant Rosalind Franklin.

The nitrogenous base can also be guanine, thymine, and cytosine.

Nucleotides are the essentially the monomers, or building blocks, of DNA. They have four different nitrogenous bases - Adenine, Cytosine, Guanine, and Thymine - that are paired to a phosphate group on one end and a 5 carbon sugar called Deoxyribose on the other end. The end with the phosphate group is referred to as the 5' end (pronounced "5 prime end") while the latter is referred to as the 3' end (pronounced "3 prime end").

Therefore, DNA is called Deoxyribonucleic Acid, as it contains Deoxyribose, is in the nucleus of the cell, and behaves as an acid because the phosphate group is acidic.

The rules for nucleotide pairing are simple - bases A and T (remembered as the "straights", since the letters have only straight lines) hydrogen bond together, while bases C and G (remembered as the curves, since the letters contain curved lines) bond together. Adenine and Guanine are called Purines, or nitrogenous bases with two organic rings. An easy way to memorize this is to associate "nine", as in Guanine and Adenine, with "pure", as in purine. On the other hand, Thymine and Cytosine are Pyrimidines, or nitrogenous bases with one organic ring. The inherent combination of Purines with Pyrimidines ensures that the rungs of the "DNA ladder" are always the same length.

Uracil is the pyrimidine replacement for Thymine in RNA.

There are other consistencies in the nature of DNA as well. In Chargaff's Rules, findings made by biochemist Erwin Chargaff in his search for what genetic material was made of, he discovered that 1) DNA base composition differs between species and 2) the percentages of A and T bases are roughly equal, as well as those of G and C bases. Rule two makes sense when considering that every A base must be paired with a T base, and likewise for G and C.

So...where exactly is this super cool molecule? It actually differs between species. For prokaryotes, or one-celled organisms, they float in cytoplasmic soup. Some may describe it as inedible spaghetti.

A chromosome "unwinded".

For eukaryotes (multi-celled organisms) they reside in the nucleus in tightly wound spools called chromosomes. Certain proteins, histones, have a positive charge that attracts negative DNA, allowing them to wrap into an "X" shape. Additionally, DNA can be in clumps called heterochromatin, whose dense format makes it hard for the sequences to be coded. Conversely, when it is being used, the clumps change into a more dispersed form called euchromatin.

The process in which genes are coded into proteins will be further explored in upcoming posts.

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Urry, Lisa, et al. Campbell Biology, 11th Ed. New York, NY, Pearson Publishing Company, 2017.

Dengler, Roni. “Scientists Have Created Four New Letters of Artificial DNA - D-Brief.” Discover Magazine, Discover Magazine, 21 Feb. 2019, blogs.discovermagazine.com/d-brief/2019/02/21/scientists-doubled-the-genetic-alphabet/#.XPhmMbzYrnE.

Photos are linked to sources.

Learn more at: https://www.youtube.com/watch?v=8kK2zwjRV0M