DNA structure

Definitions & Key takeaways

The basic structure of DNA is a double helix, which consists of two long strands of DNA wrapped around each other. Each strand consists of many nucleotides, each consisting of a sugar and phosphate group, which form the sugar-phosphate backbone of DNA, and a nitrogenous base, which determines the identity of each nucleotide. The order of the bases (A, T, C, and G) on each strand determines the DNA sequence. DNA strands are held together by hydrogen bonds between the bases.

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Buried deep within the nucleus, lies our genetic information, called DNA - which stands for deoxyribonucleic acid. DNA is made up of two strands that are coiled around one another in a double helix.
Each of the two strands that make up DNA is a polynucleotide chain - so it’s a string of nucleotides one after another. Nucleotides are organic molecules that are made up of a 5-carbon sugar, a phosphate group and a nitrogenous base - also called a nucleobase - or, simply, a “base”.
For DNA, the 5-carbon sugar is deoxyribose. Deoxyribose looks like a pentagon, and the tips of the pentagon are 4 carbons and an oxygen molecule.
The 5th carbon is outside the ring, and it binds to the phosphate group. The sugar and phosphate elements are the same for the 4 nucleotides that make up DNA - the difference comes from the nucleobase, which is attached to the first carbon of the sugar.
There are four nucleobases that make up and give DNA nucleotides their name - adenine, or A, thymine, or T, cytosine, or C and guanine, or G.
Structurally, these bases can be either purines or pyrimidines - the purines, guanine and adenine, are made up of 2 heterocyclic rings.
The pyrimidines, cytosine and thymine, are made up of a single ring. You can remember this with “CUT PYe (pie)” - because cytosine and thymine along with uracil, which is a nucleotide found in RNA, are all Pyrimidines.
The nucleotides bind to one another using their sugar and phosphate groups. The phosphate group on the 5th carbon of the sugar of one nucleotide - also called the 5’ carbon - forms a covalent bond with the 3rd carbon on the sugar of the next nucleotide - also called the 3’ carbon.
This gives each DNA strand a sugar-phosphate backbone, as well as a “direction” - one of the strands runs from the 5’ end towards the 3’ end, while the other one runs from 3’ to 5’.
This makes DNA an “antiparallel” molecule - it’s a bit like two snakes coiled up together but facing different directions.
However, to form the double helix, the the nucleotides use their bases - A, T, C, G to form hydrogen bonds with bases on the opposing strand.
Bases form bonds according to the rule of “complementary base pairing” - which states that in DNA, A always pairs with T, by means of two hydrogen bonds, while C always pairs with G, through three hydrogen bonds.
The hydrogen bonds are much weaker than the covalent bonds that hold the strands together - so they can be easily broken and reformed when DNA is being transcribed into RNA or being replicated during cell division.
Now, DNA is actually a very organized molecule, because the two strands coil around each other once every 10 base pairs.
This twisting and turning makes the DNA molecule develop major and minor grooves, which are larger or smaller spaces between the strands where proteins can bind to DNA in order to regulate its functions.
DNA is also a surprisingly long molecule - over 2 meters long when fully stretched. To make 46 of these DNA molecules - meaning one molecule for each chromosome - fit into a tiny nucleus, our cells rely on a few packaging tricks.
The first step is to take the double stranded molecule and wrap it around special proteins called histones. Histones come in groups of 8, arranged in 4 stacks of 2, and they are positively charged.
Meanwhile, the phosphate groups on the DNA strands are negatively charged. These opposite charges attract, so the double helix wraps twice around each group of 8 histones, forming a nucleosome.
Nucleosomes look a bit like beads on a string, and they can snuggle closer together and be further packaged as chromatin fibers.
Chromatin comes in two flavors - euchromatin and heterochromatin. With euchromatin, nucleosomes are more loosely packed, because it contains the genes - or regions of DNA - that the cell frequently uses to make proteins that help the cell perform its day to day job.
With heterochromatin, on the other hand, nucleosomes are more densely packed, because it contains genes that the cell rarely or never uses.
Throughout most of the cell’s life, chromatin fibers float in a loose state inside the nucleus, like DNA rich-noodles. However, when the cell is preparing for cellular division, the DNA noodles become more organized so that DNA can be replicated, and each of the two resulting daughter cells can get an identical copy of the genetic material.
Before replication, each of the chromatin noodles represents a single chromosome, which is made up of a single chromatid, that contains a single DNA molecule - so 46 chromosomes, 46 chromatids, 46 DNA molecules.
During replication, each chromatid is copied and pasted, so the amount of DNA doubles up. The two resulting chromatids are identical to each other and to the original genetic template, and they join together in the center in a region called the centromere - but they still make up a single chromosome.
So following replication, even though there are still 46 chromosomes inside the nucleus, they are made up of two chromatids each - so 92 chromatids, which correspond to 92 DNA molecules.
After the cell divides, the sister chromatids separate and each resulting daughter cell gets one chromatid made up of a single DNA molecule - so back to 46 chromosomes, 46 chromatids, 46 molecules of DNA, which once again look like chromatin noodles inside the nucleus.
Alright, as a quick recap: DNA is made up of two strands that coil one around the other to form a double helix. You CUT Py (pie) so the pyrimidines are cytosine - C, uracil - U which is relevant only for RNA, and thymine - T, and the purines are adenine - A- and guanine - G.
Nucleotides have sugar and phosphate groups which form the sugar-phosphate backbone of DNA. Nucleotides on opposite strands form hydrogen bonds to keep the two strands together: A binds with T, and C binds with G.
DNA wraps around histones to form nucleosomes. Then, the nucleosomes condense to form chromatin fibers.
Chromatin condenses to form 46 chromosomes.