What is DNA, and how does it work? You don't need a degree in genetics to understand. Here, we'll give an overview of DNA and answer questions like these: What does DNA stand for? What is it made of, and what is its function in living things? How do we study the molecular structure of DNA? And why do they talk about it so much on CSI?
What Is DNA?
DNA is the molecular basis of heredity, the inherited traits that pass between generations in a person's family tree. Embodied in the sequence of base pairs, DNA carries information between generations of living things. Present in nearly every living cell, DNA molecules are among the largest molecules known to science.
DNA is a linear biopolymer, a compound biomolecule in the form of a double helix. Polymers are made of monomers, and in DNA, these monomers are called nucleotides or nucleobases. The enzymes that handle DNA molecules are often called "polymerases," but the -ase suffix denotes an enzyme.
When not undergoing replication, DNA molecules float freely within the cytoplasm of a cell. This makes them available to the accessory molecules that organize and repair DNA. However, when cells are replicating, DNA forms organized structures called chromosomes. Members of the same species usually have DNA similar enough to have the same chromosomes and autosomes (chromosomes that don't code for sex-linked traits).
In its structure, DNA contains a pattern for producing amino acids, proteins, lipids, sugars, and every other biomolecule an organism makes. The parts of a DNA molecule that have these instructions are called genes. An individual's genome is the sum total of their genes.
What Is DNA Made Of?
DNA and RNA strands vary in length from less than two dozen base pairs to more than 200 million. Nucleotides are the combination of a five-carbon sugar, ribose, along with a phosphate group and one of five possible nucleobases: adenine, guanine, cytosine, thymine, and uracil. (Where DNA uses thymine, RNA uses uracil.)
Two nucleotides together form a base pair; the sugars and phosphates form a "backbone," along which the base pairs are strung. Fully assembled, the sequence of base pairs is a compound molecule called a nucleic acid.
Living organisms are constantly building new cells or repairing old ones. To do this, they have to make copies of their genetic material, which cells unzip, copy off, and then re-zip using functional proteins called enzymes. But enzymes often run on ATP, an energy currency that can be scarce—so instead of covalent bonds, which are more permanent because they require more energy to sunder, DNA strands are held together by delicate hydrogen bonds between the bases in a base pair.
The "double helix" of DNA refers to the physical shape of the molecule, an antiparallel coil. Imagine a zipper, wound up in a coil like an old-style telephone cord. In this analogy nucleotides are the teeth of the zipper; the sugar-phosphate backbone is the fabric to which the teeth are attached.
Because of the shape of the nucleobases—and the enzymes that handle them—there are strict rules about which nucleobase can pair with which. Adenine and guanine are double-ring molecules called purines, but cytosine, thymine, and uracil are all pyrimidines: single rings. To maintain its orderly structure, single rings on one strand can only bond with double rings on the other. This keeps the DNA molecule in the form of a ribbon of consistent width.
What Does DNA Stand For?
DNA stands for deoxyribonucleic acid. Nucleic acids are named for where they're found—inside the nuclei of cells—and for the acidic phosphate groups discovered in solution wherever DNA is present. DNA and RNA use the same five-carbon sugar, ribose, in their structure.
The deoxy- means that on one side of the base pair, there's a place where another base pair can attach itself by way of a complementary functional group, like Lego bricks, or a lock and key.
Who Discovered DNA? How and When Was It Discovered?
By the late 1800s, surgeons and medical researchers looking through their microscopes had identified something inside the nuclei of cells from bandages and tissue samples, dubbing it "nuclein." Biochemists soon discovered that nuclein was composed of a mixture of sugars, phosphate groups, and a wholly different type of biomolecule: nucleotides.
By the time World War II broke out, scientists were using X-ray diffraction, which studies the elaborate patterns in which different molecules scatter light, to study the structure of DNA.
In 1955, Frederick Sanger reported that proteins were created according to a pattern. Francis Crick attended Sanger's lectures, and in 1958, Crick published an argument that the sequence of nucleotides in a molecule of DNA was an essential template for the construction of amino acids and functional proteins. Today, scientists enjoy a huge public database of fully sequenced genomes from all branches of the tree of life, thanks to decades of work by researchers like William Astbury, Rosalind Franklin, Raymond Gosling, James Watson, and Francis Crick.
How Is DNA Used to Solve Crimes?
Almost all living cells contain DNA, and every living thing has its own unique genome. Because DNA is so specific, no two individuals' DNA will be a perfect match—even identical twins have slightly different sequences, with variations called single-nucleotide polymorphisms, or SNPs. Everywhere we go, we leave behind tiny traces of genetic material in stray hairs and on the rims of coffee cups.
It's possible to amplify a tiny amount of genetic material into a much larger sample by providing raw nucleobases and a template DNA sample to DNA replication enzymes from bacteria. Because enzymes are little molecular machines, heat cycling is enough to get them to do the work of DNA replication. The process is called the DNA polymerase chain reaction (PCR). (If it sounds familiar, it's because PCR-powered COVID testing became famous during the pandemic.)
DNA evidence can be used to identify victims of accidents or mass casualty incidents by comparing DNA samples to family members.
What Is DNA Sequencing?
DNA sequencing is the process of determining the sequence of base pairs in a sample of DNA. Scientists use various approaches to sequence a DNA sample, including Sanger sequencing, shotgun sequencing, and the nanopore method.
Different methods of DNA sequencing have different strengths. Sanger sequencing uses electrophoresis to draw DNA molecules through a gel filter, producing characteristic "fingerprints" according to their length. However, gel electrophoresis doesn't have the resolution of other methods that can sequence a genome by individual base pairs. Shotgun sequencing can reassemble a badly fragmented genome from overlapping DNA segments of various lengths—but it struggles with the larger fragments, tens of thousands of base pairs long.
Newer sequencing methods, such as the nanopore technique, can read much longer sequences of DNA. In nanopore sequencing, sensors monitor the electrical charge across a semipermeable membrane studded with carbon nanotubes that form tiny pores. As a molecule of DNA passes through a nanopore, each nucleobase causes a characteristic fluctuation in the current, resulting in a tiny flash of light in one of four colors corresponding to the four nucleobases in a molecule. By comparison, instead of measuring electrical current, Illumina sequencing uses fluorescent chemical indicators spread across a microfluidics chip.
What Is the Epigenome?
Because not all genes are "switched on," only some regions of the genome can be translated into protein. The rest are tightly coiled around spindly structures called centromeres. However, the DNA exposed to the cytoplasm is exposed to whatever else is circulating in the cytoplasm. Some small molecules will bind to active DNA, forming a kind of chemical record like writing notes in the margin of a book. These tiny chemical markers, and the genes they annotate, are collectively called the epigenome ("epi" = around)
Between generations, some epigenetic markers are stripped away, but some are preserved. It's possible to use epigenetics to draw conclusions about the biochemical environments an organism lived in by looking at which of their genes were active simultaneously.
What Does DNA Have to Do With Evolution?
Every time a cell replicates a new molecule of DNA, it has a new chance to make a mistake. Sometimes, these mistakes are beneficial; more often, they're gobbledygook that results in broken proteins and a loss of function. But they always leave a record of the change in the genetic sequence.
Because almost everything that makes or uses DNA also makes DNA repair proteins, it's possible to rewind the evolutionary clock by looking at the rate of DNA mutations that accumulate between generations in a species. DNA provides evidence of evolution and allows scientists to trace changes within species over time.
