Most people have seen the famous double helix of DNA—two strands of DNA intertwined like a spiral staircase. All living things have DNA. Simple creatures like bacteria have just one long, circular piece of DNA made up of two intertwined DNA strands. The human genome, the set of DNA in people, has many more strands of DNA.
Most of humans’ DNA is typically packaged into 46 chromosomes located in the cell’s nucleus, which is a specialised compartment for storing DNA. Each of the chromosomes in the nucleus is made up of two linear DNA strands wrapped around each other.
Human cells also contain a small amount of extrachromosomal DNA located in another part of the cell called the mitochondria. This mitochondrial DNA is more like bacterial DNA—a single long, circular piece of DNA made up of two strands of DNA.
A DNA strand is a long, thin molecule—averaging only about two nanometers (or two billionths of a meter) in width. That is so thin, that a human hair is about 40,000 times as wide. The incredible thinness of DNA strands allows them to be very tightly packed, as otherwise most DNA molecules would not fit inside of cells.
For perspective, if you stretched out every strand of DNA contained in just a single human cell end-to-end, it would measure almost two meters, or around 6.6 feet in length. If the nucleus of a cell were the size of a baseball, this would be the equivalent of 8.3 miles of DNA being stuffed inside.
Parts of a DNA Strand
Regardless of its length and location in the cell, all DNA strands share a common structure. They are all composed of building blocks called nucleotides that are linked together in a row. Nucleotides themselves are comprised of three joined parts: a sugar molecule, a phosphate group, and a nitrogenous base.
The sugars of one nucleotide link to the phosphates of the adjacent nucleotide to form the exterior of the DNA strand, known as the sugar-phosphate backbone. The interior of the DNA strand is made up of the nitrogenous bases. These bases bind together in pairs, forming weak bonds that nonetheless hold the two strands of DNA in a double helix together. Their sequence encodes an organism’s genetic information.
How Are the Two Strands of DNA Held Together?
The two strands of DNA in a double helix are held together by pairing between the nitrogenous bases in the nucleotides of each strand. The nitrogenous base of a DNA nucleotide can be one of four different molecules: adenine (A), guanine (G), thymine (T), and cytosine (C). Pairs of nitrogenous bases on opposing strands are held together by attractions called hydrogen bonds that occur in a specific pattern.
Every adenine on one DNA strand forms two hydrogen bonds with a thymine molecule on the complementary strand and vice versa. And every guanine molecule on one strand forms three hydrogen bonds with a cytosine molecule on the other and vice versa. In this way, the two DNA strands are stuck together by hydrogen bonds all along their length, forming the “steps” of the spiral staircase that is the double helix.
Why Do DNA Strands Have to Be Antiparallel?
The two complementary DNA strands that compose a double-stranded piece of DNA are described as being antiparallel to each other. The term antiparallel means that while the two strands are physically parallel to one another, they run in opposite directions— much like the right and left lanes of a street.
In other words, where the backbone of one DNA strand starts with a sugar molecule and ends in a phosphate group, the backbone of its complementary strand starts with a phosphate group and ends in a sugar molecule. The antiparallel orientation of the two DNA strands makes DNA more structurally stable and enables the complementary base pairing that holds the DNA strands together.
The direction of each DNA strand is significant to the process of copying the DNA (DNA replication) and reading the information contained in the genes of DNA (transcription), as cells can only read DNA in one direction. Just as we only read text from left to right, cells only read DNA by starting with the sugar end of the backbone and ending with the phosphate end.