DNA Bases and Structure

DNA (deoxyribonucleic acid) is a molecule that stores genetic information within living organisms and provides the instructions that cells need to function and reproduce. DNA is made of smaller subunits called nucleotides. Each nucleotide consists of three components: a five-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base. The four types of DNA nucleotides are distinguished based on which nitrogenous base they contain—adenine (A), guanine (G), thymine (T), or cytosine (C).

What Is a Strand of DNA Made of and How Is It Made?

A strand of DNA is made up of nucleotides that form a chain, creating a long, linear molecule. To form an individual strand, the sugar molecule of one nucleotide forms a covalent bond with the phosphate group of the adjacent nucleotide, forming a strong sugar-phosphate backbone.

Two DNA strands then coil together into a double helix, held together by hydrogen bonds between the nitrogenous bases. The strands can coil together because the bases pair up in a very specific way: A binds to T, and G binds to C.  Since the hydrogen bonds between bases are relatively weak, the two strands can be easily split apart so that the DNA can be copied (known as DNA replication) or used in the first step of protein synthesis (known as transcription).  

What Information Is Contained in DNA?

DNA contains all of the genetic information necessary for an organism to develop, function, and reproduce. DNA encodes this information as specific sequences of nucleotide bases.

One of the most important types of information contained in DNA is the instructions for how to build various proteins, as proteins do most of the work in cells. To follow these instructions, a cell must first copy a gene into a form of RNA known as precursor messenger RNA (pre-mRNA). This process is called transcription.

After being processed, the RNA (now called mRNA or mature mRNA) is ready to be translated into a protein that can carry out the instructions in the gene.

Genes that encode proteins only account for around two percent of our total DNA sequence. The rest of an organism’s DNA was once seen as “junk DNA,” but we now know that it can play other roles, including regulatory ones like influencing gene transcription.

How Is DNA Formed?

Copies of DNA molecules are formed through a process known as DNA replication. The first step in this process involves breaking the hydrogen bonds that connect the two strands of the DNA double helix at specific places called origins of replication, so that the double helix splits in two within the replication bubble around the origin of replication.

An enzyme called DNA polymerase then uses complementary base pairing (where A binds to T and G binds to C) to make a copy of each strand. For example, if there is an adenine (A) in the DNA strand, the DNA polymerase will add a thymine (T) to the newly forming strand—and so on, resulting in a complementary strand of DNA. Thus two copies of the DNA molecule are created from the original, in what is called semiconservative replication.

On occasion, DNA polymerase makes a mistake during replication, accidentally matching the wrong bases together. When this occurs, the polymerase pauses while other enzymes work to repair the error, and resumes its replication once the DNA mistake has been fixed.

While these error-checking mechanisms provide a high degree of accuracy, the polymerase still make mistakes: everyone is born with approximately one difference per every 100 million bases compared to their parents. These uncorrected mistakes become mutations, which can occasionally lead to disease. But they also help to explain why every human in unique.