What are deoxyribonucleic and ribonucleic acid made of, and how are these big molecules of genetic information built? This article explains the basics.

Nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the genetic material of cells.

Their names are derived from type of sugar, ribose, contained within these molecules.

Nucleic Acid Structure

Nucleotides are the building blocks of these nucleic acids. Each monomer of nucleic acid, is called a nucleotide, and consists of 3 portions:

• a pentose sugar
• one or more phosphate groups
• one of five cyclic nitrogenous bases

Phosphate-Sugar Backbone

Nucleotides are linked together by covalent bonds between phosphate of one nucleotide and sugar of next. These linked monomers become the phosphate-sugar backbone of nucleic acids. Nitrogenous bases extending from this phosphate-sugar backbone like teeth of a comb.

The Twisted “Ladder” of Nucleic Acid

Hydrogen bonds form between specific bases of two nucleic acid chains, forming a stable, double-stranded DNA molecule, which looks like a ladder. Three H bonds form between bases cytosine (C) and guanine (G), which always pair up together between two nucleic acid chains. Two H bonds form between adenine (A) and thymine (T) in DNA or adenine and uracil (U) in RNA molecules.

The structure is analogous to a ladder, with the two deoxyribose-phosphate chains as side rails and the base pairs, linked by hydrogen bonds, forming the rungs. Hydrogen bonding also twists the phosphate-deoxyribose backbones into a helix, thus typical DNA is a double helix.

Building Nucleic Acids: Replication

Before a cell divides, it must make a copy of its DNA so that both parent and daughter cell have a complete copy of genetic information. This process of copying the double-stranded DNA molecule is called replication.

Semiconservative Replication

Each DNA strand holds the same genetic information, so both strands can serve as templates for the reproduction of the opposite strand. The template strand is preserved and the new strand is assembled from nucleotides (semiconservative replication). The resulting double-stranded DNA molecules are identical.

When Does DNA Replicate?

In a cell, DNA replication must happen before cell division. Prokaryotes replicate their DNA throughout the interval between cell divisions. In eukaryotes, the timing of replication is highly regulated.

Replication Bubbles

The origin, or starting points of replication are called “bubbles.” The DNA must unwind in order to be copied. Within these unwound, or “unzipped” bubbles is an area called the replication fork, a ‘Y’-shaped region where new strands of DNA are built. The enzyme helicase catalyzes the untwisting of the DNA at the replication fork, and another enzyme, DNA polymerase, catalyzes the elongation of new DNA strand.

Prokaryotic DNA is arranged in a circular shape, and there is only one replication origin when replication starts. Despite these differences, the underlying process of replication is the same for both prokaryotic and eukaryotic DNA.

Leading and Lagging Strand

Each of DNA’s two sugar phosphate backbones runs in opposite directions. One strand runs from the 5’ end to the 3’ end, while the other runs 3’ to 5’.

DNA polymerase can only add nucleotides at the free 3’ end, forming new DNA strands in the 5’ to 3’ direction only. This results in a ‘leading strand’, which can be continuously build in one direction and a ‘lagging strand’ which, since it runs in the opposite direction, must be made in small 3’ to 5’ bits and pieces, called Okazaki fragments, which are ultimately joined together by the enzyme DNA ligase.

How Do Nucleotides Put Themselves Together Into Nucleic Acids?

The construction of a nucleic acid is an anabolic polymerization process. Anabolic reactions build bigger molecules. Polymerization is the process of taking nucleotide monomers and putting them together into polymers (large molecules composed of many monomers).

Polymerization requires monomers (building blocks) and energy and triphosphate deoxyribonucleotides provide both. These three-phosphate building blocks of DNA bring their own energy for polymerization, and the breaking of the triphosphate bond of the nucleotide contributes the energy required add it to the growing nucleic acid.

Additional Cell Biology Information

To learn more about cell biology and organic chemistry, see the Suite101 articles Nucleic Acids and Replication, What Is a Carbohydrate, What Is a Lipid, Amino Acids and Proteins, What are Organic Molecules. For more information on genetics and cell biology, you can go to the excellent websites of Science Prof Online or Cells Alive.

Sources

Bauman, R. (2005) Microbiology.

Park Talaro, K. (2008) Foundations in Microbiology.