Nucleic Acid Structure and Function

The molecule DNA is the repository of heritable genetic information. In humans, DNA is contained in 23 chromosome pairs within the nucleus. The molecule provides the basic template for replication of genetic information, RNA transcription, and protein biosynthesis to promote cellular function and survival.

Transcription of genetic information is the first step in gene expression. Transcription is the process by which DNA is used as a template to make mRNA. This process is divided into 3 stages: initiation, elongation, and termination. Transcription begins at a region known as the promoter. An enzyme called RNA polymerase “reads” the DNA template strand and creates the mRNA. Additional proteins, known as transcription factors, are required for the RNA polymerase to bind to the promoter sequence in eukaryotes. After initiating transcription, the RNA polymerase elongates the newly forming mRNA until it reaches a termination sequence.

Nucleic acids are polymers of nucleotides, organic molecules composed of a sugar, a phosphate group, and a nitrogenous base. Nucleic acids are responsible for storage, replication, and expression of genetic information. They are “acids” because of the phosphate groups that are acidic in nature and “nucleic” because they are stored in the cell’s nucleus. The 2 nucleic acids most commonly seen in eukaryotic cells are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Though chemically similar, DNA and RNA have specific biological functions to which their respective structures are tailored.

Ribonucleic acid (RNA), like deoxyribonucleic acid (DNA), is a polymer of nucleotides that is essential to cellular protein synthesis. Unlike DNA, RNA is a single-stranded structure containing the sugar moiety ribose (instead of deoxyribose) and the base uracil (instead of thymine). While DNA stores the genetic information, RNA generally carries out the instructions encoded in the DNA but RNA also executes diverse non-coding functions. There are 3 major types of RNA that perform different but collaborative roles in protein synthesis: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). During transcription, RNA is synthesized from DNA through a series of steps catalyzed by the enzyme RNA polymerase. The mRNA formed will serve as an amino acid template for protein synthesis. Translation proceeds with the tRNA transporting the corresponding amino acid based on the deciphered nucleotide sequence (codon) in the mRNA. The ribosomes, which are composed of rRNA, then facilitate the assembly of amino acids into a polypeptide. These components work together to convert the mRNA template obtained from DNA into the desired protein.

The entire DNA of a cell is replicated during the S (synthesis) phase of the cell cycle. The principle of replication is based on complementary nucleotide base pairing: adenine forms hydrogen bonds with thymine (or uracil in RNA) and guanine forms hydrogen bonds with cytosine. Replication occurs prior to cell division in the S phase of the cell cycle in order to allow 2 sets of chromosomes to be present during the metaphase of mitosis, after which they are equally partitioned into 2 new cells during separation (anaphase).