A nucleic acid is biological polymer or biopolymer that is essential to life and consists of a nitrogenous bases, 5-carbon (pentose) sugar, and phosphate groups. The two types of nucleic acids are DNA and RNA. They are “nucleic acids” because DNA is in the nucleus of eukaryotic cells and is chemically an acid. Nucleic acids carry the genetic information of all organisms and direct protein synthesis.
- All living organisms and also viruses use nucleic acids.
- The two classes of nucleic acids are 2-deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
- DNA and RNA differ according to their structure, sugar (2′-deoxyribose in DNA and ribose in RNA), and bases (adenine, thymine, guanine, cytosine in DNA and uracil instead of thymine in RNA).
- Nucleic acids code genetic information and perform other functions in cells.
Nucleic Acid Examples
There are three types of DNA and several types of RNA. Here are some examples of these nucleic acids:
- A-DNA: This is right-handed double helix.
- B-DNA: B-DNA is the most common form of DNA. It is a right-handed helix. Dehydrated B-DNA takes the A-DNA form.
- Z-DNA: This is a left-handed form of DNA that makes a zig-zag pattern.
- mRNA: mRNA or messenger RNA controls the message the cells reads from DNA and acts as a template for protein synthesis.
- tRNA: tRNA or transfer RNA brings amino acids to the ribosome for protein synthesis.
- rRNA: rRNA or ribosomal RNA form ribosomes, which control protein synthesis in cells.
So, DNA and RNA are the two classes of nucleic acids that occur in nature. But, biochemists also synthesize artificial nucleic acid analogs. The synthetic molecules mainly differ from DNA or RNA by the composition of their backbones.
Nucleic Acid Monomers
A nucleic acid is a polymer that consists of nucleotide monomers that link together. Each nucleotide consists of three parts:
- Nitrogenous base
- Pentose sugar
- Phosphate group
Each base has a ring structure and is classified by its structure and either a purine or a pyrimidine. The purines are adenine and guanine, while the pyrimidines are cytosine, thymine (in DNA), and uracil (in RNA). Purines and pyrimidines form bonds with each other, where adenine (A) bind to thymine (T) or uracil (U) and guanine (G) binds to cytosine (C).
The 5-carbon or pentose sugar is between the nitrogenous base and phosphate group. In DNA, the sugar is 2′-deoxyribose. In RNA, the sugar is ribose. The carbon atoms of the sugar are numbered 1′, 2′, 3′, 4′, and 5′. The base attaches to the 1′ carbon of the sugar, while the phosphate attaches to the 5′ carbon.
The phosphate group attaches to the pentose sugar. Together, the sugar and phosphate groups form the backbone of the DNA or RNA helix. While a nucleotide has 1, 2, or 3 phosphate groups, it only has one phosphate group in a nucleic acid.
Nucleic acids break down into smaller subunits:
(polymer) nucleic acid → (monomer) nucleotides → (components) nitrogenous base + pentose sugar + phosphate
Nucleic Acid Structure
Nucleic acids have a helical shape (with some exceptions in RNA). DNA forms a double helix, while RNA mostly forms a single helix. The phosphate of one nucleotide connects to the OH group on the 3′ carbon of the sugar of the next nucleotide. This connection is an ester linkage. The process repeats, forming a backbone of alternative phosphate and sugar subunits. The purines and pyrimidines branch off of the backbone.
The backbone has a “direction” because one end has a free sugar (the 3′ end), while the other end has a free phosphate group (the 5′ end). The two strands of a DNA helix are antiparallel, so the 3′ end of one strand is across from the 5′ end of the other strands, with the bases connected between them. By convention, chemists read the code of a nucleic acid starting with the 5′ end. So, a genetic code of guanine, thymine, adenine, cytosine is 5′-dG-dT-dA-dC-3′ or simply GTAC.
Functions of Nucleic Acids
Nucleic acids serve two main functions:
- They are responsible for heredity, which is the transmission of genetic material from one generation to the next. All cells (animal, plant, bacteria, etc.) use DNA for this purpose. Some viruses use RNA, but they still need the DNA of a host in order to replicate.
- Nucleic acids (both DNA and RNA) control protein synthesis in a cell. The DNA codes the message, while the RNA performs the actual synthesis.
References
- Alberts, B.; et al. (2002). Molecular Biology of the Cell (4th ed.). Garland Science. ISBN 0-8153-3218-1.
- Dahm, R. (2008). “Discovering DNA: Friedrich Miescher and the early years of nucleic acid research”. Human Genetics. 122 (6): 565–81. doi:10.1007/s00439-007-0433-0
- Nelson, David L.; Cox, Michael M. (2005). Principles of Biochemistry (4th ed.). New York: W. H. Freeman. ISBN 0-7167-4339-6.
- Saenger, Wolfram (1984). Principles of Nucleic Acid Structure. New York: Springer-Verlag.