In this lesson, we will learn
Nucleic acids are large biomolecules that are most important for the continuity of life. These are found in the nucleus and the cytoplasm of a cell. They are responsible for controlling the important biosynthetic cell activities as well as carrying hereditary information from one generation to the other. Therefore, nucleic acids are macromolecules of the utmost significance.
They are naturally occurring chemical compounds that are capable of being broken down to yield phosphoric acid, sugars, and a mixture of organic bases (purines and pyrimidine).
They are associated with the chromosomes. They transmit different information to the cytoplasm.
There are two main types of nucleic acids – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
DNA constitutes the genetic material in all living organisms, ranging from single-celled bacteria to multicellular mammals. In eukaryotes, it is found in the nucleus and in the chloroplasts and mitochondria. In prokaryotes, it is not enclosed in a membranous envelope, but rather free-floating within the cytoplasm. The entire genetic content of a cell is known as its genome and the study of genomes is genomics.
All the genetic or hereditary information in a cell is stored in a coded form in molecules known as DNA. Genetic or hereditary information refers to all the information that is necessary to reproduce as well as maintain a new organism. DNA is replicated and distributed to the daughter cells during cell division. Therefore, hereditary information is passed from one cell to the other and from one generation of an organism to the other.
DNA is the main genetic information store. Through transcription, the information is transmitted into RNA molecules. The process of translation of RNA leads to the synthesis of proteins. RNA helps in the expression of this information as specific patterns of protein synthesis. RNA is the genetic material of certain viruses, but it is also found in all living cells, where it plays an important role in certain processes such as the making of proteins.
In higher cells, DNA is mainly found in the nucleus as a part of the chromosomes. Small amounts of DNA are found in the cytoplasm in the chloroplasts and mitochondria. RNA is present both in the cytoplasm and in the nucleus. RNA is synthesized in the nucleus and protein synthesis takes place in the cytoplasm.
Nucleic acids are made up of sugar (pentose), phosphoric acid, and nitrogenous bases (pyrimidines and purines). A nucleic acid molecule has a linear polymer where nucleotides are joined together through a phosphodiester or a bond.
Below is an illustration of DNA nucleotide:
Below is an illustration of RNA nucleotide:
Let's discuss each of the three units of nucleic acids:
Pentose sugar
There are two basic types of sugar in nucleic acids:
The difference between the sugars in the presence of the hydroxyl group on the second carbon of the ribose and hydrogen on the second carbon of the deoxyribose. The carbon atoms of the sugar molecule are numbered as 1’, 2’, 3’, 4’ and 5’ (1’ read as “one prime”)
Phosphate group
These are connected to the carbon atom number 5 of the sugar molecule.
Nitrogenous base
The nitrogenous base are organic molecules and are so named because they contain carbon and nitrogen.
Nitrogenous bases are – Adenine (A), Guanine (G), Cytosine (C), and Thymine (T) in a DNA molecule and Uracil (U) in an RNA molecule. Uracil is found in RNA only instead of thymine in DNA. Each base is connected to the carbon atom number 1 of the sugar molecule. Nucleic acids differ with respect to the difference of the nitrogenous bases forming them.
Adenine and guanine are classified as purines. The primary structure of a purine consists of two carbon-nitrogen rings. Cytosine, thymine, and uracil are classified as pyrimidines which have a single carbon-nitrogen ring as their primary structure. Each of these basic carbon-nitrogen rings has different functional groups attached to it.
Deoxyribonucleic acid (DNA)
This forms approximately 9% of the nucleus. Chemically, it is made up of three major components: bases, sugar and phosphoric acid.
Ribonucleic acid (RNA)
RNA is mainly found in the nucleolus but it is also found on chromosomes in small amounts. Small amounts of RNA are also found in chloroplasts and mitochondria. RNA is a long chain molecule made up of repeating units of nucleotides. Ribose is the sugar component of RNA and the four bases cytosine, adenine, guanine, and uracil.
The process of making the copy from the DNA is called transcription. This is when the cell makes a copy (or "transcript") of the DNA. The copy of DNA is called RNA because it uses a different type of nucleic acid called ribonucleic acid. The DNA, which is a double helix, is transcribed or copied, into a single helix-the RNA.
Next, RNA is converted (or "translated") into a sequence of amino acids that makes up the protein. The translation process of making the new protein from the RNA instructions takes place in a complex machine in the cell called the ribosome.
Three general classes of RNA molecules are involved in expressing the genes encoded within a cell's DNA.
messenger RNA (mRNA) molecules carry the coding sequences for protein synthesis and are called transcripts;
ribosomal RNA (rRNA) molecules form the core of a cell's ribosomes (the structures in which protein synthesis takes place),
transfer RNA (tRNA) molecules carry amino acids to the ribosomes during protein synthesis.
In eukaryotic cells, each class of RNA has its own polymerase, whereas, in prokaryotic cells, a single RNA polymerase synthesizes the different classes of RNA.
Nucleic acids are carried on the chromosomes inside the cell’s nucleus. They are responsible for passing on the genetic traits from one generation to another when cells divide.
| DNA | RNA | |
| Function |
Repository of genetic information |
Involved in protein synthesis and gene regulation; carrier of genetic information in some viruses |
| Sugar | Deoxyribose | Ribose |
| Structure | Double helix | Usually single-stranded |
| Bases | C, T, A, G |
C, U, A, G |
