Nucleic Acids: Definition, Functions and Examples

We all somehow resemble our ancestors in many ways. This is because the genes of a person are passed down to their future generations. The chromosomes present in the nucleus of a living cell is responsible for the transmission of the transmission of the inherent characters. The chromosomes are made of several types of biomolecules. One of the several type of Biomolecules is Nucleic Acid.

Key Terms: Nucleic Acid, Organic chemistry, chromosomes, DNA, RNA, nucleotides, polynucleotides

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Nucleic Acids

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Nucleic acids are explained as the long-chain polymeric molecules, where the monomer (the repeating unit) is referred to as the nucleotides. Thus many times nucleic acids are being referred to as polynucleotides. The two main kinds of nucleic acids are-

1. Deoxyribonucleic acid (DNA)
2. Ribonucleic acid (RNA)

Both DNA and RNA are responsible for inheritance and transmitting some characteristics from one generation to the next. 

Chemically, DNA is made up of pentose sugar, phosphoric acid, and some cyclic bases that have nitrogen in it. The sugar moiety inside DNA molecules is β-D-2-deoxyribose. The cyclic bases having nitrogen in them include adenine (A), guanine (G), cytosine(C), and thymine (T). These bases and their arrangement in the molecules of DNA play a vital role in storing information from one generation to the next one. DNA has a double-strand helical feature where the strands are complementary to each other.

Ribonucleic Acid (RNA)

The RNA molecule is also made up of phosphoric acid, pentose sugar, and some cyclic bases containing nitrogen. RNA has β-D-ribose in it, in the form of the sugar moiety. The heterocyclic bases present in RNA include adenine (A), guanine (G), cytosine(C), and uracil (U). In RNA the fourth base is different from that of DNA, and the RNA has a single strand that many times folds back. This results in the creation of a double helix structure.

Ribonucleic Acid (RNA)

Three kinds of RNA molecules having different functions include:

• Messenger RNA (m-RNA)
• Ribosomal RNA (r-RNA)
• Transfer RNA (t-RNA)

Each of the nucleotides consists of these three parts:

1. A pentose sugar
2. A nitrogenous base
3. A phosphate group

The nitrogenous base and a pentose sugar are called as a nucleoside.

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Roles played by DNA and RNA in cells

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Nucleic acids, the macromolecules are made out of small units called nucleotides. They appear in two naturally occurring varieties including DNA and RNA. DNA is defined as genetic material found in living organisms. Many viruses use RNA, instead of DNA, as their genetic material, but they are not considered to be alive.

DNA in cells

In eukaryotes, like plants and animals, DNA is present inside the nucleus. The nucleus is a specialized, membrane-bound vault inside the cell, also it is present in certain other types of organelles(like mitochondria and the chloroplasts of plants). In prokaryotes, similar to bacteria, the DNA is not enclosed in a membranous envelope but is present in a particular cell region called the nucleoid.

In eukaryotes, DNA is mostly broken up in long, linear pieces named chromosomes; however, in prokaryotes like bacteria, chromosomes are smaller in size and mostly circular. Chromosomes include thousands of genes and every chromosome provides instructions regarding making a particular product required by the cell.

From DNA to RNA to proteins:

Many genes encode protein products, which means they encode some sequence of amino acids needed in the preparation of a particular protein. Before the same information can be used for protein synthesis, a copy of RNA(transcript) of the gene is made. This kind of RNA is being known as a messenger RNA (mRNA) as it passes on the information between DNA and the ribosomes, molecular machines that read mRNA sequences and use the same information in building Proteins. This progression from DNA to RNA to protein is being referred to as the “central dogma” of molecular biology.

From DNA to RNA to proteins

One thing to note here is that not all genes encode protein products.

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Nucleotides

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Both, DNA and RNA are polymers and are made up of monomers referred to as nucleotides. When these monomers are combined it results in a chain explained as a polynucleotide (poly = "many"). Each nucleotide is made up of three different parts including a nitrogen-containing ring structure named as a nitrogenous base; five-carbon sugar, and one phosphate group. The sugar molecule holds a central position in the nucleotide, with the base attached to one of its carbons and the phosphate group (or groups) attached to another.

Nucleotides

• Nitrogenous bases

The nitrogenous bases of nucleotides are referred to as organic (carbon-based) molecules made up of nitrogen-containing ring structures. All nucleotides in DNA include one of four possible nitrogenous bases like adenine (A), guanine (G) cytosine (C), and thymine (T).

Both, Adenine and guanine are purines and their structures include two fused carbon-nitrogen rings. However, Cytosine and thymine are pyrimidines and have a single carbon-nitrogen ring. RNA nucleotides also include adenine, cytosine, and guanine bases, but in place of thymine, they have another pyrimidine base named uracil (U).

In terms of molecular biology, the nitrogenous bases are notified by their one-letter symbols, A, T, G, C, and U. 

DNA consists of A, T, G, and C; however, RNA includes A, U, G, and C.

Nitrogenous bases

• Sugars

Along with having different bases, both DNA and RNA nucleotides also have some slightly different sugars. The sugar in DNA having 5 carbons is called deoxyribose, while in RNA, the sugar is ribose. Both of them are quite the same in structure, and just have a single difference, the second carbon of ribose belongs to a hydroxyl group, while the equivalent carbon of deoxyribose belongs to hydrogen. The carbon atoms of a nucleotide’s sugar molecule are numbered as 1′, 2′, 3′, 4′, and 5′ (1′ is read as “one prime”), the sugar is placed centrally, having its base attached to its 1′ carbon and the phosphate group (or groups) attached to its 5′ carbon.

Sugars

• Phosphate

Nucleotides might belong to one phosphate group, or a chain of up to three phosphate groups, attached to the 5’ carbon of the sugar. In many written texts, chemistry sources recommend the term “nucleotide” only for the single-phosphate case, but in terms of molecular biology, the broader definition is considered. In a cell, a nucleotide about to be added to the end of a polynucleotide chain will have a series of three phosphate groups. When the nucleotide joins the growing DNA or RNA chain, they start losing two phosphate groups. In a chain of DNA or RNA, all the nucleotides will have one phosphate group.

Structure of Nucleotides 

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