Structure and Functions of RNA Explained for Students

Ribonucleic Acid (RNA) is a vital biomolecule present in all living cells. It carries genetic information, aids in protein synthesis, and performs various regulatory and catalytic functions. Unlike DNA, RNA is mostly single-stranded, allowing it to form many three-dimensional shapes, making it versatile for cellular activities.

Properties of RNA

RNA is usually single-stranded and not a double helix like DNA. This single-stranded nature enables RNA to fold into complex three-dimensional structures, which are crucial for its function. The nucleotide of RNA contains a ribose sugar, which differs from DNA's deoxyribose by having an additional OH group on the 2' carbon atom.

This 2' OH group makes RNA more prone to hydrolysis, leading to lower stability than DNA. Due to this, RNA is typically used by cells for short-term roles, whereas DNA serves as the long-term genetic information storehouse. RNA includes the nitrogenous bases adenine (A), guanine (G), cytosine (C), and uracil (U) — with uracil replacing the thymine (T) found in DNA.

Structure of RNA

RNA is a biopolymer composed of ribonucleotides linked by phosphodiester bonds. Each ribonucleotide contains a ribose sugar, a phosphate group, and one of four nitrogenous bases (A, G, C, U). RNA strands are synthesized in the 5’ to 3’ direction using DNA as a template.

The unique structure of RNA, with its extra 2’ OH group, limits its lifespan and creates the potential for a wide range of functional three-dimensional structures, including folds and loops. These structural features allow RNA to serve as catalysts, messenger molecules, adaptors, and even structural elements within the cell.

Composition of RNA

Every RNA molecule comprises three main components:

• Ribose sugar – a five-carbon cyclic sugar with two OH groups at the 2’ and 3’ carbon positions.
• Phosphate group – bonds with the ribose sugars to form the RNA backbone via phosphodiester linkage.
• Nitrogenous bases – Adenine (A), Guanine (G), Cytosine (C), and Uracil (U). G pairs with C, and A pairs with U through hydrogen bonding in complementary regions.

Types of RNA

Among the many types of RNA found in organisms, three major types play primary roles in cells:

• Messenger RNA (mRNA): A single-stranded molecule complementary to a DNA strand, mRNA carries genetic code from the nucleus to the cytoplasm. Ribosomes read the codons on mRNA and translate them to produce proteins. Without mRNA, the information in DNA cannot be expressed as proteins.
• Ribosomal RNA (rRNA): A component of ribosomes, rRNA is synthesized in the nucleolus. Multiple forms of rRNA combine with ribosomal proteins to form the large and small subunits of ribosomes, facilitating mRNA and tRNA binding and catalyzing peptide bond formation during protein synthesis.
• Transfer RNA (tRNA): This small molecule translates the genetic code carried by mRNA into specific proteins. It has a cloverleaf structure with three arms (DHU arm for enzyme recognition, T arm for ribosome recognition, and Anticodon arm for mRNA binding). The open 3’ end attaches to the corresponding amino acid. By matching its anticodon with mRNA codons, tRNA ensures that amino acids are added in the correct order to build proteins.

Other Types of RNA

Some RNA molecules act as enzymes (ribozymes) and are found in cellular processes like RNA splicing, tRNA biosynthesis, and viral replication. Antisense RNAs are single-stranded RNAs with sequences complementary to mRNA. While they do not code for proteins, they can bind to mRNA and inhibit translation, serving as natural regulators of gene expression.

Due to their unique ability to silence specific genes, artificial antisense RNAs have been developed for therapeutic uses against diseases and faulty gene expression.

Functions of RNA

• RNA is essential for protein synthesis. Without RNA, the genetic code in DNA could not be used to build proteins needed by cells.
• mRNA is widely utilized in developing vaccines and novel medicines. It also enables the creation of cDNA libraries for research.
• rRNA forms the core of ribosomes, which are the "machines" for protein synthesis in all cells.
• Ribozymes and antisense RNAs can regulate gene expression by cleaving or inhibiting specific mRNA molecules, aiding cellular control and defense mechanisms.

Type of RNA	Main Function
Messenger RNA (mRNA)	Conveys genetic instructions from DNA to ribosome for protein synthesis
Ribosomal RNA (rRNA)	Structural and enzymatic part of ribosome; catalyzes peptide bond formation
Transfer RNA (tRNA)	Brings amino acids to ribosome; deciphers mRNA code during translation
Ribozymes/Antisense RNA	Enzymatic activity (e.g. RNA splicing) or gene silencing

Feature	RNA	DNA
Sugar	Ribose	Deoxyribose
Strands	Usually single-stranded	Double-stranded helix
Nitrogenous Bases	A, G, C, U	A, G, C, T
Stability	Less stable due to extra OH group	More stable
Primary Function	Protein synthesis, regulation	Genetic information storage

Key Points for Students

• RNA enables the use of genetic information for vital functions like protein synthesis.
• The structural differences between RNA and DNA influence their stability and cellular roles. Learn more at Difference Between DNA and RNA.
• Three main types of RNA—mRNA, tRNA, and rRNA—work together for protein synthesis.

Practice Questions

1. List and describe the main types of RNA found in cells.
2. Explain how tRNA reads the genetic code during protein synthesis.
3. Why is RNA generally less stable than DNA?
4. What role do antisense RNAs play in gene regulation?
5. Compare the structures of RNA and DNA in terms of their sugar and base content.

For deeper understanding and structured notes, visit:
RNA: Full Overview
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Difference Between DNA and RNA