Unit 4: Nucleic Acids: DNA and RNA Structure and Functions

Nucleic acids are fundamental macromolecules essential for all known forms of life. They are biopolymers, or large biomolecules, critical for the storage and expression of genetic information. The two main types of nucleic acids are Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA).

1. Salient Features and Chemical Composition of DNA and RNA

Salient Features of Nucleic Acids

Nucleic acids are polymers made up of repeating monomeric units called nucleotides. These molecules play a central role in heredity and protein synthesis.

• Genetic Material: DNA serves as the primary genetic material in most organisms, storing long-term genetic information. Some viruses use RNA as their genetic material.
• Information Storage: They carry the blueprint for all cellular structures and activities.
• Information Transfer: They are involved in transmitting genetic information from one generation to the next and within a cell (from DNA to RNA to protein).
• Protein Synthesis: RNA molecules are directly involved in the process of translating genetic information into proteins.

Chemical Composition of DNA and RNA

Each nucleotide is composed of three main components:

1. A Pentose Sugar: A 5-carbon sugar.
   • In DNA, the sugar is 2-deoxyribose (lacks an oxygen atom at the 2' carbon).
   • In RNA, the sugar is ribose (has a hydroxyl group at the 2' carbon).
2. A Nitrogenous Base: These are heterocyclic compounds containing nitrogen.
   • Purines: Double-ring structures. Adenine (A) and Guanine (G).
   • Pyrimidines: Single-ring structures. Cytosine (C), Thymine (T) (found in DNA), and Uracil (U) (found in RNA instead of Thymine).
3. A Phosphate Group: A negatively charged inorganic phosphate (PO₄^3-) group attached to the 5' carbon of the pentose sugar.

A nucleoside is formed by a pentose sugar covalently linked to a nitrogenous base. A nucleotide is formed when a phosphate group is esterified to a nucleoside.

The nucleotides are linked together by phosphodiester bonds, which form between the phosphate group of one nucleotide and the 3' hydroxyl group of the sugar of the next nucleotide, creating a sugar-phosphate backbone.

Comparison of DNA and RNA Composition

2. Watson and Crick Model of DNA; Types and Functions of DNA

Watson and Crick Model of DNA (Double Helix Model)

In 1953, James Watson and Francis Crick proposed the double helix model for the structure of DNA, which provided critical insights into how genetic information is stored and replicated. Key features of this model include:

• Double Helix: DNA consists of two polynucleotide strands coiled around a common axis, forming a right-handed double helix.
• Antiparallel Strands: The two strands run in opposite directions; one strand runs from 5' to 3', and the other runs from 3' to 5'.
• Sugar-Phosphate Backbone: The backbone of each strand is composed of alternating deoxyribose sugar and phosphate groups, located on the outside of the helix.
• Nitrogenous Bases Inside: The nitrogenous bases project inwards, perpendicular to the helix axis.
• Base Pairing: Specific pairing occurs between bases from opposite strands:
  • Adenine (A) always pairs with Thymine (T) via two hydrogen bonds.
  • Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds.
  • This is known as Chargaff's Rule (A=T and G=C).
• Major and Minor Grooves: The helical structure results in two grooves of unequal width along the surface of the molecule: a wider major groove and a narrower minor groove. These grooves are important for protein binding.
• Stability: The double helix is stabilized by hydrogen bonds between complementary base pairs and by hydrophobic interactions (base stacking) between adjacent bases.

The diameter of the DNA helix is approximately 2 nm (20 Å), and one complete turn of the helix is about 3.4 nm (34 Å), containing 10 base pairs.

Types of DNA

While B-DNA is the most common form found in living cells, DNA can exist in other conformations depending on hydration levels, salt concentration, and base sequence.

• B-DNA: The most common physiological form, a right-handed helix. Its characteristics align with the Watson and Crick model.
• A-DNA: A right-handed helix, but shorter and wider than B-DNA, with bases tilted relative to the helix axis. It forms in conditions of lower humidity.
• Z-DNA: A left-handed helix, often formed in regions with alternating purine-pyrimidine sequences (e.g., GCGCGC). It is longer and thinner than B-DNA and may play a role in gene regulation.

Functions of DNA

• Storage of Genetic Information: DNA contains the complete set of instructions for building and maintaining an organism.
• Replication: DNA can make exact copies of itself, ensuring that genetic information is faithfully passed on to daughter cells during cell division. This process is semi-conservative.
• Transcription: DNA acts as a template for the synthesis of RNA molecules, which then participate in protein synthesis.
• Mutation: Changes in the DNA sequence (mutations) are the primary source of genetic variation, driving evolution.
• Gene Expression Regulation: Specific DNA sequences act as regulatory elements, controlling when and where genes are turned on or off.

3. Structure of RNA – its Types and Functions

Structure of RNA

RNA is typically a single-stranded polynucleotide chain, but it can fold back on itself to form complex three-dimensional structures. Key structural features include:

• Single-Stranded: Unlike DNA, RNA is usually single-stranded. However, internal base pairing within the same strand can lead to localized double-helical regions and complex secondary and tertiary structures (e.g., hairpins, stem-loops).
• Ribose Sugar: Contains ribose sugar, which has a hydroxyl (-OH) group at the 2' carbon.
• Uracil Instead of Thymine: RNA contains the pyrimidine base Uracil (U) in place of Thymine (T). Uracil pairs with Adenine (A).
• Less Stable Than DNA: The 2'-hydroxyl group in ribose makes RNA more susceptible to hydrolysis under alkaline conditions compared to DNA.

Types of RNA

RNA molecules perform diverse functions within the cell, and are categorized into several types based on their structure and role:

1. Messenger RNA (mRNA):
   • Function: Carries genetic information from DNA in the nucleus to the ribosomes in the cytoplasm. It serves as a template for protein synthesis.
   • Structure: Linear sequence of nucleotides corresponding to a gene.
2. Transfer RNA (tRNA):
   • Function: Acts as an adaptor molecule, bringing specific amino acids to the ribosome during protein synthesis (translation) according to the mRNA codons.
   • Structure: Smallest RNA molecules, typically 70-90 nucleotides long. They have a characteristic cloverleaf secondary structure that folds into an L-shaped tertiary structure. Contains an anticodon loop that recognizes mRNA codons and an amino acid attachment site.
3. Ribosomal RNA (rRNA):
   • Function: A major component of ribosomes, the cellular machinery responsible for protein synthesis. rRNA molecules have catalytic activity (ribozyme) and structural roles within the ribosome.
   • Structure: Highly abundant and diverse in size. They combine with ribosomal proteins to form the large and small ribosomal subunits.
4. Other types of RNA: While mRNA, tRNA, and rRNA are the main functional types, other RNAs exist, such as small nuclear RNA (snRNA), microRNA (miRNA), and small interfering RNA (siRNA), which are involved in gene regulation and processing.

Functions of RNA

• Protein Synthesis (Translation): All types of RNA (mRNA, tRNA, rRNA) are crucial for the synthesis of proteins. mRNA carries the code, tRNA delivers amino acids, and rRNA forms the catalytic core of the ribosome.
• Catalytic Activity (Ribozymes): Some RNA molecules, particularly rRNA, can act as enzymes (ribozymes), catalyzing biochemical reactions, such as the formation of peptide bonds during protein synthesis.
• Gene Regulation: Small RNA molecules like miRNA and siRNA play significant roles in regulating gene expression by affecting mRNA stability and translation.
• Genetic Material in Viruses: In certain viruses (e.g., retroviruses, influenza virus), RNA serves as the primary genetic material instead of DNA.
• RNA Processing: snRNAs are involved in splicing, the process of removing introns from pre-mRNA.