Nucleic Acids: Structure, Function, and Properties

Nucleic Acids

Definition and Importance

Nucleic acids are large biomolecules essential for all known forms of life. They store and transmit genetic information and are involved in the synthesis of proteins.

• Nucleic acids include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• They are polymers made up of monomer units called nucleotides.

• Functions include genetic information storage, transmission, and regulation of cellular activities.

Nucleotides

Basic Components of a Nucleotide

Each nucleotide consists of three main components:

• Phosphate group

• Five-carbon sugar (either ribose in RNA or deoxyribose in DNA)

• Nitrogenous base (purine or pyrimidine)

Example: In ATP (adenosine triphosphate), the nucleotide contains a ribose sugar, adenine base, and three phosphate groups.

Formation of Nucleic Acid Polymers

Polymerization and Bonding

Nucleotides are joined together to form nucleic acid polymers through covalent bonds:

• Phosphodiester bonds link the 5' phosphate group of one nucleotide to the 3' hydroxyl group of the next.

• This forms the sugar-phosphate backbone of DNA and RNA.

Equation:

$\text{Nucleotide}_n + \text{Nucleotide}_{n+1} \xrightarrow{\text{polymerase}} \text{Nucleotide}_n-\text{Nucleotide}_{n+1}$

DNA and RNA

Structure and Roles in Living Organisms

• DNA: Stores genetic information, directs cell activities, and is passed from generation to generation.

• RNA: Involved in protein synthesis (mRNA, tRNA, rRNA), regulation, and sometimes genetic material in viruses.

Example: mRNA carries genetic instructions from DNA to ribosomes for protein synthesis.

Nitrogenous Bases

Purines and Pyrimidines

Nitrogenous bases are classified as purines or pyrimidines:

• Purines: Adenine (A), Guanine (G)

• Pyrimidines: Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA)

Base Type	DNA	RNA
Purines	A, G	A, G
Pyrimidines	C, T	C, U

Sugar Components

Ribose vs. Deoxyribose

The sugar in nucleotides determines whether the nucleic acid is DNA or RNA:

• Ribose: Found in RNA; contains a hydroxyl group (-OH) at the 2' carbon.

• Deoxyribose: Found in DNA; lacks the 2' hydroxyl group (has -H instead).

Additional info: The absence of the 2' hydroxyl group in DNA increases its stability compared to RNA.

DNA Structure

Double Helix

DNA is typically found as a double helix, consisting of two complementary strands twisted around each other.

• Strands are held together by hydrogen bonds between complementary bases.

• The double helix provides stability and allows for accurate replication.

Antiparallel Orientation

DNA strands run in opposite directions:

• One strand runs 5' to 3', the other 3' to 5'.

• This antiparallel arrangement is crucial for replication and base pairing.

Additional info: The 5' end has a phosphate group, and the 3' end has a hydroxyl group.

Base Pairing Rules

Complementary Base Pairing

Specific hydrogen bonding occurs between bases:

• DNA: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).

• RNA: Adenine (A) pairs with Uracil (U); Guanine (G) pairs with Cytosine (C).

Base	DNA Pair	RNA Pair
A	T	U
G	C	C
C	G	G
T	A	-
U	-	A

Complementary Sequence Example

Determining Complementary Strands

Given a DNA template strand, the complementary sequence is determined by base pairing rules.

• Template: 3' – GCATATGCC – 5'

• Complementary: 5' – CGTATACGG – 3'

Additional info: The complementary strand is written in the 5' to 3' direction, matching the antiparallel orientation.