Structure And Functions of DNA

 

Structure And Functions of DNA

Introduction

Deoxyribonucleic Acid (DNA) is the hereditary material of almost all living organisms. It stores, replicates, and transmits genetic information from one generation to the next and directs the synthesis of proteins that determine the structure and function of every cell.

The discovery of DNA and the elucidation of its structure revolutionized biology. The Double Helix Model proposed by James Watson and Francis Crick in 1953, based on the X-ray diffraction studies of Rosalind Franklin and Maurice Wilkins, is one of the greatest scientific achievements of the twentieth century.

What Is DNA?

Definition

DNA (Deoxyribonucleic Acid) is a long polymer of nucleotides that serves as the primary hereditary material in almost all living organisms.  It stores genetic information in the form of nucleotide sequences.

Why Is DNA Called the Hereditary Material?

DNA is called hereditary material because it:

• Stores genetic information.
• Replicates accurately before cell division.
• Passes genetic information from parents to offspring.
• Controls synthesis of proteins.
• Regulates growth, development, reproduction, and metabolism.
• Undergoes mutation, generating genetic variation.

Location of DNA

In Eukaryotic Cells

DNA is found in:

• Nucleus (major portion)
• Mitochondria
• Chloroplasts (plants)

In Prokaryotic Cells

DNA is present in:

• Nucleoid region
• Plasmids

Discovery of DNA

Scientist	Contribution
Friedrich Miescher (1869)	Discovered "nuclein" (DNA) in white blood cells
Phoebus Levene	Described nucleotide components
Erwin Chargaff	Chargaff's rules
Rosalind Franklin	X-ray diffraction photographs (Photo 51)
Maurice Wilkins	X-ray crystallography studies
James Watson & Francis Crick (1953)	Proposed Double Helix Model

 

Chemical Composition Of DNA

DNA is a polynucleotide. It is composed of repeating units called nucleotides. Each nucleotide has three components:

1. Nitrogenous base
2. Pentose sugar
3. Phosphate group

Components of a DNA Nucleotide

1. Nitrogenous Base

There are four nitrogenous bases:

Purines

• Adenine (A)
• Guanine (G)

Characteristics:

• Double-ring structure
• Larger molecules

Pyrimidines

• Cytosine (C)
• Thymine (T)

Characteristics:

• Single-ring structure
• Smaller molecules

2. Pentose Sugar

DNA contains: Deoxyribose sugar

Characteristics:

• Five-carbon sugar
• Lacks one oxygen atom at the 2′ carbon compared with ribose

3. Phosphate Group

Composed of phosphoric acid (PO₄³⁻).

Functions:

• Links adjacent nucleotides through phosphodiester bonds.
• Forms the sugar-phosphate backbone.
• Gives DNA an overall negative charge.

 

Nucleoside Vs Nucleotide

Nucleoside	Nucleotide
Nitrogenous base + Sugar	Nitrogenous base + Sugar + Phosphate
No phosphate group	Contains phosphate group
Building block precursor	Functional unit of DNA and RNA

 

DNA Polymer

DNA is formed by joining thousands to millions of nucleotides. Adjacent nucleotides are connected by 3′–5′ phosphodiester bonds. The sugar-phosphate chain forms the backbone of each strand.

Watson–Crick Model of DNA (1953)

The Watson–Crick model explains the three-dimensional structure of DNA.

Main Features

1. DNA is Double-Stranded

DNA consists of two long polynucleotide chains.

2. DNA Forms a Right-Handed Double Helix

The two strands coil around a common axis to form a right-handed helix (B-DNA).

3. Antiparallel Arrangement

The two strands run in opposite directions:

• One strand: 5′ → 3′
• Other strand: 3′ → 5′

This arrangement is called antiparallel orientation.

4. Sugar-Phosphate Backbone

The backbone lies on the outside. Nitrogenous bases project inward.

5. Complementary Base Pairing

Bases pair specifically through hydrogen bonds:

• Adenine (A) pairs with Thymine (T)
• Guanine (G) pairs with Cytosine (C)

This is called complementary base pairing.

Chargaff's Rule

Erwin Chargaff discovered that:

• Amount of Adenine = Amount of Thymine
• Amount of Guanine = Amount of Cytosine

Therefore:

A = T

G = C

Also: Purines = Pyrimidines

Hydrogen Bonds

Base pairs are held together by hydrogen bonds.

Base Pair	Number of Hydrogen Bonds
A – T	2
G – C	3

Because G–C pairs have three hydrogen bonds, GC-rich DNA is more stable and has a higher melting temperature than AT-rich DNA.

Dimensions Of DNA

According to the Watson–Crick model:

Feature	Measurement
Diameter	2 nm (20 Å)
Distance between adjacent base pairs	0.34 nm (3.4 Å)
One complete turn	3.4 nm (34 Å)
Base pairs per turn	10 (approximately 10.5 in B-DNA under physiological conditions)
Helical turns	Right-handed

convention: 10 base pairs per turn and a pitch of 3.4 nm.

DNA Grooves

The double helix has two alternating grooves:

Major Groove

• Wider
• Site for binding of many proteins, including transcription factors

Minor Groove

• Narrower
• Also participates in protein interactions

Complementary Nature Of DNA

If one strand is: 5′–ATGCCGT–3′

The complementary strand is: 3′–TACGGCA–5′

Complementary pairing ensures accurate DNA replication.

DNA Packaging

In Prokaryotes

DNA is:

• Circular
• Double-stranded
• Located in the nucleoid
• Not enclosed by a nuclear membrane

In Eukaryotes

DNA is linear and associated with histone proteins. It is packaged into:

DNA → Nucleosome → Chromatin → Chromosome

Nucleosome

The basic structural unit of chromatin.

A nucleosome consists of:

• DNA wrapped around a histone octamer (two each of H2A, H2B, H3, and H4)
• Histone H1 helps stabilize higher-order chromatin structure

Functions of DNA

DNA performs numerous essential functions.

1. Storage of Genetic Information

DNA stores hereditary information in the sequence of nucleotides. This information determines:

• Cell structure
• Cell function
• Development
• Metabolism

2. Transmission of Hereditary Information

DNA replicates before cell division. Thus, daughter cells receive identical genetic information.

During reproduction: Parents transmit DNA to offspring.

3. Control of Protein Synthesis

Genes contain instructions for protein synthesis. The sequence is:

DNA

↓

RNA

↓

Protein

Proteins determine most cellular functions.

4. Regulation of Cellular Activities

DNA regulates:

• Cell growth
• Cell division
• Differentiation
• Metabolism
• Enzyme production

5. Source of Genetic Variation

Mutations occur in DNA. These mutations create:

• New alleles
• Genetic diversity
• Evolutionary changes

6. Evolution

DNA accumulates mutations over generations. Natural selection acts upon these variations. Thus, DNA forms the molecular basis of evolution.

7. DNA Replication

DNA duplicates before cell division. Each daughter cell receives identical DNA. This ensures continuity of life.

8. DNA Repair

Cells possess repair enzymes that correct many DNA errors, maintaining genome stability.

9. Control of Cell Differentiation

Different genes are expressed in different cell types. Thus:

• Muscle cells
• Nerve cells
• Liver cells

all possess the same DNA but express different sets of genes.

Differences Between DNA and RNA

DNA	RNA
Deoxyribose sugar	Ribose sugar
Double-stranded (usually)	Single-stranded (usually)
Thymine present	Uracil replaces thymine
More stable	Less stable
Stores genetic information	Participates in protein synthesis
Located mainly in nucleus (also mitochondria/chloroplasts)	Found in nucleus and cytoplasm

 

Relationship Between DNA, Gene, And Chromosome

DNA

↓

Genes (segments of DNA)

↓

Chromosomes (DNA packaged with proteins)

↓

Genome (complete genetic material)

Central Dogma of Molecular Biology

Francis Crick proposed the Central Dogma:

DNA

↓

RNA

↓

Protein

This explains the flow of genetic information in cells.

Summary Flow Chart

DNA

↓

Stores Genetic Information

↓

Replication

↓

Transmission to Daughter Cells

↓

Transcription

↓

RNA

↓

Translation

↓

Protein

↓

Expression of Traits

Importance of DNA in Modern Biology

DNA is the basis of:

• Genetics
• Biotechnology
• Genetic engineering
• DNA fingerprinting
• Gene therapy
• Forensic science
• Evolutionary biology
• Personalized medicine
• Disease diagnosis

HIGH-YIELD FACTS

·       DNA is the hereditary material in almost all living organisms.

·       DNA is composed of nucleotides, each containing a nitrogenous base, deoxyribose sugar, and phosphate group.

·       Purines: Adenine (A), Guanine (G).

·       Pyrimidines: Cytosine (C), Thymine (T).

·       Watson and Crick proposed the double helix model in 1953.

·       DNA strands are antiparallel.

·       A pairs with T by 2 hydrogen bonds.

·       G pairs with C by 3 hydrogen bonds.

·       Diameter of DNA = 2 nm.

·       Distance between adjacent base pairs = 0.34 nm.

·       One helical turn = 3.4 nm.

·       Approximately 10 base pairs per turn.

·       DNA is packaged around histone proteins to form nucleosomes in eukaryotes.