DNA Structure and Function: Molecular Basis of Inheritance
DNA Structure and Function: Molecular Basis of Inheritance
DNA structure and function are fundamental concepts in genetics and molecular biology. DNA stores hereditary information and provides the instructions required for the growth, development, functioning, and reproduction of living organisms.
For students studying high school biology, AP Biology, introductory college biology, genetics, or molecular biology, understanding how DNA is organized and how its structure allows it to store and transmit genetic information is essential.
In this guide, we explain the molecular basis of inheritance in a simple, step-by-step format. Topics include DNA packaging in eukaryotes, nucleotides, nitrogenous bases, Chargaff's rules, the Watson and Crick model, DNA dimensions, different forms of DNA, melting temperature, and DNA length calculations.
Packaging of DNA in Eukaryotes
In eukaryotic cells, DNA is associated with histone proteins to form a compact chromatin structure.
DNA is negatively charged because of its phosphate groups, whereas histone proteins contain positively charged amino acid residues. This electrostatic interaction helps DNA associate with histones and become efficiently packaged inside the nucleus.
Histone Octamer
The core of a nucleosome contains a histone octamer consisting of:
- 2 × H2A
- 2 × H2B
- 2 × H3
- 2 × H4
DNA wraps around the histone octamer to form a nucleosome.
Approximately 146 base pairs (bp) of DNA are wrapped around the histone core. Nucleosomes are connected by linker DNA and form higher levels of chromatin organization.
In the traditional model of chromatin organization, nucleosomes can form a 30-nm chromatin fiber, often described as the solenoid model.
DNA Structure and Function
DNA stands for deoxyribonucleic acid. It is a nucleic acid and a polymer made up of repeating nucleotide units.
DNA → Nucleic acid → Polynucleotide
DNA and RNA are both polynucleotides.
What Is a Nucleotide?
A nucleotide consists of three components:
Nitrogenous base + Pentose sugar + Phosphate
What Is a Nucleoside?
A nucleoside consists of:
Nitrogenous base + Pentose sugar
Therefore:
Nucleotide = Nucleoside + Phosphate
Adjacent nucleotides in a polynucleotide chain are connected through phosphodiester bonds.
Pentose Sugar in DNA and RNA
Two important pentose sugars occur in nucleic acids: ribose and deoxyribose.
Ribose
- Present in RNA.
- Contains an –OH group at the 2′ carbon.
Deoxyribose
- Present in DNA.
- Has a hydrogen atom instead of an –OH group at the 2′ carbon.
- This gives DNA its characteristic deoxyribose sugar.
Nitrogenous Bases in DNA and RNA
Nitrogenous bases are classified into two major groups: purines and pyrimidines.
Purines
Purines have a two-ring structure.
- Adenine (A)
- Guanine (G)
Pyrimidines
Pyrimidines have a single-ring structure.
- Cytosine (C)
- Thymine (T) — found in DNA
- Uracil (U) — found in RNA
Purines = A + G
Pyrimidines = C + T + U
Chargaff's Rules of Base Equivalence
Chargaff's rules describe the base composition of double-stranded DNA.
In double-stranded DNA:
A = T
G = C
Therefore:
A + G = T + C
Adenine pairs with thymine, while guanine pairs with cytosine.
Base Ratios
For double-stranded DNA:
A/T = 1
G/C = 1
The relative proportion of AT and GC base pairs can vary among organisms, making base composition a useful characteristic when comparing genomes.
A commonly used measure is:
(A + T) / (G + C)
Discovery of DNA
In 1869, Friedrich Miescher isolated a phosphorus-containing substance from cell nuclei that he called nuclein.
This work contributed to the later identification and study of nucleic acids and ultimately to our understanding of DNA as the genetic material.
Watson and Crick Model of DNA
The double-helical structure of DNA was proposed by James Watson and Francis Crick in 1953, building on important experimental evidence including X-ray diffraction data.
The common physiological form of DNA is known as B-DNA, which has a right-handed double-helical structure.
Main Features of the DNA Double Helix
- DNA consists of two polynucleotide chains.
- The two chains are coiled around a common axis.
- The overall structure is a double helix.
- The two DNA strands are antiparallel.
- One strand runs from 5′ → 3′, while the other runs from 3′ → 5′.
- The sugar-phosphate backbone is located toward the outside.
- The nitrogenous bases project toward the inside of the helix.
- The two strands are held together by hydrogen bonds.
- The strands are complementary because of specific base pairing.
Complementary Base Pairing
Adenine (A) pairs with Thymine (T) through two hydrogen bonds.
Guanine (G) pairs with Cytosine (C) through three hydrogen bonds.
Therefore:
A = T
G ≡ C
Dimensions of B-DNA
The important dimensions of B-DNA are:
- Diameter of DNA helix: approximately 2 nm
- Distance between successive base pairs: approximately 0.34 nm
- Pitch of one complete turn: approximately 3.4 nm
- Base pairs per turn: approximately 10
Therefore:
10 × 0.34 nm = 3.4 nm
10 base pairs × 0.34 nm per base pair = 3.4 nm per turn
Forms of DNA
DNA can adopt several structural forms depending on environmental and molecular conditions.
| DNA form | Helix | Base pairs/turn | Approx. diameter |
|---|---|---|---|
| A-DNA | Right-handed | ~11 | ~2.3 nm |
| B-DNA | Right-handed | ~10 | ~2.0 nm |
| Z-DNA | Left-handed | ~12 | ~1.8 nm |
| C-DNA | Right-handed | ~9–10 | ~1.9 nm |
B-DNA is the predominant DNA form under normal physiological conditions.
Z-DNA is distinctive because it has a left-handed helical structure.
Functions of DNA
DNA performs several essential biological functions. Two useful ways to describe its information-processing roles are autocatalytic and heterocatalytic functions.
1. Autocatalytic Function
DNA can make a copy of itself through DNA replication.
DNA → DNA
Replication ensures that genetic information can be passed to daughter cells and, ultimately, across generations.
2. Heterocatalytic Function
DNA contains information that can be used to produce RNA through transcription.
DNA → RNA
DNA Melting Temperature (Tm)
Melting temperature (Tm) is the temperature at which approximately half of a DNA population is present in the single-stranded state under specified experimental conditions.
Effect of GC Content on Tm
Guanine and cytosine form three hydrogen bonds in standard Watson-Crick base pairing, whereas adenine and thymine form two.
Therefore, DNA with a higher GC content generally requires a higher temperature to separate its strands.
High GC content → Higher Tm
High AT content → Lower Tm
In laboratory applications, Tm also depends on factors such as salt concentration and DNA sequence composition.
Length of DNA
The approximate length of a DNA molecule can be calculated from its number of base pairs and the average distance between adjacent base pairs.
Length of DNA = Number of base pairs × Distance between successive base pairs
For the commonly used B-DNA approximation:
Length = Number of base pairs × 0.34 nm
Examples of Genome Size
- φX174 bacteriophage: approximately 5,386 nucleotides; its genome is single-stranded DNA.
- Escherichia coli: approximately 4.7 × 106 base pairs.
- Human haploid genome: approximately 3.2 × 109 base pairs.
Why DNA Is an Effective Genetic Material
DNA is particularly well suited to store hereditary information because it is chemically stable, can be accurately replicated, and contains sequence information that can be transmitted to new cells.
Its complementary double-stranded structure provides a mechanism for accurate copying, while occasional changes in DNA sequence provide a molecular basis for genetic variation.
Frequently Asked Questions About DNA Structure
What is DNA?
DNA, or deoxyribonucleic acid, is a nucleic acid that stores hereditary information in cells and many viruses.
What are the four bases in DNA?
The four major nitrogenous bases in DNA are adenine, thymine, guanine, and cytosine.
What are Chargaff's rules?
Chargaff's rules state that in double-stranded DNA, the amount of adenine equals thymine and the amount of guanine equals cytosine.
How many hydrogen bonds are between A and T?
Adenine and thymine form two hydrogen bonds.
How many hydrogen bonds are between G and C?
Guanine and cytosine form three hydrogen bonds.
What is the diameter of B-DNA?
The diameter of B-DNA is approximately 2 nm.
How many base pairs are present in one turn of B-DNA?
Approximately 10 base pairs occur in one complete turn of B-DNA.
What is the difference between a nucleotide and a nucleoside?
A nucleotide contains a nitrogenous base, sugar, and phosphate. A nucleoside contains only a nitrogenous base and sugar.
Why does DNA with high GC content have a higher Tm?
Guanine and cytosine form three hydrogen bonds, compared with two between adenine and thymine. Higher GC content generally increases the thermal stability of double-stranded DNA.
Quick Revision: DNA Structure and Function
- Nucleotide = Sugar + Base + Phosphate
- Nucleoside = Sugar + Base
- Purines = A + G
- Pyrimidines = C + T + U
- A = T
- G = C
- A–T = 2 hydrogen bonds
- G–C = 3 hydrogen bonds
- B-DNA diameter = 2 nm
- 10 bp = approximately 3.4 nm
- 1 bp = approximately 0.34 nm
- Replication: DNA → DNA
- Transcription: DNA → RNA
- High GC content → Higher Tm
- B-DNA → Right-handed
- Z-DNA → Left-handed
Related Biology Topics: Genetics, DNA replication, transcription, genetic material, chromosomes, nucleosomes, molecular biology, heredity, and inheritance.
This article is intended as an educational biology study guide for students learning genetics and molecular biology.
