Structure and Function of DNA

Mr. Arvind Kumar

Introduction

  • DNA, or Deoxyribonucleic Acid, is the fundamental molecule that carries genetic information in all living organisms, from the simplest bacteria to complex plants and animals, including humans.
  • It is often referred to as the “blueprint of life” because it contains the instructions for the growth, development, functioning, and reproduction of organisms.

Features of DNA:

  1. Chemical Composition:
    DNA is a polymer made up of repeating units called nucleotides. Each nucleotide consists of:

    • A phosphate group,

    • A deoxyribose sugar, and

    • A nitrogenous base (Adenine (A), Thymine (T), Cytosine (C), or Guanine (G)).

  2. Double Helix Structure:
    The DNA molecule is composed of two long strands that form a double helix. These strands are held together by complementary base pairs: Adenine pairs with Thymine and Cytosine pairs with Guanine through hydrogen bonds. This helical structure was first discovered by James Watson and Francis Crick in 1953, based on the X-ray crystallography data produced by Rosalind Franklin.

  3. Genetic Code:
    The sequence of nitrogenous bases (A, T, C, G) in DNA determines the genetic instructions that guide the development of an organism. A specific sequence of three bases, called a codon, codes for a specific amino acid, which in turn makes up proteins—key components of cells that perform various functions.

  4. Location in Cells:

    • In eukaryotic cells, DNA is primarily located in the nucleus and is organized into chromosomes.

    • In prokaryotic cells (like bacteria), DNA is found in the cytoplasm in a region called the nucleoid. They typically have a single, circular DNA molecule.

    • Mitochondria (in animal and plant cells) and chloroplasts (in plant cells) also contain small amounts of DNA.

  5. Replication:
    DNA is able to replicate itself, ensuring that genetic information is passed on to the next generation during cell division. This process is highly accurate but involves several enzymes (like DNA polymerase) and several key steps, such as unwinding the DNA, copying the strands, and proofreading for errors.

 

Classification of DNA

Seven Criteria for DNA Classification

1. Number of Base Pairs per Turn:

  • The number of base pairs that make one complete twist or turn in the helical structure of DNA.

  • For example, in B-DNA (the most common form of DNA), there are about 10 base pairs per turn. This number can vary in different forms of DNA (e.g., A-DNA and Z-DNA).

2. Coiling Pattern:

  • The way in which the DNA strands are wound around each other.

  • B-DNA is the most common coiling pattern, forming a right-handed helix.

  • Other coiling patterns include A-DNA (which is right-handed but more tightly coiled) and Z-DNA (a left-handed helix).

3. Location:

  • DNA can be classified based on its location within the cell.

    • Nuclear DNA: Found in the nucleus of eukaryotic cells and typically arranged into chromosomes.

    • Mitochondrial DNA: Found in the mitochondria and is inherited maternally.

    • Chloroplast DNA: Found in the chloroplasts of plant cells.

    • Plasmid DNA: Found in bacteria and some eukaryotes, usually existing in small circular forms outside the main chromosome.

    • Prokaryotic DNA: Found in the cytoplasm of prokaryotes (bacteria) and often forms a single circular chromosome.

4. Structure:

  • DNA can be classified based on its structural form:

    • Linear DNA: Found in the chromosomes of eukaryotes, where it is organized into long, straight strands.

    • Circular DNA: Found in prokaryotes (bacteria) and organelles like mitochondria and chloroplasts, where the DNA is a closed loop.

    • Supercoiled DNA: In certain organisms, like prokaryotes and in the nucleus, DNA is supercoiled to save space and regulate gene expression.

5. Nucleotide Sequences:

  • The specific order of the nitrogenous bases (A, T, C, G) in a DNA molecule determines the genetic information it carries.

  • DNA can be classified based on the uniqueness of its nucleotide sequence:

    • Unique DNA sequences: Found in functional genes and regulatory regions.

    • Repetitive DNA sequences: Found in areas like satellite DNA, which do not code for proteins but have structural or regulatory roles.

6. Coding and Non-Coding DNA:

  • Coding DNA: DNA that contains genes, which code for proteins. This is the part of DNA that carries the instructions for building proteins and performing cellular functions.

    • Exons: Coding regions of genes that are expressed.

  • Non-Coding DNA: DNA that does not code for proteins but may have regulatory or structural roles.

    • Introns: Non-coding regions within genes.

    • Regulatory regions: Sequences that control the expression of genes, such as promoters and enhancers.

    • Satellite DNA: Repetitive sequences that may play structural roles in chromosomes (like at centromeres).

7. Number of Strands:

  • DNA can be single-stranded or double-stranded:

    • Double-Stranded DNA (dsDNA): The typical structure in most organisms, where two complementary strands of DNA twist into a double helix.

    • Single-Stranded DNA (ssDNA): Found in some viruses and also as an intermediate in processes like DNA replication and transcription.

 

Structure of DNA

The structure of DNA (Deoxyribonucleic Acid) is central to its function in storing genetic information, guiding cellular activities, and enabling reproduction. DNA’s structure is both simple and highly organized, and it has been described as a double helix. Here’s an in-depth look at its structure:

1. Basic Components of DNA:

DNA is a long molecule made up of repeating units called nucleotides. Each nucleotide consists of three parts:

  • A phosphate group: A phosphorus atom bonded to four oxygen atoms.

  • A deoxyribose sugar: A five-carbon sugar (ribose with one oxygen atom removed), which forms the backbone of the DNA strand.

  • A nitrogenous base: One of four possible bases—Adenine (A), Thymine (T), Cytosine (C), or Guanine (G). These bases carry the genetic information.

2. The Double Helix:

DNA has a double-stranded structure, often referred to as the double helix, which was famously described by James Watson and Francis Crick in 1953. The structure consists of two long chains of nucleotides twisted around each other in a spiral shape.

  • Two complementary strands: The two strands of DNA are held together by hydrogen bonds between the nitrogenous bases. These strands are anti-parallel, meaning they run in opposite directions.

    • One strand runs in the 5′ to 3′ direction (where 5′ and 3′ refer to the carbon atoms in the sugar backbone).

    • The other strand runs in the 3′ to 5′ direction.

  • Sugar-phosphate backbone: The backbone of each strand is formed by alternating deoxyribose sugars and phosphate groups, with the nitrogenous bases extending inward from the sugar molecules.

3. Base Pairing (Complementary Base Pairing):

The nitrogenous bases of the two strands form specific pairs:

  • Adenine (A) pairs with Thymine (T) via two hydrogen bonds.

  • Cytosine (C) pairs with Guanine (G) via three hydrogen bonds.

This complementary base pairing ensures that the genetic code is preserved and accurately copied during DNA replication.

4. Major and Minor Grooves:

As the DNA strands twist into the double helix, the helical shape creates regions of space between the strands, known as grooves:

  • Major groove: The larger gap between the strands.

  • Minor groove: The smaller gap between the strands.

These grooves are important because they provide access points for regulatory proteins and enzymes to interact with the DNA, such as during processes like transcription, replication, and repair.

5. Helical Structure:

The double helix is a right-handed helix, meaning it twists in a clockwise direction. The helix has about 10 base pairs per turn of the spiral.

  • The twisting of the DNA molecule helps to pack the long DNA strands into a compact structure, which is essential for fitting into the cell nucleus.

6. DNA Packaging:

In eukaryotic cells, DNA is packaged into structures called chromosomes. To achieve this, DNA is wrapped around histone proteins, forming nucleosomes. This further coils into higher-order structures to fit inside the cell nucleus.

  • Nucleosomes: Composed of DNA wrapped around histone proteins, they resemble beads on a string.

  • Chromatin: The nucleosomes fold into more compact structures, forming chromatin.

  • Chromosomes: During cell division, chromatin further condenses to form visible chromosomes, which carry the genetic information.

7. DNA Replication and Repair:

DNA replication is the process through which a cell copies its DNA before division. The double-helix structure of DNA is crucial for this process:

  • The two strands of DNA separate.

  • Each strand serves as a template for synthesizing a complementary strand.

The accuracy of this process is enhanced by proofreading mechanisms in DNA polymerase, which helps prevent mutations. DNA repair mechanisms also maintain the integrity of the genetic material by fixing damaged bases or strands.

8. Stability and Flexibility:

The structure of DNA is stable enough to store genetic information reliably over generations, yet it is flexible enough to allow for processes like transcription, replication, and recombination. The structure of the double helix also protects the genetic material from environmental damage.

Function of DNA

1. Stores Genetic Information:

DNA holds all the information needed to create and maintain an organism. This includes instructions for building proteins, which are essential for the body to work.

2. Replicates Itself:

DNA can make copies of itself, which is crucial when cells divide. This ensures that every new cell has the same genetic information.

3. Protein Production:

DNA tells cells how to make proteins. Proteins are needed for almost every function in the body, like building tissues, repairing cells, and carrying out chemical reactions.

4. Passes Information to Offspring:

DNA is passed from parents to children, which is why offspring inherit traits from their parents, like eye color or height.

5. Mutations and Evolution:

Sometimes, DNA can change (mutate), and this can lead to new traits. Over time, these changes can help organisms adapt to their environment and evolve.

6. Regulates Body Functions:

DNA controls which genes are turned on or off, helping cells know when and how to do their job.

7. DNA Repair:

DNA has built-in systems to fix itself when it gets damaged, helping to prevent problems like diseases or errors in the genetic code.

8. Protects Chromosomes:

At the ends of chromosomes, DNA has protective caps called telomeres that keep the chromosomes from getting damaged during cell division.