Discovery of DNA & its Structure

Introduction, Discovery of DNA and Structure 

Introduction 

Cell – The cell is the fundamental structural and functional unit of all living organisms.  

All organs in our body and all parts of the plant are composed of thousands of cells. Different cells in our body perform different functions. Every cell has its own life. All organisms’ lives start as a single cell called the egg. The weak and old cells die and are replaced by new cells. 

Structure of a cell  

A generalized cell consists of three essential parts: 

1. Cell membrane (plasma membrane) 

2. Cytoplasm 

3. Nucleus 

1. Cell membrane – Cell membrane is the living part of the cell. Each cell is surrounded by a membrane called plasma membrane or cell membrane. The cell membrane has fine pores through which the substances may enter or leave the cell. The cell membrane is selective and only allows certain substances to pass through it.  

Cytoplasm – It is a semi-liquid substance that is present in most of the parts of the cell within the cell membrane. It contains both living and non-living parts.

3. Nucleus – The basic unit of a cell, which is a small spherical mass located nearly at the center of the cytoplasm, is called the nucleus.  

• It is an important part of the cell which regulates and coordinates various life processes of the cell.  
• It contains both the living and non-living parts.  
• It is the largest cell organelle.  

• A nucleus is mostly spherical and has a nuclear membrane that is filled with a dense fluid called nucleoplasm.  
• The nucleoplasm contains certain threadlike structures called chromatin fibers. 

The nucleus plays an important role in cell division. During cell division, the chromatin fibers become thick and ribbon-like structures called chromosomes.

• The chromosomes carry genetic characters from the parents to the offspring.  
• Chromosomes are made of chromatin.  
• The chromatin is composed of hereditary units called genes.  
• Genes are made of a complex chemical substance called DNA (deoxyribonucleic acid).  

Cell organelles – These are little organs present inside the cell which have a definite shape, a definite structure and a definite function. The organelles are living parts present in the entire body performing specific functions. 

Discovery of DNA 

• DNA was first recognized and identified by the Swiss biologist Johannes Friedrich Miescher in 1869 during his research on white blood cells. 
• The double helix structure of a DNA molecule was later discovered by James Watson and Francis Crick. They also won a Nobel Prize in the year 1962. They found that DNA is responsible for storing genetic information in living organisms. 

How does DNA relate to the cell? 

DNA is defined as deoxyribonucleic acid, which is a molecule that contains instructions an organism needs to develop, reproduce and live. These instructions are found inside every cell and are passed from generation to generation. It contains the instructions to construct other components of the cell. Nearly every cell in a person’s body has the same DNA, and most of the DNA is located in the cell nucleus. 

DNA Structure 

The double helix structure of DNA is like a twisted ladder with two DNA strands that are antiparallel. DNA is a nucleic acid i.e., they are a naturally occurring chemical compound. Nucleic acid is the basic building block and is made up of nucleotides. 

Each nucleotide comprises a sugar molecule, phosphate groups and nitrogen bases. Deoxyribose is the sugar that forms the backbone of the DNA molecule. Each strand of DNA is formed by grouping sugar and phosphate groups and each sugar molecule is attached to a phosphate group and a nitrogen base. 

There are four naturally occurring nitrogen bases such as adenine (A), thymine (T), cytosine (C) and guanine (G) where adenine and guanine are called purines and thymine and cytosine are called pyrimidines. The order in which they form determines the DNA instructions otherwise called genetic code. These nitrogen bases form hydrogen bonds in the opposite strands resulting in a ladder-like structure. Each strand forms a right-handed coil and a single turn consists of ten nucleotides.