DNA (deoxyribonucleic acid) is a double-stranded, helical molecule that stores and transmits genetic information in living organisms. The structure of DNA was first proposed by James Watson and Francis Crick in 1953, based on X-ray crystallography data collected by Rosalind Franklin and Maurice Wilkins. The structure of DNA is composed of nucleotides, which are the basic building blocks of DNA.
Structure of DNA: A DNA molecule consists of two complementary strands of nucleotides that are held together by hydrogen bonds between their nitrogenous bases. The nitrogenous bases present in DNA are adenine (A), thymine (T), guanine (G), and cytosine (C). Adenine always pairs with thymine, and guanine always pairs with cytosine. These base pairs form the “rungs” of the DNA ladder, while the sugar-phosphate backbone forms the “sides” of the ladder. The sugar in DNA is deoxyribose, which is why it is called deoxyribonucleic acid.
Function of DNA: The main function of DNA is to store and transmit genetic information from one generation to the next. This information is encoded in the sequence of nucleotides in the DNA molecule. The sequence of nucleotides in DNA determines the sequence of amino acids in proteins, which in turn determines the structure and function of the proteins. DNA also plays a role in gene expression, which is the process by which genetic information is used to produce proteins.
Types of DNA
There are several types of DNA, including:
- B-DNA: This is the most common form of DNA found in cells. It is a right-handed, double-stranded helix that forms a regular spiral shape.
- A-DNA: This is a right-handed, double-stranded helix that is shorter and wider than B-DNA. It is often found in DNA-RNA hybrids and in certain conditions where the water content is low.
- Z-DNA: This is a left-handed, double-stranded helix that has a zigzag pattern. It is usually found in regions of DNA that are rich in guanine-cytosine (GC) base pairs and may play a role in gene regulation.
- Mitochondrial DNA (mtDNA): This is a circular DNA molecule found in the mitochondria of cells. It is inherited from the mother and is used to produce proteins required for cellular respiration.
- Chloroplast DNA (cpDNA): This is a circular DNA molecule found in the chloroplasts of plant cells. It is used to produce proteins required for photosynthesis.
- Satellite DNA: This is a type of non-coding DNA that is composed of short, repetitive sequences that are tandemly arranged. It is found in the centromeres and telomeres of chromosomes and may play a role in chromosome structure and function.
- Transposable elements: These are mobile genetic elements that can move around the genome. They are found in both prokaryotic and eukaryotic cells and can cause mutations or changes in gene expression.
- Telomeric DNA: This is the repetitive DNA sequence found at the ends of chromosomes. It helps protect the genetic information by preventing the loss of important DNA sequences during cell division.
Each of these types of DNA has a unique structure and function that is essential to the proper functioning of cells and organisms.