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Pg:- 64,65

Ch:- 4

DNA, or deoxyribonucleic acid, is a molecule that carries the genetic instructions essential for the growth, development, functioning, and reproduction of all living organisms and many viruses. Often referred to as the “blueprint of life,” DNA provides the information necessary to build and maintain cells and pass on traits from one generation to the next.

Structure of DNA:

DNA is a long, double-stranded molecule that forms a helical structure, often described as a "double helix." Its shape resembles a twisted ladder, with two strands running in opposite directions (antiparallel).

1. Nucleotides: DNA is composed of repeating units called nucleotides. Each nucleotide consists of three parts:

   • Phosphate Group: Provides structural stability.

   • Deoxyribose Sugar: A five-carbon sugar that holds the phosphate and nitrogenous base in place.

   • Nitrogenous Base: There are four types of nitrogenous bases in DNA – adenine (A), thymine (T), cytosine (C), and guanine (G). These bases are crucial for storing genetic information.

2. Base Pairing: The nitrogenous bases pair up in a specific manner to form the "rungs" of the ladder. Adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). This pairing is due to hydrogen bonding, with A-T pairs forming two hydrogen bonds and C-G pairs forming three. The order of these base pairs constitutes the genetic code.

3. Double Helix: The two strands of DNA coil around each other to form a double helix. This structure provides stability to the molecule and helps protect the genetic information within.

The function of DNA:

The primary role of DNA is to store and transmit genetic information. It achieves this by encoding the instructions for making proteins, which perform virtually all functions in a cell.

1. Storage of Genetic Information: DNA contains all the genetic information necessary for building an organism. The sequence of bases (A, T, C, and G) encodes instructions for the development, functioning, and behavior of cells.

2. Replication: Before a cell divides, its DNA must replicate to ensure that each daughter cell receives a complete set of genetic instructions. During DNA replication, the double helix unwinds, and each strand serves as a template for the formation of a new complementary strand. This semi-conservative process produces two identical DNA molecules.

3. Gene Expression: Genes, specific segments of DNA, contain instructions to make proteins. Through the processes of transcription and translation, DNA is converted into functional proteins:

   • Transcription: A gene's DNA sequence is copied into messenger RNA (mRNA) in the nucleus. This mRNA then carries the genetic code out of the nucleus and into the cytoplasm.

   • Translation: The mRNA sequence is used as a template to build proteins, with the help of ribosomes and transfer RNA (tRNA), which assemble amino acids in the correct order according to the mRNA’s sequence.

4. Inheritance: DNA ensures that genetic traits are passed from one generation to the next. During reproduction, DNA is transmitted from parent to offspring, with specific sequences contributing to inherited characteristics.

DNA and Genetic Code:

The sequence of bases along a DNA strand forms the genetic code. This code is read in groups of three bases, called codons. Each codon specifies a particular amino acid, which are the building blocks of proteins. For example, the codon ATG codes for the amino acid methionine, and TGG codes for tryptophan. The sequence of codons in a gene determines the sequence of amino acids in the protein it encodes.

Variability and Mutation:

DNA is highly stable, but it can undergo changes or mutations. Mutations are changes in the DNA sequence that can be caused by errors during replication, exposure to chemicals, radiation, or viral infections. Some mutations can have no effect, while others may alter a protein’s structure or function, leading to potential diseases or variation in traits.

DNA in Forensics, Medicine, and Biotechnology

1. Forensics: DNA profiling is widely used in forensic science to identify individuals based on their unique DNA patterns.

2. Medicine: DNA research has enabled advancements in genetic testing, gene therapy, and personalized medicine, allowing for treatments tailored to an individual's genetic makeup.

3. Biotechnology: Techniques like genetic engineering and CRISPR gene editing allow scientists to modify DNA, leading to innovations in medicine, agriculture, and industry.