DNA (Deoxyribonucleic acid)
DNA is genetic material (Genetic material is that substance that controls the formation and expression of traits in an organism and can replicate and pass on from a cell to its daughter cell and from one generation to the next).
DNA is located in the nucleus (in the form of chromatin or chromosome) in eukaryotic cells. And it is located in the cytoplasm (in the form of nucleoid) in prokaryotic cells.
Structure of DNA
DNA is the macromolecule that consists of two complementary strands of deoxyribonucleotides that run antiparallel and are held together by hydrogen bonds between their opposite nitrogen bases.
- The double helix structure or secondary structure of DNA was proposed by James Watson and Francis Crick in 1953.
- James Watson and Francis Crick suggested that the DNA molecule is made up of two chains of nucleotides.
- Each in a helix, one chain that went up and another that went down, which is antiparallel, which is called the double helix of DNA.
- James Watson and Francis Crick used Chargaff’s rules about the 1:1 base ratios to add to the model, determining that matching base pairs A + T and C + G, interlocked in the centre of the double helix, keeping a constant distance between the chains.
- DNA consists of four types of basic units called nucleotides. Each nucleotide is made up of a pentose sugar(deoxyribose), a phosphate group and a nitrogenous base.
- A subunit composed of only sugar and nitrogen base is known as a nucleoside.
Erwin chargaff formulated important generalizations about DNA structure, these generalizations are called Chargaff’s rules in his honour. They are summarized as:
- The purines and pyrimidines are always in equal amounts, i.e., A+G=T+C.
- The amount of adenine is always equal to that of thymine, and the amount of guanine is always equal to that of cytosine, i.e., A=Tand G=C. However, the amount of A+T is not necessarily equal to G+C.
- The base ratio A+T/G+C may vary from one species to another but is constant for given species.
Packing of DNA Helix in Eukaryotes
It is carried out with the help of lysine and arginine-rich basic proteins called histones. The unit of compaction is the nucleosome.
- There are five types of histone proteins – H1, H2A, H2B, H3, and H4. Four of them (H2A, H2B, H3, and H4) occur in pairs to produce histone octamer, called the nu-body or core of nucleosome.
- DNA connecting two adjacent nucleosomes is called inter bead or linker DNA. It bears H1 histone protein (called plugging protein and act as marker protein).
- Nucleosome and linker DNA together constitute chromatosome.
- The nucleosome constitutes the repeating unit of a structure in the nucleus called chromatin.
- Some regions of chromatin are loosely packed (and stain light) and are referred to as euchromatin.
- Some are more densely packed and stain dark is called heterochromatin.
- Euchromatin is transcriptionally active chromatin, whereas heterochromatin is inactive.