In this article, author has explained the introduction, structure and functions of DNA. author has also explained the characteristics of double helix structure of DNA and replication of DNA.
DNA is a nucleic acid that is a polymer of nucleotides and is responsible for the transmission of hereditary information from parent to offspring.
DNA also plays an essential role in the synthesis of proteins. The DNA present in the nucleus of a cell (nuclear DNA) codes for the genome of the majority of organisms. The DNA present in mitochondria is known as mitochondrial DNA. It is also inherited from mothers to children. Mitochondrial DNA has about 16,000 base pairs. Plastids also have their own DNA which play role in photosynthesis.
Structure of DNA
DNA has a helix structure. It is composed of double strands. Each strand is made of nucleotides. Nucleotides are building blocks of DNA. Each nucleotide consists of three components; a nitrogenous base, a pentose sugar, and a phosphate group.
Four types of nitrogenous bases are present in the DNA; adenine, thymine, guanine, and cytosine. Adenine links to thymine through two hydrogen bonds while guanine links to cytosine through three hydrogen bonds. Therefore, the guanine-cytosine pair is stronger than the adenine-thymine bond. Adenine always forms a bond with thymine while cytosine always forms a bond with guanine. This is called complementary base pairing.
The sugar is deoxyribose in DNA. The phosphate group consists of a phosphorus atom attached to four oxygen atoms.
DNA is a polymer of deoxyribonucleotides. The monomeric units in DNA are deoxythymidylate (dTMP), deoxyadenylate (dAMP), deoxyguanylate (dGMP), deoxycytidylate (dCMP). The monomeric units are linked by phosphodiester bonds. The phosphodiester bond links the 5 carbon of one sugar to the 3 carbon of the second sugar.
The sugar (deoxyribose) and phosphate group form the backbone of DNA. The nucleotides of opposite strands of DNA are attached by hydrogen bonds. The hydrogen bond is much weaker than the phosphodiester bond but there are many hydrogen bonds so, collectively they are stronger. It forms a helix-like structure. The strands are helically twisted and each turn consists of 10 nucleotides. The two strands of DNA are antiparallel means the 5 end of one strand is parallel with the 3 ends of the other strand.
Characteristics of the double helix structure
- The double helical structure of DNA was discovered by James Watson and Francis Crick in 1953.
- DNA is a right-handed helical structure. It consists of two strands that twist around each other.
- The two strands of DNA are anti-parallel. One strand runs in 5 to 3 directions while the other in 3 to 5 directions.
- The diameter of the double helix is 20Å (2 nm).
- Each turn of the helix is 34Å consisting f 10 base pairs. The distance between the base pair is 3.4Å.
- The hydrophilic sugar-phosphate backbone is present on the outside while hydrophobic bases are present in the core.
- The two strands of DNA are not identical but complementary to each other. The base pairing is complementary base pairing.
- The bonds present between the two strands are hydrogen bonds. These bonds are formed between complementary base pairs. The Guanine-cytosine bond is 50% stronger than the adenine-thymine bond.
- The hydrogen bond is formed between purine and pyrimidine. The arrangement of base pairs possible in DNA is A-T, T-A, G-C, and C-G.
- The complementary base pairing in DNA is according to Chargaff’s rule. The content of adenine is always equal to thymine and the content of guanine is always equal to cytosine.
- One of the two strands which contain genetic information is called a template or sense strand while the other strand is the antisense strand.
- The double helix has also grooves; major (wide) grooves and minor (narrow) grooves along the phosphodiester bond. the proteins interact with DNA through these grooves without disrupting the other structure.
Functions of DNA
- DNA is a hereditary substance that carries all the genetic information. It is responsible for the transmission of genetic information from one generation to the next.
- DNA consists of small segments called genes. Gene further consists of three nucleotides segment called a codon.
- A codon is a segment of double-stranded DNA that is responsible for coding other polymers like proteins.
- DNA is involved in the synthesis of proteins through RNA.
- DNA has a major role in the identification of criminals. Through DNA, it becomes easy to reach the actual criminal.
- DNA fingerprinting has a major role in the identification of dead bodies. It is also used in criminal investigations.
- DNA is used in paternity tests for the identification of the parents of the child.
The replication of DNA is an important process that occurs during cell division. The replication occurs in three stages;
The replication of DNA begins at a point known as the origin of replication. The two strands of DNA are separated by the enzyme DNA helicase. It forms a replicating fork.
Another enzyme DNA polymerase reads the nucleotides on the sense/template strand and adds the complementary base pairs. For example, if adenine is present on the sense strand then thymine is added as complementary base pair.
When base pairs are added to the strand, gaps are formed between them. These gaps are known as Okazaki fragments.
The termination is initiated by the termination sequence present on the opposite side of the origin of replication. A substance known as a terminus utilization substance binds with the termination sequence. This stops the movement of DNA polymerase thus, induces the termination.