Structure of DNA and DNA replication

 

Structure of DNA:

DNA (Deoxyribonucleic Acid) is the molecule that carries genetic information in living organisms and some viruses. It has a unique double-helix structure composed of two complementary strands. Each strand consists of a backbone made of alternating sugar (deoxyribose) and phosphate groups, with nitrogenous bases attached to the sugars.

1. Double Helix: DNA is shaped like a twisted ladder or a spiral staircase. The sides of the ladder are the sugar-phosphate backbones, and the rungs are pairs of nitrogenous bases.

2. Nucleotide: The basic unit of DNA is the nucleotide, which consists of three components:

   • A phosphate group.
   • A deoxyribose sugar.
   • A nitrogenous base.

3. Nitrogenous Bases: There are four types of nitrogenous bases:

   • Adenine (A).
   • Thymine (T).
   • Cytosine (C).
   • Guanine (G).

   These bases pair specifically: Adenine always pairs with Thymine (A-T) and Cytosine always pairs with Guanine (C-G). These pairs are held together by hydrogen bonds (A-T has two bonds, C-G has three bonds).

4. Antiparallel Orientation: The two strands of DNA run in opposite directions, meaning one strand runs 5' to 3', while the other runs 3' to 5'. This antiparallel arrangement is crucial for DNA replication and function.

DNA Replication:

DNA replication is the process by which a cell copies its DNA before cell division. It ensures that each new cell receives an exact copy of the DNA.

1. Initiation:

   • Replication begins at specific sites called origins of replication.
   • Helicase enzymes unwind the DNA double helix, creating a replication fork by breaking the hydrogen bonds between the base pairs.

2. Elongation:

   • Primase synthesizes a short RNA primer to provide a starting point for DNA synthesis.
   • DNA polymerase adds nucleotides to the 3' end of the RNA primer, synthesizing the new strand in the 5' to 3' direction.
   • One strand, called the leading strand, is synthesized continuously toward the replication fork.
   • The other strand, called the lagging strand, is synthesized discontinuously in short segments called Okazaki fragments, which are later joined by DNA ligase.

3. Proofreading and Correction:

   • DNA polymerase has proofreading abilities to correct errors during replication by removing mismatched nucleotides and replacing them with the correct ones.

4. Termination:

   • Once the entire DNA molecule has been replicated, replication is completed. The result is two identical DNA molecules, each with one original (parental) strand and one newly synthesized strand. This is called semi-conservative replication, as each new DNA molecule conserves one of the original strands.

Summary:

• DNA has a double-helix structure composed of two antiparallel strands with complementary base pairs.
• DNA replication is a semi-conservative process that ensures the genetic information is accurately passed on to daughter cells. It involves unwinding the helix, synthesizing new strands using old strands as templates, and proofreading to correct errors.