DNA (Deoxyribonucleic Acid) and Its Replication
What is DNA?
DNA (Deoxyribonucleic Acid) is the molecule that carries the genetic instructions for all living organisms and many viruses. It is a long chain of nucleotides, which contain a phosphate group, a sugar molecule (deoxyribose), and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). DNA's structure is a double helix, where two strands are coiled around each other, held together by hydrogen bonds between complementary bases (A pairs with T, C pairs with G).
Structure of DNA:
- Double helix: Two strands of nucleotides twist around each other to form a spiral.
- Nucleotides: Building blocks of DNA, composed of:
- Phosphate group
- Deoxyribose sugar
- Nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).
- Complementary base pairing: A pairs with T (via 2 hydrogen bonds) and C pairs with G (via 3 hydrogen bonds).
DNA Replication:
DNA replication is a biological process by which DNA makes an exact copy of itself during cell division. It is essential for cell growth, repair, and reproduction. This process occurs in three main steps: initiation, elongation, and termination.
1. Initiation:
- Origin of replication: DNA replication begins at specific locations called origins of replication, where the two strands of the DNA double helix are separated to form a replication "bubble."
- Helicase enzyme: This enzyme unwinds the DNA strands by breaking the hydrogen bonds between the base pairs, creating two single strands that serve as templates for replication.
- Single-strand binding proteins (SSBs): These proteins stabilize the separated strands and prevent them from rejoining.
- Topoisomerase: This enzyme prevents the DNA from becoming too tightly coiled ahead of the replication fork by cutting and rejoining the DNA.
2. Elongation:
- Primase enzyme: Primase synthesizes short RNA primers on each DNA template. These primers provide the starting point for DNA polymerase, the enzyme responsible for adding new nucleotides.
- DNA Polymerase: This enzyme adds nucleotides to the 3' end of the RNA primer, following the rules of complementary base pairing (A pairs with T, C pairs with G). DNA polymerase can only synthesize DNA in the 5' to 3' direction.
- Since the DNA strands are antiparallel, replication on the two strands happens differently:
- Leading strand: The strand that is synthesized continuously in the 5' to 3' direction.
- Lagging strand: The strand that is synthesized discontinuously in short fragments, called Okazaki fragments, because it runs in the 3' to 5' direction. These fragments are later joined together.
3. Termination:
- RNA primer removal: Once the new DNA strand is formed, the RNA primers are removed by a special enzyme (DNA polymerase I in prokaryotes, or a similar enzyme in eukaryotes) and replaced with DNA nucleotides.
- DNA Ligase: The Okazaki fragments on the lagging strand are joined together by the enzyme DNA ligase, sealing the gaps between the fragments to create a continuous strand.
- Telomeres: At the ends of linear chromosomes in eukaryotes, telomeres protect the DNA from being degraded during replication. An enzyme called telomerase helps extend the telomeres in some cells (e.g., germ cells, stem cells).
Replication is Semi-conservative:
This means that each new DNA molecule consists of one old (parental) strand and one newly synthesized strand. This method helps ensure genetic consistency across generations of cells.
Summary of DNA Replication Steps:
- Initiation:
- Helicase unwinds the DNA.
- SSBs stabilize the strands.
- Topoisomerase prevents over-twisting.
- Primase adds RNA primers.
- Elongation:
- DNA polymerase synthesizes new strands (leading and lagging).
- Okazaki fragments form on the lagging strand.
- Termination:
- RNA primers are replaced by DNA.
- Ligase seals gaps in the DNA.
- Telomerase extends telomeres, if necessary.
This accurate replication process ensures that genetic information is faithfully passed from one generation of cells to the next.
