DNA Replication and Operons: Mechanisms in Prokaryotes and Eukaryotes
DNA Replication in Prokaryotes: Initiation and Elongation
Initiation Phase: In prokaryotes, DNA replication begins with the initiation phase.
Elongation Phase: This phase involves the synthesis of a new DNA strand. Several DNA polymerase enzymes are involved, primarily DNA polymerase I, II, and III. These enzymes have a dual function:
- Polymerase Activity (5′-3′): Nucleotides bind to the DNA, recognizing the template strand and selecting complementary deoxyribonucleotides. The energy for forming new bonds comes from the hydrolysis of two phosphate groups.
- Exonuclease Activity (3′-5′): Mismatched nitrogenous bases and RNA fragments synthesized by primase are removed.
DNA polymerase III synthesizes the leading strand continuously in the 5′-3′ direction. It requires a primer synthesized by primase to initiate synthesis. The other strand, called the lagging strand, is synthesized in short, discontinuous fragments known as Okazaki fragments (1000-2000 nucleotides long). These fragments are later joined together.
DNA polymerase I eliminates primers and corrects errors during replication due to its exonuclease activity, filling gaps with DNA nucleotides. Error correction is also facilitated by the methylation of adenines. Finally, DNA ligase seals all the DNA fragments together.
Eukaryotic DNA Replication: Key Differences
DNA replication in eukaryotes occurs within the nucleus and differs from prokaryotic replication due to its complexity:
- Eukaryotic chromosomes contain long DNA molecules. To shorten the replication process, replication is initiated at multiple sites called replicons. However, the replication velocity is slower.
- Eukaryotes have five types of DNA polymerase instead of three. The size of Okazaki fragments is smaller.
- Eukaryotic DNA is associated with histones, proteins that are duplicated during replication.
DNA replication extends to the telomeres. Telomere shortening is addressed by telomerase, which elongates the telomeres. Telomere shortening and telomerase activity are linked to aging and cell death.
Operons: Gene Regulation in Prokaryotes
An operon is a cluster of genes that interact to regulate transcription. It consists of:
- Structural Genes: Encode proteins involved in the same metabolic process, resulting in a polycistronic mRNA.
- Promoter: A nucleotide sequence where RNA polymerase binds to initiate transcription.
- Operator: A nucleotide sequence located between the promoter and structural genes.
- Inductor: A substrate whose presence induces gene expression.
- Regulated Gene: Encodes a repressor protein. When the repressor binds to the operator, RNA polymerase cannot bind to the DNA, preventing transcription.
This mechanism is typical of prokaryotes. The lactose operon (lac operon) is a well-known example. It regulates the synthesis of enzymes involved in lactose metabolism and consists of three structural genes (lacZ, lacY, and lacA) and lactose as an inducer. When lactose is absent, RNA polymerase does not bind to DNA, and transcription does not occur. When lactose is present, it binds to the repressor, forming an inactive repressor-inducer complex, initiating transcription to synthesize the necessary enzymes.