DNA Replication Process

DNA Replication

• DNA replication is a semi-conservative process.
• Replication in bacterial cells like E. coli requires DNA-dependent DNA polymerase.
• This enzyme is essential for polymerizing deoxyribonucleotides.
• E. coli replicates its 4.6 × 106 bp DNA in 18 minutes, with an average rate of 2000 bp per second.
• DNA polymerase needs to operate quickly and accurately to avoid mutations.
• The energy needed for replication comes from deoxyribonucleoside triphosphates, which also serve as substrates.
• Replication occurs within a small opening of the DNA helix called the replication fork.
• Replication occurs in the 5' to 3' direction.
• Because of this, replication is continuous on one strand (3' to 5') and discontinuous on the other (5' to 3').
• Discontinuous fragments are joined by the enzyme DNA ligase.
• DNA polymerase cannot initiate replication.
• Replication originates at a specific region called the origin of replication.
• DNA replication in eukaryotes occurs during the S phase of the cell cycle.
• Proper coordination between DNA replication and cell division is essential to prevent chromosomal anomalies like polyploidy.

Transcription

• Transcription is the process of copying genetic information from DNA to RNA.
• It follows the principle of complementarity, with uracil replacing thymine.
• Only a segment of DNA, and one strand, is copied into RNA during transcription.
• Both DNA strands aren't copied to avoid coding for different protein sequences and the formation of double-stranded RNA.

Transcription Unit and the Gene

• A gene is the functional unit of inheritance and is located on DNA.
• A cistron is a segment of DNA coding for a polypeptide.
• Eukaryotic structural genes are monocistronic and have split coding sequences called exons.
• Introns are non-coding sequences that are removed from mature RNA.
• Regulatory sequences that control gene expression are sometimes referred to as regulatory genes.

Types of RNA

• In bacteria, there are three major types of RNA: mRNA, tRNA, and rRNA.
• mRNA provides the template for protein synthesis.
• tRNA carries amino acids and reads the genetic code.
• rRNA plays structural and catalytic roles in translation.
• In bacteria, a single RNA polymerase is responsible for transcribing all types of RNA.
• RNA polymerase binds to the promoter and initiates transcription.
• Transcription proceeds in a template-dependent fashion following the rule of complementarity.

Genetic Code

• The genetic code translates genetic information from nucleotides to amino acids during protein synthesis.
• There is no direct complementarity between nucleotides and amino acids.
• George Gamow proposed that a combination of three nucleotides, called codons, code for each amino acid.
• There are 64 possible codons (4 x 4 x 4).
• Har Gobind Khorana's chemical methods helped to synthesize RNA molecules with defined base combinations.

Operon Concept

• An operon is a unit of genetic function in bacteria.
• The lac operon in E. coli consists of three genes: lac Z, lac Y, and lac A.
• These genes regulate lactose metabolism.
• The lac operon is regulated by a repressor protein.
• The repressor binds to the operator region and prevents transcription.
• In the presence of lactose, the repressor is inactivated, allowing RNA polymerase to transcribe the operon.
• The lac operon can also be regulated by its substrate, lactose.
• The regulation of the lac operon by the repressor is called negative regulation.
• The lac operon is also subject to positive regulation.

Human Genome Project

• The Human Genome Project aimed to sequence the entire human genome.
• This project revealed the complete DNA sequence of a human individual.
• The sequence provides insights into human genetics and disease.