Origin of Replication in DNA

Initiation of DNA Replication

• Bacterial chromosomes and plasmids have one origin of replication, whereas archaeal and eukaryotic chromosomes have multiple sites.
• A molecule lacking this sequence will not be replicated.

Process of DNA Replication

• The process requires the coordinated action of many enzymes and proteins, including DNA polymerases, helicases, and primases.
• DNA polymerases synthesize DNA in the 5' to 3' direction, using one parent strand as a template.
• Primases synthesize short stretches of RNA complementary to the exposed templates.
• RNA primers are required at the origin of replication.

Process of DNA Synthesis

• DNA polymerase adds nucleotides onto the 3' end of the new strand, powering the reaction with the energy released from high-energy phosphate bonds.
• DNA polymerases add nucleotides only onto an existing nucleotide strand, so they cannot initiate synthesis.

Frameshift Mutation

• The addition of a nucleotide pair to the DNA results in a shift in the reading frame when the sequence is transcribed and translated.
• Deletion of a single nucleotide pair in the DNA would have a similar effect.

Transposons (Jumping Genes)

• Transposons are pieces of DNA that can move from one location to another in a cell's genome.
• The gene into which a transposon jumps is insertionally inactivated, meaning that the inserted DNA disrupts the function of the gene.
• Most transposons contain transcriptional terminators, which stop the expression of downstream genes in the same operon.

Barbara McClintock's Discoveries

• Barbara McClintock discovered transposons and their role in colour variation in corn kernels.
• She observed that the kernel colours were not inherited in a predictable manner and concluded that segments of DNA were moving into and out of genes involved in kernel colour.

Induced Mutations

• Induced mutations are genetic changes that occur due to an influence outside of a cell, such as exposure to a chemical or radiation.
• Chemical mutagens can cause base substitutions or frameshift mutations.

Homologous Recombination

• The molecular mechanisms involved in homologous recombination are still not fully understood.
• Donor DNA replaces a homologous segment of recipient DNA, and the DNA it replaced is degraded.

DNA-Mediated Transformation

• DNA-mediated transformation involves the uptake of "naked" DNA by recipient cells.
• Naked DNA is free in the cells' surroundings and is not contained within a cell or a virus.
• The process requires competence, a specific physiological state that allows the cell to take up DNA.

F' Donors

• Hfr strains can revert to F+ because the process of F plasmid integration is reversible.
• F' plasmids are formed when an error occurs during excision, and a piece of the bacterial chromosome is removed along with the F plasmid DNA.

The Mobile Gene Pool

• The mobile gene pool refers to the genes that vary considerably among different strains of a species.
• These genes can move from one DNA molecule to another, carried on mobile genetic elements including plasmids, transposons, genomic islands, and phage DNA.

The Core Genome

• The core genome refers to the genes that are conserved between strains of a species.
• The genes in the core genome vastly outnumber the sequences in the mobile gene pool.

Plasmids

• Plasmids are common in the microbial world and are found in many bacteria, archaea, and some eukarya.
• Plasmids are double-stranded DNA molecules with an origin of replication and can be replicated independently of the chromosome before the cell divides.