13. Bacterial genetics. Carriers of genetic information: Bacterial chromosomes- Structure, replication and function:
Covers the structure of the bacterial chromosome, including its circular double-stranded DNA, supercoiling by topoisomerases such as DNA gyrase, and the nucleoid, with E. coli given as a size example. It then walks through semi-conservative replication step by step, from the origin through helicase, primase, polymerase and ligase to termination, and ends with how bacterial genes are organised into operons and controlled, using the lac operon as the classic example.
Topic 13 of 113 in the MU Sofia syllabus for Microbiology (Dental Medicine, Year 2). The free AceMed note covers it in 2 sections, about 504 words (4 min of reading), with 13 flashcards.
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Bacterial genetics. Carriers of genetic information: Bacterial chromosomes- Structure, replication and function:
Bacterial Genetics:
Bacterial Genetics:
Structure of the Bacterial Chromosome
The bacterial chromosome is typically a single molecule.
It is circular (i.e., has no free ends). An exception is Borrelia, which has linear chromosomes.
It is double-stranded DNA.
The DNA is superspirilized or supercoiled, meaning it is folded many times. This supercoiling is necessary to save space and energy and is steered by enzymes called topoisomerases. DNA gyrase and topoisomerase IV are topoisomerases found only in bacteria. DNA gyrase twists the DNA.
It is located in a discrete area in the cytoplasm called the nucleoid, and it is not surrounded by a nuclear membrane. The lack of a nuclear membrane allows transcription and translation to occur simultaneously.
Bacterial DNA is not associated with histones (proteins that maintain DNA conformation in eukaryotes). Eukaryotic chromosomes are diploid and have histones. Bacterial chromosomes are haploid.
Compared to eukaryotic genomes, the bacterial chromosome is much smaller in length and number of base pairs. For example, E. coli has a chromosome of about 1.4 micrometres in length and 5 million base pairs, coding for around 4288 different proteins.
Replication of the Bacterial Chromosome
Bacterial DNA replication is a process of identical duplication.
It is semi-conservative, meaning each new double strand conserves one old strand.
Replication starts at a specific point called the origin of replication (Ori C).
The process is similar to eukaryotic DNA replication.
Helicase unwinds the DNA double helix at the origin, creating a replication fork.
Primase synthesises RNA primers to start the process.
DNA polymerase synthesises the new DNA copy in the 5' to 3' direction. It extends the primer.
Replication involves both a leading strand (synthesised continuously) and a lagging strand (synthesised discontinuously as Okazaki fragments).
DNA ligase joins the Okazaki fragments together on the lagging strand.
Replication continues throughout the entire fission cycle.
In bacteria, the resolution of circular DNA concatemers by topoisomerase IV is involved in termination.
Rapidly growing bacteria can copy their entire genome in approximately 25 minutes.
Function (Control) of the Bacterial Chromosome
Bacterial genes are organised in operons.
An operon is a group of one or more functionally related genes that are controlled collectively by a single promoter and terminator. They have common regulation.
Unlike eukaryotic DNA, bacterial genes typically do not have introns or exons.
Transcription and translation are coupled because there is no nuclear membrane. Ribosomes can bind to mRNA as it is being synthesised.
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