prokaryotes

• The structure and organisation of prokaryotic genome includes the description of the structure of bacterial chromosome, bacterial plasmid, and the organisation of DNA at the level of prokaryotic genome.
• Gene expression in prokaryotes can be regulated through the concept of simple operons, including the role of regulatory genes, and distinguishing between inducible and repressible systems.
• Prokaryotic genome can be compared with eukaryotic genome in terms of structure, organisation, number of nucleotides, packing of DNA, linearity/circularity, and presence/absence of introns.
• A prokaryote is a unicellular organism that lacks a membrane-bound nucleus (karyon), mitochondria, or any other membrane-bound organelle.
• Prokaryotes can be divided into two main groups of organisms, archaea and bacteria.
• DNA structure, organisation and regulation of genetic expression in bacteria is the main example for studying the genetics of prokaryotes.
• Nucleosomes undergo further packing to form 30 nm solenoids.
• Multi-levels of packing in DNA include the basic level, which is a "beads-on-a-string" model where DNA winds twice around a histone octamer to form 10 nm nucleosomes.
• These solenoids undergo further packing to form 300 nm looped domains.
• These looped domains undergo further packing to form 1400 nm metaphase chromosomes.
• DNA supercoiling refers to the supercoiling of the looped domain.
• Bacteria are small, unicellular microorganisms with an average size of 1 µm in diameter and 0.1 to 10 µm in length.
• Bacterial DNA is a single circular DNA molecule of about 5 x 10^6 base pairs and of length 1mm.
• Some bacteria possess protective layers called capsules and/or slime layers.
• Bacteria possess a cell wall which gives shape and rigidity to the bacteria.
• Genetic control in prokaryotes can be regulated through the concept of simple operons, including the role of regulatory genes, and can distinguish between inducible and repressible systems.
• Eukaryotes have transcription and translation processes separated in time and location, with transcription taking place in the nucleus and translation in the cytoplasm.
• The translation process in prokaryotes usually begins even before the transcription has been completed, with ribosomes starting to translate the mRNA template which are still being synthesised.
• In bacteria, structural proteins with related functions are usually encoded from genes found together within the genome in a block called an operon and are transcribed together under the control of a single promoter, resulting in the formation of a polycistronic mRNA transcript.
• The operon’s regulatory region includes both the promoter and the operator.
• A structural gene is a gene that codes for any RNA or protein product other than a regulatory factor.
• Bacterial cells have DNA sequences called operons.
• In prokaryotes, structural genes of related function are often organised together on the genome and transcribed together under the control of a single promoter.
• If a repressor binds to the operator, then the structural genes will not be transcribed.
• Alternatively, activators may bind to the regulatory region, enhancing transcription.
• These genes are regulated as one unit by a single promoter and an operator.
• Regulation of the transcription of all of the structural genes encoding the enzymes that catalyze the many steps in a single biochemical pathway can be controlled simultaneously because they will either all be needed at the same time, or none will be needed.
• Genetic activities such as replication and transcription that require strand separation would be enhanced by supercoiling as it causes tension between the two DNA strands, facilitating the separation of the two strands.
• While the polycistronic mRNA is translated as one mRNA, each gene contains their own start and stop codons which results in different polypeptides.
• These structural genes are transcribed together as one long mRNA molecule which is also known as a polycistronic mRNA.
• In E. coli, all of the structural genes that encode enzymes needed to use lactose as an energy source lie next to each other in the lactose (or lac) operon under the control of a single promoter, the lac promoter.
• An operon is defined by a region on a bacterial chromosome where a group of structural genes which code for proteins involved in the same metabolic pathway are grouped together.
• The control of gene expression in prokaryotes can be regulated at the transcriptional level through an inducible system of gene regulation, such as the lac operon.
• Plasmids are not usually attached to the plasma membrane and sometimes are lost to one of the daughter cells during division.
• Plasmids are additional genetic materials that may carry genes which code for functional products that provide a selective advantage, such as establishing mating capabilities, providing resistance to antibiotics, providing tolerance to toxic metals, providing new metabolic abilities.
• DNA supercoiling involves DNA being twisted, further twisting of DNA double helix leads to a secondary conformation change.
• Plasmids are not required for cell growth and reproduction.
• Plasmids are usually inherited and passed on to daughter cells through the process of binary fission.
• Many bacteria possess plasmids in addition to the bacterial chromosome.
• Formation of loop domains involves segments of bacterial DNA folded into structures that resembles a loop, this region is called the ‘loop domain’.