This article contains information about the mysterious and complex molecular structures of various cells, most often bacteria, - plasmids. Here you will find information about their structure, purpose, methods of replication, general characteristics and much more.
What are plasmids
Plasmids are DNA molecules that are small in size and are physically separated from genomic type chromosomes. Have the ability to autonomous replication process. Basically, plasmids are found in bacterial organisms. Externally, it is a molecule having a ring double-stranded appearance. It is extremely rare that plasmids can be found in archaea and eukaryotic organisms.
As a rule, bacterial plasmids contain genetic information that can increase the body's resistance to external factors that adversely affect the state of the organism in which they are located. In other words, plasmids can reduce the effectiveness of antibiotics due to the increased resistance of the bacterium itself. Often there is a process of transfer of plasmids from bacteria to bacteria. Plasmids are structural elements that are a means of efficiently transferring genetic information in a horizontal way.
D. Lederberg, a molecular biologist, a scientist from the United States, introduced the concept of plasmid in 1952.
Dimensional values of plasmids and their numbers
Plasmids are structures with a wide variety of sizes. The smallest forms can contain about two thousand paired bases or less, while the other, largest forms of plasmids, contain several hundred thousand paired types. Knowing this allows you to draw a line between megaplasmids and mini-chromosomes. There are bacteria that can contain various types of plasmids. Moreover, the total amount of their genetic material may exceed the size of the material of the host cell.
The number of copies of plasmids located in one cell can vary greatly. For example, in one cell there can be only a couple, while in another, the number of plasmids of the same type reaches tens or hundreds. Their number is due to the replication nature.
Plasmids are cellular structural elements capable of autonomous replication. That is, they can replicate on their own without being subjected to chromosome control. At the same time, the chromosome can control the plasmids themselves. In the case of strict control, the number of replicated plasmids is usually small, about 1-3. Small plasmids are more often exposed to a weakened type of control and can produce more copies.
Replication process
Bacterial plasmids are able to autonomously replicate. However, this process is subject to chromosome control to varying degrees. This is due to the lack of some necessary genes. In view of this, cellular enzymes are included in the process of plasmid replication.
The replication stage is divided into the stage of initiation, elongation and termination. DNA polymerase will begin to replicate only after it is seeded with a primer. First, the chain opens and RNA priming occurs, followed by one of the chains breaking and the free 3`-OH end is formed.
Most often, the initiation step occurs under the influence of protein catalysts encoded by the plasmid. Sometimes these same proteins can enter into the process of primer processing.
Elongation occurs with the help of a holoenzyme of DNA polymerase III (sometimes I) and some cellular proteins consisting in replicom.
Termination of replication can begin only if certain conditions exist.
Principles of Replication Control
Replication mechanisms are controlled at the stage of replication initiation. This allows you to keep the number of plasmids in a strict amount. Molecules capable of carrying it out include:
- RNAs having the opposite polarity.
- DNA is a sequence (iteron).
- RNAs having opposite polarity and proteins.
These mechanisms determine the frequency of repeating cycles of plasmid reconstruction within the cell; they also fix any deviations from the norm of the frequency.
Types of Replication Mechanisms
There are three mechanisms for plasmid replication:
- The theta mechanism consists of the stage of unwinding of 2 parental chains, synthesis of an RNA primer on each chain, replication initiation due to an increase in the covalent type of rRNA on both chains and synthesis of the corresponding DNA chain on the parent chains. Despite the fact that the synthesis process occurs simultaneously, one of the chains is the leader, and the other is behind.
- Substitution of a chain is the replacement of one of the parent DNA with a newly synthesized DNA chain. As a result of this mechanism, single-chain type ring DNA and double-stranded supercoiled DNA are formed. DNA from a single chain will be restored later.
- Rolling Ring Replication Mechanism — Represents the breaking of single-stranded DNA using the Rep protein. As a result of this, a 3`-OH group is formed, which will act as a primer. This mechanism proceeds with the help of various proteins of the host cell, for example, DNA helicase.
Transmission methods
Plasmids enter the cell using one of two paths. The first way is to establish contact between the host cell and the cell that does not contain plasmids as a result of the conjugation process. There are conjugative plasmids in gram-positive and gram-negative bacteria. The first method also includes transmission at the time of transduction or transformation. The second way is carried out artificially, by introducing plasmids into the cell, while the body must survive the expression of the genes of the host cell, that is, acquire the competence of the cell.
Functions Performed
The role of plasmids, as a rule, is to impart certain properties to the host cell. Some of them can practically not affect the phenotypic characteristics of their host, while others can cause the carrier to display properties that give it superiority over other similar cells. This superiority will help the host cell better survive the harmful conditions of the environment in which it lives. In the absence of such plasmids, the cell will either grow and develop poorly, or completely die.
Plasmids are a multifunctional compound cell. They perform a huge number of functions:
- Transport of genetic information during the course of conjugation. This is usually done by F plasmids.
- Bacteriocinogenic plasmids control protein synthesis, which can lead to the death of other bacteria. This is mainly done by Col plasmids.
- Hly plasmid synthesizes hemolysin.
- Resistance to heavy metals.
- R-plasmid - increases resistance to antibiotic agents.
- Ent-plasmid - allows enterotoxins to be synthesized.
- Some of them increase the degree of resistance to ultraviolet radiation.
- Plasmids of colonization antigens allow bacterial adhesion to pass on the cell surface inside the animal organism.
- Certain of their representatives are responsible for the cut of the DNA chain, that is, for restriction, as well as modification.
- CAM plasmids cause camphor cleavage, XYL plasmids cleave xylene, and SAL plasmids cause salicylate.
The most studied species
The most well-studied person has studied the properties of plasmids F, R and Col.
F-plasmid is the most famous conjugative plasmid. It is an episode consisting of one hundred thousand double type bases. It has its own point of replication origin and break point. Like other conjugative plasmids, it encodes proteins that can counteract the process of attaching pili of other bacterial organisms to the wall of a particular cell.
In addition to standard information, it contains the tra and trb loci, which organize a common, integral operon containing thirty-four thousand paired bases. The genes located in this operon are responsible for various aspects of conjugation.
R-plasmid (factor) - is a DNA molecule and has a circular shape. The plasmid DNA contains information that is responsible for the course and implementation of the process of replication and transfer of resistant properties into the recipient cell. They also determine the level of cell resistance to certain antibiotics. Some of the R plasmids are conjugative. R-factor transmission occurs as a result of transduction and standard cell division. They are capable of being transmitted between distinct species or even families.
It is this form of plasmid that often causes problems in the treatment of diseases of a bacterial nature when using the currently known antibiotic agents.
Col plasmids are responsible for the synthesis of colicin, a special protein that can inhibit the development and reproduction of all bacteria except the carrier itself.
Classification characteristic
The entire classification system is built in accordance with some properties of plasmids:
- Replication methods and its flow mechanism.
- The presence of a common circle of carriers.
- Features copying.
- Topological characteristics of plasmids.
- Compatibility.
- Non / conjugative plasmids.
- The presence of a marker gene located on the plasmid.
However, any way of classifying them contains a replication initiation point.
Scopes of plasmids
The function of plasmids when used by humans is to create a cloned copy of DNA. The plasmids themselves act as a vector. The replication ability of plasmids allows you to recreate recombinant DNA in a host cell. They found widespread use in genetic engineering. In this branch of science, plasmids are created artificially to transfer information of a genetic type or any manipulation of genetic material.
The concept of these cellular components is also found in the gaming industry ("BioShock"). Plasmids perform the function of special substances that can give the body unique properties. It is important to know that game plasmids have practically nothing to do with real ones. In a game made in the genre of a shooter with RPG elements called Bioshock, plasmids are a genetic modification of certain properties of the body, their change and the way of giving supernormal abilities.