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    if the gene for cell wall material is mutated in the bacterial dna, what happens to the structure of cell in the bacteria?

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    Bacterial DNA Mutations

    Genomes of bacteria exist on a single double-stranded circular DNA molecule that contains approximately 4000 kb of DNA and are regulated by operons. A mutation is a change in the nucleotide sequence and can create new cellular functionalities or lead to the dysfunction of others. Mutations can occur spontaneously or be caused by exposure to mutation-inducing agents.[1]

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    Bacterial DNA Mutations

    Shelby Watford; Steven J. Warrington.

    Author Information

    Last Update: April 14, 2022.

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    Introduction

    Genomes of bacteria exist on a single double-stranded circular DNA molecule that contains approximately 4000 kb of DNA and are regulated by operons. A mutation is a change in the nucleotide sequence and can create new cellular functionalities or lead to the dysfunction of others. Mutations can occur spontaneously or be caused by exposure to mutation-inducing agents.[1]

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    Function

    While the majority of bacterial genes exist on one circular chromosome, there are other genetic elements within the bacterial genome. Elements like plasmids, transposons, integrons, or gene cassettes are shorter sequences that mainly contribute to recombination events. Bacterial DNA replication and transcription co-occur and utilize the same template DNA. Replication forks proceed bi-directionally with a single origin of replication, OriC.

    Bacterial genes with similar functions often share one promoter (RNA polymerase binding site) and are transcribed simultaneously; this system is called an operon. Typical operons consist of several structural genes that code for the enzymes required for the pathway. Regulation occurs through transcription factors binding to a short sequence of DNA between the promoter region and the structural genes called an operator.[2]

    A mutation is a change in the nucleotide sequence of a short region of a genome, and phenotypic results may vary on the severity and location of the mutation. Mutations can result from errors during DNA replication or induced by exposure to mutagens (like chemicals and radiation). Spontaneous mutations occur at a rate of 1 in 10^5 to 10^8 and contribute to random population variation.[3] Since bacteria are haploid for the majority of their genes and have short generation turnover, phenotypic variation due to point mutations can occur relatively quickly.

    Results of mutations can produce changes in structural or colony characteristics or loss in sensitivity to antibiotics. Some potential consequences of mutations are as follows:

    Auxotrophs: have a mutation that leaves an essential nutrient process dysfunctional.

    Resistant mutants: can withstand the stress of exposure to inhibitory molecules or antibiotics secondary to acquired mutation.

    Regulatory mutants have disruptions on regulatory sequences like promotor regions.

    Constitutive mutants: continuously express genes that usually switch on and off as in operons.

    Spontaneous Mutations

    Spontaneous mutations occur without mutation induction and are the result of errors during DNA replication. When DNA Pol III synthesizes a new strand of DNA, occasionally, a nucleotide will be mispaired, added, or omitted.[4] Thus, a point mutation will occur. For example, when nucleotides are mispaired, it will appear that one nucleotide substitutes for another leading to one mutated granddaughter DNA strand. Two separate malfunctions must happen in the bacteria's DNA replication machinery for this to occur:[5]

    DNA pol III pairs an incorrect complementary nucleotide base onto the parent strand in the replication fork

    The chemical activity of the mispairing is not enough to slow the polymerase portion of DNA polymerase so that the exonuclease can remove the mispair

    Studies with show that spontaneous mutations occur 20 times more often on the lagging strand than the leading strand.[6]

    DNA bases can exist in many different forms, referred to as tautomers. Nucleotide bases dominantly exist in the keto (C-O) and amino (C-NH2) forms, while the imino (C≡NH) and enol (C-OH) occur rarely. Tautomerization, during DNA replication, will alter nucleotide base pair formation. For example, assume that thymine undergoes keto-enol tautomerization during replication. This enol species will preferentially bind to guanine during the first replication cycle. Due to the semiconservative nature of DNA replication, at the end of the 2nd round of replication, there will be (3) A-T base pairs and (1) G-C in the locus of mutation.[7]

    The mechanism is as follows:

    T – A --> Tautomerization --> T' – A --> replication 1 --> T' – G and A –T

    T – G --> Replication 2 --> T – A and G – C

    (enol form of thymine indicated as T')[8]

    Errors in DNA replication can result in the addition of erroneous nucleotides or the deletion of template nucleotides. For example, loci with a high number of short repeat nucleotides are prone to polymerase slippage. During replication, the DNA Pol III temporarily dissociates from the template strand. The DNA polymerase may relocate a few repeats upstream or downstream of its original locus along with its newly synthesized strand.  Slip strand mispairing can result in addition/deletion mutations because some nucleotides are replicated twice while others do not replicate. If the repeats are not in a multiple of three, the mutation can result in a frameshift (A shift in the coding sequence downstream of the mutation). These mutations lead to loss of normal protein functionality. Slip-strand mispairings can increase the variation in short tandem repeats (STRs) in a bacterial population and are useful in genetic testing. When an addition or deletion occurs, the potential genomic outcomes are as follows:[9]

    स्रोत : www.ncbi.nlm.nih.gov

    3.1: Horizontal Gene Transfer in Bacteria

    Horizontal gene transfer enables bacteria to respond and adapt to their environment much more rapidly by acquiring large DNA sequences from another bacterium in a single transfer. Horizontal gene …

    3.1: Horizontal Gene Transfer in Bacteria

    Last updated Apr 9, 2022

    3: Bacterial Genetics

    3.2: Bacterial Quorum Sensing, Pathogenicity Islands, and Secretion Systems (Injectosomes)

    Gary Kaiser

    Community College of Baltimore Country (Cantonsville)

    Learning Objectives

    After completing this section you should be able to perform the following objectives.

    Compare and contrast mutation and horizontal gene transfer as methods of enabling bacteria to respond to selective pressures and adapt to new environments.

    Define horizontal gene transfer and state the most common form of horizontal gene transfer in bacteria.

    Briefly describe the mechanisms for transformation in bacteria.

    Briefly describe the following mechanisms of horizontal gene transfer in bacteria:

    generalized transduction

    specialized transduction

    Briefly describe the following mechanisms of horizontal gene transfer in bacteria:

    Transfer of conjugative plasmids, conjugative transposons, and mobilizable plasmids in Gram-negative bacteria

    F+ conjugation Hfr conjugation

    Describe R-plasmids and the significance of R-plasmids to medical microbiology.

    Bacteria are able to respond to selective pressures and adapt to new environments by acquiring new genetic traits as a result of mutation, a modification of gene function within a bacterium, and as a result of horizontal gene transfer, the acquisition of new genes from other bacteria. Mutation occurs relatively slowly. The normal mutation rate in nature is in the range of 10-6 to 10-9 per nucleotide per bacterial generation, although when bacterial populations are under stress, they can greatly increase their mutation rate. Furthermore, most mutations are harmful to the bacterium. Horizontal gene transfer, on the other hand, enables bacteria to respond and adapt to their environment much more rapidly by acquiring large DNA sequences from another bacterium in a single transfer.

    Horizontal gene transfer, also known as lateral gene transfer, is a process in which an organism transfers genetic material to another organism that is not its offspring. The ability of Bacteria and Archaea to adapt to new environments as a part of bacterial evolution most frequently results from the acquisition of new genes through horizontal gene transfer rather than by the alteration of gene functions through mutations. (It is estimated that as much as 20% of the genome of Escherichia coli originated from horizontal gene transfer.)

    Horizontal gene transfer is able to cause rather large-scale changes in a bacterial genome. For example, certain bacteria contain multiple virulence genes called pathogenicity islands that are located on large, unstable regions of the bacterial genome. These pathogenicity islands can be transmitted to other bacteria by horizontal gene transfer. However, if these transferred genes provide no selective advantage to the bacteria that acquire them, they are usually lost by deletion. In this way the size of the bacterium's genome can remain approximately the same size over time.

    There are three mechanisms of horizontal gene transfer in bacteria: transformation, transduction, and conjugation. The most common mechanism for horizontal gene transmission among bacteria, especially from a donor bacterial species to different recipient species, is conjugation. Although bacteria can acquire new genes through transformation and transduction, this is usually a more rare transfer among bacteria of the same species or closely related species.

    Transformation

    Transformation is a form of genetic recombination in which a DNA fragment from a dead, degraded bacterium enters a competent recipient bacterium and is exchanged for a piece of DNA of the recipient. Transformation usually involves only homologous recombination, a recombination of homologous DNA regions having nearly the same nucleotide sequences. Typically this involves similar bacterial strains or strains of the same bacterial species.

    A few bacteria, such as Neisseria gonorrhoeae, Neisseria meningitidis, Hemophilus influenzae, Legionella pneomophila, Streptococcus pneumoniae, and Helicobacter pylori tend to be naturally competent and transformable. Competent bacteria are able to bind much more DNA than noncompetent bacteria. Some of these genera also undergo autolysis that then provides DNA for homologous recombination. In addition, some competent bacteria kill noncompetent cells to release DNA for transformation.

    Figure 3.1.1 3.1.1

    : Pairing of Homologous DNA molecules and Exchange of DNA Segments by way of Rec A Protein. 1) A DNA endonuclease inserts a nick in one strand of the donor DNA. 2) The nicked strand is separated from its partner strand by proteins functioning as a helicase. Molecules of single-stranded binding protein (yellow) then bind. 3) Rec A protein then binds to the single-strand fragment and promotes base pairing of the donor DNA with the recipient DNA (crossing over). 4) The linked molecules are separated by resolvases, enzymes that cut and rejoin the cross-linked DNA molecules.

    During transformation, DNA fragments (usually about 10 genes long) are released from a dead degraded bacterium and bind to DNA binding proteins on the surface of a competent living recipient bacterium. Depending on the bacterium, either both strands of DNA penetrate the recipient, or a nuclease degrades one strand of the fragment and the remaining DNA strand enters the recipient. This DNA fragment from the donor is then exchanged for a piece of the recipient's DNA by means of RecA proteins and other molecules and involves breakage and reunion of the paired DNA segments as seen in (Figure

    स्रोत : bio.libretexts.org

    Conjugation, transformation & transduction

    Mechanisms that generate variation in prokaryote populations. Transduction, transformation, conjugation, transposable elements.

    Key points:

    In transformation, a bacterium takes up a piece of DNA floating in its environment.

    In transduction, DNA is accidentally moved from one bacterium to another by a virus.

    In conjugation, DNA is transferred between bacteria through a tube between cells.

    Transposable elements are chunks of DNA that "jump" from one place to another. They can move bacterial genes that give bacteria antibiotic resistance or make them disease-causing.

    Introduction

    When you hear the word "clone," what do you think of? Maybe Dolly the sheep, or experiments carried out in molecular biology labs. But it's also true that the bacteria around you—on your skin, in your gut, growing on your kitchen sink—are "cloning" themselves all the time!

    Bacteria reproduce by splitting in two via binary fission. Binary fission makes clones, or genetically identical copies, of the parent bacterium. Since the "child" bacteria are genetically identical to the parent, binary fission doesn't provide an opportunity for genetic recombination or genetic diversity (aside from the occasional random mutation). This contrasts with sexual reproduction.

    Still, genetic variation is key to the survival of a species, allowing groups to adapt to changes in their environment by natural selection. That's true for bacteria as well as plants and animals. So it's not too surprising that prokaryotes can share genes by three other mechanisms: conjugation, transformation, and transduction.

    Transformation

    In transformation, a bacterium takes in DNA from its environment, often DNA that's been shed by other bacteria. In a laboratory, the DNA may be introduced by scientists (see biotechnology article). If the DNA is in the form of a circular DNA called a plasmid, it can be copied in the receiving cell and passed on to its descendants.

    Left: plasmid taken up by transformation.

    Right: linear DNA fragment taken up by transformation and swapped into the bacterial chromosome by homologous recombination.

    Image modified from "Conjugation," by Adenosine (CC BY-SA 3.0). The modified image is licensed under a CC BY-SA 3.0 license.

    Why would this be important? Imagine that a harmless bacterium takes up DNA for a toxin gene from a pathogenic (disease-causing) species of bacterium. If the receiving cell incorporates the new DNA into its own chromosome (which can happen by a process called homologous recombination), it too may become pathogenic.

    Transduction

    In transduction, viruses that infect bacteria move short pieces of chromosomal DNA from one bacterium to another "by accident."

    Yep, even bacteria can get a virus! The viruses that infect bacteria are called bacteriophages. Bacteriophages, like other viruses, are the pirates of the biological world—they commandeer a cell's resources and use them to make more bacteriophages.

    However, this process can be a little sloppy. Sometimes, chunks of host cell DNA get caught inside the new bacteriophage as they are made. When one of these "defective" bacteriophages infects a cell, it transfers the DNA. Some bacteriophages chop the DNA of their host cell into pieces, making this transfer process more likely

    ^1 1

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    .

    Virus infects cell by injecting its DNA. Bacterial DNA is fragmented and viral DNA is replicated. New viral particles are made and exit the cell. One contains host DNA instead of viral DNA. When this virus infects a new host, it injects the bacterial DNA, which can recombine with the chromosome of the new hows.

    Image modified from "Conjugation," by Adenosine (CC BY-SA 3.0). The modified image is licensed under a CC BY-SA 3.0 license.

    Archaea, the other group of prokaryotes besides bacteria, are not infected by bacteriophages but have their own viruses that move genetic material from one individual to another.

    Conjugation

    In conjugation, DNA is transferred from one bacterium to another. After the donor cell pulls itself close to the recipient using a structure called a pilus, DNA is transferred between cells. In most cases, this DNA is in the form of a plasmid.

    An F+ donor cell contains its chromosomal DNA and an F plasmid. It has a rodlike pilus. A recipient F- cell has only a chromosome and no F plasmid.

    The donor cell uses its pilus to attach to the recipient cell, and the two cells are pulled together.

    A channel forms between the cytoplasms of the two cells, and a single strand of the F plasmid is fed through.

    Both of the cells now have an F plasmid and are F+. The former recipient cell is now a new donor and can form a pilus.

    Image modified from "Conjugation," by Adenosine (CC BY-SA 3.0). The modified image is licensed under a CC BY-SA 3.0 license.

    Donor cells typically act as donors because they have a chunk of DNA called the fertility factor (or F factor). This chunk of DNA codes for the proteins that make up the sex pilus. It also contains a special site where DNA transfer during conjugation begins

    स्रोत : www.khanacademy.org

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