Causative Agents For Mutations

1. The properties of bacteria can change either through mutations, changes in the chemical structure of DNA, the gain of DNA from other cells, or by changes in gene expression.

2. Bacteria contain only a single set of genes (haploid) so any changes in DNA are expressed rapidly.

Gene Mutation

8.2 Spontaneous Mutations

1. Spontaneous mutations are changes in the nucleotide sequences in DNA that occur without the addition of agents known to cause mutations.

2. These mutations are rare and occur at a characteristic frequency for each gene.

3. They are stable but on rare occasion can undergo a change back to the non-mutant form—a reversion.

Base Substitution

1. Base substitutions usually occur during DNA replication; point mutations occur when only one base pair changes. (Figure 8.1)

Removal or Addition of Nucleotides (Figure 8.2)

1. Frame shift mutations involve the addition or deletion of nucleotides, rendering all genes downstream of a stop codon and in the same operon non-functional.

Transposable Elements (Jumping Genes) (Figure 8.24)

1. Certain genes, called transposons, have the ability to move to any other location in the genome.

2. Introduction of a transposon into another gene inactivates that gene. (Figure 8.3)

8.3 Induced Mutations (Table 8.1)

Chemical Mutagens

1. Chemical mutagens frequently alter hydrogen-bonding properties of purines and pyrimidines, increasing the frequency of mutations.

2. Base analogs with different hydrogen-bonding properties can be incorporated into DNA in place of the usual purines and pyrimidines. (Figure 8.4)

3. Intercalating agents are planar molecules that insert into the double helix and push nucleotides apart, resulting in a frameshift mutation.

Transposition

1. An insertion mutation results when a transposon integrates into a recipient cell's genome.

Radiation

1. Ultraviolet irradiation results in thymine dimers due to the formation of covalent bonds between adjacent thymine molecules on the same strand of DNA. (Figure 8.5)

2. X rays cause single-strand breaks, double-strand breaks, and alterations to the DNA bases.

8.4 Repair of Damaged DNA (Table 8.2)

Repair of Errors in Base Incorporation

1. DNA polymerase has a proofreading function.

2. In mismatch repair, an endonuclease cuts out the damaged single-stranded fragment and a new DNA strand is synthesized. (Figure 8.6)

Repair of Thymine Dimers

1. In light repair, a photoreactivating enzyme breaks the bonds of the thymine dimer, thereby restoring the original molecule. (Figure 8.7a)

2. In excision or dark repair, the damaged single-stranded segment is excised by an endonuclease. A new strand is synthesized by DNA polymerase. (Figure 8.7b)

SOS Repair

1. SOS repair is a last ditch repair mechanism in which about 20 enzymes are induced by damaged DNA, including a new DNA polymerase which bypasses the damaged DNA but without proofreading the DNA it synthesizes. Consequently, the synthesized DNA contains many mutations.

2. SOS repair accounts for the mutagenic activity of UV irradiation.

8.5 Mutations and Their Consequences

1. Genes mutate independently of one another, and the chance that two mutations will occur within the same cell is the product of the individual mutation rates.

2. Mutations provide a mechanism for altering the population of an organism to adapt to a changing environment—a process called natural selection.

8.6 Mutant Selection

Direct Selection

1. Direct selection involves inoculating cells onto a medium on which the mutant but not the parent can grow; these are the easiest kinds of mutants to isolate. (Figure 8.8)

Indirect Selection

1. Indirect selection is required when the desired mutant does not grow on a medium on which the parent grows.

2. Replica plating involves the simultaneous transfer of all the colonies on one plate to another and the comparison of the growth of individual colonies on both plates. (Figure 8.9)

3. Penicillin enrichment increases the proportion of mutants in a population by killing growing bacteria in a medium on which only non-mutants will grow. (Figure 8.10)

Conditional Lethal Mutants

1. Conditional lethal mutations cannot be overcome by adding growth factors to the medium. (Figure 8.11)

Testing of Chemicals for Their Cancer-Causing Ability

1. The Ames test measures whether a suspected carcinogen increases the frequency of reversion; a positive test indicates the subject chemical is a mutagen and therefore a likely carcinogen. (Figure 8.12)

Mechanisms of Gene Transfer

8.7 DNA-Mediated Transformation (Table 8.3)

1. DNA-mediated transformation involves the transfer of "naked'' DNA. (Figure 8.14)

Natural Competence

1. Natural competence is the ability of a cell to take up DNA.

2. DNA enters the cell as a single-stranded molecule and is integrated by replacing recipient cell genes via homologous recombination.

Artificial Competence

1. Cells can be made artificially competent by electroporation; in this process, the cells are treated with an electric current, which makes holes in the cell envelope through which DNA can pass. (Figure 8.15)

8.8 Transduction

1. Transduction involves the transfer of bacterial DNA by a bacteriophage. (Figure 8.16)

8.9 Conjugation

1. Conjugation requires cell-to-cell contact. (Figure 8.17)

Plasmid Transfer

1. The donor cells synthesize a sex pilus encoded on a F plasmid, which recipient cells do not have; the F plasmid is transferred from an F+ to an F: cell. (Figure 8.18)

Transfer of Plasmids Other than F

1. The F plasmid can often mobilize and transfer other plasmids to the recipient cell.

Chromosome Transfer

1. Chromosome transfer occurs when the F plasmid integrates into a chromosome and the resulting cell can transfer a portion of the chromosome into a recipient cell. (Figure 8.20)

F' Donors

1. In an F' donor, the F plasmid excises a fragment from the chromosome and transfers it as the F plasmid is transferred.

8.10 Plasmids (Table 8.4)

1. Plasmids are extrachromosomal replicons that code for non-essential information.

Review Questions 217

R plasmids

1. R plasmids code for antibiotic resistance; many are self-transmissible to other bacteria. (Figure 8.22)

8.11 Genetic Transfer of Virulence Factors

1. Genes coding for virulence factors are transferred between bacteria by transformation, transduction, and conjugation.

2. Genes coding for virulence factors are frequently grouped together in pathogenicity islands.

8.12 Gene Movement Within the Same Bacterium— Transposable Elements

Transposons and Transfer of Genes to Unrelated Bacteria (Figure 8.23)

1. Transposable elements often transfer antimicrobial-resistance genes from a chromosome to a plasmid. Plasmids can be readily transferred by conjugation to unrelated bacteria.

Structure of Transposons

1. There are different types of transposons, which differ in complexity; all have inverted repeat sequences at their ends.

2. An insertion sequence (IS) contains a gene that codes for the enzyme transposase. (Figure 8.24a)

3. A composite transposon consists of one or more genes flanked by insertion sequences. (Figure 8.24b)

8.13 Barriers to Gene Transfer

Restriction of DNA

1. DNA entering unrelated bacteria is recognized as foreign and is degraded by a class of deoxyribonucleases, called restriction enzymes, that recognize specific nucleotide sequences. (Figure 8.25)

Modification Enzymes

1. The DNA inside a cell is not cleaved, because it is methylated at the potential cleavage sites by a modification enzyme that is paired with the restriction enzyme to form a restriction-modification system. (Figure 8.26)

8.14 Importance of Gene Transfer to Bacteria

1. Gene transfer allows bacteria to survive changing environments by providing cells with a set of new genes.

2. Mutations only result in a single gene being modified.

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  • Lalli
    What are mutations and its causitive agents?
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    What Is Mutation,its Causative Agent And Effects?
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    What Are The Agents Of Mutation?
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    What are the causative factor of mutation?
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    What are the specific causative agents of mutation?
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  • gorbadoc brandybuck
    What are the causation agent of mutation?
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    What are the causative facters for mutation?
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  • melody
    What is the name purine purine and pyrimidine pyrimidine mutation?
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