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Radiation

Exposure to either ionizing or nonionizing radiation can each induce mutations in DNA, although by different mechanisms. Strong ionizing radiation like X-rays and gamma rays can cause single- and double-stranded breaks in the DNA backbone through the formation of hydroxyl radicals on radiation exposure ( [link] ). Ionizing radiation can also modify bases; for example, the deamination of cytosine to uracil, analogous to the action of nitrous acid. K.R. Tindall et al. “Changes in DNA Base Sequence Induced by Gamma-Ray Mutagenesis of Lambda Phage and Prophage.” Genetics 118 no. 4 (1988):551–560. Ionizing radiation exposure is used to kill microbes to sterilize medical devices and foods, because of its dramatic nonspecific effect in damaging DNA, proteins, and other cellular components (see Using Physical Methods to Control Microorganisms ).

Nonionizing radiation, like ultraviolet light, is not energetic enough to initiate these types of chemical changes. However, nonionizing radiation can induce dimer formation between two adjacent pyrimidine bases, commonly two thymines, within a nucleotide strand. During thymine dimer formation, the two adjacent thymines become covalently linked and, if left unrepaired, both DNA replication and transcription are stalled at this point. DNA polymerase may proceed and replicate the dimer incorrectly, potentially leading to frameshift or point mutations.

a) Ionizing radiation (such as X-rays or gamma-rays) create double stranded breaks in DNA (breaks in the backbone). B) Non-ionizing radiation (such as ultraviolet light) causes Ts on the same strand of DNA to bind to each other rather than to the As across from them. This causes a kink in the DNA strand.
(a) Ionizing radiation may lead to the formation of single-stranded and double-stranded breaks in the sugar-phosphate backbone of DNA, as well as to the modification of bases (not shown). (b) Nonionizing radiation like ultraviolet light can lead to the formation of thymine dimers, which can stall replication and transcription and introduce frameshift or point mutations.
A Summary of Mutagenic Agents
Mutagenic Agents Mode of Action Effect on DNA Resulting Type of Mutation
Nucleoside analogs
2-aminopurine Is inserted in place of A but base pairs with C Converts AT to GC base pair Point
5-bromouracil Is inserted in place of T but base pairs with G Converts AT to GC base pair Point
Nucleotide-modifying agent
Nitrous oxide Deaminates C to U Converts GC to AT base pair Point
Intercalating agents
Acridine orange, ethidium bromide, polycyclic aromatic hydrocarbons Distorts double helix, creates unusual spacing between nucleotides Introduces small deletions and insertions Frameshift
Ionizing radiation
X-rays, γ-rays Forms hydroxyl radicals Causes single- and double-strand DNA breaks Repair mechanisms may introduce mutations
X-rays, γ-rays Modifies bases (e.g., deaminating C to U) Converts GC to AT base pair Point
Nonionizing radiation
Ultraviolet Forms pyrimidine (usually thymine) dimers Causes DNA replication errors Frameshift or point
  • How does a base analog introduce a mutation?
  • How does an intercalating agent introduce a mutation?
  • What type of mutagen causes thymine dimers?

Dna repair

The process of DNA replication is highly accurate, but mistakes can occur spontaneously or be induced by mutagens. Uncorrected mistakes can lead to serious consequences for the phenotype. Cells have developed several repair mechanisms to minimize the number of mutations that persist.

Questions & Answers

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namugenyi Reply
never recap or bend a sharp objects
benita
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Joy Reply
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Sadiya Reply
list three categories of symbiotic relationships.
mary Reply
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dranimva Reply
microbial intoxication results when a person ingests a toxin or a poisonous substance that has been produced by a microbe while infectious disease results when a pathogen colonize the body and subsequently cause disease.
mary
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It is mainly used for media preparation and product testing purpose.
Nakaweesi
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nuur
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Shamsuddeen Reply
study of living organisms that are too small to be visible with naked eye
Marah
yes
Ahmed
microbiology is the study of microscopic organisms, either unicellular(single cell), multicellular( cell colony), or acellular (lacking cell).
munachimso
is the word Atypical or a typical bacteria. .am confused pliz help
MUWANGUZI Reply
typical bacteria
Zulpha
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MUWANGUZI
atypical means that it has some characters from bacteria not all characters ...but tybical means that it has all the characters that bacteria have
Reham
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MUWANGUZI
some examples please
MUWANGUZI
thank you Reham
MUWANGUZI
1) typical bacteria contain a cell wall whereas atypical bacteria usually do not contain a cell . 2) typical bacteria can be either Gram-positive or Gram-negative while atypical bacteria remain colorless with Gram staining. 3) cells of typical bacteria are large ,while cells of the atypical small
Marah
Example of atypical : Mycoplasma pneumoniae , chlamydophila pneumoniae , legionella
Marah
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Jauharah Reply
is the study of organisms which can't be viewed by our necked eyes
Egumat
Because it preexisting causing secondary infection after collateral damage of normal microbota
Rafaa Reply
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Antagonism: the combined action is less than that of the more effective agent when used alone). All these effects may be observed in vitro (particularly in terms of bactericidal rate) and in vivo والله اعلم
Lenovo
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which of the following microorganisms are classified as eukaryotic?
Semugab Reply
how do I see the list?
Melissa
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Melissa
dear tell us the choices
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mr semugab give us the list please
Nambi
Fungi
munachimso
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bacteriology,viriology,micrology
Egumat
understanding of contributed to attempts to treat contain disease
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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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