<< Chapter < Page Chapter >> Page >

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

Types of micro organisms
Jennifer Reply
Who discovered plant and animal cell
Jennifer
Robert Hooke
Martha
Father of microbiology
Jennifer Reply
Antonie van Leeuwenhoek
Satarupa
Why are mitochondria and chloroplasts unable to multiply outside of a host cell?
Iqra Reply
iqra mitochondria arises from the division of existing mitochondria and they are fused together. they move around inside the cell with the interactions of the cytoskeleton that's why mitochondria unable to multiply outside the host cells chloroplasts has its own separate DNA from the plant cell.
Rana
short not on medical microbiology
Massah Reply
Hy
Iqra
hope you fine
Arif
love you
Arif
Assalamualaikum
Suhaib
arai yeh micro kai sth kis ko love hua
Umer
icrobiology is the scientific study of these microorganisms. Microorganisms are those organisms that are too small to see with the naked eye and include things like bacteria, fungi, and viruses
Suhaib
iqra iqbal
Arif
What is gram syaining?
Arif
to differentiate between gram positive and gram negative bacty
Umer
True
Arif
where you are from
Arif
i m frm sopore..kashmir
Umer
i m frm baramulla .. Kashmir
Umer
and u
Umer
umer
Arif
and i m from afghanistan lovely country
Arif
yes
Umer
thanks
Arif
medical microbiology means study of microorganisms which are beneficial or harmful for the body. mostly which are studied under microscope by structure and shape appearance. e.g bacteria, viruses, bacteriophage, fungus hyphae and some blood and other body fluids parasites e.g plasmodium etc.
Rana
arif bhai gram staining in which we are given different dyes to the bacterial cell wall and cytoplasm and nucleus.e.g giemsa stain
Rana
Gram staining uses crystal violet to stain cell walls iodine as a mordant and a fuchsin or safranin counterstain to mark all bacteria
Umer
Gram staining uses crystal violet to stain cell walls iodine as a mordant and a fuchsin or safranin counterstain to mark all bacteria
Umer
Gram staining uses crystal violet to stain cell walls iodine as a mordant and a fuchsin or safranin counterstain to mark all bacteria
Umer
Omar baba crystal violet iodine 70% alcohol fuchicine or safranine after every step washing must be done
Rana
hello
Cristy
*Fuchsin stain
Cristy
*Safranin
Cristy
thanks
Ganesh
difference between epidermophyton trichophyton and microsporum
Ankita Reply
plz answer the question
Iqra
epidermophyton type of fungi causes superficial and cutaneous mycoses trichophyton is also fungi type including parasitic varieties causes dermatophytosis microsporum is also type of fungi causes dermatophytosis.
Rana
ok
Iqra
Is this app perfect for preparation of exam?
Ankita
Ankita I totally disagree with your respected opinion. it's just minor helpful
Rana
disagree by me ?
Ankita
what is infectious
Fred Reply
Many poeple are die due to covid-19 virus
AMINU Reply
they become sick due to millions bacteria, fungi and some insects that make plants as their host.
charles Reply
what are microbes and what are their effects to humans
charles Reply
they are organism that cannot be seen with the naked eye
Angel
classify microorganisms.
Satarupa Reply
protozoa' bacteria' virus' algai' archea'
Cabdifitaax
protozoa
Ayesha
Thank you
Satarupa
bacteria,fungi, protozoa and virus
Angel
mutant having a requirement for a certain growth factor is called a(n)
Purushoth Reply
what is atome
Marie Reply
is the smallest part of any material that cannot be broken up by chemical means
Feisal
How does forest tree become sick
Emmanuel
Aseptate hyphae are seen in
lab Reply
yes
Ahmed
w
Ahmed
hello
Apdikrim
what is ADT
Feisal
hii
Salman
how does this work bro
Feisal
CHEMISTRY
Apdikrim
biology
Ahmed
I.m student
Ahmed
I.m question what is difference between arthropod and insect?
Ahmed
Arthropod is a large division of jointed-foot Invertebrates such as Insects, centipedes and others so Insect is a part of the division or the phylum
hilina
thanks
Ahmed
ur welcome
hilina
describe binary fission
Queen Reply
in which parent cell devides into two daughter cells
Iqra
single parent cell
Tean
who is the father of microbiology
Mary Reply
Antonie van Leeuwenhoek , he is father of microbiology and Louis Pasteur is father of modern microbioloy.
Arshad
Anton van leeuwenhoek ( He descovered bacteria in 1679)
Satarupa
what is the contriyo sachilden and sachwan in cell thoery
Iqra

Get the best Microbiology course in your pocket!





Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Microbiology' conversation and receive update notifications?

Ask
Brooke Delaney
Start Exam
Marion Cabalfin
Start Quiz
Jesenia Wofford
Start Quiz