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Molecular Biology Sabah Linjawi ١
Lecture 8
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Molecular Biology Sabah Linjawi ٢
Mutagenesis
A process by which the genetic information of an organism is changed in a stable manner
either in nature or experimentally by the use of chemicals or radiation
There are many different ways that DNA can be changed
1- Substitution
A substitution is a mutation that exchanges one base for another (CTG GAG to CTG GGG)
1-change a codon to one that encodes a different amino acid and cause a small change in the protein produced. For
example, sickle cell anemia is caused by a substitution in the beta-hemoglobin gene, which alters a single amino acid in the protein produced.
2-change a codon to one that encodes the same amino acid
and causes no change in the protein produced. These are
called silent mutations.
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Molecular Biology Sabah Linjawi ٣
Types of mutations
3-change an amino-acid-coding to a single "stop" codon and
cause an incomplete protein. This can have serious effects since the incomplete protein probably won't function.
2- Insertion
Insertions are mutations in which extra base pairs are inserted into a new place in the DNA. (CTG GAG to CTG GTG GAG)
3- Deletion
Deletions are mutations in which a section of DNA is lost, or deleted (CTG GAG to CTAG)
4- Frameshift
Since protein-coding DNA is divided into codons three bases long
insertions and deletions can alter a gene so that its message is no longer correctly parsed.
These changes are called frameshifts (CTG GAG TTT ATC to
TGG AGT TTA TCA)
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Molecular Biology Sabah Linjawi ٤
Types of mutations
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Replication fidelity
The accuracy or fidelity of DNA replication can be measured relatively easily and has been for many different types of organisms.
It can be expressed in a number of different ways, for example, errors per nucleotide incorporated.
It varies between prokaryotes, eukaryotes, unicellular and multicellular organisms.
A typical value given by some genetics textbooks is 1 error/genome/1000 bacterial replications.
It would be 10-3 instead of 10-8 to 10-10, if there were no checks on the system.
Checks include a pre-synthesis check to verify the incoming base is correct.
The proofreading check verifies that the correct base was incorporated.
The 3’ to 5’ exonuclease activity assists when it is found that the base incorporated was incorrect.
It removes the incorrect base so that the polymerase can try again to get it right.
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Molecular Biology Sabah Linjawi ٦
Physical mutagens
Absorption of high-energy ionizing radiation such as X- rays and γ-rays causes the target molecules to lose electrons
These electrons can cause extensive chemical
alterations to DNA, including strand breaks and base and sugar destruction
Nonionizing radiation causes molecular vibrations or
promotion of electrons to higher energy levels within the target molecules
This can lead to the formation of new chemical bonds
The most important from causing DNA damage is UV
light which produces pyrimidine dimers from adjacent
pyrimidine bases
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Molecular Biology Sabah Linjawi ٧
Chemical mutagens
The chemical agents which may cause mutation are called mutagens. Most of them are also carcinogens.
Acridines (e.g., proflavin) are positively charged molecules. They may be inserted between two DNA strands, thereby altering DNA's structure and rigidity. As a result, DNA replication will not be faithful.
Alkylating agents are chemicals that add an alkyl group (CnH2n+1) to another molecule. Alkylation of a base may change the normal base pairing. For example, the alkylating agent EMS converts guanine to 7- ethylguanine which pairs with thymine. The mispairing will lead to
mutation. Some alkylating agents may also cross-link DNA, resulting in chromosome breaks.
Nitrous acid is a deaminating agent that converts cytosine to uracil, adenine to hypoxanthine, and guanine to xanthine. The hydrogen-
bonding potential of the modified base is altered, resulting in mispairing.
Hydroxylamine and free radicals may modify base structures, resulting in mispairing.
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Molecular Biology Sabah Linjawi ٨
Chemical mutagens
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Directed mutagenesis
Directed mutagenesis, also known as directed mutation or adaptive mutation
is a hypothesis that organisms can respond to environmental stresses or advantages through directing mutations to certain genes or areas of the genome.
It has long been observed in nature with a classic example being sex change, with animals
They responding to their environmental conditions by changing their DNA between male and female for survival advantage.
Examples include, the clown fish and frogs, such as the Common Reed Frog.
If a base analog or modified base whose base pairing properties are different from the parent base is not removed by a DNA repair
mehchanism before passage of a replication fork, then an incorrect base will be incorporated
A second round of replication fixes the mutation permanently in the DNA
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Directed mutagenesis
Clownfish
can change their DNA between male or
female depending on the environmental conditions and advantages.
African Cichlid Fish can change their DNA between male or female depending on the environmental
advantages.
Common Reed Frog.
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Indirect mutagenesis
Most lesions in DNA are repaired by error- free direct reversal or excision repair
mechanisms before passage of a replication fork
If this is not possible, an error-prone form of translation DNA synthesis may take place involving specialized DNA polymerases and one or more incorrect bases become
incorporated opposite the lesion
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References
www.biology.ewu.edu/aHerr/Genetics/.../ch10notes.
html
en.wikipedia.org/wiki/Mutagenesis –
evolution.berkeley.edu/evolibrary/article/0.../mutatio ns_03
www.web-books.com/MoBio/Free/Ch7F6.htm
en.wikipedia.org/wiki/Directed_mutagenesis
Molecular Biology. P.C. Turner, A.G. Mclennan, A.D.
Bates & M.R.H. White.School of Biological Sciences, University of Liverpool, Liverpool, UK. Second edition.
BIOS Scientific Publishers, 2000.
