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Mutations Mutations in simple words are

“change in DNA”

Any sudden change occurring in hereditary material is called as mutation

Term mutation was given by Devries in 1901 while studying evening primerose Oenothera lamarckiana

Most of these were chromosomal variations Resultant effects –

Positive Neutral Negative

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In multicellular organism, two broad categories of mutations:

Somatic mutations & germ line mutations Somatic mutations

Arise in the somatic cells Passed on to other cells through the process

of mitosis Effect of these mutations depends on the

type of the cell in which they occur & the developmental stage of the organism

If occurs early in development, larger the clone of the mutated cells

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Germ line mutations

They occur in the cells that produce gametes

Passed on to future generations

In multicellular organisms, the term mutation is generally used for germ line mutations

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Base substitution is of two types:

Transition: Purine is replaced with a purine

Pyrimidine is replaced with a pyrimidine

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Transversions:

A purine is replaced by a pyrimidine

or a pyrimidine is replaced by a purine

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Missense mutation: a base is substituted that alters a codon in the mRNA resulting in a different amino acid in the protein product

TCAAGT

UCA

TTAAAT

UUA

Ser Leu

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Nonsense mutation: changes a sense codon into a nonsense codon. Nonsense mutation early in the mRNA sequence produces a greatly shortened & usually nonfunctional protein

TCA

AGT

UCA

TGA

ACT

UGA

Ser

Stop codon

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Silent mutation: alters a codon but due to degeneracy of the codon, same amino acid is specified

TCA

AGT

UCA

TCG

AGC

UCG

Ser Ser

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Insertions & deletions: 2nd major class of gene mutation Addition or the removal, respectively, of

one or more nucleotide pair Usually changes the reading frame, altering

all amino acids encoded by codonsfollowing the mutation

Also called as frame shift mutations Additions or deletions in the multiples of

three nucleotides will lead to addition or deletion of one or more amino acids

These mutations are called in-frame insertions and deletions, respectively.

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Run-on mutation

Stop codon lost so protein is extra long

(can also produce nonsense and run-

ons)

Summary of Mutation Types

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Causes of MutationsA) Spontaneous errors (due to enzyme)

B) Induced errors by mutagenic agents

UV radiation

X – rays

Chemical Agents

C) Transposable elements (Transposons)*

Mutagen: Agent that causes mutations

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*Transposable element A transposable

element (TE, transposon or retrotransposon) is a DNA sequence that can change its position within the genome, sometimes creating or reversing mutations and altering the cell's genome size.

Barbara McClintock's discovery of these jumping genes earned her a Nobel prize in 1983.

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On the basis of Causative agent of mutation:

Spontaneous: Mutations that result from natural changes in DNA (1 in 109 bp)

Occur in the absence of a mutagen

Induced: Results from changes caused By

environmental chemicals & radiations Any environmental agent that increases

the rate of mutation above the spontaneous is called a mutagen such as chemicals & radiations

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Ionizing Radiation: UV

UV radiation causes thymine dimers, which block replication.

Light-repair separates thymine dimers

Sometimes the “repair job” introduces the wrong nucleotide, leading to a point mutation.

17Figure 8.20

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Since replication errors and a variety of mutagens canalter the nucleotide sequence, a microorganism mustbe able to repair changes in the sequence that might befatal.

DNA is repaired by several different mechanismsbesides proofreading by replication enzymes (DNApolymerases can remove an incorrect nucleotideimmediately after its addition to the growing end ofthe chain). Repair in E. coli is best understood and isbriefly described in this section.

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Excision Repair

Excision repair is a general repair system thatcorrects damage that causes distortions in the doublehelix.

A repair endonuclease or uvrABC endonucleaseremoves the damaged bases along with some bases oneither side of the lesion.

The resulting single-stranded gap, about 12nucleotides long, is filled by DNA polymerase I, andDNA ligase joins the fragments.

This system can remove thymine dimers and repairalmost any other injury that produces a detectabledistortion in DNA.

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Besides this general excision repair system, specializedversions of the system excise specific sites on the DNAwhere the sugar phosphate backbone is intact but thebases have been removed to form apurinic orapyrimidinic sites (AP sites).

Special endonucleases called AP endonucleasesrecognize these locations and nick the backbone at thesite.

Excision repair then commences, beginning with theexcision of a short stretch of nucleotides.

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Another type of excision repair employs DNAglycosylases.

These enzymes remove damaged or unnatural basesyielding AP sites that are then repaired as above.

Not all types of damaged bases are repaired in this way,but new glycosylases are being discovered and theprocess may be of more general importance than firstthought.

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Removal of Lesions

Thymine dimers and alkylated bases often are directlyrepaired.

Photoreactivation is the repair of thymine dimersby splitting them apart into separate thymines withthe help of visible light in a photochemical reactioncatalyzed by the enzyme photolyase.

Because this repair mechanism does not remove andreplace nucleotides, it is error free.

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Sometimes damage caused by alkylation is repaireddirectly as well.

Methyls and some other alkyl groups that have beenadded to the O–6 position of guanine can be removedwith the help of an enzyme known as alkyltransferaseor methylguanine methyltransferase.

Thus damage to guanine from mutagens such asmethyl-nitrosoguanidine can be repaired directly.

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Postreplication Repair

Despite the accuracy of DNA polymerase action andcontinual proofreading, errors still are made during DNAreplication.

Remaining mismatched bases and other errors are usuallydetected and repaired by the mismatch repair system inE. coli.

The mismatch correction enzyme scans the newlyreplicated DNA for mismatched pairs and removes astretch of newly synthesized DNA around the mismatch.

A DNA polymerase then replaces the excised nucleotides,and the resulting nick is sealed with a ligase.

Postreplication repair is a type of excision repair.

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Successful postreplication repair depends on the ability ofenzymes to distinguish between old and newly replicatedDNA strands.

This distinction is possible because newly replicated DNAstrands lack methyl groups on their bases, whereas olderDNA has methyl groups on the bases of both strands.

DNA methylation is catalyzed by DNAmethyltransferases and results in three differentproducts: N6-methyladenine, 5-methylcytosine, and N4-methylcytosine.

After strand synthesis, the E. coli DNA adeninemethyltransferase (DAM) methylates adenine bases ind(GATC) sequences to form N6-methyladenine.

For a short time after the replication fork has passed, thenew strand lacks methyl groups while the template strandis methylated. The repair system cuts out the mismatchfrom the unmethylated strand.

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Recombination Repair

In recombination repair, damaged DNA for whichthere is no remaining template is restored.

This situation arises if both bases of a pair are missingor damaged, or if there is a gap opposite a lesion.

In this type of repair the recA protein cuts a piece oftemplate DNA from a sister molecule and puts it intothe gap or uses it to replace a damaged strand.

Although bacteria are haploid, another copy of thedamaged segment often is available because either ithas recently been replicated or the cell is growingrapidly and has more than one copy of itschromosome.

Once the template is in place, the remaining damagecan be corrected by another repair system.

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The recA protein also participates in a type of induciblerepair known as SOS repair.

In this instance the DNA damage is so great thatsynthesis stops completely, leaving many large gaps. RecAwill bind to the gaps and initiate strand exchange.

Simultaneously it takes on a proteolytic function thatdestroys the lexA repressor protein, which regulates thefunction of many genes involved in DNA repair andsynthesis.

As a result many more copies of these enzymes areproduced, accelerating the replication and repair processes.The system can quickly repair extensive damage caused byagents such as UV radiation, but it is error prone and doesproduce mutations.

However, it is certainly better to have a few mutations thanno DNA replication at all.

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References

Fig 1 http://academic.pgcc.edu/~kroberts/Lecture/Chapter%207/mutation.html

Fig 2 http://www.nature.com/nprot/journal/v6/n10/fig_tab/nprot.2011.378_F2.html

Fig 3 http://www-personal.ksu.edu/~bethmont/mutdes.html

Fig 4 www.motifolio.com

Genome 2 By T.A.Brown

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Thank You

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