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7.S: Mutation and Repair of DNA (Summary) - Biology

7.S: Mutation and Repair of DNA (Summary) - Biology


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Summary: Causes of Transitions and Transversions

Table 7.1 lists several causes of mutations in DNA, including mutagens as well as mutator strains in bacteria. This can also be a source of mutant alleles.

Table. 7.1. Summary of effects of various agents that alter DNA sequences (mutagens and mutator genes)
Agent (mutagen, etc.)ExampleResult
Nucleotide analogsBrdUTPtransitions, e.g. A:T to G:C
Oxidizing agentsnitrous acidtransitions, e.g. C:G to T:A
Alkylating agentsnitrosoguanidinetransitions, e.g. G:C to A:T
Frameshift mutagensBenz(a)pyrenedeletions (short)
Ionizing radiationX-rays, g-raysbreaks and deletions (large)
UVUV, 260 nmY-dimers, block replication
Misincorporation:
Altered DNA Pol IIImutD=dnaQ; e subunit of DNA PolIIItransitions, transversions and frameshifts in mutant strains
Error-prone repairNeed UmuC, UmuD, DNA PolIIItransitions and transversions in wild-type during SOS
Other mutator genesmutM, mutT, mutYtransversions in the mutant strains

Additional Readings

  • Friedberg, E. C., Walker, G. C., and Siede, W. (1995) DNA repair and mutagenesis, ASM Press, Washington, D.C.
  • Kornberg, A. and Baker, T. (1992) DNA Replication, 2nd Edition, W. H. Freeman and Company, New York.
  • Zakian, V. (1995) ATM-related genes: What do they tell us about functions of the human gene? Cell 82: 685-687.
  • Kolodner, R. (1996) Biochemistry and genetics of eukaryotic mismatch repair. Genes & Development10:1433-1442.
  • Sutton MD, Smith BT, Godoy VG, Walker GC. (2000) The SOS response: recent insights into umuDC-dependent mutagenesis and DNA damage tolerance.Annu Rev Genet34:479-497.
  • De Laat, W. L., Jaspers, N. C. J. and Hoeijmakers, J. (1999) Molecular mechanism of nucleotide excision repair. Genes & Development13: 768-785. This review focuses on nucleotide excision repair in mammals.

DNA repair

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DNA repair, any of several mechanisms by which a cell maintains the integrity of its genetic code. DNA repair ensures the survival of a species by enabling parental DNA to be inherited as faithfully as possible by offspring. It also preserves the health of an individual. Mutations in the genetic code can lead to cancer and other genetic diseases.

Successful DNA replication requires that the two purine bases, adenine (A) and guanine (G), pair with their pyrimidine counterparts, thymine (T) and cytosine (C). Different types of damage, however, can prevent correct base pairing, among them spontaneous mutations, replication errors, and chemical modification. Spontaneous mutations occur when DNA bases react with their environment, such as when water hydrolyzes a base and changes its structure, causing it to pair with an incorrect base. Replication errors are minimized when the DNA replication machinery “proofreads” its own synthesis, but sometimes mismatched base pairs escape proofreading. Chemical agents modify bases and interfere with DNA replication. Nitrosamines, which are found in products such as beer and pickled foods, can cause DNA alkylation (the addition of an alkyl group). Oxidizing agents and ionizing radiation create free radicals in the cell that oxidize bases, especially guanine. Ultraviolet (UV) rays can result in the production of damaging free radicals and can fuse adjacent pyrimidines, creating pyrimidine dimers that prevent DNA replication. Ionizing radiation and certain drugs, such as the chemotherapeutic agent bleomycin, can also block replication, by creating double-strand breaks in the DNA. (These agents can also create single-strand breaks, though this form of damage often is easier for cells to overcome.) Base analogs and intercalating agents can cause abnormal insertions and deletions in the sequence.

There are three types of repair mechanisms: direct reversal of the damage, excision repair, and postreplication repair. Direct reversal repair is specific to the damage. For example, in a process called photoreactivation, pyrimidine bases fused by UV light are separated by DNA photolyase (a light-driven enzyme). For direct reversal of alkylation events, a DNA methyltransferase or DNA glycosylase detects and removes the alkyl group. Excision repair can be specific or nonspecific. In base excision repair, DNA glycosylases specifically identify and remove the mismatched base. In nucleotide excision repair, the repair machinery recognizes a wide array of distortions in the double helix caused by mismatched bases in this form of repair, the entire distorted region is excised. Postreplication repair occurs downstream of the lesion, because replication is blocked at the actual site of damage. In order for replication to occur, short segments of DNA called Okazaki fragments are synthesized. The gap left at the damaged site is filled in through recombination repair, which uses the sequence from an undamaged sister chromosome to repair the damaged one, or through error-prone repair, which uses the damaged strand as a sequence template. Error-prone repair tends to be inaccurate and subject to mutation.

Often when DNA is damaged, the cell chooses to replicate over the lesion instead of waiting for repair ( translesion synthesis). Although this may lead to mutations, it is preferable to a complete halt in DNA replication, which leads to cell death. On the other hand, the importance of proper DNA repair is highlighted when repair fails. The oxidation of guanine by free radicals leads to G-T transversion, one of the most common mutations in human cancer.

Hereditary nonpolyposis colorectal cancer results from a mutation in the MSH2 and MLH1 proteins, which repair mismatches during replication. Xeroderma pigmentosum (XP) is another condition that results from failed DNA repair. Patients with XP are highly sensitive to light, exhibit premature skin aging, and are prone to malignant skin tumours because the XP proteins, many of which mediate nucleotide excision repair, can no longer function.


7.S: Mutation and Repair of DNA (Summary) - Biology

DNA Mutation and Repair

A mutation, which may arise during replication and/or recombination, is a permanent change in the nucleotide sequence of DNA. Damaged DNA can be mutated either by substitution, deletion or insertion of base pairs. Mutations, for the most part, are harmless except when they lead to cell death or tumor formation. Because of the lethal potential of DNA mutations cells have evolved mechanisms for repairing damaged DNA.

There are three types of DNA Mutations: base substitutions, deletions and insertions.

1. Base Substitutions

Single base substitutions are called point mutations, recall the point mutation Glu -----> Val which causes sickle-cell disease. Point mutations are the most common type of mutation and there are two types.

Transition: this occurs when a purine is substituted with another purine or when a pyrimidine is substituted with another pyrimidine.

Transversion: when a purine is substituted for a pyrimidine or a pyrimidine replaces a purine.

Point mutations that occur in DNA sequences encoding proteins are either silent, missense or nonsense.

Silent: If abase substitution occurs in the third position of the codon there is a good chance that a synonymous codon will be generated. Thus the amino acid sequence encoded by the gene is not changed and the mutation is said to be silent.

Missence: When base substitution results in the generation of a codon that specifies a different amino acid and hence leads to a different polypeptide sequence. Depending on the type of amino acid substitution the missense mutation is either conservative or nonconservative. For example if the structure and properties of the substituted amino acid are very similar to the original amino acid the mutation is said to be conservative and will most likely have little effect on the resultant proteins structure / function. If the substitution leads to an amino acid with very different structure and properties the mutation is nonconservative and will probably be deleterious (bad) for the resultant proteins structure / function (i.e. the sickle cell point mutation).

Nonsense: When a base substitution results in a stop codon ultimately truncating translation and most likely leading to a nonfunctional protein.

A deletion, resulting in a frameshift, results when one or more base pairs are lost from the DNA (see Figure above). If one or two bases are deleted the translational frame is altered resulting in a garbled message and nonfunctional product. A deletion of three or more bases leave the reading frame intact. A deletion of one or more codons results in a protein missing one or more amino acids. This may be deleterious or not.

The insertion of additional base pairs may lead to frameshifts depending on whether or not multiples of three base pairs are inserted. Combinations of insertions and deletions leading to a variety of outcomes are also possible.

Errors in DNA Replication

On very, very rare occasions DNA polymerase will incorporate a noncomplementary base into the daughter strand. During the next round of replication the missincorporated base would lead to a mutation. This, however, is very rare as the exonuclease functions as a proofreading mechanism recognizing mismatched base pairs and excising them.

Errors in DNA Recombination

DNA often rearranges itself by a process called recombination which proceeds via a variety of mechanisms. Occasionally DNA is lost during replication leading to a mutation.

Chemical Damage to DNA

Many chemical mutagens, some exogenous, some man-made, some environmental, are capable of damaging DNA. Many chemotherapeutic drugs and intercalating agent drugs function by damaging DNA.

Gamma rays, X-rays, even UV light can interact with compounds in the cell generating free radicals which cause chemical damage to DNA.

Damaged DNA can be repaired by several different mechanisms.

Mismatch Repair

Sometimes DNA polymerase incorporates an incorrect nucleotide during strand synthesis and the 3' to 5' editing system, exonuclease, fails to correct it. These mismatches as well as single base insertions and deletions are repaired by the mismatch repair mechanism. Mismatch repair relies on a secondary signal within the DNA to distinguish between the parental strand and daughter strand, which contains the replication error. Human cells posses a mismatch repair system similar to that of E. coli, which is described here. Methylation of the sequence GATC occurs on both strands sometime after DNA replication. Because DNA replication is semi-conservative, the new daughter strand remains unmethylated for a very short period of time following replication. This difference allows the mismatch repair system to determine which strand contains the error. A protein, MutS recognizes and binds the mismatched base pair.

Another protein, MutL then binds to MutS and the partially methylated GATC sequence is recognized and bound by the endonuclease, MutH. The MutL/MutS complex then links with MutH which cuts the unmethylated DNA strand at the GATC site. A DNA Helicase, MutU unwinds the DNA strand in the direction of the mismatch and an exonuclease degrades the strand. DNA polymerase then fills in the gap and ligase seals the nick. Defects in the mismatch repair genes found in humans appear to be associated with the development of hereditary colorectal cancer.

Nucleotide Excision Repair (NER)

NER in human cells begins with the formation of a complex of proteins XPA, XPF, ERCC1, HSSB at the lesion on the DNA. The transcription factor TFIIH, which contains several proteins, then binds to the complex in an ATP dependent reaction and makes an incision. The resulting 29 nucleotide segment of damaged DNA is then unwound, the gap is filled (DNA polymerase) and the nick sealed (ligase).

Direct Repair of Damaged DNA

Sometimes damage to a base can be directly repaired by specialized enzymes without having to excise the nucleotide.

Recombination Repair

This mechanism enables a cell to replicate past the damage and fix it later.

Regulation of Damage Control

DNA repair is regulated in mammalian cells by a sensing mechanism that detects DNA damage and activates a protein called p53. p53 is a transcriptional regulatory factor that controls the expression of some gene products that affect cell cycling, DNA replication and DNA repair. Some of the functions of p53, which are just being determined, are: stimulation of the expression of genes encoding p21 and Gaad45. Loss of p53 function can be deleterious, about 50% of all human cancers have a mutated p53 gene.

The p21 protein binds and inactivates a cell division kinase (CDK) which results in cell cycle arrest. p21 also binds and inactivates PCNA resulting in the inactivation of replication forks. The PCNA/Gaad45 complex participates in excision repair of damaged DNA.

Some examples of the diseases resulting from defects in DNA repair mechanisms.


The DNA Repair Pathways

A variety of endogenous and exogenous DNA-damaging agents such as UV light, ionizing radiation (IR) and chemotherapeutic agents can lead to DNA lesions, including mismatches, single-strand breaks (SSBs), double-strand breaks (DSBs), chemical modifications of the bases or sugars, and interstrand or intrastrand cross-links. If the damage is not corrected, it will cause genomic instability and mutation, which is one of the cancer hallmarks (Hanahan and Weinberg, 2011). In order to prevent this situation, cells have evolved a series of mechanisms called DNA damage response (DDR) in order to deal with such lesions. DDR is a complex network that functions in different ways to target various DNA lesions, including signal transduction, transcriptional regulation, cell-cycle checkpoints, induction of apoptosis, damage tolerance processes, and multiple DNA repair pathways (Figure 1) (Giglia-Mari et al., 2011 Tian et al., 2015).

FIGURE 1. DNA damage response. DNA damage is caused by endogenous agent oxygen species (ROS) or exogenous agents such as UV light, ionizing radiation (IR) and chemotherapy agents. DNA damage response (DDR) is induced to deal with the lesions, including signal transduction, transcriptional regulation, cell-cycle checkpoints, induction of apoptosis, multiple DNA repair pathways as well as damage tolerance processes. DNA repair pathways include nuclear and mitochondrial DNA repair pathways. Direct repair, BER, MMR and recombinational repair (HR and NHEJ) are existence in both nuclear and mitochondrial repair systems. NER has been reported only appearance in nucleus, and the existence of TLS pathway in mitochondria is unknown. NDNA, nuclear DNA MtDNA, mitochondrial DNA BER, base excision repair HR, homologous recombination repair NHEJ, non-homologous end joining MMR, mismatch repair TLS, translesion synthesis NER, nucleotide excision repair.

In mammalian cells, the two main organelles containing DNA are nucleus and mitochondria. Nuclear DNA (nDNA) repair systems are divided into the following major pathways: 1) direct reversal, which mainly repairs the lesion induced by alkylating agents, 2) base excision repair (BER), aiming at DNA breaks (SSBs) and non-bulky impaired DNA bases, 3) nucleotide excision repair (NER), correcting bulky, helix-distorting DNA lesions, 4) mismatch repair (MMR), repair of insertion/deletion loops (IDLs) and base-base mismatch, 5) recombinational repair, which is further divided into homologous recombination repair (HRR) and non-homologous end joining (NHEJ), primarily functioning at DNA double strand breaks, 6) alternative nonhomologous end joining (alt-NHEJ, MMEJ), involved in repair of DSBs, 7) translesion synthesis (TLS), which is more likely to be a DNA damage tolerance mechanism (Jackson and Bartek, 2009 Hosoya and Miyagawa, 2014). Mitochondrial DNA (mtDNA) repair pathways, including the direct reversal, BER, MMR, TLS and double-strand break repair (DSBR), can repair damaged DNA to maintain mitochondria genetic integrity, protect mtDNA against oxidative damage, and promote cell survival (Ohta, 2006 Saki and Prakash, 2017).


DNA Repair System (With Diagram) | Mutation

Pyrimidine di­nners (induced by UV rays) can be monomerised again by DNA photolyases in presence of visible light. Cleavage of the cyclobutane ring of pyri­midine dimers by DNA photolyases restores the original DNA structure (Fig. 13.15A). Photolyases have chromophores which absorb blue light to provide energy for the reaction.

Photo-reactivation is specific for pyrimidine dimers and an example of direct reversal and is error-free.

Mutagenic effect of alkylating agents is protected through direct reversal afforded by alkyltransferase enzyme. This inducible protein specifically removes an alkyl group from the O 6 position of guanine and transfers it to protein itself, causing inactivation of the protein (Fig. 13.15B).

Type # 2. Excision Repair:

In nucleotide excision repair, an endonuclease makes nicks on either side of the lesion, which is then removed to leave a gap. This gap is filled by a DNA poly­merase, and DNA ligase makes the final phosphodiester bond (Fig. 13.16A). In base excision repair, the lesion is removed by a specific DNA glycosylase.

The resulting AP site is cleaved and expanded to a gap by an AP endonuclease plus exonuclease. Thereafter, the process is like nucleotide excision repair (Fig. 13.16B).

Type # 3. Mismatch Repair:

Replication errors which escape proofreading have a mismatch in the daughter strand. Hemi-methylation of the DNA after replication allows the daughter strand to be distinguished from the parental strand. The mismatched base is removed from the daughter strand by an excision repair mechanism (Fig. 13.17).

Type # 4. Double-Strand Break Repair:

Double- strand breaks are repaired simply by bringing the ends back together called non-homologous end joining. This is accomplished by DNA ligase under the direction of multi-component protein complex (Fig. 13.18). Alternative repair mecha­nism relies on nucleotide sequences of homo­logous piece of DNA, such as sister chromatid or homologous chromosome, called homology- directed recombination.

Type # 5. SOS Repair:

SOS response is initiated by interaction of Rec A protein with Lex A repres­sor. Damage activates Rec A protein which brings about proteolytic degradation of Lex A protein. Thus all operons, to which Lex A is bound, are induced (Fig. 13.19). This may include a number of genes with SOS box coding for repairing enzymes. This facilitates increased capacity to repair DNA damage.


Single-strand viruses show higher mutation rates than double-strand viruses

Single-strand DNA viruses tend to mutate faster than double-strand DNA viruses, although this difference is based on work with bacteriophages, as no mutation rate estimates have been obtained for eukaryotic single-strand DNA viruses [1]. Within RNA viruses, there are no obvious differences in mutation rate among Baltimore classes (Fig.  2 a). The mechanisms underlying these differences are not well understood. One possible explanation for the differences between single and double-strand viruses is that single-strand nucleic acids are more prone to oxidative deamination and other types of chemical damage. Elevated levels of reactive oxygen species (ROS) and other cellular metabolites during viral infections can induce mutations in the host cell and in the virus. For instance, ethanol is likely to synergize with virus-induced oxidative stress to increase the mutation rate of HCV [21]. Differences among single- and double-strand DNA viruses may also be explained in terms of their access to post-replicative repair. Work with bacteriophage ϕX174 has provided interesting clues on this issue. In enterobacteria, methyl-directed mismatch repair (MMR) is performed by MutHLS proteins and Dam methylase. Dam methylation of GATC sequence motifs is used to differentiate the template and daughter DNA strands and is thus required to perform mismatch correction [22]. Mismatches are recognized by MutS, which interacts with MutL and leads to the activation of the MutH endonuclease, which excises the daughter strand. However, the genome of bacteriophage ϕX174 has no GATC sequence motifs, even if approximately 20 such sites are expected by chance. As a result, the ϕX174 DNA cannot undergo MMR. This contributes to explaining the relatively high mutation rate of this virus, which falls on the order of 10 𢄦  s/n/c, a value three orders of magnitude above that of Escherichia coli and highest among DNA viruses [23]. Avoidance of GATC motifs may be a consequence of selection acting on mutation rate, but also of other selective factors. For instance, inefficient methylation of the phage DNA may render it susceptible to cleavage by MutH, therefore imposing a selection pressure against GATC sequence motifs [24].

As opposed to bacteriophage ϕX174, the link between post-replicative repair and mutation rate is still unclear in eukaryotic viruses. Numerous studies have shown that viruses interact with DNA damage response (DDR) pathways by altering the localization or promoting the degradation of DDR components [25, 26]. For instance, the adenoviral E4orf6 protein promotes proteasomal degradation of TOPBP1, a DDR component [27]. DDR activation can occur as an indirect consequence of cellular stress due to the infection per se or as a part of an antiviral response, which would be in turn counteracted by viruses. Although DNA viruses tend to promote genomic instability in the host cell, it remains to be shown whether DDR dysregulation can determine DNA virus mutation rates.


Abstract

Reactive oxygen species are a constant threat to DNA as they modify bases with the risk of disrupting genome function, inducing genome instability and mutation. Such risks are due to primary oxidative DNA damage and also mediated by the repair process. This leads to a delicate decision process for the cell as to whether to repair a damaged base at a specific genomic location or better leave it unrepaired. Persistent DNA damage can disrupt genome function, but on the other hand it can also contribute to gene regulation by serving as an epigenetic mark. When such processes are out of balance, pathophysiological conditions could get accelerated, because oxidative DNA damage and resulting mutagenic processes are tightly linked to ageing, inflammation, and the development of multiple age-related diseases, such as cancer and neurodegenerative disorders.

Recent technological advancements and novel data analysis strategies have revealed that oxidative DNA damage, its repair, and related mutations distribute heterogeneously over the genome at multiple levels of resolution. The involved mechanisms act in the context of genome sequence, in interaction with genome function and chromatin.

This review addresses what we currently know about the genome distribution of oxidative DNA damage, repair intermediates, and mutations. It will specifically focus on the various methodologies to measure oxidative DNA damage distribution and discuss the mechanistic conclusions derived from the different approaches. It will also address the consequences of oxidative DNA damage, specifically how it gives rise to mutations, genome instability, and how it can act as an epigenetic mark.


Nucleic Acid Sensing and Immunity - Part B

Abstract

DNA repair is a critical cellular process required for the maintenance of genomic integrity. It is now well appreciated that cells employ several DNA repair pathways to take care of distinct types of DNA damage. It is also well known that a cascade of signals namely DNA damage response or DDR is activated in response to DNA damage which comprise cellular responses, such as cell cycle arrest, DNA repair and cell death, if the damage is irreparable. There is also emerging literature suggesting a cross-talk between DNA damage signaling and several signaling networks within a cell. Moreover, cell death players themselves are also well known to engage in processes outside their canonical function of apoptosis. This chapter attempts to build a link between DNA damage, DDR and signaling from the studies mainly conducted in mammals and Drosophila model systems, with a special emphasis on their relevance in overall tissue homeostasis and development.


7.S: Mutation and Repair of DNA (Summary) - Biology

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DNA Mismatch Repair Mechanism

Both the prokaryotic and eukaryotic mismatch repair system follows the same pathway to recover the mismatched DNA:

Mismatch Recognition

  • In prokaryotic MMR: The MutS protein complex detects the DNA lesion and combines with an ATP molecule, and gets activated.
  • In eukaryotic MMR: The MSH protein complex recognizes the non-matched bases in a DNA strand and gets activated by an ATP molecule.

Recruitment of Related Proteins

  • In prokaryotic MMR: The activated ATP-MutS complex recruits MutL (acts as a carrier protein). Then, MutS and MutL complex activates the MutH complex. MutS, MutL and MutH form the ternary complex, which loads the DNA helicase-II or UvrD enzyme onto the DNA lesion site.
  • In eukaryotic MMR: The activated ATP-MSH complex recruits PCNA and polymerase-δ. Then, ATP dependent conformation change occurs in the mobile clamp. The conformational changes recruit the binding of the MutLa complex. The PCNA (Proliferating cell nuclear antigens) activates the MutLa complex and creates a nick in a daughter strand.

Excision and Replacement

  • In prokaryotic MMR: Exonuclease-I is activated by the MutSa protein, which removes the mispaired bases. Then, RDA protein displaces the mismatched base. MutSa and MutLa terminate the exonuclease-I activity.
  • In eukaryotic MMR: The MutLa complex promotes the digestion of the mismatched bases by the exonuclease activity-I with the association of RPA (Recognition replication protein A).

Gap Filling

  • In prokaryotic MMR: The DNA polymerase III, through its 5’-3’ activity, forms new bases. Single stranded binding proteins (SSB) also bind to the new DNA strand and prevents the new DNA to become double-stranded. Finally, DNA ligase fills the gaps in the DNA strand.
  • In eukaryotic MMR: The RFC (Replication factor-C) enzyme plays an important in the synthesis of new DNA bases. Replication factor-C enzyme attaches at the 3’ primer site and activates the DNA polymerase activity to form new bases. Single stranded binding proteins (SSB) prevents the new DNA to become double-stranded. Finally, DNA ligase fills the gap in the DNA strand.

Types of DNA Mismatch Repair

The mismatch repair is generally of two types, namely:


Prokaryotic Mismatch Repair

It consists of four essential components:

  1. MutS: It is the most crucial protein complex whose function is to recognize the mismatched bases in the DNA. MutS protein complex is the first enzyme, which initiates the process of MMR by recognizing the non-specific sequences in the DNA. It consists of two specific sites that are sterically distant from each other, as both can affect each other’s function.
    • DNA binding site: It helps in associating the MutS protein complex to the DNA lesion site.
    • ATPase or Dimerization site: It brings conformational changes in the MutS protein.
  2. MutL: It acts as an “intermediateprotein” complex, which links the MutS protein and endonuclease enzyme. Thus, it is associated with the two activities (recognition and excision of mismatched bases). Firstly, MutL binds with the activated MutS and eventually activates the endonuclease enzyme, i.e. MutH. MutL performs another function by recruiting and loading helicase enzyme (UvrD) at the DNA mismatched site.
  3. MutH: It belongs to the type-II family of restriction endonucleases. MutH creates a nick in the hemimethylated GATC site. MutS, MutL and an ATP molecule trigger MutH-endonuclease activity. MutH protein also consists of a C-terminal that functions as a molecular attachment site where the other two enzymes (MutS and MutL) communicate and stimulate the MutH activity.
  4. UvrD: It refers to the “DNA helicase II” enzyme complex, which functions to unwind the DNA lesion site. MutL helps in loading UvrD protein at the mismatched site and stimulates the intrinsic helicase activity of an UvrD enzyme. The SSB proteins and UvrD enters the ss-DNA via nick created by the MutH complex.
  5. DNA specific exonucleases: Exonuclease enzymes like Exo-I, Exo-X and 5’-3’ exonuclease perform excision of the newly formed DNA between the nick and mismatch.

Eukaryotic Mismatch Repair

It consists of the following components:

  1. MSHs protein: It shows homology with the MutS enzyme of prokaryotic MMR. In yeasts and mammals, MSH2 to MSH6 protein complexes are found. MSH enzyme is a heterodimer protein complex, which consists of two domains:
    • MutSa: It contains two types of subunits, namely, MSH2 and MSH6. MSH2 contributes 80-90% of the cellular level. MSH6 recognizes the mismatched bases, particularly base-base mismatches and insertion/deletion loops as well.
    • MutSb: It contains two types of subunits, namely, MSH2 and MSH3. It mainly repairs the insertion/deletion mispairs.
  2. MLH protein: It resembles a MutL enzyme of prokaryotic MMR. MLH protein consists of four highly conserved proteins (MLH1, MLH3, PMS1 and PMS2) in yeasts and mammals. It also acts as a heterodimer protein complex and consists of three subunits:
    • MutLa: It consists of two subunits MLH1, PMS2. MutLa coordinates with the Mut-S complex to repair the damage.
    • MutLb: It consists of two subunits, MLH1, PMS 1. MutLb functions are not known. : It consists of two subunits MLH1, MLH3. MutLg repairs an insertion or deletion loop damage.
  3. Accessory proteins: It includes enzymes like PCNA (Proliferating cell nuclear antigens), Recognition replication protein A (RPA), Replication factor C (RPC), Binding exonuclease-I etc.

Functional Role

DNA mismatch repair involves a simple mechanism of excising mismatched bases along with 3,000 base pairs. Other than the excision of mismatched bases, it also performs some other functions, according to a recent study.

MMR proteins repair the post replication repair of mismatched DNA. Other function like promotion of meiotic cross over, DNA damage response and diversification of immunoglobulins. Suppression of homologous recombination by antirecombinational activity between divergent sequences.


Watch the video: The different types of mutations. Biomolecules. MCAT. Khan Academy (July 2022).


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