What are the three types of changes that happen to DNA
Deoxyribonucleic acid 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 office, are harmless except when they atomic number 82 to jail cell death or tumor formation. Because of the lethal potential of DNA mutations cells take evolved mechanisms for repairing damaged DNA.
Types of Mutations
At that place are three types of DNA Mutations: base of operations substitutions, deletions and insertions.
1. Base of operations Substitutions
Single base substitutions are called point mutations, think the point mutation Glu -----> Val which causes sickle-prison 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 exist generated. Thus the amino acrid sequence encoded by the factor is non changed and the mutation is said to be silent.
Missence: When base substitution results in the generation of a codon that specifies a unlike 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 acrid the mutation is said to be bourgeois and will near likely accept piddling result on the resultant proteins structure / function. If the exchange leads to an amino acid with very different structure and backdrop the mutation is nonconservative and will probably exist deleterious (bad) for the resultant proteins construction / function (i.eastward. the sickle cell point mutation).
Nonsense: When a base substitution results in a end codon ultimately truncating translation and near likely leading to a nonfunctional poly peptide.
2. Deletions
A deletion, resulting in a frameshift, results when ane or more base pairs are lost from the DNA (run across Effigy above). If ane 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 poly peptide missing one or more amino acids. This may be deleterious or not.
3. Insertions
The insertion of additional base of operations pairs may pb 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 besides possible.
Causes of Mutations
Errors in DNA Replication
On very, very rare occasions Deoxyribonucleic acid polymerase will comprise a noncomplementary base into the daughter strand. During the next round of replication the missincorporated base of operations would lead to a mutation. This, nevertheless, is very rare as the exonuclease functions equally a proofreading mechanism recognizing mismatched base pairs and excising them.
Errors in DNA Recombination
DNA oft rearranges itself by a process called recombination which proceeds via a diversity 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 ecology, are capable of damaging DNA. Many chemotherapeutic drugs and intercalating amanuensis drugs role by dissentious DNA.
Radiations
Gamma rays, Ten-rays, even UV lite tin can interact with compounds in the cell generating free radicals which cause chemic damage to DNA.
Dna Repair
Damaged DNA tin 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 right it. These mismatches too equally unmarried base insertions and deletions are repaired by the mismatch repair machinery. Mismatch repair relies on a secondary signal within the Dna to distinguish betwixt the parental strand and daughter strand, which contains the replication error. Human being cells posses a mismatch repair organization 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 girl strand remains unmethylated for a very short flow of time following replication. This deviation allows the mismatch repair organisation to determine which strand contains the mistake. A poly peptide, MutS recognizes and binds the mismatched base pair.
Another protein, MutL and so 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 management 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 exist associated with the development of hereditary colorectal cancer.
Nucleotide Excision Repair (NER)
NER in human cells begins with the formation of a circuitous of proteins XPA, XPF, ERCC1, HSSB at the lesion on the Deoxyribonucleic acid. The transcription cistron TFIIH, which contains several proteins, and 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 (Deoxyribonucleic acid polymerase) and the nick sealed (ligase).
Direct Repair of Damaged Deoxyribonucleic acid
Sometimes damage to a base tin can be direct 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 Impairment Control
Deoxyribonucleic acid repair is regulated in mammalian cells by a sensing mechanism that detects Dna harm 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 beingness adamant, are: stimulation of the expression of genes encoding p21 and Gaad45. Loss of p53 office can exist deleterious, about 50% of all human cancers have a mutated p53 cistron.
The p21 poly peptide binds and inactivates a cell division kinase (CDK) which results in cell wheel 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.
Xeroderma pigmentosum
Cockayne's syndrome
Hereditary nonpolyposis colorectal cancer
© Dr. Noel Sturm 2019
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Source: http://www2.csudh.edu/nsturm/CHEMXL153/DNAMutationRepair.htm
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