<unk>  changes to a genome can be caused by small<unk> s that affect a single base pair, or large<unk> s that affect entire chromosomes and are divided into two classes, spontaneous<unk> s and induced<unk> s. Spontaneous<unk> s Depurination<unk>  results in one normal strand and one shortened strand after replication. Most spontaneous<unk> s are the result of naturally occurring lesions to DNA and errors during DNA replication without direct exposure to an agent. A few common spontaneous<unk> s are: * Depurination- The loss of a purine (A or G) base to form an apurinic site. An apurinic site, also known as an AP site, is the location in a genetic sequence that does not contain a purine base. During replication, the affected double stranded DNA will produce one doubled-stranded daughter containing the missing purine, resulting in an unchanged sequence. The other strand will produce a shorter strand, missing the purine and its complementary base. * Deamination- The amine group on a base is changed to a keto group. This results in cytosine being changed to uracil and adenine being changed to hypoxanthine which can result in incorrect DNA replication and repair. * Tautomerization- The hydrogen atom on a nucleotide base is repositioned causing altered hydrogen bonding pattern and incorrect base pairing during replication. For example, the keto tautomer of thymine normally pairs with adenine, however the enol tautomer of thymine can bind with guanine. This results in an incorrect base pair match. Similarly there are amino and imino tautomers of cytosine and adenine that can cause incorrect base pairing with other nucleotides. Induced<unk> s Induced<unk> s are any lesions in DNA caused by an agent or mutagen. Mutagens often demonstrate<unk> al specificity, meaning they cause predictable changes in the DNA sequence. A few common mutagens that induce<unk> s are: * Ultraviolet light (UV)- Causes pyrimidine (T or C) nucleotide bases on the same strand to covalent join together forming a pyrimidine dimer. Thymine-thymine dimers are the most common<unk>  caused by UV light. Since dimers cause a disruptive kink in DNA structure, polymerases often have trouble reading the region, slowing down DNA replication. * Base Analogs- Chemical compounds that are sufficiently similar in structure and chemistry to the nitrogenous bases of DNA, such that they are able to be incorporated in the sequence. These analogs do not have the same pairing properties of normal bases, therefore they can pair incorrectly with nucleotides during replication. 5- bromouracil (5-BU) is a common analog to thymine, however the enol form of 5-BU is still able to bind with adenine. The ionized form, on the other hand, pairs with guanine. * Intercalating agents- Chemical compounds that place themselves between stacked nitrogenous bases in DNA, causing a frameshift<unk> . Some intercalating agents, like daunorubicin, are capable of blocking replication and transcription, making them incredibly toxic to proliferating cells. * Reactive Oxygen Species (ROS)- Highly reactive oxygen- containing molecules that are capable causing DNA strand breaks and many damaging effects to cellular components. * Alkylating agents- Compounds that attach an alkyl group to the four bases. When an alkyl group is added to guanine, it can lead to the incorrect pairing with thymine and disrupt the accuracy of replication.
Postzygotic mutation
Robertsonian translocation
Uniparental disomy
Germline mosaicism
Caudal duplication
Genomic imprinting
Sex-chromosome dosage compensation
Isodisomy