Excision endonuclease, also known as excinuclease or UV-specific endonuclease, is a

nuclease In biochemistry, a nuclease (also archaically known as nucleodepolymerase or polynucleotidase) is an enzyme capable of cleaving the phosphodiester bonds that link nucleotides together to form nucleic acids. Nucleases variously affect single and ...

(enzyme) which excises a fragment of

nucleotides Nucleotides are Organic compound, organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both o ...

during

DNA repair DNA repair is a collection of processes by which a cell (biology), cell identifies and corrects damage to the DNA molecules that encode its genome. A weakened capacity for DNA repair is a risk factor for the development of cancer. DNA is cons ...

. The excinuclease cuts out a fragment by hydrolyzing two

phosphodiester bond In chemistry, a phosphodiester bond occurs when exactly two of the hydroxyl groups () in phosphoric acid react with hydroxyl groups on other molecules to form two ester bonds. The "bond" involves this linkage . Discussion of phosphodiesters is d ...

s, one on either side of the lesion in the DNA. This process is part of "

nucleotide excision repair Nucleotide excision repair is a DNA repair mechanism. DNA damage occurs constantly because of chemicals (e.g. Intercalation (biochemistry), intercalating agents), radiation and other mutagens. Three excision repair pathways exist to repair single ...

", a mechanism that can fix specific types of damage to the

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

in the

G1 phase The G1 phase, gap 1 phase, or growth 1 phase, is the first of four phases of the cell cycle that takes place in eukaryotic cell division. In this part of interphase, the cell synthesizes Messenger RNA, mRNA and proteins in preparation for subsequ ...

of the eukaryotic

cell cycle The cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell (biology), cell that causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA (DNA re ...

. Such damage may include thymine dimers created by UV rays as well as the bulky distortions in DNA caused by oxidized

benzopyrene A benzopyrene is an organic compound with the formula C20H12. Structurally speaking, the colorless isomers of benzopyrene are pentacyclic hydrocarbons and are fusion products of pyrene and a phenylene group. Two isomeric species of benzopyrene ...

s from sources such as cigarette smoke. A deficiency of excinuclease occurs in a rare

autosomal recessive In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the Phenotype, effect of a different variant of the same gene on Homologous chromosome, the other copy of the chromosome. The firs ...

disease called

xeroderma pigmentosum Xeroderma pigmentosum (XP) is a genetic disorder in which there is a decreased ability to repair DNA damage such as that caused by ultraviolet (UV) light. Symptoms may include a severe sunburn after only a few minutes in the sun, freckling in su ...

. This disease can cause light-skin, extreme freckling and facial lesions, as well as preventing the repair of pyrimidine dimers. Diagnosis of this disease is done by measuring the enzyme's level in

white blood cells White blood cells (scientific name leukocytes), also called immune cells or immunocytes, are cells of the immune system that are involved in protecting the body against both infectious disease and foreign entities. White blood cells are genera ...

in a blood sample. Symptoms in children include extreme UV sensitivity, excessive freckling, multiple

skin cancer Skin cancers are cancers that arise from the Human skin, skin. They are due to the development of abnormal cells (biology), cells that have the ability to invade or metastasis, spread to other parts of the body. It occurs when skin cells grow ...

s and

corneal ulcer Corneal ulcer, often resulting from keratitis is an inflammatory or, more seriously, infective condition of the cornea involving disruption of its epithelial layer with involvement of the corneal stroma. It is a common condition in humans part ...

ations. Typically, these symptoms are seen during a child's first sun exposure.

Notes

DNA repair {{Enzyme-stub