Tm-06 Dna Repair

DNA damage, repair and recombination

TM-06

DNA REPAIR

CP 6.3: menjelaskan perbaikan DNA

FLASH 3. DNA Repair

DNA REPAIR

Proses perbaikan dapat dikelompokkan menjadi kategori sebagai berikut didasarkan atas tipe kerusakan DNA: 1) Photoreactivation Repair: repairs thymine dimers caused by UV light2) Excision Repair: repaires thymine dimers and other single strand errors3) Proffreading and Mismatch Repair: corrects incorrect base insertion during

replication4) Postreplication Repair: repaires incorrect base insertion after replication is

completed5) Double Strand Break Repair: repairs breaks across both strands of a double

helix Penjelasan lebih rinci dapat dilihat di FLASH 3. DNA Repair.


DNA REPAIR


Introduction

DNA molecules may acquire mutations as a consequnce of errors in DNA replication or as consequences of exposure to mutagens.

Living systems have evolved a variety of complex repair systems. Without these repair systems, mutations would accumulate and

cells (and organisms) would cease to survive (1)

DNA REPAIR


There are a number of known repair systems. We will cover each of these in this module. Repair systems:

1. Photoreactivation repair2. Excision repair

a. Base excision repairb. Nucleotide excision rapair

3. Proofreading and mismatch repair4. Postreplication repair5. SOS repair6. Double-strand break repair (2)

DNA REPAIR


UV Radiations as a Mutagen UV light is mutagenic, as function of the creation of thymine

dimers (two adjacent thymine molecules become crosslinked and do not pair properly).

In order to replicate a DNA strand without error, these dimers must be corrected. (3)

DNA REPAIR


Photoreactivation Repair The first important discovery concerning UV repair was by Albert Kelner in

1949. He showed that UV-induced damage in E. coli could be partially reversed by

exposure to visible light after irradiation. Kelner showed that only blue light was necessary to correct radiation damage. The phenomenon is called “photoreactivation repair”. (4)

DNA REPAIR


Blue Light and High Temperature Kelner further showed that the blue-light effect was temperature

sensitive. Temperature sensitivity suggests that a protein is involved in the process

(protein structure is disrupted at higher temperature leading to loss function). (5)

DNA REPAIR


Photoreactivation Enzyme Photoreactivation repair is dependent on a protein called the

photo-reactivation enzyme (PRE). The enzyme cleave the bonds between two adjacent thymine

dimers. Although PRE can assosiate with thymine dimers in the dark, it

requires a photon of light to cleave the dimer. (6)

DNA REPAIR


Excision Repair Excision repair is a widespread, multistep process in which

damaged DNA is removed and resynthesized using the opposite strand as a template.

Excision repair was originally discovered because of its ability to repair thymine dimers, but it can be used for any damaged segment of DNA.

There are two types of excision repair: base excision repair (BER) and nucleotide excision repair (NER).

Both types of excision repair involve removal of nucleotides and gap filling by DNA polymerase I. (7)

DNA REPAIR


Nucleotide Excision Repair The steps in nucleotide excision repair are:

1. The distortion or error is recognized.2. The damaged area is removed by a nuclease.3. DNA polymerase I fills the gap by inserting nucleotides complementary to

the other strand. 4. Ligase seals the final nick in the backbone that remains at the 3’-OH end

of the base inserted.

DNA REPAIR


Base Excision Repair Base excision repair (BER) involves an extra level of enzyme activity prior to excision of

the damaged area. Base alterations may result from spontaneous

hydrolysis of a base or mutations induced by mutagenic chemicals.

The first step in BER involves recognition of the damaged base by enzymes called DNA glycosylases.

This is a group of enzymes that are spesific to different kinds of DNA damaged.

The enzyme cuts the bond between the base and sugar creating an apyrimidinic (AP) site.

A sugar with a missing base is then recognized by an enzyme called an AP endonuclease.

The enzyme cuts the sugar-phosphate backbone. Finally, the nick in the DNA backbone creates a

distortion in the helix, which is recognized by the excision-repair system that replaces the damaged nucleotide (as described earlier).(8)

DNA REPAIR


Two Individuals with Xeroderma pigmentosum Xeroderma pigmentosum (XP) is a human disease characterized

by skin lessions and multiple tumors. Patients with XP are deficient in one of the several genes involved

in nucleotide excision repair. Examination of their cells has helped elucidate details of the

excision repair process. (9)

DNA REPAIR


Cell Fusion Studies using a technique known as somatic cell hybridization have been used to

identify the number of different genes involved in xeroderma pigmentos. (10)

DNA REPAIR


Nucleotide Excision Repair Determined by Study of XP Human somatic hybrid studies have identified at least 7 genes involved in

nucleotide excision repair and have elucidated details of the process shown here.

Remember that each of these proteins is the product of at least one gene and a defect in any of these proteins will reduce or eliminate nucleotide excision repair, leading to xeroderrma pigmentosum. (11)

DNA REPAIR


Correction of Error During Replication DNA polymerase is not prefect. The enzyme occasionally inserts an incorrect base. DNA polymerase has the ability to reverse and correct the error

by cutting out the incorrect nucleotide and replacing it. This is called proofreading repair. (12)

DNA REPAIR


A Mismatch Missed by Proofreading Even after proofreading repair, some errors persist. To cope with these errors, a mechanism called mismatch repair may be

acivated. A special problem exists with a mismatch. Which strand is correct and which is the mutation? (13)

DNA REPAIR


Mismatch Repair in Bacteria At least in bacteria, the template strand is methylated and a methyl group is

added to adenine residues. The newly synthesized strand remains temporarily unmethylated. A repair enzyme binds to the unmethylated strand. A nuclease nicks the DNA strand. The DNA is unwound, degraded, and replaced until the mismatch is reached

and removed. (14)

DNA REPAIR


Repair Occurring After Replication Sometimes, damaged DNA escapes all repair systems. When this happens, a

final post-replication repair system may function. When DNA polymerase encounters a lesion such as a thymine dimer during

replication, it first stalls, then skips over the lesion and continues, leaving a gap in the newly synthesized strand.

A protein complex, RecA, directs a recombination event from the undamaged parent strand.

This leaves a gap in the parent strand that can be filled by the repair process involving DNA polymerase I as seen earlier.

Post-replication repair is sometimes called homologous recombination repair, a more general category. (15)

DNA REPAIR


Undamaged DNA: SOS Repair System Repressed Sometimes, in the bacterium E. coli, another system exists to repair DNA damage missed

by other repair systems. This is called the SOS repair system (after the well-known distress call). The SOS repair system creates conditions that allow DNA polymerase to replicate across

a damaged area. This system is error prone and sometimes the wrong base is inserted, a phenomenon

known as SOS mutagenesis. A large number of gene product including LexA and RecA are involved in this process.

DNA REPAIR


Undamaged DNA: SOS Repair System Repressed When the LexA protein product is produced it prevents the transcription of the recA and

other repair genes involved in the SOS repair system. Howover, if RecA protein is present, if binds to the single stranded DNA in the area of

damage, and activates the cleavage of the LexA repressor from the repair genes. When RecA binds at the site of DNA damage, the regulatory capacity of the LexA protien

is disrupted. SOS repair genes are transcribed and their protein products move to the repair site. RecA protein appears to form a bridge at the lesion, and the SOS proteins convert the

lesion to an error-prone site. This allows DNA polymerase to complete the replication across the gap. (16)

DNA REPAIR


Double Strand Break Repair Some mutagenic agents create breaks across both strands of the doule helix.

Ionizing radiation (X-rays and gamma rays, for ezample) is known to do this. In mammals, a specialized form of repair, DNA double-strand break repair

(DSB) repairs this type of DNA damage. The system mediates re-annaeling of the damaged DNA. (17)

DNA REPAIR


Homologous Recombination Repair Breaks across both strands of DNA

can also be repaired by a poorly understood process called homologous recombination repair in which the undamaged strand.

This occurs in the late S/G2 part of the cell cycle.

This process is not well understood. When damage is severe, a non-

homologous portion of the chromosome may be used to repair the breaks.

Clearly, this process can lead to mutations. (18)

DNA REPAIR


Module Summary | DNA Repair 1. It is clear that DNA repair is an extremely important process and a wide variety of

mechanisms exist to ensure genetic integrity. When these systems fail, the result is mutation leading to abnormal cell function, cancers, and cell death. A number of genetic diseases are thought to be associated with defective repair processes.

2. Repair processes can be roughly divided into the following categories based on the type of DNA damage that they repair:a. Photoreactivation Repair: repairs thymine dimers caused by UV lightb. Excision Repair: repairs thymine dimers and other single-strand errors.c. Proofreading and Mismatch Repair: corrects incorrect base insertion during

replication.d. Postreplication Repair: repairs incorrect base insertion after replication is completed.e. Double Strand Break Repair: repairs breaks across both strands of a double helix.

3. While DNA repair is essential to all life, ensuring the fidelity of genetic information, the occasional failure of repair and subsequent mutation is essential for evolution of life to new forms. (19)

Tm-06 Dna Repair

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