Part III, Module III.1, Lesson III.1.2

Part III, Module III.1, Lesson III.1.2
Biological Effects of Ionising Radiation IAEA Post Graduate Educational Course in Radiation Protection and Safety of Radiation Sources Biological Effects of Ionizing Radiation Effects of Radiation at the Molecular and Cellular Level Part III: Biological Effects of Ionising Radiation Module III.1: Effects of Radiation at the Molecular and the Cellular Level Lesson III.1.2: Effects of Radiation on Cells Learning objectives: Upon completion of this lesson, the students will be able to: understand DNA and chromosomal structure explain DNA metabolic process describe damage to DNA by ionising radiation (major types and mechanism) understand DNA repair process and mechanism Activity: lecture Duration: 1 hour Materials and equipment needed: none References: 1. Cell and Molecular Biology, E.D.P. De Robertis V., De Robertis E.M.F., Info- Med Ltd; Hong Kong (1998). 2. Physical and chemical mechanisms in molecular radiation biology. Glass W.A. and Varma M.N. Eds, Plenum Press, New York 1991. 3. Kiefer J. Biological Radiation effects. Springer Verlag, Berlin, Heidelberg 1990; 444. 4. IAEA Regional Basic Professional Training Course on Radiation Protection, India, Training Materials 5. IAEA Regional Basic Professional Training Course on Radiation Protection, Germany, Training Materials. Effects of Radiation on Cells Lecture IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation Introduction This lecture presents information about structure of DNA, and mechanism of DNA damage by ionising radiation It covers the topics describing DNA and chromosomal structure, stages of DNA metabolic process, damage to DNA by ionising radiation, major types and mechanism of the damage, DNA repair process and mechanism Lecture notes: This lecture presents materials about structure of DNA, mechanism of DNA damage by ionising radiation. We will cover the topics describing DNA and chromosomal structure, stages of DNA metabolic process, damage to DNA by ionising radiation, major types and mechanism of the damage, DNA repair process and mechanism. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation Content Chromosomal structure DNA structure DNA metabolic process Damage to DNA by ionising radiation (major types and mechanism) DNA repair process and mechanism DNA repair and human disorders Lecture notes: The following topics are covered in the lesson: Chromosomal structure DNA structure DNA metabolic process Damage to DNA by ionising radiation (major types and mechanism) DNA repair process and mechanism DNA repair and human disorders. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation Overview Cell- as a principal target to ionising radiation Effects of radiation on Cells: Effects on DNA Chromosomal aberrations Cell inactivation or cell killing Cell transformation Cell mutation Lecture notes: Cell is the principal target of ionising radiations. Ionising radiation can cause breaks in the strands and substitution of the wrong base or loss / rearrangement of bases (mutation) either by direct interaction with the DNA molecule, or by the formation of free radicals within the cell. The DNA within a cell is constantly being damaged both by radiation and by chemicals and the cell has very effective repair mechanisms which remove damaged portions and replace them or mend breaks in the chromosomes. Damage to the DNA on the chromosomes can lead to cancer induction (malignant transformation), non malignant mutation or cell death. If damage occurs when the cell is replicating, the strands of DNA separate and the damaged strand can no longer replicate or be repaired because it is no longer linked to the undamaged strand. If cells receive a lot of damage in a short time it is possible that the repair mechanisms will be saturated and unable to cope. A common cause of mutation is believed to be replication in the presence of damage and the incorrect repair of that damage. In humans, inherited genetic defects of the DNA repair systems increase the probability of cancer, for example, Ataxia telangiectasia results in a loss of the repair of X-ray induced damage and hence a higher sensitivity to radiation and an impairment in the processing of radiation damage. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Properties of genetic material
Part III, Module III.1, Lesson III.1.2 Biological Effects of Ionising Radiation Properties of genetic material Genetic material must be able to: Be replicated Control expression of traits or phenotypes Change or to mutate in a controlled way Lecture notes: The genetic material must have three general properties: It must be able to be replicated, in order to be in each cell after cell division. It must be able to control expression of traits or phenotypes. Since we know that traits are determined by proteins that act in an organism, and that these proteins are determined by their sequences of amino acids, the genetic material must be able to encode the sequence of proteins. It must be able to change or to mutate in a controlled way, in order to ensure survival of a species in a changing environment. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Chromosomes as a deposit of hereditary material
Part III, Module III.1, Lesson III.1.2 Biological Effects of Ionising Radiation Chromosomes as a deposit of hereditary material Lecture notes: A variety of chromosome types, as defined by relative size and shape, are found in the nucleus of each cell. Furthermore, there are usually two copies of each type of chromosome in each cell. The cell is called diploid. All cells of an organism, excluding germ cells and red blood cells, and all organisms of the same species, have the same number of chromosomes. The germ cells (i.e. sperm and egg) have exactly half of the number of chromosomes as are found in non-germ or somatic cells of an organism. The fertilization of an egg with a sperm cell produces a diploid cell called zygote, which has again the same number of chromosomes as a somatic cell of that organism. The nucleus of most human cells contains 2 sets of chromosomes, 1 set given by each parent. Each set has 23 single chromosomes - 22 autosomes and an X or Y sex chromosome. A normal female has a pair of X chromosomes and a normal male has an X and Y pair in each cell. Chromosomes are composed of about 40% Deoxyribonucleic acid (DNA) and 60% protein. The protein component of chromatin are histones and non-histone chromosomal proteins. The five types of histones fall into two main groups, the nucleosomal histones and the H1 histones. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation Histones Histones that form nucleosomes (H2A, H2B, H3 et H4) - highly positively-charged proteins able to bind to DNA and one another, in the form of a nucleosomes H1 histone responsible for linking the nucleosomes within the 30 nm chromatin fiber Lecture notes: In eukaryotes, DNA is strongly bound to an equal mass of histones. The five types of histones are divided into two main groups: histones that form nucleosomes (H2A, H2B, H3 et H4) are highly positively-charged proteins that are thus able to bind to DNA and one another, in the form of a nucleosomes. the H1 histone, which appears to be responsible for linking the nucleosomes within the 30 nm chromatin fiber. Some DNA regions, even those several hundred nucleotide-base pairs long, have no nucleosomes. These sites are thus hypersensitive to nucleases. It is thought that most of these sites are regions from which a nucleosome was by a DNA-binding protein specific to a sequence and involved in regulating eukaryote genes. There are also non-histone chromosomal proteins which are DNA-binding proteins, specific to a sequence. They seem to play many different roles: folding the DNA molecule into distinct, initiating DNA replicating gene transcription, etc. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation Nucleosome Nucleosome Lecture notes: Histones are responsible for packing the large DNA molecules (nearly 5 cm) into a cell nucleus (a few m). The fundamental packing unit is known as the nucleosome with a diameter of 5 nm. 146 base pairs (bp) of DNA are coiled around one nucleosome, and each nucleosome is separated from the next nucleosome by a region of linker DNA with approximately 60 bp. The nucleosomes are packed upon each other to generate regular arrays, called chromatin fibers with a diameter of 30 nm. Histone H1 is responsible for packing nucleosomes into the 30 nm-fiber. The 30 nm-fiber is folded into loops and maintained in that configuration by DNA-binding proteins. The length of DNA in each loop is between 20,000 and 80,000 bp. The average length of one loop is about 400 nm. Upon chromatin condensation during mitosis, the DNA loop structure is coiled up to form a much more condensed structure. The two daughter DNA molecules of a metaphase stage chromosome are separately folded to give a bipartite structure consisting of the two daughter chromatids held together at a structure called centromere. Chromosomes can be seen under a light microscope and, when stained with certain dyes, reveal a pattern of light and dark bands reflecting regional variation in the amounts of A and Т vs G and C. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation DNA structure Lecture notes: Classically, the diameter of the nucleus is 5 m. The packing of DNA into a 30 nm chromatin thread yields a human chromosome approximately 0.1 cm long, which indicates that the presence of several levels of folding. Studies of the relatively easily observable chromosomes of some animal species (namely, E. coli and Drosophila) have shown that the chromosomes have domains in loops, which extend radially from the principal axis of the chromosome. These are maintained by DNA-binding proteins that staple together two DNA regions specifically recognized as the loop base. Most of these loops contain between and pairs of nucleotides. A chain of DNA is a long non-branching polymer composed of only four sub-units: the purine bases adenine (A) and guanine (G), and the pyrimidine bases thymine (T) and cytosine ( C ). The bases are arranged in two linear arrays (or strands) held together by hydrogen bonds centrally and linked externally by covalent bonds to sugar-phosphate residues. The conformation proposed by WATSON and CRICK requires that the bases of each strand be very close to one another and thus that they be paired: a large purine base (A or G, each with a double ring), opposite a small pyrimidine base (C or T, each with a single ring), maintained by two or three hydrogen bonds. Although of the DNA bases varies greatly among species, the double helix structure requires that, quantitatively, [A] = [T] and [C] = [G]. Chemically, DNA is fairly inert. The information it contains is expressed indirectly by other molecules: DNA directs the synthesis of specific RNA molecules (transcription) and proteins (translation) that in turn determine the cell’s biophysical and biochemical properties. The sequence of the bases defines the genetic code; each gene has a unique sequence, although certain common sequences exist in control and structural DNA elements. Damage to DNA may affect any one of its components, but it is the loss or alteration of base sequence that has genetic consequences. A section of DNA that codes for one protein is referred to as a gene although the “message” from several genes can be carried by a single piece of Ribonucleic acid (RNA). Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation RNA Is a single stranded DNA Is produced in the nucleus by transcription from opened DNA molecule Involved in the translation Types of RNA: Messenger Transfer Ribosomal Lecture notes: For all practical purposed RNA is single stranded DNA, i.e., a sugar-phosphate chain with one of four nitrogenous bases attached to each sugar group. It is produced in the nucleus by a process (transcription) in which certain sections of the DNA molecule split. One side of the opened DNA molecule serves as a template for the production of RNA. The RNA then migrates out of the nucleus into the cytoplasm where it I involved in protein production (translation). The sequence of bases on the RNA molecule determinates which types of proteins are produced by specifying the order in which the amino acids are joined. There are three types of RNA: messenger, transfer, and ribosomal RNA. 1. Messenger RNA (mRNA) is a copy of a gene by having a sequence complementary to one strand of DNA and identical to the other strand. The mRNA acts as a shuttle to carry the information stored in DNA within the nucleus to the cytoplasm where the ribosomes translate the message into protein. 2. Transfer RNA (tRNA) is a small RNA with a very specific secondary and tertiary structure. It binds an amino acid at one end and hybridizes to mRNA at the other end. It acts as an adapter to carry the amino acids of a protein to the appropriate place as coded for by the mRNA. 3. Ribosomal RNA (rRNA) is one of the structural components of the ribosome. It has sequence complementary to regions of the mRNA so that ribosomes know where to bind mRNA and to start translation into protein. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation DNA metabolic process Condensation Replication Transcription Recombination Repair Lecture notes: Although the maintenance of overall chromosome structure is crucial for DNA metabolic processes such as condensation, transcription, replication, recombination and repair, it is the DNA polymer itself that is the source of cellular information and, thereby, physiological control. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation DNA replication Lecture notes: DNA is replicated semiconservatively, i.e. the two strands unwind and each act as a template for new strands. Each new double-strand is half comprised of molecules from the old strand. The unwound DNA helix, with each strand being synthesized into a new double-helix, is called the replication fork. DNA is synthesized from 5` to 3`. A triphosphate is required to provide energy for the bond between a newly attached nucleotide and the growing DNA strand. The triphosphate is broken down into a monophosphate and an inorganic pyrophosphate. The leading strand is synthesized directly from 5' to 3', as the helix unwinds. The lagging strand is synthesized in fragments (100 to 1000 bases long), called Okazaki fragments. After synthesis these fragments are legated to form the new strand. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Mechanism of DNA replication
Part III, Module III.1, Lesson III.1.2 Biological Effects of Ionising Radiation Mechanism of DNA replication Mother cell DNA as a model Necessity of complementary to the original strand Each DNA nucleotide must be recognizable by a non-polymerized nucleotide Two chains of the helix must be separated so that the groups of hydrogen-bond donors and acceptors are accessible for pairing Lecture notes: Cell division requires that DNA be replicated precisely, with the mother cell DNA as its model. In the replication process, DNA is copied by pairing bases complementary to the original strand. For this replication, each DNA nucleotide must be recognizable by a non-polymerized nucleotide: therefore the two chains of the helix must be separated so that the groups of hydrogen-bond donors and acceptors are accessible for pairing. It was shown in 1956 that DNA polymerase, a multi-enzyme complex, was responsible for polymerizing the deoxyribonucleoside triphosphate groupings. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Enzymes involved in DNA replication
Part III, Module III.1, Lesson III.1.2 Biological Effects of Ionising Radiation Enzymes involved in DNA replication Topoisomerase Helicase DNA polymerase III Primase Ligase Single-stranded DNA binding proteins Lecture notes: 1. Topoisomerase is responsible for the initiation of unwinding of the DNA and breaking tension of coiled and supercoiled DNA by nicking and legating. 2. Helicase accomplishes further unwinding after topoisomerase action. 3. DNA polymerase III proceeds along a single-stranded molecule of DNA, recruiting free dNTP's to hydrogen bond with their appropriate complementary base on the template strand (A with Т and G with C), and to form a covalent phosphodiester bond with the previous nucleotide of the same strand. DNA polymerase III cannot start synthesizing DNA de novo on a single-stranded template. It needs a primer with a 3'OH group onto which it can attach a dNTP. DNA polymerase is a holoenzyme with proofreading activities. 4. Primase is part of the primasome. The enzyme attaches a small RNA primer to single-stranded DNA. This RNA acts as primer for DNA polymerase III. Thereafter the RNA primer is removed by RNase H and the gap is filled in by DNA polymerase I. 5. Ligase catalyzes the formation of phosphodiester bonds between adjacent 3'OH and 5'phosphate. 6. Single-stranded DNA binding proteins are important to maintain the stability of the replication fork. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation DNA transcription Transcription is analogous to DNA replication. Difference from replication: RNA is synthesized with DNA as template, and different enzymes are involved Stages: Initiation of transcription Termination of transcription Lecture notes: Transcription is analogous to DNA replication. However, RNA is synthesized with DNA as template and different enzymes are involved. The RNA polymerase is a holoenzyme. There are three RNA polymerases in eukaryotic cells. RNA polymerase I makes ribosomal RNA, RNA polymerase II makes mostly messenger RNA and RNA polymerase III makes transfer RNA, ribosomal RNA and a few other small RNA molecules. Each eukaryotic RNA polymerase has its own promoters and terminators that bind the transcription enzyme units and regulate the rate at which they work. Initiation of transcription: RNA polymerase is directed to the start site of transcription by one of its subunits, which recognizes a specific DNA sequence at the beginning of genes. This sequence is called promoter. It is an unidirectional sequence on one strand of the DNA that tells RNA polymerase both where to start and in which direction to continue synthesis. RNA polymerase then stretches open the DNA double helix at that point in the DNA and starts synthesis of RNA complementary to one strand of DNA. The DNA strand from which it copies is called antisense or template strand, and the other DNA strand, to which it is identical, the sense or coding strand. The RNA polymerase recruits rNTPs in the same way that DNA polymerase recruits dNTPs. Synthesis proceeds in the 5' to 3' direction. Termination of transcription: The RNA polymerase continues to add nucleotides until it encounters a second special sequence in the DNA, the termination signal, at which point the polymerase releases both the DNA template and the newly synthesized RNA chain. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Transcription of DNA to messenger RNA
Part III, Module III.1, Lesson III.1.2 Biological Effects of Ionising Radiation Transcription of DNA to messenger RNA Lecture notes: RNA processing: All RNA polymerase II transcripts in the nucleus are known as heterogenous nuclear RNA (hnRNA). Many of the hnRNA molecules are destined to leave the nucleus as mRNA. Before they leave, however, they undergo a series of covalent modifications: The 5' end of the RNA molecule is capped. This cap structure is later recognized by ribosomes. To the 3' end of most transcripts a poly-A tail ( A residues) is added by poly-A polymerase. The site of poly-adenylation is created either by a cleavage of the growing RNA chain or by termination. HnRNA often contains long insertions of non-coding RNA sequences, copied from regions of a gene known either as intervening sequences or as introns. These intron sequences are spliced out (RNA splicing) of each RNA transcript in order to convert the transcript to a mRNA molecule. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation DNA translation Lecture notes: Each protein is synthesized by a process in which mRNA becomes attached to large ribonucleoprotein particles called ribosomes. Translation begins with the binding of a small ribosomal subunit to an mRNA molecule. A unique initiator tRNA molecule positions the small ribosomal subunit over a special start codon on the mRNA. A large ribosomal subunit is added to complete the ribosome (Initiation of translation). After completion of the ribosome, the elongation phase begins. Each amino acid is added to the carboxyl-terminal end of the growing polypeptide by means of a cycle of three sequential steps: aminoacyl-tRNA binding, followed by peptide bond formation, followed by ribosome translocation. The ribosome progresses from codon to codon in the 5' to 3' direction along the mRNA until one of the three stop codons is reached. A release factor the binds the stop codon, terminating translation and releasing the complete polypeptide from the ribosome. Many proteins are modified post-translationally, i.e. by phosphorylation, glycosylation etc. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Damage to DNA by ionising radiation
Part III, Module III.1, Lesson III.1.2 Biological Effects of Ionising Radiation Damage to DNA by ionising radiation Major types of damage: base damage, single strand breaks, and double stand break Lecture notes: The consensus seems to be that radiation affects cells primarily by damage to the chromosomal DNA. Studies wherein the nucleus and cytoplasm are irradiated separately indicate that the nucleus is up to 100 times more sensitive than the cytoplasm. Since most cells possess only one or two copies of each DNA molecule, damage here will have a greater effect than damage to a molecule for which thousands of copies exist e.g., enzymes. There are three major types of damage to the chromosomes base damage, single strand breaks, and double stand break. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation DNA base damage The most common effect of radiation on DNA Easily repaired But may be severe enough to lead to cell death Often meant by the term mutation or genetic mutation Lecture notes: DNA base damage is the most common effect of radiation on DNA. Primarily due to interactions of free radicals with the nitrogenous bases. For example, the free radical might induce the determination of the nitrogenous base cytosine converting it to uracil. In most cases this damage is easily repaired. The “classical” (rather slow) repair mechanism operates in the following manner. First, the damage section is excised by an endonuclease. Next, the excised segment is recynthesized by a polymerase using the undamaged strand as a template. Finally ligases attach the newly synthesized segment in place. Faster, less accurate repair mechanism also exist. If the damage remains unrepaired, the cell may survive and reproduce although its function may be impaired. Or, the effect on the cell metabolism may be severe enough to lead to cell death. DNA base damage is often (although not always) what it meant by the term mutation. It is also often referred to as a genetic (vs chromosomal) mutation. Base damage is less likely than chromosomal mutation to result in cell death. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation Single strand breaks Break or nick of the desoxyribose-base bond or the phosphate-deoxyribose bond in a single strand of DNA Result in a partial separation of the affected strand, over several nucleotides Requires approximately eV per break Single strand breaks Lecture notes: A single strand lesion is a break or nick of the desoxyribose-base bond or the phosphate-deoxyribose bond in a single strand of DNA. It should result in a partial separation of the affected strand, over several nucleotides. Requires approximately eV per break. Is not considered to be as important as either of the other two types of damage. The number of single strand lesions increases linearly with dose over a vast dose range (0.2 to Gy). Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation Double strand breaks Separate degradation of both strands of DNA, for a distance not farther than several nucleotides Can be produced by the same event or by two separate events If unrepaired, result in broken chromosomes Lecture notes: A double strand lesion is the separate degradation of both strands of DNA, for a distance not farther than several nucleotides. It can be produced either by the same particle, with enough energy to create two successive lesions, or by two particles, one after the other, in a short enough period for the second to occur before the first has been repaired. The induction of double strand lesions in the nucleus has been found to be linear with dose for all Linear Energy Transfers (LET) studied, whatever the analytic technique. In addition, the formation of double strand lesions differs qualitatively and quantitatively as LET increases (for example, between gamma rays and fission neutrons). This may be explained by the diminution of the cell's capacity to repair breaks, in either aerobic or anoxic conditions, as and when LET augments. Moreover, for intermediate and high doses, the quantity of double lesions formed seems practically independent of the position of the cell cycle. Straight breaks of both strands are probably not the only phenomenon responsible for forming double strand lesions. In particular, DNA repair systems are very sensitive to the condition of the strand around any lesion. Simple base damage defectively repaired can also be transformed into a break. Double strand breaks Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation DNA repair process Mechanism of synthesis of repair enzymes Repair rate influence cell survival Lecture notes: The importance of the DNA repair process for cell survival has led to the development of mechanisms allowing the synthesis of repair enzymes on an emergency basis when major DNA lesions occur. This system, called the SOS response, has been shown in E. coli, and its existence is suspected in eukaryotes. Blockage of DNA replication by a lesion in the strand results in the production of a large amount of RecA, a protein that triggers proteolytic activity. RecA is thought to promote transcription of more than 15 different genes that code proteins that play a role in the SOS response. It may cause the insertion of any base, at random, to fill in the lesion, or it may promote genetic recombination. The increased rate of DNA repair increases cell survival, but on the other hand, the repairs are less accurate and thus raise the rate of mutation. This mutation may, of course, be a better adapted cell. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Mechanism of DNA repair
Part III, Module III.1, Lesson III.1.2 Biological Effects of Ionising Radiation Mechanism of DNA repair Excision Recognition and elimination of the damaged part of a DNA strand Resynthesis reconstructing the original information from the undamaged strand Complete reconstruction Lecture notes: The basic mechanism for DNA repair involves three stages: the damaged part of a DNA strand is recognized and eliminated by DNA repair enzymes called endonucleases, which hydrolyze the phosphodiester bonds that link the damaged nucleotides to the rest of the molecule, leaving a gap in the DNA. This is known as excision. The details of the excision stage depend on the type of DNA lesion. - Depurination is by far the most common DNA lesion: it leaves a deoxyribose sugar with one missing base. This simple sugar is recognized by an AP endonuclease, which cuts the DNA-phosphodiester backbone at the modified site. After the enzymatic excision of the sugar-phosphate residue, a complete DNA sequence is restored by the mechanism described above. - Deaminated cytosine and adenine bases, alkylated bases, bases with open rings, bases in which a double carbon - carbon bond is accidentally converted into a simple carbon - carbon bond: a similar repair mechanism uses different enzymes from the DNA glycosylase family, each of which recognizes only a single basic type of damaged DNA and catalyzes its elimination, by hydrolysis. 2) another enzyme, DNA polymerase, attaches itself to the 3’- OH end of the cut strand and fills in the gap nucleotide after nucleotide, by reconstructing the original information, from the undamaged strand. This mechanism is called resynthesis. 3) the cut left in the damaged chain when DNA polymerase has filled in the gap is welded by a third enzyme, DNA ligase, which completes the reconstruction process. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

DNA repair and human disorders
Part III, Module III.1, Lesson III.1.2 Biological Effects of Ionising Radiation DNA repair and human disorders Xeroderma pigmentosum Fanconi’s anemia Ataxia teleangiectasia Lecture notes: Several congenital diseases in humans are known to be due to defective functioning of DNA repair systems. Three of these disorders are particularly characteristic. Xeroderma pigmentosum is an autosomal recessive disease affecting both sexes equally. It is characterized by intense erythema of zones of exposed skin, accompanied by telangiectases (lesions from capillary dilations), mucous degeneration, mental retardation, and a predisposition to cancer, especially skin cancer. In most cases, death occurs before adulthood, with prognosis associated with tumor development. The cells seem to be normally sensitive to ionizing radiation, but excessively sensitive to UV. An alteration of the excision-re-synthesis system has been shown; it affects endonuclease and is expressed by the non-repair of pyrimidine dimers. The three enzymes that follow endonuclease in the excision-re-synthesis system function normally, thereby explaining the normal repair of strand breaks due to, for example. X-rays. Another autosomal recessive disease is Fanconi’s anemia. This anemia is characterized by bone marrow decay (aplasia) affecting all cell lines, multiple anatomic lesions, mental retardation, and a predisposition to leukemia and other cancers. Here, again, the patient rarely reaches adulthood. Patients with Fanconi's anemia have been shown to be more than normally sensitive to ionizing radiation, to UV, and to some chemical products. Chromosomal studies have shown that lesions induced at any point in the cell cycle lead to the maximum rate of aberrations, indicating that no DNA repair takes place. Ataxia teleangiectasia is primarily autosomal recessive, with rare dominant cases. Its principal characteristics include cerebral ataxia (inability to coordinate some voluntary movements), telangiectases (lesions from capillary dilations), especially ocular, respiratory infections, and a predisposition to chronic lymphopathy. Patients with this disease are 2 to 3 times more sensitive to ionizing radiations than are normal subjects, with increased radiosensitivity in all cell lines. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Biological Effects of Ionising Radiation Summary This lecture presented materials about structure of DNA, and mechanism of DNA damage by ionising radiation The materials of the lecture should be learned at the introductory stage of radiobiology learning Comments are welcomed Lecture notes: Let’s summarize the main subjects we did cover in this session. This lecture presented materials about structure of DNA, mechanism of DNA damage by ionising radiation. We did cover the topics describing DNA and chromosomal structure, stages of DNA metabolic process, damage to DNA by ionising radiation, major types and mechanism of the damage, DNA repair process and mechanism. The material covered in the lecture should be learned at the introductory stage of learning or refreshing radiobiology knowledge, as this material presents the ground for understanding the basics of radiation damage at the cellular level. Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Where to Get More Information
Part III, Module III.1, Lesson III.1.2 Biological Effects of Ionising Radiation Where to Get More Information Cell and Molecular Biology, E.D.P. De Robertis V., De Robertis E.M.F., Info-Med Ltd; Hong Kong (1998). Physical and chemical mechanisms in molecular radiation biology. Glass W.A. and Varma M.N. Eds, Plenum Press, New York 1991. Kiefer J. Biological Radiation effects. Springer Verlag, Berlin, Heidelberg 1990; 444. IAEA Regional Basic Professional Training Course on Radiation Protection, Germany, Training Materials. IAEA Regional Basic Professional Training Course on Radiation Protection, India, Training Materials Module III.1 - Effects of Radiation at the Mollecula and the Cellular Level IAEA Post Graduate Educational Course in Radiation Protection and Safty of Radiation Sources

Part III, Module III.1, Lesson III.1.2

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Part III, Module III.1, Lesson III.1.2

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