Chapter 14 Flashcards
Genetic Mutation, DNA repair, and Transposition
Intro
DNA stores, replicates, transmits and decodes info
Change in DNA seq give rise to variation
- result in phenotypic variability, adaptation to
environmental changes and evolution
Gene mutations
- new source of alleles and genetic variation in population
- source of genetic changes that can lead to cell death,
genetic disease and cancer.
Mutations
- Alteration in DNA sequence
- Any bp change in sequence
- Single bp substitution
- Deletion of insertion of bp
- major alteration in chromosomal structure
may occur in somatic or germ cells
may occur in coding of noncoddoing region
Mutation classification by molecular change
- Point mutation or base substitution: change from one base pair to another
- Missense mutation: results in new triplet code for different amino acid
- Nonsense mutation: results in triplet code for stop codon (translation terminated prematurely)
- Silent mutation: New triplet code still codes for same amino acid.
Base substitutions (Point Mutations)
- Transitions: Pyrimidine replaces pyrimidine, or purine replaces purine.
- Transversions: Purine and pyrimidine are interchanged.
- AG (purine) CT (pyrimidines)
Frameshift mutations
- Result from insertions or deletions of base pair
- Loss or addition of nucleotide causes shift in reading frame
- Frame of triplet reading during translation is altered
Mutations classified by phenotype
- Loss-of-function mutations
- Gain-of-function mutations
- Visible (morphological) mutations
- Nutritional (biochemical) mutations
- Behavioral mutations
- Regulatory mutations
- Lethal mutations
- Conditional/temperature-sensitive mutations
Loss-of-function mutation
reduces/eliminates function of gene product
Null mutation
results in complete loss of function
Dominant mutation
results in mutant phenotype in diploid organism
Dominant gain-of-function mutation
results in gene with enhanced, negative, or new function
Visible (morphological) mutations
Alter normal/wild-type visible phenotype
Nutritional (biochemical) mutations
Cause loss in ability to synthesize amino acid or vitamin
Gain-of-function mutations
Result in a gene product with enhanced/new function
Biochemical mutations
Can have effect on well-being and survival of affected individual
Behavioral mutations
Affect behavior patterns of organisms. For example, the mating behavior of the fruit fly may be impaired if it cannot beat its wings.
Regulatory mutations
Affect regulation of gene expression
Recessive autosomal mutation
Occurs in somatic cell of diploid organism–Is unlikely to result in detectable phenotype
X-linked recessive mutations
- Arise in gametes of homogametic female
- May be expressed in hemizygous male offspring
Lethal mutations
- Interrupt essential process and result in death
- Various inherited biochemical disorders
Conditional mutations
- Dependent on organism’s environment
Temperature-sensitive mutation - Gene product functions at one temperature but not another
Neutral mutation
- Occurs in protein-coding region or in any part of genome
- Vast majority of mutations likely to occur in large portions of genome that do not contain genes
- Do not affect gene products or gene expression
- The effect on genetic fitness of organism is neither beneficial nor detrimental.
- Neutral effect on genetic fitness of organism
Classification Based on Location of Mutation
- Somatic mutations occur in any cell except germ cells; they are not heritable.
- Germ-line mutations occur in gametes; they are inherited.
- Autosomal mutations occur within genes located on autosomes.
- X-linked and Y-linked mutations occur within genes located on X and Y chromosome, respectively.
Spontaneous and Induced Mutations
Spontaneous mutations
– Happen naturally and randomly
– Usually linked to normal biological or chemical processes in organism
— alter structure of nitrogenous bases
– Occur during enzymatic process of DNA replication
DNA Replication Errors and Slippage
Mutations arise from replication
– Replication is imperfect.
– DNA polymerase occasionally inserts incorrect nucleotides.
– Misincorporated nucleotides persist after proofreading.
– Errors due to mispairing predominantly lead to point mutations.
Replication slippage
- If loop occurs in template strand during replication, DNA polymerase misses looped out nucleotides, and small insertions and deletions occur.
- Replication slippage is more common in repeat sequences
1. Hot spots for DNA mutation
2. Contributes to hereditary diseases
(Fragile-X, Huntington disease)
Tautomeric Shifts
- Tautomers
- Purines and pyrimidines exist in tautomeric forms
(alternate chemical forms)
- Increase chance of mispairing during DNA replication - Tautomeric shifts
- Can change the bonding structure, allowing noncomplementary base pairing
- May lead to permanent base-pair changes and mutations (Figure14-2 and Figure14-3)
Depurination and Deamination
- DNA base damage
- Depurination and deamination
- Common causes of spontaneous mutations
- Lead to new base pairing and mutations
Depurination and Deamination
- Depurination
– Loss of nitrogenous bases (usually purine—guanine or adenine), leads to apurinic site (without purine) - Deamination
- Amino group in cytosine or adenine converted to uracil
(adenine converted to hypoxanthine)
- Result: change in base pairing of original bases
A=T converted to G=C
Figure14-4
Mutagens
Mutagens: Natural or artificial agents that induce mutations –All cells are exposed to a plethora of mutagens. –Fungal toxins –Cosmic rays –Ultraviolet light –Industrial pollutants –Medical X rays –Chemicals in tobacco smoke
Base analogs (mutagenic chemicals)
–Can substitute for purines or pyrimidines during nucleic acid biosynthesis
–Increase tautomeric shifts
–Increase sensitivity to UV light (mutagenic)
–Example: 5-Bromouracil behaves as thymine analog. (Figure 14-5)
Alkylating, Intercalating, and Adduct-Forming Agents
Alkylating agents –Donate alkyl group (CH3 or CH3CH3) to amino or keto groups in nucleotides –Alter base-pairing affinity –Transition mutations result –Example: mustard gas
UV light
- UVlight
– Electromagnetic spectrum (Figure 14-7)
– Purines and pyrimidines absorb UV at 260 nm. - UV radiation creates pyrimidine dimers.
– Two identical pyrimidines that distort DNA conformation – Errors can be introduced during DNA replication. (Figure14-8)
Ionizing radiation
– Energy of radiation varies intensely with wavelength.
– Mutagenic: X rays, gamma rays, cosmic rays
– Penetrates deeply into tissues
– Causes ionization of molecules
Free radicals
–Stable molecules transformed into free radicals (chemical species containing 1 or 1+ unpaired electrons)
–Free radicals directly/indirectly affect DNA.
- Alter purines and pyrimidines
- Break phosphodiester bonds
- Produce deletions, translocations, and fragmentation
Oxidative damage to DNA
- Due to by-products of normal cellular processes
- Exposure to high-energy radiation
- Superoxides (O2–)
- Hydroxyl radicals (OH)
- Hydrogen peroxide (H2O2)
DNA Repair
Repair systems counteract spontaneous and induced DNA damage.
- DNA repair system: maintains integrity of genetic material
- Repair systems counteract genetic damage that would result in genetic diseases and cancer.
Proofreading and Mismatch Repair
- DNA polymerase “proofreads,” removes, and replaces incorrectly inserted nucleotides.
- Mismatch repair (if proofreading fails) becomes activated.
- Mismatches are detected, cut, and removed (endonuclease and exonuclease). Correct nucleotide is inserted by DNA polymerase.
Defective Mismatch Repair
Strong link between defective mismatch repair, and cancer has been documented.
- Hereditary nonpolyposis colon
- Leukemia
- Lymphoma
- Tumors of ovary, prostate, and endometrium
Photoreactivation repair (PRE)
– Cleaves bonds between thymine dimers (T-T), reversing effect of UV radiation on DNA (Figure 14-10)
– Enzyme must absorb photon of light to cleave dimer.
Humans and other organisms lack photoreactivation repair.
Base and nucleotide excision repair (BER & NER)
- Light-independent DNA repair mechanisms exist in all prokaryotes and eukaryotes and involve excision repair.
- Exonuclease recognizes and cuts distortion/error.
- DNA polymerase inserts complementary nucleotides in missing gap.
- DNA ligase seals final “nick.”
Excision repair (2 types)
Two types of excision repair
- Base excision repair (BER)
- Corrects DNA containing a damaged DNA base (f 14-11)
- DNA glycosylase recognizes altered base. - Nucleotide excision repair (NER)
- Repairs bulky lesions that alter/distort double helix (Figure14-12)
Nucleotide Excision Repair and Xeroderma Pigmentosum
Xeroderma pigmentosum (XP) – Rare genetic disorder due to defects in NER pathways – Affected individuals exhibit severe skin abnormalities, skin cancers, and developmental and neurological defects. – Individuals have a 2000-fold higher rate of cancer. (Figure 14-13)
The Ames Test Is Used to Assess the Mutagenicity of Compounds
Ames test
– Uses different strains of Salmonella typhimurium that are able to reveal presence of specific mutations
– Assay measures frequency of reverse mutations in mutant gene.
– Used extensively during development of industrial and pharmaceutical chemical compound
– Many known carcinogens shown by Ames test to be strong mutagens
– More than 60 compounds found in cigarette smoke test positive and cause cancer in animals.
Transposable Elements Move within the Genome and May Create Mutations
Transposable elements (transposons) –“Jumping genes” can move w/in and b/w chromosomes – Insert themselves into various locations w/in genome – Found in all organisms; precise function still unknown
Transposable Elements in Humans
Human transposable elements
– Half of human genome is composed of transposable elements.
– LINEs and SINEs: long interspersed elements and short interspersed elements
0.2% of detectable human mutations may be due to transposable element insertions.–Transposons may contribute to variability that underlies evolution.
Transposons, Mutation, and Evolution
Transposons can have a wide range of effects on genes.
– Insertions can lead to translation disruptions.
–Transposon promoters and enhancers can have effects on nearby genes.
–Transposons can cause aberrant splicing and early transcription translation.
– Recombination between transposons can lead to chromosomal rearrangements, resulting in phenotypic changes or disease.
Polygenic and Monogenic Diseases
Polygenic
–Most human genetic diseases are polygenic, caused by variations in several genes.
Monogenic diseases
–Single base-pair change in one of approximately 20,000 human genes may lead to serious inherited disorders. (Table 14.1)
