genetics exam 4 Flashcards
what is epigenetics?
the transmission of info from one cell generation to the next without altering/involving DNA sequences
changes in gene expression that can be passed from cell to cell and are reversible but DOES NOT involving changing DNA seq
what are epigenetics effects due to?
- histone modifications/nucleosome remodeling (involves methylation and acetylation)
- DNA methylation
what is significant about DNA methylation?
DNA methylation is HERITABLE
methylated DNA seqs are inherited during cell division (daughter cells carry the same modification patterns as the mother cell)
pattern of one copy of gene being methylated and the other not is maintained in resulting offspring
explain the molecular model for inheritance of DNA methylation
de novo methylation is an INFREQUENT and HIGHLY REGULATED event
begin with parent’s methylated DNA –> undergoes DNA replication where each daughter DNA strand gets one of the parent’s methylated DNA, making hemimethylated DNA strands –> maintenance methylase targets hemimethylated DNA and will methylated the unmethylated daughter strands
explain modified histone inheritance of cells from generation to generation
old modified histones distributed randomly among 2 daughter DNA molecules
old modified histones serve as TEMPLATES for mod of new histones
self-perpetuating state
epigenetic inheritance = patterns of chromatin mod are passed onto next cell generation
how are genes/chromosomes targeted for epigenetic regulation?
- targeting gene for epigenetic mod by a TRANSCRIPTION FACTOR –> transcription factor recognizes specific gene sequences and binds to them, transcription factor will recruit other proteins (like histone-modifying enzymes and DNA methyltransferase) which will lead to changes in chromatin structure or DNA methylation, changes alter expression of gene and are maintained in subsequent cell divisions
- targeting gene for epigenetic mod by NONCODING RNA –> non-coding RNA recognizes specific gene sequences and binds to them, non-coding RNA will recruit other proteins (like histone-mod enzymes and DNA methyltransferase) which leads to changes in chromatin structure or DNA methylation, changes alter expression of this gene and are maintained in subsequent cell divisions
what are the 2 categories of epigenetic gene regulation?
- during the life of the organism = epigenetic gene regulation or epigenetic marks may occur as a programmed developmental change (cell differentiation)
- trans-generational = epigenetic patterns inherited from parents
genomic imprinting = one copy of a gene is INHERITED IN AN INACTIVE state
what can result in epigenetic changes?
environmental agents like temp, diet, toxins
these can alter epigenetic marks
what are the mechanisms to maintain epigenetic marks?
- DNA methylation = hemimethylated DNA becomes fully methylated via maintenance methylation
- histone modifications = histones recruit chromatin-mod enzymes and chromatin-remodeling complexes to daughter chromatids
why are epigenetic marks important?
they tell our cells whether to turn a gene on or off
when are epigenetic marks of adults erased?
mostly erased at gametogenesis/fertilization
adults have stable epigenetic marks in all cells –> fusion of gametes involves resetting which erase epigenetic marks so that the fertilized egg can develop into any type of cell (few or no epigenetics from parents) –> fetal development is when new germ cells develop in fetus and new epigenetic marks are established as cells differentiate
there are exceptions to this rule like imprinted genes
what is genomic imprinting?
a phenomenon in which a segment of DNA is inherited in a SILENCED STATE (in adult, one copy is silent and the other is active)
depending on how the genes are marked, the offspring expresses either MATERNALLY-inherited or PATERNALLY-inherited allele = monoallelic expression –> even if a person is mut/wt heterzygous, if the wt copy is silenced and only the mut is expressed then the person displays the disease
what are the stages of imprinting?
- establishment of imprint during embryogenesis (in parent)
- maintenance of imprint during embryogenesis and in adult somatic cells (in offspring)
- erasure and re-establishment of imprint in the germ cells of offspring
review slide 16 about Igf2 imprinting lecture 31
explain the stages of imprinting in relation to Igf2
- establishment of imprint
imprinting of Igf2 gene occurs during gametogenesis, sperm carries the Igf2 allele and egg carries Igf2- allele, only paternal allele will be expressed in offspring - maintenance of imprint
after fertilization, imprint pattern is maintained throughout development –> in this example, maternal Igf2- will NOT be expressed in somatic cells - erasure and re-establishment
in germ-line cells, imprint is erased –> female mouse produces eggs in which the gene is silenced (has Igf2 and Igf2- but they’re all silenced) and male produces sperm in which gene can be transcribed into mRNA (has Igf2 and Igf2- that are all transcribed)
what are some examples of human diseases associated with imprinted genes?
Prader-Willi syndrome and Angelman syndrome
how do you differentiate between paternal and maternal imprinting?
paternal imprinting (paternal gene silenced) = paternally-inherited allele is inherited in SILENT state –> half the progeny of affected females will be affected regardless of their gender
ex: if offspring inherits wt or mut from mother then they will pass it on to 50% of their children, but if they inherit wt or mut from father, none of the genes will be expressed (50% offspring will be carriers)
maternal imprinting (maternal gene silenced) = maternally-inherited allele is inherited in a SILENT state –> half the progeny of affected males will be affected regardless of their gender
affected individuals are HETERZYGOTES
SAME GENDER as parents will be carriers while opp genders will pass on to 50% of their offpsring
explain the effects of diet on genetics in mice
researchers fed pregnant YELLOW mouse a METHYL-RICH diet –> most of her pups were brown and stayed healthy for life (lots of methylation)
decrease methylation of agouti gene and only fed with BPA –> her pups were yellow
2 mice have same mutation in agouti gene –> its promoter is VERY SENSITIVE to methylation (when highly expressed (no Me++), agouti affects coat color, obesity, diabetes, and tumorigenesis
what are non-coding RNAs?
some genes DO NOT encode polypeptides, but are transcribed into non-coding RNAs (ncRNAs)
in most cell types, ncRNAs are more abundant than mRNAs (in human cell –> only about 20% of transcription involves production of mRNAs whereas 80% of it is associated with making ncRNAs)
ncRNAs can bind to different types of molecules
RNA molecules can form stem-loop structures which may bind to pockets on surface of proteins
ncRNAs can have multiple binding sites (RNA acts as a scaffold)
what are some common binding interactions between ncRNA and other molecules?
- ncRNA-DNA binding
- ncRNA-protein binding
- ncRNA-mRNA binding
- ncRNA-small molecule binding
what are the functions of ncRNAs?
they can function as:
- scaffold = ncRNA binds a group of proteins
- alteration of protein function or stability (ncRNA binds to a protein and alters that protein’s structure –> ability of protein to act as a catalyst, to bind to another molecule, or stability of the protein)
- guide = ncRNA binds to a protein and guides it to a specific site in the cell
some are housekeeping RNAs like telomerase RNAs, rRNAs, tRNAs, components of splicing machineries, guide RNAs, ribozymes and components of secretory machinery
how does ncRNA act as a guide?
it can direct a protein to a specific DNA/other RNA seq
ncRNA guides protein to a site in the DNA
this is the kind of function that tracrRNA+crRNA provides in the CRISPR/Cas9 system (Cas9 is directed to a particular DNA seq in a DNA molecule by base pairing of guide RNA with the DNA target
what are the ncRNA lengths?
long ncRNAs (lncRNAs) are longer than 200 nt –> this includes many of the housekeeping RNAs and some long, regulatory RNAs
small regulatory RNAs (short ncRNAs) are shorter than 200 nt –> microRNAs, siRNAs and piRNAs fall into this catergory (usually 20-25 nt) –> these 3 classes of sncRNAs silence gene expression
how do small regulatory RNAs (miRNA, piRNA and siRNA) regulate gene expression?
there are 3 mechanisms
- translation repression of target seqs = bind to mRNA and prevent its translation
- mRNA destruction of target seqs = bind to mRNA and target it for destruction
- silencing chromatin = bind to/near the DNA seq and cause it to be heterochromatinized
what is the cellular roles of ncRNAs?
researchers used antisense RNA to inhibit mRNA translation (RNA complementary to mRNA)
researchers also introduced sense RNA which also inhibited mRNA expression (RNA os SAME SEQ and POLARITY as mRNA)
explain the use of mRNA in C. elegans
one experiment involved an mRNA encoded by gene called mex-3
an abundant mRNA in early embryos of C. elegans –> make sense and antisense mex-3 RNA in vitro using cloned genes for mex-3 –> inject either mex-3 antisense RNA or a mix of mex-3 sense and antisense RNA into gonads of C. elegans –> RNA is taken up by eggs and early embryos then you see what happened to the mRNA –> used in situ hybridization for this
how does the experiment mRNA and mex-3 work?
- incubate and then subject early embryos in situ hybridization (a laboratory technique that finds and localizes specific DNA or RNA sequences in biological samples)
- in this method, labeled probe is added that is complementary to mex-3 mRNA
- probe is labeled with green fluorescent dye
- if cells express mex-3, the mRNA in cells will bind to the probe and become labeled with green
- after incubation with a labeled probe, the cells are washed to remove unbound probe
- observe embryos under a microscope
the intensity of green color is proportional to [mRNA] –> control is really green (high levels of mex-3 mRNA), injected with mex-3 antisense RNA is moderately green (mRNA levels decreased), injected with dsRNA (no mRNAs detected)
this data indicates that dsRNA is more POTENT at SILENCING mRNA than antisense RNA = RNA interference (RNAi) which dsRNA causes the silencing of mRNA
what are examples of RNAi?
gene silencing mechanisms mediated by miRNAs, siRNAs, or piRNAs
RNAi is found in most eukaryotic species
- microRNAs (miRNAs) are transcribed from ENDOGENOUS eukaryotic genes (as many as 60% of human genes may be regulated by microRNA)
- small interfering RNAs (siRNAs) include those that originate from EXOGENOUS sources (not normally made by cells and those encoded on genome
- PIWI-interacting RNAs (piRNAs) are less widely distributed, mostly in ANIMAL GONADS and target transposable elements
what is significant about hte 3 types of RNAi?
they differ mostly in ORIGIN/BIOGENESIS
all 3 silence genes, associated with conserved set of proteins and use base complementarity to id mRNA targets
what are small RNAs processed from?
larger precursors
- miRNAs = pri-miRNA is made –> folds up on itself to form a HAIRPIN recognized by RNA processing enzymes, cleaved to 70-nt pre-miRNA and exported, cut by dicer to 20-25 bp
- siRNA (long dsRNA produced) –> these are either encoded or sometimes produced in response to viral infections, processed into smaller fragments by dicer enzymes
- long dsRNAs are artificially introduced into cell will also be processed to siRNAs
what do small RNAs form?
they associate with a group of proteins to form silencing complexes –> RNA-Induced Silencing Complex (RISC) [binds to mRNA and causes degradation] or RITS (RNA-induced initiation of transcriptional silencing) [silences mRNA]
what do RISC and RITS contain?
- dsRNA molecule that is 20-25 bp long produced from pre-miRNAs and pre-siRNAs by dicer (one strand will be degraded and the other will be retained)
- proteins
- piRNAs associate with PIWI proteins to form complexes known as piRNA-induced silencing complexes (piRISCS)
- all complexes are directed to their target site by base pairing with small RNA
review slide 17 of lecture 32
what are the silencing mechanisms of the small RNAs?
siRNA derived from dsRNA of viral replication and miRNA from pre-miRNA (this is easily manipulated) will form RISC or RITS –> RITS or RISC complex is GUIDED to mRNA target by complementarity between small RNA and mRNA
piRNAs cause transcriptional silencing of transposable elements via piRISCs complexes
RITS can lead to DNA/histone methylation while RISC leads to mRNA degradation and translation block
what is the function and benefits of RNAi?
RNAi is important form of GENE REGULATION (silences the expression of specific mRNAs but will NEVER INCREASE gene expression)
widely used by plants to prevent viral infections
siRNAs can be introduced into a cell or produced in cell by genetic engineering to target specific genes for silencing
this is huge therapeutic value
what was the first FDA approved RNA-based therapeutic?
Patisiran is a ds siRNA that specifically binds to a genetically conserved seq in 3’-untranslated region of mut and wt transthyretin (TRR) mRNA
used for treatment of neuropathy of hereditary transthyretin-mediated amyloidosisin
what are viruses?
replication particles with nucleic genomes
they must infect LIVING cells to proliferate
they can vary with regard to their structure and ability to INFECT DIFF HOSTS
how do viruses differ?
- host range = cell that is infected by virus is called host cell –> host range is number of species that a particular virus can infect
- structure = all viruses have NUCLEIC ACID GENOME (DNA or RNA) surrounded by a PROTEIN CAPSID
- some viruses have VIRAL ENVELOPE which is derived from plasma membrane of host cell and contains VIRAL SPIKE GLYCOPROTEINS
- genome composition = genome of virus can be DNA or RNA, can be ss or ds, and can carry a few of many genes
describe the viral reproductive cycles
series of steps that results in the production of new viruses, cycle occurs after a virus has infected a host cell, details of cycles vary from one type of virus to another but they all follow 5-6 basic steps
- attachment = virus attaches to surface of a host cell
- entry = virus or viral genome enters host cell
- integration = some but not all viruses integrate their genome into the genome host cell
- synthesis of viral components = viral proteins and RNA or RNA are made by host cell
- viral assembly = viral components assemble into virus particles
- release = viruses released from host cell
AEISAR
what does latency mean in viruses?
viruses may remain INACTIVE or LATENT, during which new viruses ARE NOT MADE
some bacteriophages are latent when they are in LYSOGENIC CYCLE
HIV usually remains latent for A LONG TIME after RNA genome is reverse transcribed into DNA and integrated into host chromosome
some viruses (like herpesviruses) can remain latent as EPISOMES = genetic element that can replicate independently of host genome
how does HIV work?
HIV causes AIDS, infects human T cells (important in immunity)
HIV has RNA genome, retrovirus
during cycle, HIV RNA is reverse transcribed into DNA and integrated into a host chromosome (may remain latent for a long time) –> after activation, new HIV particles are made by transcription of HIV and production of new HIV proteins that assemble at plasma membrane and bud from cell
HIV carries 9 genes, but some of them encode polyproteins that are cut into multiple proteins by proteolytic processing
explain the structure of HIV
has 2 copies of RNA genome that is surrounded by CAPSID –> capsid is surrounded by ENVELOPE that is derived from host cell plasma membrane and is studded with CIRAL SPIKE GLYCOPROTEINS
many of the HIV proteins are contained within the virus including REVERSE TRANSCRIPTASE and INTEGRASE (needed during early steps in viral reproductive cycle)
what is the genome of HIV?
9 genes of HIV can be divided into 5 categories
- gag = proteins used for VIRAL ASSEMBLY and CAPSID FORMATION, gene encodes polyprotein that is cleaved into 4 diff proteins (matrix protein, capsid protein, nucleocapsid protein, and p6’-role in release of virions from membranes of infected cells)
- pol = enzymes needed for VIRAL REPLICATION AND VIRAL ASSEMBLY, gene encodes a polyprotein that is cleaved into 3 enzymes (HIV protease, reverse transcriptase, and integrase)
- vif, vpu = proteins that promote INFECTIVITY and BUDDING
- vpr, rev, tat, nef = proteins with REGULATORY FUNCTIONS
- env = proteins that are part of VIRAL ENVELOPE –> gene encodes a polyprotein that is cleaved into 2 proteins (gp41 and gp120)
what allows HIV to infect host cells?
they can only infect host cells that have the required CD4 marker plus the coreceptor (CCR5 or CXCR4)
what is the HIV reproductive cycle?
spike glycoproteins on the HIV bind to receptors on the host cell membrane, the viral envelope fuses with the membrane, releasing the capsid and its contents
after HIV enters host cell, ssRNA is reverse transcribed into dsDNA by REVERSE TRANSCRIPTASE –> process begins when a host cell tRNA binds to the HIV RNA (tRNA acts as a primer) –> in series of steps, portions of viral RNA are reverse transcribed into DNA, viral DNA is used as templated to make complementary DNA strand to give dsDNA –> during this process, viral RNA is degraded by RNase H (component of reverse transcriptase) –> dsDNA is integrated into a random position in genome by HIV encoded integrase protein
when host cell receives signal to become active, provirus uses host RNA poly to create copies of HIV genome and shorter strands of mRNA which are translated –> assembly of virus particle takes place on INNER SURFACE of cell membrane (in macrophages and vacuoles)
HIV enzyme called protease cuts the long chains of HIV proteins into small proteins, the smaller HIV proteins come together with copies of HIV’s RNA genetic material and a new virus particle is assembled
gag precursor (Pr55gag) is major virion component, gets cleaved into 4 major proteins (matrix, capsid, nucleocapsid, and p6gag) and associates into virions spontaneously –> newly assembled virus buds out from host cell –> during budding, new virus gets covered by cell’s cells outer envelope (studded with glycoproteins to help virus bind to CD4 and coreceptors)
what are therapies for HIV?
viral life cycle have been targeted for antiviral therapy
combo therapy of use of multiple anit-HIV drugs like one protease inhibitor with 2 reverse transcriptase inhibitors
what is significant about +ve or -ve strand viruses?
+ve strand viruses have RNA genome that can be translated (mRNA like)
-ve RNA viruses CANNOT be translated
RNA synthesis has no…
proofreading mechanism
which influenza virus causes the most infections?
types A, B, and C but A causes the most infections
what kind of virus is the influenza virus?
orthomyxovirus
explain the components of the influenza virus
NEGATIVE (means cannot be translated), ssRNA
has 8 segmented genomes in A and B
3 proteins = M2, hemagglutinin, and neuraminidase in host cell-derived membrane on surface of virion
matrix protein M1 underneath, RNA segments covered in nucleoprotein
virus attaches to and multiples in the cells of the RESPIRATORY TRACT
describe the cycle of infection by influenza
- virus binds to the a respiratory epi cell by hemagglutinin spikes and fuses with the membrane
- virus is endocytosed into a vacuole and uncoated to release its 8 nucleocapsid segments into the cytoplasm
- nucleocapsid are transported into nucleus where the - sense RNA is transcribed into + sense strand that will be translated into viral proteins that make up the capsid and spikes
- (+) sense RNA is used to synthesize glycoprotein spikes inserted into host membrane
- release of mature virus occurs when viral parts gather at the cell membrane and are budded off with an envelope containing spike
- (+) sense RNA stands are used to synthesize new - sense RNA stands –> these are assembled into nucleocapsids and transported out of the nucleus to the cell membrane
why are influenza glycoproteins important?
- hemagglutinin (H) = 18 diff subtypes, most IMPORTANT VIRULENCE FACTOR, binds to host cells
- neuraminidase (N) = 11 subtypes, HYDROLYZES MUCUS and ASSISTS VIRAL BUDDING and RELEASE
H and N are used to classify the influenza viruses (H1N1)
both glycoproteins frequently undergo genetic changes, producing new variant viruses
what is antigenic DRIFT?
viral RNA poly is ERROR-PRONE, resulting in constant MUTATIONS and results in AA subs in the viral proteins
what is antigenic SHIFT?
one of the genes or RNA stands is sub with a strand from ANOTHER influenza virus of a different animal host by REASSORTMENT
what is needed for influenza to be infective?
it needs all 8 RNA segments to be present to be an infectious virus
what happens to the RNA segs when a cell is infected by 2 different influenzas?
RNA replication will produce a mix of the 8 RNA segs from the avian virus and from the human virus
the packaged virus will contain a mix of human and avian RNA segs
this is known as REASSORTMENT and produced new combos of HA and NA (antigenic shift)
what are the 3 reservoirs of influenza viruses?
aquatic birds
pigs
humans
when reassortment occurs, the new virus will have to be very diff from all existing viruses (worldwide epidemics may be a consequence since the entire pop is susceptible to the virus)
explain the components of coronaviruses
large RNA viruses with spaced spikes on envelopes
+ve strand RNA (can be translated into mRNA), non -segmented, large RNA genomes
common in domesticated animals
7 coronaviruses have been id as causes of human disease, ex: MERS, SARS, SARS-2CoV
virus has spikes, nucleocapsids on its RNA viral genome which is enclosed in an envelope
what is the reproductive cycle of coronaviruses?
- binds to host cell receptor ACE-2
- once endocytosed, its viral genome is released
- the RNA genome is replicated
- the viral RNA is transcribed
- translation of the viral proteins using host machinery
- assembly of the viral particle
- formation of the mature virion
- exocytosis of the new virion
what components of the coronoviruses are targets for vaccine development?
spike protein is a common target for anntibody and vaccine development
another strategy = inject indvs with DNA/RNA encoding viral proteins
- live attenuated
- whole inactivated
- RNA
- DNA
- recombo subunits
- recombo viral vectors
what is a mutation?
a heritable change in gene material
mutations provide allelic variations
mutations are more likely harmful than beneficial
what is a point mutation?
change in a single base pair/adjacent base pairs
it can involve a base substitution
this includes transitions and transversions
mutations may also involve addition or deletions of short DNA seqs
what is a transition?
a change of a pyrimidine (C, T) to another pyrimidine or a purine (A, G) to another purine
transitions are more common than transversions
what is a transversion?
a change of pyrimidine to a purine or vice versa
what are silent mutations?
base subs that DO NOT ALTER the AA seq of the polypep (due to degeneracy of genetic code)
what are missense mutations?
base subs in which AA change does occur
ex: sickle cell anemia from glutamic acid to valine
if the sub AA has no detectable effect on protein function, the mutation is NEUTRAL
this can occur if new AA has similar chem to the AA it replaced
what are nonsense mutations?
base subs that change a normal codon to a STOP codon (can result in shorter proteins)
what are frameshift mutations?
addition or deletion of number of nucleotides that is NOT DIVISIBLE BY 3
shifts reading rame so that translation of mRNA resutls in completely diff AA seq downstream of mutation
what is the frameshift mutation of HIV?
caused by a 32 bp deletion in CCR5 gene, results in an inactive (undetectable) protein and near complete resistance to HIV-2 infection (present in 13% of N. Europeans)
what are the effects of gene mutations outside of the coding seqs?
they can still affect phenotype
mutations in the core promoter can change levels of gene expression
- up promoter mutations = increase expression
- down promoter mutations = decrease expression
- regulatory element/operator site = may disrupt ability of gene to be properly regulated
- 5’UTR/3’UTR = may alter ability of mRNA to be translated, may alter mRNA stability
- splice recognition seq = may alter ability of pre-mRNA to be properly spliced
what are forward mutations?
changes the wt genotype into some new variation
what are reverse mutations?
changes mutant allele back to wt
also called reversion
what are deleterious mutations?
decrease the chances of survival
most extreme = lethal mutations
what are beneficial mutations?
enhance survival or reproductive success of an organism
environment can affect whether a given mutation is deleterious or beneficial
what are conditional mutations?
they affect phenotype only under a defined set of conditions
ex: temp-sensitive mutation
what are the 2 animal cell types?
germ-line cells = cells that give rise to gametes
somatic cells = all other cells
what are germ line mutations?
occur directly in sperm or egg or in one of their precursor cells
can be passed to next gen where mutation will be found in whole body
what are somatic mutations?
those that occur directly in a body cell or in one of its precursor cells, NOT PASSED ONTO NEXT GEN
result in patches of affected area, size of patch will depend on timing of mutation (earlier mutation = larger patch)
indv with somatic regions that are genotypically diff is called GENETIC MOSAIC
when do mutations arise?
arise spontaneously and continuously during culture, early arising mutants will give rise to LOTS OF MUTANT COLONIES
late arising mutants will give rise to FEW RESISTANT COLONIES
number of resistant colonies in cultures should FLUCTUATE
what were the 2 possibilities of mutations for T1-resistant mutants?
- mutants arise spontaneously and continuously aka spontaneous mutation (if T1-resistant mutants are are already in culture before T1 exposure, then there will be variation and fluctuation which supports the hypothesis that resistance to T1 arises from pre-existing random mutations)
- T1-resistance mutants arise ONLY AFTER exposure to T1 (in this case, see no fluctuation, any variation among the cultures and aliquots from same large culture should be the SAME) aka induced mutation
mutations were continuous and random, they were pre-existing and selected by environmental agent
what is the physiological adaptation theory?
predicts that the number of tonr (T one resistance) bacteria is very low unless there is a selection, when the numbers created increase
what is the random mutation theory?
predicts that the number of tonr bacteria will fluctuate in diff bacterial pops and will occur without selection
explain the process of the random mutation theory in relation to resistance of E. coli to infection by bacteriophage T1
- place indv bacterial cells onto growth media
- incubate overnight to allow formation of bacterial colonies = master plate
- press velvet cloth onto master plate and then lift gently to obtain a replica of each bacterial colony –> press replica onto 2 secondary plates that contain T1 phage –> incubate overnight to allow growth of mutant cells
resistant cells were in the same location on both plates
mutations had randomly occurred in absence of selection by T1
became observable with selection for T1 resistance
supports random mutation theory
what are spontaneous mutations?
result from abnormalities in cell/biological processes
ex: errors in DNA replication, DNA damage from normal cell processes
underlying cause originates with the cell
what are induced mutations?
caused by environmental agents
agents that are known to alter DNA structure are termed mutagens (can be chemical or physical agents
