Unit 8 - Control of Gene Expression Flashcards
1
Q
what causes gene mutations?
A
DNA replicational errors in interphase generally by substitution, addition or deletion of bases from the normal DNA sequence
occur naturally
2
Q
what increases the rate of mutations?
A
mutagenic agents:
UV light
ionising radiation
chemical carcinogens e.g. tobacco tar & asbestos
3
Q
define mutagenic agent
A
a factor that increases the rate of gene mutations
4
Q
most mutations have…
A
a negative/neutral impact (selected against) on the organism but some are selected for (as they create beneficial alleles) by natural selection
5
Q
what are the 3 types of base substitution mutations?
A
silent mutation
mis-sense mutation
non-sense mutation
6
Q
describe silent mutation
A
base substitution - one base substituted for another
new codon codes for same amino acid
because DNA code is degenerate
so no effect on 1y structure
so no effect on 2y or 3y structure
so protein function unaffected
7
Q
describe mis-sense mutation
A
base substitution - one base substituted for another
new codon codes for a different amino acid
so different 1y structure
so different 2y & 3y structure due to different H bonds, ionic bonds or disulfide bonds b/w R groups
so different specific 3d shape & function
8
Q
describe non-sense mutation
A
base substitution - one base substituted for a stop codon
causes premature translation
shorter 1y structure
so different 2y & 3y structure
different H bonds, ionic bonds & disulfide bonds formed b/w R groups
different specific 3d shape
loss of function - no ESCs formed if about enzymes
9
Q
describe addition & deletion mutations
A
addition - a base is added to the DNA sequence
deletion - a base is removed from the DNA sequence
causes frameshift, shifting the last base of each codon into the next codon to produce a different sequence downstream of the mutation
addition causes frameshift to the right
deletion causes frameshift to the left
so different 1y structure
different 2y & 3y structure
NB addition/deletion of bases in multiples of 3 does not cause frameshift so is less detrimental to the overall protein function
10
Q
describe base duplication mutation
A
one or more bases are repeated, causing frameshift to the right
different 1y, 2y, 3y etc.
11
Q
describe inversion of bases mutation
A
a group of bases becomes separated from the DNA sequence & re-joins at the same position but in the inverse order
different 1y, 2y, 3y etc.
12
Q
describe translocation of bases mutation
A
bases are separated from the DNA sequence on one chromosome & re-join on a different chromosome
affect gene expression & phenotype
can lead to cancer
13
Q
summary of protein synthesis
A
all cells (apart from gametes & RBCs) have the same DNA/genes but they express the genes differently = control of gene expression
transcription factors control which genes are transcribed
splicing
siRNA can destroy mRNA molecules = controls how much mRNA is translated
activity of a protein can be altered by other enzymes - secondary messenger model
14
Q
all cells in the body…
A
have the same DNA/genes but have different structures & functions
they are specialised (differentiated) for a specific function & only transcribe & translate the proteins they need
15
Q
sperm + egg =
A
zygote
16
Q
describe totipotent stem cells
A
a fertilised egg/zygote
early cells derived from zygote by mitosis for a limited time in a mammalian embryo
unspecialised
capable of differentiation into any specialised cell
able to divide for long periods = self-renewal
17
Q
how do totipotent stem cells develop?
A
translate only part of their DNA, leading to cell specialisation
18
Q
describe pluripotent stem cells
A
embryonic stem cells in humans
can give rise to most cell types needed for a foetus to develop
cannot form placental cells
19
Q
what is the medical use for pluripotent stem cells?
A
can divide an unlimited number of times (self-renewal)
& can be used to treat a variety of human disorders e.g. genetic disorders like type 1 diabetes (B cells in pancreas) & paralysis (nerve cells)
20
Q
describe multipotent stem cells
A
adult stem cells e.g. in bone marrow
can differentiate into a limited range of specialised cells
21
Q
describe unipotent cells & e.g.
A
cells that can only divide to form one cell type e.g. formation of cardiomyocytes
22
Q
why is specialisation irreversible?
A
most animal adult cells are specialised & unable to divide
(stem cells replace them by mitosis)
although cells retain all the genes of an organism, many genes are permanently switched off (not expressed)
23
Q
why is it controversial to use pluripotent embryonic stem cells?
A
right to life
embryo cannot consent
human rights argument
24
Q
what is an induced pluripotent stem cell?
A
stem cell that is capable of differentiating into a wide range of specialised cells
made from patient’s adult cells
25
describe induced pluripotent stem cells
generated by using appropriate protein transcription factors
similar to embryonic stem cells
capable of self-renewal
could replace embryonic stem cells in medical research & treatment
would not have the same ethical & rejection (iPS made from patient's adult cells) issues associated with the use of embryonic stem cells
26
how is transcription of target genes controlled?
transcription of target genes can be stimulated or inhibited when specific transcription factors move from the cytoplasm to the nucleus
27
why might embryonic pluripotent stem cells cause harm to recipient?
might differentiate into the wrong types of cells
might divide out of control, leading to a tumour/cancer
28
why are embryonic pluripotent stem cells suitable to treat damage?
they divide
& differentiate into spec. cells
so can replace any type of cell
29
describe the control of gene expression
every somatic cell in an organism contains the same DNA & genes (same genome)
some genes are permanently expressed (switched on) in all cells e.g. genes coding for respiration enzymes
other genes are never expressed (permanently switched off) e.g. insulin gene in skin cells
some genes are switched on & off as & when needed = control of gene expression
e.g. lactase enzyme
30
describe human haemoglobin as an example of the control of gene expression
see booklet for graph
shift in gene expression around the time of birth from gamma globulin to beta globulin
human haemoglobin has 4 globulin polypeptide chains (4y)
adult form has 2 alpha & 2 beta polypeptides
foetal form has 2 alpha & 2 gamma polypeptides
31
describe & explain the structure of foetal haemoglobin
foetal form has 2 alpha & 2 gamma polypeptides
higher affinity for O2
so higher saturation of O2 in placenta
32
how are genes expressed (switched on & off) in a eukaryote?
by transcription factors
33
why don't prokaryotic cells have regulation of transcription by transcription factors?
lack a nucleus that separates transcription & translation
34
define transcription factor
protein usually found in cytoplasm that stimulate transcription
35
how do transcription factors work?
1. TF move from cytoplasm into nucleus, where they bind to specific DNA base sequence or promotor region
2. RNA polymerase binds to DNA-TF complex
3. RNA polymerase moves along gene, catalysing the formation of phosphodiester bonds b/w adjacent RNA nucleotides by condensation
so gene is expressed & mRNA is transcribed
36
describe the action of an inhibitor molecule
the inhibitor molecule binds to the transcription factor so it cannot bind to DNA
so the gene not expressed
37
describe how non lipid soluble hormones can control gene expression
e.g. glucagon - a protein hormone so not lipid soluble
binds to receptor at CSM - secondary messenger model (see unit 6)
38
describe how lipid soluble hormones can control gene expression
e.g. oestrogen - a steroid hormone
1. oestrogen is lipid-soluble so can simply diffuse through the phospholipid bilayer of CSM
2. in cytoplasm, oestrogen binds to complementary receptor on the TF
3. this binding causes a change in 3y structure of the DNA binding site on the TF
4. TF enters nucleus (via nuclear pores) & binds with the specific DNA base sequence/promotor region
5. this stimulates RNA polymerase to join & transcription begins, mRNA is made
39
not all cells have oestrogen receptors
40
how can the translation of mRNA be inhibited?
breaking mRNA down before it is translated into a polypeptide at a ribosome
RNA interference is carried out by small interfering RNA (siRNA)
41
how does siRNA work?
1. double-stranded RNA is cut into shorter lengths by an endonuclease enzyme (cuts DNA at spec. sequences) to form siRNA
2. these form single strands of siRNA & bind to an enzyme
3. siRNA strand with enzyme attached pairs up with complementary bases on a section of mRNA strand
4. the enzyme cuts mRNA into smaller sections (by hydrolysing phosphodiester bonds) so mRNA cannot be translated into a functioning protein
5.the gene is not expressed & the mRNA sections are hydrolysed into RNA mononucleotides
42
describe the importance of RNA interference
- defence mechanism against viruses (HIV) as it inhibits the translation of viral proteins by host cell's ribosomes
- useful research tool - can be used to silence particular genes of interest to see what happens without them e.g. in biochemical pathway
- effective treatment for certain diseases e.g. Huntington's disease involves the production of a faulty protein - RNAi can reduce the translation of this protein & delay symptoms
- if targeted at cancer cells, it could also reduce the expression of proteins that cause rapid rate of division of tumour cells
43
define proteome
full range of proteins that a cell is able to produce
44
define genome
the complete set of genes in a cell or organism
45
define epigenome
the collection of chemical modifications applied to DNA & histones without changing DNA base sequence
this can change over time
heritable
46
what is phenotype determined by?
a combination of genetic & environmental factors
environment influences gene expression, which affects phenotype
47
describe the heritability of changes in gene expression due to environmental stimuli
they have, until recently, been assumed to be uninheritable
we are beginning to understand that, in some cases, environmental stimuli can cause heritable changes in gene function without changes in DNA base sequence (i.e. without mutation)
= some of the epigenome can be inherited
48
define epigenetics
heritable changes in gene function without changes in DNA base sequence
49
what do epigenetic changes involve?
chemical tagging of DNA or their associated histone proteins, which affects the likelihood of particular genes being transribed/expressed
50
what do chemical tags of the epigenome do?
affect the shape & packing density of DNA-histone complexes/chromatin (nucleosomes)
this can alter the physical accessibility of genes for TF & RNA polymerase, needed for transcription
51
what is the effect of tightly packed vs loosely packed nucleosomes?
tightly packed: make DNA & genes less accessible - genes are inactive (switched off/not expressed)
loosely packed: make DNA & genes more accessible - genes are active (switched on/expressed)
52
chromatin
nucleosome
DNA + associated histone proteins
single DNA-histone complex
chromatin is a series of nucleosomes of variable packing density
53
what causes the chemical tags to be added & describe the process
environmental stimuli
starts in embryonic development & continues throughout life
when stem cells differentiate to become specialised cells, many genes are switched on & off by epigenetic mechanisms
stimuli can arise from internal or external environment
54
describe acetylation
acetyl group is transferred to a molecule
deacetylation is removal of acetyl group from a molecule
decreased acetylation of histones increases the +ve charge on histones so increases the attraction b/w histones & phosphate groups (-ve) on DNA
this makes DNA-histone complex/chromatin more tightly packed so DNA is not accessible to TF & RNA polymerase
TF cannot initiate transcription so gene is not expressed & protein is not made
55
describe methylation
methyl group transferred to a molecule
demethylation is removal of methyl group
methyl group is added to cytosine bases of DNA at CpG site
increased methylation of DNA inhibits transcription of genes by making the DNA-histone complex/chromatin pack more tightly together so it prevents the binding of TF & RNA polymerase to the DNA
transcription not activated so gene is not expressed (protein is not made)
56
draw table of effects of epigenetic factors on the DNA-histone complex
see booklet
57
why do tumours form?
uncontrolled cell division
genes involved in the normal regulation of the cell cycle are altered, either by mutation or dysregulation (either overexpressed or under expressed)
58
why are the causes of most cancers complex?
varying genetic predisposition (family history & inheritance of specific alleles/genes)
environmental & lifestyle risk factors are linked to cancer development e.g. smoking, UV radiation etc.
59
compare benign & malignant tumours
see table in booklet
60
what are most cancers initially caused by?
mutation in proto-oncogenes/oncogenes
& tumour suppressor genes
61
describe proto-oncogenes/oncogenes
oncogenes result from a mutation in proto-oncogenes
in normal cells, proto-oncogenes encode proteins that promote cell division
they are normally switched on in cells in response to growth factors e.g. hormones
62
what are growth factors?
proteins that bind to receptors on CSM that stimulate cell growth, or differentiation
63
describe what mutations in proto-oncogenes (or their promotor regions) can cause
cause proto-oncogenes to be permanently overexpressed/switched on
which causes receptors from growth factors being permanently activated (even without binding to growth factor)
excess growth factor produced, stimulating too much cell division
64
describe tumour suppressor genes
encode proteins that:
inhibit/slow down cell division
repair DNA replication errors
stimulate apoptosis (programmed cell death e.g. in response to DNA damage)
65
describe what mutations in tumour suppressor genes (or their promotor regions) can cause
cause TSG to be permanently under expressed (switched off), therefore reducing a cell's ability to inhibit cell division
66
summary table of proto-oncogenes vs tumour suppressor genes:
effect on cell division
effect on apoptosis
if mutated,
see booklet
67
describe how epigenetic changes can cause uncontrolled cell division/cancer
1. hypermethylation of DNA in tumour suppressor gene promotor regions
- too many methyl groups added
- chromatin becomes more tightly packed/condensed
- transcription is inhibited so gene is not expressed
this leads to uncontrolled cell division
2. hypomethylation of DNA in oncogenes
- not enough methylation
- chromatin becomes less tightly packed
- transcription is stimulated so gene is expressed
this leads to uncontrolled rapid cell division
68
describe the role of epigenetics in the treatment of disease
epigenetic changes are reversible
so they are good targets for therapeutic drugs
drugs that prevent methylation can cause genes to be upregulated i.e. decrease methylation on TSGs
e.g. azacitidine
or increased methylation of oncogenes
inhibitor drugs cause increased acetylation of histone proteins, which can upregulate genes i.e. increased acetylation of TSGs
69
what is the problem with targeting epigenetic changes to treat disease?
must target only the inappropriate epigenetic changes - must make the drug specific
70
describe the role of oestrogen in breast cancer
1. oestrogen alters the expression of target genes as they have the oestrogen receptor
2. oestrogen binds, which changes the shape of receptor, allowing the DNA binding site on TF to bind to the promotor region & stimulate transcription
3. in oestrogen-receptor positive breast cancer, the cancer cells have oestrogen receptors on TFs to which the oestrogen can bind, increasing the rate of cell division --> oestrogen can cause overexpression of oncogenes in breast tissue
71
how do drugs against breast cancer work?
binds to the oestrogen receptor, which prevents oestrogen binding so TF cannot bind to DNA promotor region
prevents transcription (RNA pol. not stimulated) of these genes so reduces rate of cell division
aromatase inhibitors block aromatase which is enzyme that synthesises oestrogen --> decreased oestrogen production --> decreased binding of TF --> decreased transcription --> decreased rate of cell division
72
describe genome projects
Human Genome Project - international effort to sequence the human genome, identifying & mapping all human genes
DNA sequencing is now mainly automated & cheap
73
how is information about the proteome derived from the genome & why is it easier in prokaryotes & viruses?
sequencing a genome --> information about the AA sequences of protein in the proteome
prokaryotes & viruses are easier to sequence as they generally have smaller genomes than eukaryotes
deriving protein sequences from their genome is also easier because: they do not have non-coding regions of DNA within genes (introns)
the have fewer regulatory genes
74
how can determining the genome/proteome of a pathogen help the development of vaccines?
identification of proteins that can act as antigens for the immune system
identify proteome
these genes can be used to synthesis the antigens & use them in a vaccine
75
what does recombinant DNA technology involve?
the transfer of fragments of DNA from one organism or species to another
76
what does a transgenic organism contain?
DNA from a different species
77
why does recombinant DNA technology work?
genetic code is universal
similar transcription & translation processes means that the gene transferred can function correctly
78
what are the 3 steps for how a transgenic bacterium is made inc. the 3 methods for step 1?
1. produce a fragment of DNA containing the gene to be transferred
a. conversion of mRNA to cDNA
b. restriction endonucleases used to cut DNA
c. make in a 'gene machine'
2. amplification/cloning of the DNA fragment
3. transformation - getting the DNA fragment into the target organism
79
describe the conversion of mRNA to cDNA
1. find a cell that expresses the gene of interest e.g. insulin gene expressed by B cells
2. isolate mRNA that codes for ____ (insulin)
3. add reverse transcriptase & DNA nucleotides to produce a strand of complementary DNA (cDNA) using mRNA as template
4. remove/digest mRNA strand
5. add DNA polymerase to produce 2nd strand of cDNA = double-stranded DNA copy of the gene of interest (insulin gene)
80
what are the advantages of using conversion of mRNA to cDNA?
it produces a copy of the human (insulin) gene without introns - already spliced
this is vital if gene is inserted into prokaryote as they cannot remove introns themselves
mRNA is easier to extract as there are many copies in the cytoplasm
if the gene is expressed in that cell
81
describe how restriction endonucleases are used to cut DNA
1. isolate DNA/chromosome containing gene of interest
2. restriction endonucleases cut DNA at specific DNA base sequences by breaking PDE bonds
3. the 2 ends formed can be blunt or sticky (more useful)
4. ligase enzymes join sticky ends of different pieces of DNA together if the same restriction endonuclease was used on both pieces
5. gene will still contain introns
82
what are restriction endonucleases (RE)?
enzymes that cut DNA at specific DNA base sequences by breaking phosphodiester bonds
each restriction endonuclease cuts DNA at a specific restriction site
83
why are pieces of DNA only joined if cut by the same restriction endonuclease?
overhanging ends are complementary
so will form H bonds by complementary base pairing
84
what are restriction endonucleases important for?
insertion of DNA fragments into plasmid vectors
DNA tech. techniques e.g. DNA fingerprinting & electrophoresis
85
describe making a gene in a 'gene machine'
machines can synthesise any sequence of DNA bases required
the DNA sequence of the gene of interest must be known - can be found by DNA sequencing or can work backwards from AA sequence of the protein
can include relevant restriction sites --> helps insert DNA section into plasmid vector
no introns included
86
describe in vivo cloning of the DNA fragment
1. DNA fragment inserted into bacterial plasmid
2. plasmid inserted into bacterium
3. bacteria cultured on growth medium - divide quickly by binary fission
87
what is in vitro cloning by polymerase chain reaction (PCR)?
method of DNA amplification
artificial form of DNA replication
amplified DNA can be used for analytical processes
88
what 'ingredients' does PCR require?
DNA template - the sequence to be amplified
DNA primers - 2 are needed, 1 for each strand of template DNA
taq DNA polymerase - produces new DNA strands complementary to the template strands by forming PDE bonds b/w nucleotides
DNA nucleotides - joined by DNA polymerase to make new DNA strands
89
what are DNA primers & why are they necessary for PCR?
short, single-stranded DNA with specific, complementary sequence to the flanking regions of the DNA sequence of interest
necessary because DNA polymerase can only begin DNA synthesis from a double-stranded DNA section
primers prevent separated strands from re-joining together
90
what is taq DNA polymerase & why is it used in PCR?
obtained from thermophile taq bacterium that can survive at high temp.
allows PCR to be run continuously without being denatured at the high temp. used to denature DNA
= speeds up process
91
describe the process of PCR
1. denaturation of DNA:
the mixture is heated to 95C
which breaks H bonds b/w complementary base pairs
so the 2 DNA strands separate
2. annealing of primers
mixture is cooled to 68C
which allows primers to anneal by forming H bonds with complementary base sequence at the end of each DNA strand
this allows DNA polymerase to work & prevents separated DNA strands from re-joining
3. elongation
mixture is heated to 72C
which is the optimum temp. for tag DNA polymerase, which joins free-floating DNA nucleotides to form new DNA strands
this cycle repeats to increase the # of DNA copies exponentially
92
PCR machines do this cycle automatically
93
describe the process of transformation/getting the DNA fragment into target organisms
1. plasmid is removed from bacteria & many copies made using PCR
2. promotor & terminator regions are added to the gene of interest
3. DNA fragment & the plasmid are mixed with specific RE to produce complementary sticky ends using the same RE
4. ligase is added to join the plasmid DNA & DNA sequence of interest together
5. marker genes can be added
6. this produces recombinant plasmid vectors which are inserted into the bacterium by transformation
94
what are the functions of the promotor & terminator regions?
promotor: region of DNA where RNA polymerase binds to initiate transcription once TF has bound
terminator: region of DNA that signals to RNA polymerase to stop transcription
95
what are common vectors for recombinant DNA?
bacterial plasmids
genetically modified viruses - can deliver DNA fragment into cells & can incorporate DNA into genome of target cells --> used in human gene therapy
96
what are selective marker genes?
marker genes are additional genes added to the vector/plasmids that are used to help identify host cells that have successfully taken up the vector/plasmids
97
describe the use of selective marker genes
often genes that provide antibiotic resistance (AR) to bacterium
cells that successfully take up the recombinant plasmid will gain both the gene of interest & the gene for AR
growing bacteria on agar containing the antibiotic kills bacteria that lack the recombinant plasmid
other marker genes include genes for fluorescent proteins that give off visible light under UV
98
define DNA probe
a short, single-stranded DNA with complementary base sequence to a particular target DNA sequence
99
how can DNA probes be labelled?
1. the addition of fluorescent dye
2. the addition of radioactive marker
100
define hybridisation
DNA probes will form H bonds with a complementary base sequence on another single-stranded DNA molecule
101
describe how DNA probes are used to test whether a specific DNA base sequence is present in a sample
/locate a harmful/mutant allele
1. DNA is extracted from patient's cells & purified
2. PCR is used to amplify the DNA
3. DNA is digested using restriction endonucleases
4. restriction fragments are separated using gel electrophoresis, forming bands of different sized DNA fragments
5. the DNA bands are transferred to a solid membrane & treated to make them single-stranded
6. labelled DNA probes are added to the membrane
7. if the target allele is present, the DNA probes will hybridise/anneal/bind with H bonds to their complementary base sequence
8. excess probes are washed off the membrane & processed to observe the labels - fluorescence/radioactivity
102
define electrophoresis
the process of separating DNA fragments by size/length
103
describe the process of electrophoresis
1. agarose gel is place in a tank, submerged in an electrolyte that conducts electricity
2. DNA samples (that have been digested by restriction endonucleases & amplified by PCR) are added to wells in the gel
3. one well contains a mixture of DNA fragments with known lengths, which are used as a 'ruler' to which the sample fragments are compared & their lengths estimated
4. an electric current is applied to the tank, which causes DNA fragments to move towards the +vely charged anode as the -ve phosphate groups in DNA are attracted
5. shorter DNA fragments move through the gel more quickly than longer fragments (friction) so over time, the fragments are separated into mass order
6. chemical dye is added & binds to DNA so the DNA bands are visible.
radioactively or fluorescently labelled DNA probes can be added to locate base sequences of interest
104
what does each well in electrophoresis contain?
DNA from a different source e.g. patients, family members etc.
105
how are DNA probes used in genetic screening?
to detect & locate harmful/mutant alleles that are associated with inherited diseases
to identify individuals with inherited alleles associated with increased risk of developing cancer (e.g. mutated TSG)
to screen for other health risks
106
how are is genetic screening used for personalised medicine?
to detect which mutated oncogenes are present in a patient's cancer cell --> identify best treatment
drug response: patients with a specific genotype/alleles of particular genes respond to some medication better than others - genetic screening helps doctors decide which drug will be most effective
107
what does genetic counselling involve?
providing info & advice to patients based on their genetics
108
how does genetic screening inform genetic counselling?
provide info about the genetic disease
if a person is a carrier of a recessive allele, they can be advised about the risk of having offspring that are affected by genetic disease
patients can be advised about their increased risk for a specific type of cancer & lifestyle choices that can mitigate the genetic risk --> sometimes leads to pre-emptive medical intervention
109
what are tandem repeats?
a section of DNA in which a sequence of bases is repeated more than once
110
define variable number tandem repeats (VNTRs)
tandem repeats that show significant variation/polymorphism
111
where are VNTRs located?
at many loci in the genome, mainly outside of protein-coding genes, where DNA sequences are more variable
112
define short tandem repeats (STRs)
type of VNTR in which the length of the repeated base sequence is short (2-10 base pairs)
they vary in length between individuals, but do not affect the individual's phenotype
113
what is the probability of 2 individuals having the same VNTRs?
very low
114
describe the process of genetic fingerprinting (restriction fragment length polymorphism - RFLP analysis)
1. DNA from an individual is digested with restriction endonucleases, forming fragments of varying length
2. these DNA fragments are separated by electrophoresis, where shorter fragments travel further than longer fragments, & stain is added to ensure the DNA banding pattern is visible
3. DNA from another source is treated with the same restriction endonucleases & its fragments are separated by electrophoresis
4. the DNA banding patterns from each source are compared & similarities observed
115
what is the variation in lengths of restriction fragments b/w RFLP analysis based on?
the different lengths of VNTR sequences b/w restriction sites
116
describe the process of genetic fingerprinting with PCR
PCR primer design allows specific sequences of DNA to be amplified quickly
primers are complementary to the end points of the sequence of interest so only that section of DNA is amplified
if the DNA sequence is known, machines can produce the complementary primers
117
what are the benefits of using PCR-based techniques for genetic fingerprinting?
quicker
can give results from a small DNA sample even if the whole genome is not available
118
what are the applications of DNA fingerprinting?
genetic relationships & variability
forensic science
medical diagnosis - some STRs occur within genes & are associated with increased risk of developing diseases if present in high numbers
animal & plant breeding
