3.8 Flashcards
(90 cards)
1
Q
What is a gene mutation
A
A change in the base sequence of DNA, it can arise spontaneously during DNA replication (interphase)
2
Q
What is a mutagenic agent
A
A factor that increases the rate of mutation eg ultraviolet light
3
Q
Explain how a gene mutation can lead to the production of a non-functional protein or enzyme
A
1-Changes sequence of base triplets in DNA so changed sequence of codons on mRNA
2- So changed amino acid sequence in the encoded polypeptide
3-Do changes the position of the hydrogen/ionic/ disulphide bridges
4- changes the tertiary structure
5- which changes the shape of the enzymes active site so enzyme substrate complexes cant form
4
Q
Describe substitution
A
A base/ nucleotide are added to the DNA base sequence
5
Q
Describe Addition
A
1 or more bases are added to the base sequence
6
Q
Describe deletion
A
1 or more bases are lost from the base sequence
7
Q
Describe duplication
A
A sequence of DNA bases are repeated
8
Q
Describe inversion
A
A sequence of DNA bases detaches from the DNA sequence the rejoins in the same position but reverse order
9
Q
Describe translocation
A
A sequence of DNA bases detach and is inserted at a different location within the same or different chromosome
10
Q
Explain why not all gene mutation saffect the order of amino acids
A
Some substitutions change only 1 triplet code which can still code for the same amino acid as they are degenerate
Some occur in introns which do not code for an amino acid as they are removed during splicing
11
Q
Explain why a change in the amino acid sequence is not always harmful
A
May not change tertiary structure of protein if bonds don’t change, may positively change the properties of the protein giving the organism a selective advantage
12
Q
Explain what is meant by a frame shift
A
Occurs when mutations change the number of nucleotides to a number not divisible by 3
This shifts the way the genetic code is read so all the triplets downstream from the codon change
13
Q
Explain how mutations can lead to production of shorter polypeptides
A
Deletion or translocation means missing codons so missing amino acids
Substitution addition deletion duplication inversion or translocation mean premature stop codon is created so amino acids missing at the end of the polypeptide
14
Q
What are stem cells
A
Undifferentiated unspecialised cells capable of dividing via mitosis to replace themselves indefinitely and can differentiate into specialised cells
15
Q
Describe how stem cells become specialised during development
A
Stimuli lead to activation of some genes due to transcription factors
So mRNA is transcribed to form proteins
These proteins modify cells permanently and determine the structure and function
16
Q
Describe totipotent cells
A
Occur for a limited time in early mammalian embryos
Can divide and differentiate into any type of body cell (including extra embryonic cells like placenta）
17
Q
Describe pluripotent cells
A
Found in mammalian embryos after first few divisions
Can divide and differentiate into most cell types (not placenta)
18
Q
Describe multipotent cells
A
Found in mature mammals
can divide and differentiate into a limited number of cells.
19
Q
Describe unipotent cells
A
Found in mature mammals and can divide and differentiate into one type of cell
20
Q
Explain how stem cells can be used in the treatment of human disorders
A
Transplanted into patients to divide into unlimited numbers, then differentiate into required healthy cells to replace faulty
eg type 1 diabetes by creating healthy islets of lanagahan to produce insulin
21
Q
Explain how induced pluripotent stem cells iPS are produced
A
1-Obtain adult somatic cells from patient
2- Add specific protein transcription factors associated with pluripotency to cells so they express genes associated with pluripotency
3-Culture cells to allow them to divide via mitosis
22
Q
Evaluate the use of stem cells in treating human disorders
A
For:
-Can divide and differentiate into required human cells so could relieve human suffering and improve quality. of life
-Embryos are often left over from IVF so would be destroyed otherwise
-iPS unlikely to be rejected as made with own cells
=iPS can be made without destruction of embryos and adult can give permission
Against
-Ethical issues with embryonic stem cells as obtaining them require destruction of an embryo and potential life
-Immnosupressant drugs required
-Cells could divide out of control and form cancer or tumours
23
Q
What are transcription factors
A
Proteins which regulate transcription of specific target genes in eukaryotes
by binding to specific DNA base sequences on a promoter region
24
Q
Describe how transcription can be regulated using transcription factors
A
=Transcription factors move from cytoplasm to nucleus
=Bind to DNA at a specific DNA base sequence on a promotor region
=This stimulates or inhibits the binding transcription of target genes by helping or preventing RNA polymerase
25
Explain how oestrogen affects transcription
Oestrogen is a lipid-soluble steroid hormone so diffuses into
cell across the phospholipid bilayer
2. In cytoplasm, oestrogen binds to its receptor, an inactive
transcription factor, forming an oestrogen-receptor complex
3. This changes the shape of the inactive transcription factor,
forming an active transcription factor
4. The complex diffuses from cytoplasm into the nucleus
5. Then binds to a specific DNA base sequence on the promoter
region of a target gene
6. Stimulating transcription of target genes forming mRNA by
helping RNA polymerase to bind
26
Explain why oestrogen only affects target cells
Other cells do not have oestrogen receptors
27
Describe what is meant by epigenetics
Heritable changes in gene function / expression without changes to the base sequence of DNA
● Caused by changes in the environment (eg. diet, stress, toxins)
28
Describe what is meant by epigenome
All chemical modification of DNA and histone proteins - methyl groups on DNA and acetyl groups on histones
29
Summarise the epigenetic control of gene expression in eukaryotes
To inhibit transcription: increase methylation, decrease acetylation
To allow transcription: decrease methylation and increase acetylation
30
Explain how methylation and acetylation can inhibit transcription
1. Increased methylation of DNA - methyl
groups added to cytosine bases in DNA
2. So nucleosomes (DNA wrapped around
histone) pack more tightly together
3. Preventing transcription factors and
RNA polymerase binding to promoter
1. Decreased acetylation of histones
increases positive charge of histones
2. So histones bind DNA (negatively
charged) more tightly
3. Preventing transcription factors and
RNA polymerase binding to promoter
31
Explain the relevance of epigenetics on disease development and treatment
● Environmental factors (eg. diet, stress, toxins) can lead to epigenetic changes
● These can stimulate / inhibit expression of certain genes that can lead to disease development
○ Increased methylation of DNA OR decreased acetylation of histones inhibits transcription
○ Decreased methylation of DNA OR increased acetylation of histones stimulates transcription
● Diagnostic tests can be developed that detect these epigenetic changes before symptoms present
● Drugs can be developed to reverse these epigenetic changes
32
What is RNA interference (RNAi)?
● Inhibition of translation of mRNA produced from target genes, by RNA molecules eg. siRNA, miRNA
● This inhibits expression of (silencing) a target gene
33
Describe the regulation of translation by RNA interference
1. Small interfering RNA (siRNA) or micro-RNA (miRNA) is incorporated into
/ binds to a protein, forming an RNA-induced silencing complex (RISC)
○ siRNA synthesised as double-stranded RNA → 1 strand incorporated
○ miRNA synthesised as a double-stranded hairpin bend of RNA →
both strands incorporated
2. Single-stranded miRNA / siRNA within RISC binds to target mRNA with a
complementary base sequence
3. This leads to hydrolysis of mRNA into fragments which are then
degraded OR prevents ribosomes binding
4. Reducing / preventing translation of target mRNA into protein
34
Describe how tumours and cancers form
● Mutations in DNA / genes controlling mitosis can lead to
uncontrolled cell division
● Tumour formed if this results in mass of abnormal cells
○ Malignant tumour = cancerous, can spread by metastasis
○ Benign tumour = non-cancerous
35
main characteristics of benign tumours
Usually grow slowly (cells divide less often)
Cells are well differentiated / specialised
Cells have normal, regular nuclei
Well defined borders and often surrounded by a
capsule so do not invade surrounding tissue
Do not spread by metastasis (as cell adhesion
molecules stick cells together)
Can normally be removed by surgery
and they rarely return
36
Main characteristics of malignant tumours
Usually grow faster (cells divide more often)
Cells become poorly differentiated / unspecialised
Cells have irregular, larger / darker nuclei
Poorly defined borders and not encapsulated so can
invade surrounding tissues (growing projections)
Spread by metastasis - cells break off and spread to
other parts of the body, forming secondary tumours
(due to lack of adhesion molecules)
Can normally be removed by surgery combined with
radiotherapy / chemotherapy but they often return
37
Describe the function of tumour suppressor genes
Code for proteins that:
● Inhibit / slow cell cycle (eg. if DNA damage detected)
● OR cause self-destruction (apoptosis) of potential
tumour cells (eg. if damaged DNA can’t be repaired)
38
Explain the role of tumour suppressor genes in the development of tumours
● Mutation in DNA base sequence → production of non-functional protein
○ By leading to change in amino acid sequence which changes protein tertiary structure
● Decreased histone acetylation OR increased DNA methylation → prevents production of protein
○ By preventing binding of RNA polymerase to promoter region, inhibiting transcription
● Both lead to uncontrolled cell division (cell division cannot be slowed)
39
Describe the function of (proto-)oncogenes
Code for proteins that stimulate cell division
(eg. through involvement in signalling pathways
that control cell responses to growth factors)
40
Explain the role of oncogenes in the development of tumours
● Mutation in DNA base sequence → overproduction of protein OR permanently activated protein
○ By leading to change in amino acid sequence which changes protein tertiary structure
● Decreased DNA methylation OR increased histone acetylation → increases production of protein
○ By stimulating binding of RNA polymerase to promoter region, stimulating transcription
● Both lead to uncontrolled cell division (cell division is permanently stimulated)
41
Suggest why tumours require mutations in both alleles of a tumour
suppressor gene but only one allele of an oncogene
● One functional allele of a tumour suppressor gene can produce enough protein to slow the cell cycle
OR cause self-destruction of potential tumour cells → cell division is controlled
● One mutated oncogene allele can produce enough protein to lead to rapid / uncontrolled cell division
42
Explain the relevance of epigenetics in cancer treatment
● Increasing DNA methylation OR decreasing histone acetylation of oncogene
○ To inhibit transcription / expression
● Decreasing DNA methylation OR increasing histone acetylation of tumour suppressor gene
○ To stimulate transcription / expression
43
Explain the role of increased oestrogen concentrations in the development
of some (oestrogen receptor-positive) breast cancers
1. Some breast cancers cells have oestrogen receptors, which are inactive transcription factors
2. If oestrogen concentration is increased, more oestrogen binds to oestrogen receptors,
forming more oestrogen-receptor complexes which are active transcription factors
3. These bind to promoter regions of genes that code for proteins stimulating cell division
4. This increases transcription / expression of these genes, increasing the rate of cell division
44
Suggest how drugs that have a similar structure to oestrogen help treat
oestrogen receptor-positive breast cancers
Drugs bind to oestrogen receptors (inactive transcription factors), preventing binding of oestrogen
● So no / fewer transcription factors bind to promoter regions of genes that stimulate the cell cycle
45
Define ‘genome’
The complete set of genes in a cell
46
Define ‘proteome’
The full range of proteins that a cell can produce (coded for by the cell’s DNA / genome)
47
What is genome sequencing and why is it important?
● Identifying the DNA base sequence of an organism’s genome
● So amino acid sequences of proteins that derive from an organism’s genetic code can be determined
48
Explain how determining the genome of a pathogen could allow vaccines to
be developed
● Could identify the pathogen’s proteome
● So could identify potential antigens (proteins that stimulate an immune response) to use in the vaccine
49
Suggest some other potential applications of genome sequencing projects
● Identification of genes / alleles associated with genetic diseases / cancers
○ New targeted drugs / gene therapy can be developed
○ Can screen patients, allowing early prevention / personalised medicine
● Identification of species and evolutionary relationships
50
Explain why the genome cannot be directly translated into the proteome in
complex organisms
● Presence of non-coding DNA (eg. introns within genes do not code for polypeptides)
● Presence of regulatory genes (which regulate expression of other genes, eg. by coding for miRNA)
51
Describe how sequencing methods are changing
● They have become automated (so are faster, more cost-effective and can be done on a larger scale)
● They are continuously updated
52
What is recombinant DNA technology?
Transfer of DNA fragments from one organism or species, to another
53
Explain why transferred DNA can be translated within cells of recipient
(transgenic) organisms
1. Genetic code is universal
2. Transcription and translation mechanisms are universal
54
Describe how DNA fragments can be produced
using restriction enzymes
1. Restriction enzymes cut DNA at specific base ‘recognition
sequences’ either side of the desired gene
○ Shape of recognition site complementary to active site
2. Many cut in a staggered fashion forming ‘sticky ends’
(single stranded overhang)
55
Describe how DNA fragments can be produced from mRNA
1. Isolate mRNA from a cell that readily synthesises the protein coded for by the desired gene
2. Mix mRNA with DNA nucleotides and reverse transcriptase → reverse transcriptase uses
mRNA as a template to synthesise a single strand of complementary DNA (cDNA)
3. DNA polymerase can form a second strand of DNA using cDNA as a template
56
Suggest two advantages of obtaining genes from mRNA rather than directly
from the DNA removed from cells
● Much more mRNA in cells making the protein than DNA → easily extracted
● In mRNA, introns have been removed by splicing (in eukaryotes) whereas DNA contains introns
○ So can be transcribed & translated by prokaryotes who can’t remove introns by splicing
57
Describe how fragments of DNA can be produced using a gene machine
● Synthesises fragments of DNA quickly & accurately from scratch without need for a DNA template
○ Amino acid sequence of protein determined, allowing base sequence to be established
● These do not contain introns so can be transcribed & translated by prokaryotes
58
Name an in vitro and in vivo technique used to amplify DNA fragments
● In vitro (outside a living organism) - polymerase chain reaction
● In vivo (inside a living organism) - culturing transformed host cells eg. bacteria
59
Explain how DNA fragments can be amplified by PCR
Reaction mixture: DNA fragment, DNA polymerase (eg. taq polymerase), primers and DNA nucleotides
1- Mixture heated to 95 to separate the strands by breaking the hydrogen bonds between the bases
2-Mixture is cooled to 55, allows for the primers to bind to template strand by a hydrogen bond
3- Mixture heated to 72, nucleotides align next to complementary exposed bases, DNA polymerase joins adjacent nucleotides by forming phosphodiester bonds.
60
Explain the role of primers in PCR
● Primers are short, single stranded DNA fragments
● Complementary to DNA base sequence at edges of region to be copied / start of desired gene
● Allowing DNA polymerase to bind to start synthesis (can only add nucleotides onto pre-existing 3’ end)
● Two different primers (forward and reverse) are required (as base sequences at ends are different)
61
Suggest one reason why DNA replication eventually stops in PCR
There are a limited number of primers and nucleotides which are eventually used up
62
Summarise the steps involved in amplifying DNA fragments in vivo
1. Add promoter and terminator regions to DNA fragments
2. Insert DNA fragments & marker genes into vectors (eg. plasmids) using
restriction enzymes and ligases
3. Transform host cells (eg. bacteria) by inserting these vectors
4. Detect genetically modified (GM) / transformed cells / organisms by identifying
those containing the marker gene (eg. that codes for a fluorescent protein)
5. Culture these transformed host cells, allowing them to divide and form clones
63
Explain why promoter and terminator regions are added to DNA fragments
that are used to genetically modify organisms
Promoter- allows transcription to start by allowing RNA polymerase to bind to DNA
Can be selected to ensure gene expression happens only in specific cell types
○ Eg. in gland cells of a mammal so the protein can be easily harvested
Terminator- Ensure transcription stops at the end of a gene, by stopping RNA polymerase
64
What are the role of vectors in recombinant DNA technology?
To transfer DNA into host cells / organisms eg. plasmids or viruses (bacteriophage)
65
Explain the role of enzymes in inserting DNA fragments into vectors
1. Restriction endonucleases / enzymes cut vector DNA
○ Same enzyme used that cut the gene out so vector DNA & fragments
have ‘sticky ends’ that can join by complementary base pairing
2. DNA ligase joins DNA fragment to vector DNA
○ Forming phosphodiester bonds between adjacent nucleotides
66
Describe how host cells are transformed using vectors
● Plasmids enter cells (eg. following heat shock in a calcium ion solution)
● Viruses inject their DNA into cells which is then integrated into host DNA
67
Explain why marker genes are inserted into vectors
● To allow detection of genetically modified / transgenic cells / organisms
○ If marker gene codes for antibiotic resistance, cells that survive antibiotic exposure = transformed
○ If marker gene codes for fluorescent proteins, cells that fluoresce under UV light = transformed
● As not all cells / organisms will take up the vector and be transformed
68
Suggest how recombinant DNA technology can be useful
Medicine- ● GM bacteria produce human proteins (eg. insulin for type 1 diabetes) → more ethically
acceptable than using animal proteins and less likely to cause allergic reactions
● GM animals / plants produce pharmaceuticals (‘pharming’) → cheaper
● Gene therapy (see below)
Agriculture-
● GM crops resistant to herbicides → only weeds killed when crop sprayed with herbicide
● GM crops resistant to insect attack → reduce use of insecticide
● GM crops with added nutritional value
● GM animals with increased growth hormone production
Industry-
GM bacteria produce enzymes used in industrial processes and food production
69
Describe gene therapy
● Introduction of new DNA into cells, often containing healthy / functional alleles
● To overcome effect of faulty / non-functional alleles in people with genetic disorders eg. cystic fibrosis
70
Suggest some issues associated with gene therapy
Effect is short lived as modified cells (eg. T cells) have a limited lifespan → requires regular treatment
● Immune response against genetically modified cells or viruses due to recognition of antigens
● Long term effect not known - side effects eg. could cause cancer
○ DNA may be inserted into other genes, disrupting them → interfering with gene expression
71
Suggest why humanitarians might support recombinant DNA technology
● GM crops increase yields → increased global food production → reduced risk of famine / malnutrition
● Gene therapy has potential to cure many genetic disorders
● ‘Pharming’ makes medicines available to more people as medicines cheaper
72
Suggest why environmentalists and anti-globalisation activists might
oppose recombinant DNA technology
● Recombinant DNA may be transferred to other plants → potential herbicide resistant ‘superweeds’
● Potential effects on food webs eg. affect wild insects → reduce biodiversity
● Large biotech companies may control the technology and own patents
73
What are DNA probes?
● Short, single stranded pieces of DNA
● With a base sequence complementary to bases on part of a target allele / region
● Usually labelled with a fluorescent or radioactive tag for identification
74
Suggest why DNA probes are longer than just a few bases
● A sequence of a few bases would occur at many places throughout the genome
● Longer sequences are only likely to occur in target allele
75
What is DNA hybridisation?
● Binding of a single stranded DNA probe
to a complementary single strand of DNA
● Forming hydrogen bonds / base pairs
76
Explain how genetic screening can be used to locate specific alleles of genes
1. Extract DNA and amplify by PCR
2. Cut DNA at specific base sequences (either side of target gene) using restriction enzymes
3. Separate DNA fragments / alleles (according to length) using gel electrophoresis
4. Transfer to a nylon membrane and treat to form single strands with exposed bases
5. Add labelled DNA probes which hybridise / bind with target alleles (& wash to remove unbound probe)
6. To show bound probe, expose membrane to UV light if a fluorescently labelled probe was used
OR use autoradiography (expose to X-ray film) if a radioactive probe was used
77
What is gel electrophoresis?
● A method used to separate nucleic acid (DNA / RNA) fragments OR proteins
● According to length / mass (number of bases / amino acids) AND charge (DNA is negatively charged due
to phosphate groups and protein charge varies based on amino acid R groups)
78
Explain how gel electrophoresis can be used to separate DNA fragments
1. DNA samples loaded into wells in a porous gel and
covered in buffer solution (which conducts electricity)
2. Electrical current passed through → DNA is negatively
charged so moves towards positive electrode
3. Shorter DNA fragments travel faster so travel further
79
How can data showing results of gel electrophoresis be interpreted?
● Run a standard with DNA fragments / proteins of known lengths under the same conditions
● Compare to position of unknown DNA fragments / proteins to estimate their size
● Shorter DNA fragments/ proteins travel further / faster
80
Describe examples of the use of labelled DNA probes
● Screening patients for heritable conditions (eg. cystic fibrosis)
● Screening patients for drug responses (some alleles code for enzymes involved in drug
metabolism that enable better responses to certain drugs)
● Screening patients for health risks (some alleles predispose patients eg. to high blood cholesterol)
81
Describe the role of a genetic counsellor
1. Explain results of genetic screening, including consequences of a disease
2. Discuss treatments available for genetic condition
3. Discuss lifestyle choices / precautions that might reduce risk of a genetic
condition developing eg. regular screening for tumours or a mastectomy
4. Explain probability of condition / alleles being passed onto offspring →
enable patients to make informed decisions about having children
82
What is personalised medicine?
● Medicine tailored to an individual's
genotype / DNA
● Increasing effectiveness of treatment
eg. by identifying the particular
mutation / allele causing cancer and
treating it with tailored drugs
83
Evaluate the screening of individuals for genetically determined conditions
and drug responses
✓ Some people could be heterozygous / carriers (eg. in families with a history of a disease)
✓ Can enable these people to make lifestyle choices to reduce chances of diseases
developing, to prevent suffering / death
✓ Allows people to make informed decisions about having their own biological children
✓ Allows use of personalised medicines, increasing effectiveness of treatment
X Screening for incurable diseases or diseases that develop later in life (where nothing positive can be done in response) may lead to depression
X May cause undue stress if patient does not develop the disease
X Could lead to discrimination by insurance companies / employers
X Many diseases are rare
X Many are caused by many genes so would need too many probes (expensive)
84
What are variable number tandem repeats (VNTRs)?
● Repeating sequences of nucleotides / bases (eg. GATA)
● Found within non-coding sections of DNA at many sites throughout an organism’s genome
85
Why are VNTRs useful in genetic fingerprinting?
● Probability of two individuals having the same VNTRs is very low
● As an organism’s genome contains many VNTRs and lengths at each loci differ between individuals
86
Explain how genetic fingerprinting can be used to analyse DNA fragments
1. Extract DNA from sample (eg. blood cells) and amplify by PCR
2. Cut DNA at specific base sequences / recognition sites (either side of VNTRs) using restriction enzymes
3. Separate VNTR fragments according to length using gel electrophoresis (shorter ones travel further)
4. Transfer to a nylon membrane and treat to form single strands with exposed bases
5. Add labelled DNA probes which hybridise / bind with complementary VNTRs (& wash to remove
unbound probe)
6. To show bound probe, expose membrane to UV light if a fluorescently labelled probe was used
OR use autoradiography (expose to X-ray film) if a radioactive probe was used
87
Compare and contrast genetic fingerprinting with genetic screening
● Both use PCR to amplify DNA sample
● Both use electrophoresis to separate DNA fragments
● Both use labelled DNA probes to visualise specific DNA fragments
● Genetic fingerprinting analyses VNTRs whereas genetic screening analyses specific alleles of a gene
88
Explain how genetic fingerprinting can be used to determine genetic
relationships
● More closely related organisms have more similar VNTRs, so more similarities in genetic fingerprints
● Paternity testing - father should share around 50% of VNTRs / bands with child (due to inheritance)
89
Explain how genetic fingerprinting can be used to determine genetic
variability within a population
Differences in VNTRs arise from mutations, so more differences show greater diversity within a population
90
Explain the use of genetic fingerprinting in the fields of forensic science,
medical diagnosis, animal and plant breeding
Forensic science
● Compare genetic fingerprint of suspects to genetic fingerprint of DNA at crime scene
● If many bands match, the suspect was likely present at the crime scene
Medical diagnosis
● Some VNTR patterns are associated with an increased risk of certain genetic
disorders eg. Huntington’s
Animal and plant breeding
● Shows how closely related 2 individuals are, so that inbreeding can be avoided
● Breed pairs with dissimilar genetic fingerprints
