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Bio Aqa > Control Of Gene Expression > Flashcards

Control Of Gene Expression Flashcards

1
Q

What is a gene mutation?

A

● A change in the base sequence of DNA
● Can arise spontaneously during DNA replication (interphase)

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2
Q

What is a mutagenic agent?

A

A factor that increases rate of mutation, eg. ultraviolet (UV) light or alpha particles.

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3
Q

Explain how a gene mutation can lead to the production of a non-functional
protein or enzyme (general)

A
  1. Changes sequence of base triplets in DNA so changes sequence of codons on mRNA
  2. So changes sequence of amino acids in the encoded polypeptide
  3. So changes position of hydrogen / ionic / disulphide bonds (between amino acids)
  4. So changes tertiary structure (shape) of protein
  5. Enzymes- active site changes shape so substrate can’t bind, enzyme-substrate complex can’t form
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4
Q

Substitution

A

A base / nucleotide is replaced by a different base / nucleotide in DNA

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5
Q

Addition

A

1 or more bases / nucleotides are added to the DNA base sequence

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6
Q

Deletion

A

1 or more bases / nucleotides are lost from the DNA base sequence

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7
Q

Duplication

A

A sequence of DNA bases / nucleotides is repeated / copied

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8
Q

Inversion

A

A sequence of bases / nucleotides detaches from the DNA sequence,
then rejoins at the same position in the reverse order
polypeptide.

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9
Q

Translocation

A

A sequence of DNA bases / nucleotides detaches and is inserted at a
different location within the same or a different chromosome

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10
Q

Explain why not all gene mutations affect the order of amino acids

A

● Some substitutions change only 1 triplet code / codon which could still code for the same amino acid
○ As the genetic code is degenerate (an amino acid can be coded for by more than one triplet)
● Some occur in introns which do not code for amino acids

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11
Q

Explain why a change in amino acid sequence is not always harmful

A

● May not change tertiary structure of protein (if position of ionic / disulphide / H bonds don’t change)
● May positively change the properties of the protein, giving the organism a selective advantage

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12
Q

Explain what is meant by a frameshift

A

Explain what is meant by a frameshift
● A frameshift occurs when gene mutations (eg. addition, deletion,
duplication or translocation) change the number of nucleotides /
bases by any number not divisible by 3
● This shifts the way the genetic code is read, so all the DNA triplets
/ mRNA codons downstream from the mutation change
● The sequence of amino acids encoded changes accordingly and
the effects on the encoded polypeptide are significant Explain what is meant by a frameshift
● A frameshift occurs when gene mutations (eg. addition, deletion,
duplication or translocation) change the number of nucleotides /
bases by any number not divisible by 3
● This shifts the way the genetic code is read, so all the DNA triplets
/ mRNA codons downstream from the mutation change
● The sequence of amino acids encoded changes accordingly and
the effects on the encoded polypeptide are significant
If a multiple of 3 bases is added / removed there won’t be a frameshift, but extra / less triplets will result in extra /
less amino acids in the encoded polypeptide.
A frameshift could also lead to production of a stop codon (that doesn’t code for amino acids so terminates
translation), resulting in a shorter polypeptide.

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13
Q

What are stem cells

A

Undifferentiated / unspecialised cells capable of:
1. Dividing (by mitosis) to replace themselves indefinitely
2. Differentiating into other types of (specialised) cell

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14
Q

Describe how stem cells become specialised during development

A

● Stimuli lead to activation of some genes (due to transcription factors- see 8.2.2)
● So mRNA is transcribed only from these genes and then translated to form proteins
● These proteins modify cells permanently and determine cell structure / function

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15
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 eg. placenta)

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16
Q

Describe pluripotent cells

A

● Found in mammalian embryos (after first few cell divisions)
● Can divide AND differentiate into most cell types (every cell type in the body but not placental cells)

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17
Q

Describe unipotent cells, using an example

A

● Found in mature mammals
● Can divide AND differentiate into just one cell type

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18
Q

What are transcription factors?

A

● Proteins which regulate (stimulate or inhibit) transcription of specific target genes in eukaryotes
● By binding to a specific DNA base sequence on a promoter region

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19
Q

Describe how transcription can be regulated using transcription factors

A

Describe how transcription can be regulated using transcription factors
Transcription factors move from cytoplasm to nucleus
Bind to DNA at a specific DNA base sequence on a promoter region (before / upstream of target gene)
This stimulates or inhibits transcription (production of mRNA) of target gene(s) by helping or preventing RNA polymerase binding

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20
Q

Explain how oestrogen affects transcription

A

1)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

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21
Q

Explain why oestrogen only affects target cells

A

Other cells do not have oestrogen receptors.

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22
Q

Describe what is meant by epigenetics

A

● Heritable changes in gene function / expression without changes to the base sequence of DNA
● Caused by changes in the environment (eg. diet, stress, toxins)

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23
Q

Describe what is meant by epigenome

A

All chemical modification of DNA and histone proteins - methyl groups on DNA and acetyl groups on histones.

24
Q

What 2 things happen for epigenetic control of gene expression to inhibit transcription

A

Methylation of DNA is increased
Acetylation of histones is decreased
So dna not very tightly wound up, chromatin open

25
Q

What 2 things happen for epigenetic control of gene expression to allow transcription

A

Methylation of DNA is decreased
Acetylation of histones is increased
So dna very tightly wound up, chromatin closed

26
Q

How does increased methylation of dna inhibit transcription

A
  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
27
Q

How does decreased acetylation of histones inhibit transcription

A
  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
28
Q

Explain the relevance of epigenetics on disease development and treatment

A

● 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

29
Q

What is RNA interference (RNAi)?

A

● Inhibition of translation of mRNA produced from target genes, by RNA molecules eg. siRNA, miRNA
● This inhibits expression of (silencing) a target gene

30
Q

RNA interference happens in…

A

This happens in eukaryotes and some prokaryotes.

31
Q

Describe the regulation of translation by RNA interference

A
  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
32
Q

Describe how tumours and cancers form

A

● 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

33
Q

Compare characteristics of benign and malignant tumours

A
  • benign grow slowly with cells dividing less often vs malignant which grow quicker with cells dividing more often
  • benign tumours have cells which are well differentiated and specialised vs malignant tumours have cells which become poorly differentiated and unspecialised
  • benign have normal regular nuclei vs malignant tumours that have irregular larger and darker nuclei
  • benign tumours have well defines borders and are often surrounded by a capsule so dont invade surrounding tissue vs malignant Poorly defined borders and not encapsulated so can
    invade surrounding tissues (growing projections)
  • benign Poorly defined borders and not encapsulated so can
    invade surrounding tissues (growing projections)
    Do not spread by metastasis (as cell adhesion
    molecules stick cells together) vs malignant Spread by metastasis- cells break off and spread to
    other parts of the body, forming secondary tumours
    (due to lack of adhesion molecules)
  • benign Can normally be removed by surgery
    and they rarely return, Can normally be removed by surgery
    and they rarely return
    Can normally be removed by surgery combined with
    radiotherapy / chemotherapy but they often return
34
Q

Explain the role of tumour suppressor genes in the development of tumours

A

● 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)

35
Q

Describe the function of proto-oncogenes

A

Code for proteins that stimulate cell division
(eg. through involvement in signalling pathways
that control cell responses to growth factors)

36
Q

An oncogene is a …

A

An oncogene is a mutated / abnormally expressed form of the corresponding proto-oncogene.

37
Q

Explain the role of oncogenes in the development of tumours

A

● 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)

38
Q

Suggest why tumours require mutations in both alleles of a tumour
suppressor gene but only one allele of an oncogene

A

● 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

39
Q

Explain the relevance of epigenetics in cancer treatment

A

Drugs could reverse epigenetic changes that caused cancer, preventing uncontrolled cell division. For example:
● 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

40
Q

Explain the role of increased oestrogen concentrations in the development
of some (oestrogen receptor-positive) breast cancers

A
        1. Some breast cancers cells have oestrogen receptors, which are inactive transcription factors
          If oestrogen concentration is increased, more oestrogen binds to oestrogen receptors,
          forming more oestrogen-receptor complexes which are active transcription factors
          These bind to promoter regions of genes that code for proteins stimulating cell division
          This increases transcription / expression of these genes, increasing the rate of cell division
41
Q

Suggest how drugs that have a similar structure to oestrogen help treat
oestrogen receptor-positive breast cancers

A

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

42
Q

Define genome

A

the complete set of genes in a cell

43
Q

Define proteome

A

The full range of proteins that a cell can produce (coded for by the cells DNA/genome)

44
Q

What is genome sequencing and why is it important?

A

● 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

45
Q

Explain how determining the genome of a pathogen could allow vaccines to be developed

A

● Could identify the pathogen’s proteome
● So could identify potential antigens (proteins that stimulate an immune response) to use in the vaccine

46
Q

Suggest some other potential applications of genome sequencing projects

A

● 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

47
Q

Explain why the genome cannot be directly translated into the proteome in
complex organisms

A

● 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)

48
Q

Describe how sequencing methods are changing

A

● They have become automated (so are faster, more cost-effective and can be done on a larger scale)
● They are continuously updated

49
Q

What are DNA probes?

A

● 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

50
Q

Suggest why DNA probes are longer than just a few bases

A

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

51
Q

What is DNA hybridisation?

A

● Binding of a single stranded DNA probe
to a complementary single strand of DNA
● Forming hydrogen bonds / base pairs

52
Q

Explain how genetic screening can be used to locate specific alleles of genes

A
  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
53
Q

What is gel electrophoresis?

A

● 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)

54
Q

Explain how gel electrophoresis can be used to separate DNA fragments

A
  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
  2. Shorter DNA fragments travel faster so travel further
55
Q
A

Bio Aqa (12 decks)

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