Topic 8 - The control of gene expression Flashcards
1
Q
What are mutations?
A
Any change to a base sequence of DNA
2
Q
How can mutations be caused?
A
Errors during DNA replication
3
Q
What can increase the rate of mutations?
A
Mutagenic agents
4
Q
What are the types of mutations that can occur?
A
Substitution
Deletion
Addition
Duplication
Inversion
Translocation
5
Q
What is a substitution mutation?
A
One or more bases are swapped for another
6
Q
What is a deletion mutation?
A
One or more bases are removed
7
Q
What is an addition mutation?
A
One or more bases are added
8
Q
What is a duplication mutation?
A
One or more bases are repeated
9
Q
What is an inversion mutation?
A
A sequence of bases is reversed
10
Q
What is a translocation mutation?
A
A sequence of bases is moved from one location of the genome to another
This could be movement within the same chromosome or movement to a different chromosome
11
Q
What does the order of DNA bases determine?
A
The sequence of amino acids in a particular polypeptide
If a mutation occurs in a gene, the sequence of amino acids in the polypeptide that it codes for could be changed
12
Q
What effect can the change in the amino acid sequence have?
A
Change in amino acid sequence of a polypeptide may change the final 3D shape of the protein, which could mean that it doesn’t work properly
13
Q
What things can different mutations cause?
A
-Increase likelihood of developing certain cancers
-Can cause genetic disorders - inherited disorder caused by abnormal genes or chromosomes
14
Q
What happens if a gamete containing a mutation for a type of cancer or genetic disorder is fertilised?
A
Mutation will be present in the new foetus formed - these are called hereditary mutation as they are passed on to the offspring
15
Q
Why don’t all mutation affect the order of amino acids in a protein?
A
The degenerate nature of the genetic code means that some amino acids are coded for than more than one DNA triplet
This means that not all types of mutation will always result in a change to the amino acid sequence of the polypeptide
16
Q
Provide some examples of mutagenic agents
A
UV radiation
Ionising radiation
Some chemicals
Some viruses
17
Q
What are the different ways in which mutagenic agents can increase the rate of mutations?
A
- Acting as a base:
Chemicals called base analogs can substitute for a base during DNA replication, changing the base sequence in the new DNA - Altering bases
Some chemicals can delete or alter bases - Changing the structure of DNA
Some types of radiation can change the structure of DNA, which causes problems during DNA replication
18
Q
Give an example of how acting as a base increases rate of mutations?
A
5-bromouracil is a base analog that can substitute for thymine
It can pair with guanine (instead of adenine), causing a substitution mutation in the new DNA
19
Q
Give an example of how altering bases increases rate of mutations?
A
Alkylating agents can add an alkyl group to guanine, which changes the structure so that it pairs with thymine (instead of cytosine)
20
Q
Give an example of how changing the structure of DNA increases rate of mutations?
A
UV radiation can cause adjacent thymine bases to pair up together
21
Q
What are acquired mutations?
A
Mutations that occur in individual cells after fertilisation
22
Q
What happens if acquired mutations occur in the genes that control the rate of cell division?
A
Uncontrolled cell division
23
Q
What results from a cell dividing uncontrollably?
A
Tumor
24
Q
What is a tumour?
A
Mass of abnormal cells
25
What are cancers?
Tumours that invade and destroy surrounding tissue
26
What are the two types of gene that control cell division?
Tumour suppressor genes
Proto-oncogenes
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What are tumour-suppressor genes?
Slow cell division by producing proteins that stop cells dividing or cause them to self-destruct (apoptosis)
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What happens if a mutation occurs in a tumour suppressor gene?
Gene will be inactivated
Protein it codes for isn't produced and the cells divide uncontrollably resulting in a tumour
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What are proto-oncogenes?
Stimulate cell division by producing proteins that make cells divide
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What happens if a mutation occurs in a proto-oncogene?
Gene can become overactive
This stimulates the cells to divide uncontrollable resulting in a tumour
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What is a mutated proto-oncogene called?
An oncogene
32
What are the two types of tumours?
Malignant
Benign
33
What is a malignant tumour?
Cancers
Grow rapidly
Invade and destroy surrounding tissues
Cells can break off the tumours and spread to other parts of the body in the bloodstream or lymphatic system
34
What is a benign tumour?
Not cancerous
Usually grow slower than malignant tumour Often covered in fibrous tissue that stops cells invading other tissues
Benign tumours are often harmless, but they can cause blockages and put pressure on organs
Some benign tumours can become malignant
35
List 6 ways in which a tumour cell can differ from normal cells
- Nucleus is larger and darker than in normal cells. Sometimes the cells have more than one nucleus
-Have an irregular shape
-Don't produce all the proteins needed to function correctly
-Have different antigens on their surface
-Don't respond to growth regulating processes
-Divide by mitosis more frequently than normal cells
36
What is methylation?
Adding a methyl (-CH3) group onto something
37
What is methylation important for?
An important method of regulating gene expression
38
What does methylation do?
Can control whether or not a gene is transcribed (copied into mRNA) and translated (turned into a protein)
39
What happens when tumour suppressor genes are hypermethylated?
Genes are not transcribed
So the proteins they produce to slow cell division aren't made
This means that cells are able to divide uncontrollably by mitosis and tumours can develop
40
What happens when proto-oncogenes are hypermethylated?
Causes them to act as oncogenes - increasing the production of the proteins that encourage cell division
This stimulates cells to divide uncontrollable, which causes the formation of tumours
41
What can increased exposure to oestrogen be the result of?
Starting menstruation earlier than usual
Starting the menopause later than usual
Taking oestrogen-containing drugs, such as HRT
42
What does increased exposure to oestrogen thought to do?
Increase a woman's risk of developing breast cancer
43
Give three theories as to how oestrogen can contribute to the development of some breast cancer
-Oestrogen can stimulate certain breast cells to divide and replicate. The fact that more cell divisions are taking place naturally increases the chance of mutations occurring, and so increases the chance of cells becoming cancerous
-Oestrogen's ability to stimulate division could also mean that if cells do become cancerous, their rapid replication could be further assisted by oestrogen, helping tumours to form quickly
-Other research suggests that oestrogen is actually able to introduce mutations directly into the DNA of certain breast cells, again increasing the chance of these cells becoming cancerous
44
How can genetic factors be risk factors for cancer?
Some cancers are linked with specific inherited alleles
If you inherit that allele you're more likely to get that type of cancer
45
How can environmental factors be risk factors for cancer?
Exposure to radiation, lifestyle choices such as smoking, increased alcohol consumption, and a high-fat diet have all been linked to an increased chance of developing some cancers
46
What are stem cells?
Unspecialised cells that can develop into other types of cell
47
What two things can stem cells do?
Self-renew
Differentiate
48
Why is it important that stem cells can self-renew?
To ensure a constant supply
Maintains stem cell pool
49
Why is it important that a stem cell can differentiate?
Replaces dead or damages cells throughout the life of the organism
50
What are totipotent cells?
A fertilised egg has the potential to form every type of human cell
As the fertilised cell divides specific genes are turned off so that only certain genes are made
Protein synthesis is prevented either by preventing transcription or translation
Once cells are specialised only the relevant genes will be translated from the DNA
51
What is a pluripotent stem cell?
They can specialise into any cell in the body but lose the ability to become the cells that make up the placenta
52
What are multipotent stem cells?
Make a restricted range of related cell types
e.g. bone marrow stem cells make red and white blood cells and platelets
53
What are unipotent stem cells?
Can only differentiate into one type of cell e.g. cardiac muscle stem cells
54
Summarise how cells become specialised through gene expression
1. Gene expressed
2. mRNA transcribed and translated into proteins
3. Proteins modify the cell
4. Cell becomes specialised for a particular function
OR
1. Gene switched off
2. mRNA not transcribed or translated
3. Proteins not produced
55
What are cardiomyocytes?
Heart muscle cells that make up a lot of the tissue in our hearts
56
What is bone marrow?
Contains stem cells that can become specialised to form any type of blood cell
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What is a bone marrow transplant?
Can be used to replace faulty bone marrow in patients that produce abnormal blood cells
The stem cells in the transplanted bone marrow divide and specialise to produce healthy blood cells
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What are three sources of stem cells?
Adult stem cells
Embryonic stem cells
Induced pluripotent stem cells (iPS cells)
59
How can adult stem cells be used as a source of human stem cells?
Obtained from body tissues of an adult
e.g. adult stem cells are found in bone marrow
Can be obtained in simple operation - low risk high discomfort
Not as flexible as embryonic stem cells - can only specialise into limited range of cells, not all cell body types (multipotent)
60
Explain how for some patients bone marrow transplants can be an effective treatment?
Produce healthy blood cells
No faulty/cancerous blood cells
Stem cells divide/replicate
61
How can embryonic stem cells be used as a source of human stem cells?
Obtained from embryos at early stage of development
Embryos created in laboratory using IVF - egg cells fertilised by sperm outside of womb
Once embryos are 4-5 days old, stem cells are removed from them and rest of embryo destroyed
Embryonic stem cells can divide an unlimited number of times and develop into all types of body cells (pluripotent)
62
How can induced pluripotent stem cells be used as a source of human stem cells?
iPS cells are created by scientists in lab
Process involves reprogramming specialised adult body cells so that they become pluripotent
The adult cells are made to express a series of transcription factors that are normally associated with pluripotent stem cells
The transcription factors cause the adult body cells to express genes that are associated with pluripotency
63
Explain a way in which transcription factors can be introduced to adult stem cells in iPS
By infecting them with a specially-modified virus
The virus has the genes coding for the transcription factors within its DNA
When the virus infects the adult cell, these genes are passed into the adult cell's DNA, meaning that the cell is able to produce transcription factors
64
What are the the similarities and differences between iPS and embryonic?
IPS
No need for embryos
Can uncontrollably divide
Differentiate into anything
No chance of rejection
Adult cell
Can self renew
Can divide indefinitely
Have a gene added
Embryonic
Needs embryo - unethical
Can uncontrollably divide
Differentiate into anything
Used within 14 days
Chance of rejection
Embryo
Can self renew
Can divide indefinitely
Don't have gene added
65
What are the advantages of using iPS over gene therapy?
Use of iPS cells long-term
Less chance of rejection
Single treatment
Harm/side effects from using viruses (in gene therapy)
66
What are the ethical considerations surrounding stem cells from embryos?
-Procedure results in destruction of an embryo that could become a fetus if placed in a womb
-At the moment of fertilisation an individual is formed that has the right to life
67
Why is iPS better than embryos for stem cells ethically?
As flexible as embryonic stem cells
Less ethical issues as they're adult cells
iPS cells could be made from patient's own cells - wouldn't be rejected
68
What are three benefits of stem cell therapy?
- Could save many lives - e.g. many people waiting for organ transplants die before a donor becomes available. Stem cells could be used to grow organs for those people awaiting transplants
- Might be possible to make stem cells genetically identical to patient's own cells. These could be used to grow some new tissue or an organ that the patient's body wouldn't reject
- They could improve the quality of life for many people - e.g. that could be used to replace damaged cells in the eyes of people who are blind
69
What is transcription?
When a gene is copied from DNA into messenger RNA using RNA polymerase
70
What is the transcription of genes controlled by?
Transcription factors
71
What is the role of transcription factors
In eukaryotes, they move from the cytoplasm to the nucleus
Control expression of genes by controlling the rate of transcription
72
What are promotors?
In the nucleus, transcription factors bind to specific DNA sites called promotors
73
What are activators?
Transcription factors that stimulate or increase the rate of transcription
e.g. they help RNA polymerase bind to the start of the target gene and activate transcription
74
What are repressors?
Transcription factors that inhibit or decrease the rate of transcription
e.g. they bind to the start of the target gene, preventing RNA polymerase from binding, stopping transcription
75
What does oestrogen do in transcription?
A steroid hormone that can affect transcription by binding to a transcription factor called an oestrogen receptor, forming an oestrogen-oestrogen receptor complex
76
Are all cells affected by oestrogen?
No as not all cell types have oestrogen receptors
77
Where does the oestrogen-oestrogen receptor complex go?
Complex moves from the cytoplasm into the nucleus
78
What happens to the complex in the nucleus?
Binds to specific DNA sites near the start of the target gene
The complex can act as an activator of transcription
e.g. helping RNA polymerase bind to the start of the target gene
79
What happens at RNAi?
Where small, double-stranded RNA molecules stop mRNA from target genes being translated into proteins
80
What is RNAi?
RNA interference
81
What are the molecules involved in RNAi called?
siRNA = small interfering RNA
miRNA = microRNA
82
Describe how RNAi works?
Once mRNA has been transcribed, it leaves the nucleus to the cytoplasm
In the cytoplasm, double-stranded siRNA associates with enzymes and unwinds, forming single-stranded siRNA
One of the resulting single strands of siRNA is selected and the other strand is degraded
The single strand of siRNA binds to target mRNA
Base sequence of siRNA is complementary to the base sequence in section of the target mRNA
The enzymes associated with the siRNA cut the mRNA into fragments - so it can no longer be translated
The fragments then move into a processing body which contains 'tools' to degrade them
83
What does miRNA do in mammals?
Isn't usually fully complementary to the target mRNA
Makes it less specific than siRNA and so it may target more than one mRNA molecule
When miRNA is first transcribed, it exists as long, folded strand
It is processed into a double strand and then into two single strands, by enzymes in the cytoplasm
Like siRNA, one strand associates with proteins and binds to target mRNA in the cytoplasm
Instead of the proteins associated with miRNA cutting mRNA into fragments, the miRNA-protein complex physically blocks the translation of the target mRNA
The mRNA is then moved into a processing body, where it can either be stored or degraded
When it's stored it can be returned and translated at another time
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