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8- The control of gene expression > Gene expression > Flashcards

Gene expression Flashcards

1
Q

List the six types of gene mutation

A

1) Insertion/addition
2) Substitution
3) Deletion
4) Duplication
5) Inversion
6) Translocation

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2
Q

What is a substitution mutation

A

A mutation in which a nucleotide in a section of a DNA molecule is replaced by another nucleotide that has a different base

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3
Q

What is a key example of a disease caused by a substitution mutation

A

Sickle cell anaemia- here one amino acid in the protein is changed which significantly affects its tertiary structure and therefore function.

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4
Q

Describe the three possible consequences of a substitution mutation

A

1) One of the three stop codons is produced
2) A codon which codes for a different amino acid is produced
3) A different codon which codes for the same amino acid is produced

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5
Q

Explain the effect of a substitution mutation causing one of the stop codons to be coded for

A
  • If one of the stop codons is coded for, the production of the polypeptide coded for by the section of DNA would be stopped prematurely.
  • The final protein would almost always be significantly different and the protein could not perform its normal function.
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6
Q

Explain the possible effects of a substitution mutation causing a codon that codes for a different amino acid to be produced

A
  • The formation of a codon for a different amino acid would result in the final polypeptide differing in a single amino acid.
  • This could have very little effect if the amino acid is not significant in forming the proteins tertiary structure.
  • However, if the amino acid is important in the formation of the proteins tertiary structure, the polypeptide may differ in shape and hence its function may be impaired.
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7
Q

Explain the effect of a substitution mutation causing a different codon to be codes for but that codes for the same amino acid

A
  • As the DNA code is degenerate, a substitution mutation that causes a different codon in mRNA could still code for the same amino acid.
  • This mutation would have no effect on the polypeptide produced.
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8
Q

What type of mutations are the most severe/dangerous

A

Mutations that cause a frame shift.

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9
Q

What is a deletion mutation and what is its effect

A
  • A deletion mutation is one where a nucleotide base is deleted from the DNA sequence.
  • This causes a frame shift to the left, meaning that all of the triplets after it will be shifted by a base.
  • This is very dangerous when the deletion occurs at the start of a DNA sequence.
  • It has a lesser effect at the end of a DNA sequence but can still be dangerous.
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10
Q

What is an addition mutation and what is its effect

A
  • An addition/insertion mutation is when an extra base becomes inserted into the sequence.
  • This causes a frame shift to the right which can be very dangerous to an organism.
  • If three (or any multiple of three) bases are inserted, this will not cause a frame shift.
  • Addition mutations are more dangerous if they occur at the start of the DNA sequence.
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11
Q

What is a duplication mutation and what is its effect

A
  • One or more bases are repeated.
  • This creates a frame shift to the right (which has the same effects as an addition mutation).
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12
Q

What is an inversion mutation and what is its effect

A
  • An inversion mutation is where a group of bases become separated from the DNA sequence and rejoin in the same position but in the inverse order.
  • The base sequence of this portion is therefore reversed and affects the amino acid sequence that results.
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13
Q

What is a translocation mutation and what is its effect

A
  • A group of bases become separated from the DNA sequence on one chromosome and become inserted into the DNA sequence of a different chromosome.
  • Translocations often have significant effects on gene expression leading to an abnormal phenotype.
  • These effects include the development of certain forms of cancer and also decreased fertility.
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14
Q

Give some examples of genes that are permanently expressed in all cells

A
  • The genes that code for essential chemicals such as the enzymes involved in respiration.
  • The genes that code for proteins involved in transcription, translation, membrane synthesis and tRNA synthesis.
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15
Q

What stops a cell from producing the proteins for the genes that are not expressed

A

A variety of stimuli ensure genes for the other proteins are not expressed by:
1) Preventing transcription and so preventing the production of mRNA
2) Preventing translation

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16
Q

What are stem cells

A

Cells which retain the ability to differentiate into other cells in mature mammals.

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17
Q

What is self-renewal and why is it necessary

A
  • Self-renewal is the process by which stem cells divide to form an identical copy of themselves.
  • This is needed as stem cells are constantly differentiating into specialised cells and so constantly need to be replaced.
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18
Q

List and describe the sources of stem cells in mammals

A

1) Embryonic stem cells: they come from embryos in the early stages of development. They can differentiate into any type of cell in the initial stages of development.
2) Umbilical cord blood stem cells: these are derived from the umbilical cord blood and are similiar to adult stem cells.
3) Placental stem cells: these are found in the placenta and develop into specific types of cells.
4) Adult stem cells: these are found in the body tissues of the foetus through to adult. They are specific to a particular tissue or organ within which they produce the cells to maintain and repair tissues throughout an organisms life.

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19
Q

List the four types of stem cells

A

1) Totipotent stem cells
2) Pluripotent stem cells
3) Multipotent stem cells
4) Unipotent stem cells

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20
Q

Describe what totipotent stem cells are

A
  • Totipotent stem cells are found in the early embryo and can differentiate into any type of cell.
  • Since all body cells are formed from a zygote, it follows that the zygote is totipotent
  • As the zygote divides and matures, its cells develop into more specialised cells called pluripotent stem cells.
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21
Q

Describe what pluripotent stem cells are

A
  • Pluripotent stem cells are found in embryos and can differentiate into almost any type of cell.
  • Examples of pluripotent stem cells are embryonic stem cells and fetal stem cells.
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22
Q

Describe what multipotent stem cells are

A
  • Multipotent stem cells are found in adults and can differentiate into a limited number of specialised cells.
  • They usually develop into cells of a particular type.
  • For example stem cells in the bone marrow can produce any type of blood cell.
  • Examples of multipotent cells are adult stem cells and umbilical cord stem cells.
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23
Q

Describe what unipotent stem cells are

A
  • Unipotent stem cells can only differentiate into a single type of cell.
  • They are derived from multipotent stem cells and are made in adult tissue.
  • An example of unipotent stem cells are cardiomyocytes which are heart muscle cells that can divide to produce new heart tissue, and so repair damage to the heart muscle.
24
Q

What are induced pluripotent stem cells (iPS cells) and how are they produced

A
  • iPS cells are a type of pluripotent cell that is produced from unipotent stem cells.
  • The unipotent cell may be almost any body cell.
  • These body cells are then genetically altered in a laboratory to make them acquire the characteristics of embryonic stem cells which are a type of pluripotent cell.
  • making the unipotent cell acquire the new characteristics involves inducing genes and transcriptional factors within the cell to express themselves.
25
Q

Why do iPS cells have great promise

A
  • They are capable of self-renewal
  • This means that they can potentially divide to produce a limitless supply.
  • As such, they could replace embryonic stem cells in medical research and treatment and so overcome many of the ethical issues surrounding the use of embryos in stem cell research.
26
Q

List some human cells that could be produced from stem cells and the diseases this could treat

A
  • Heart muscle cells: heart damage eg. After a heart attack.
  • Skeletal muscle cells: Muscular dystrophy
  • B cells of the pancreas: Type 1 diabetes
  • Nerve cells: Parkinson’s, MS, strokes, Alzheimers disease, paralysis
  • Blood cells: leukaemia, inherited blood disorders
  • Skin cells: burns and wounds
  • Bone cells: osteoporosis
  • Cartilage cells: osteoarthritis
  • Retina cells: macular degeneration
27
Q

What is oestrogen

A

A steroid hormone

28
Q

Describe how the regulation of transcription controls the expression of a gene

A
  • For transcription to begin the gene in switched on by specific molecules that move from the cytoplasm into the nucleus.
  • These molecules are called transcriptional factors.
  • Each transcriptional factor has a site that binds to a specific base sequence of the DNA in the nucleus.
  • When it binds, it causes this region of DNA to begin the process of transcription.
  • Messenger RNA (mRNA) is produced and the information it carries is then translated into a polypeptide.
  • When a gene is not being expressed, the site on the transcriptional factor that binds to DNA is not active.
  • As the site on the transcriptional factor binding to DNA is inactive it cannot cause transcription and polypeptide synthesis.
29
Q

Describe how oestrogen (and other hormones like it) controls gene expression

A
  • Oestrogen is a lipid soluble molecule and therefore diffuses easily through the phospholipid portion of the cell-surface membranes.
  • Once inside the cytoplasm of the cell, oestrogen binds with a site on a receptor molecule of the transcriptional factor.
  • The shape of the site and the shape of the oestrogen molecule are complementary.
  • By binding with the site, the oestrogen changes the shape of the DNA binding site on the transcriptional factor, which can now bind to DNA (it is activated)
  • The transcriptional factor can now enter the nucleus through a nuclear pore and bind to specific base sequences on DNA.
  • The combination of the transcriptional factor with DNA stimulates transcription of the gene that makes up the portion of DNA.
30
Q

What is epigenetics

A
  • The process by which environmental factors cause heritable changes in gene function without changing the base sequence of DNA.
  • it is a relatively new scientific field that provides explanations as to how environmental influences such as stress, diet, toxins etc can subtly alter the genetic inheritance of an organisms offspring.
31
Q

Summarise what the epigenome is

A

All of the chemical modifications to all histone proteins and DNA (except base changes) in an organism.

32
Q

Describe what the epigenome is

A
  • DNA is wrapped around proteins called histones
  • Both DNA and histones are covered in chemicals, sometimes called tags.
  • These chemical tags form a second layer known as the epigenome.
  • The epigenome determines the shape of the DNA-histone complex.
  • It keeps genes that are inactive in a tightly packed arrangement and therefore ensures that they cannot be read (remain switched off).
  • It unwraps active genes to that the DNA is exposed and can easily be transcribed.
  • The epigenome of a cell is the accumulation of the signals it has received during its lifetime and it therefore acts like a cellular memory.
33
Q

What is the difference between the DNA code and epigenome and why is it the case

A
  • The DNA code is fixed whereas the epigenome is flexible.
  • This is because its chemical tags respond to environmental changes.
  • Factors like diet and stress can cause the chemical tags to adjust the wrapping and unwrapping of the DNA and so switch genes on and off.
34
Q

Describe how the epigenome works in responding to signals

A
  • In early development, the signals come from within the cells of the fetus and the nutrition provided by the mother is important in shaping the epigenome at this stage.
  • After birth, and throughout life, environmental factors affect the epigenome, as do signals from within the body such as hormones.
  • These factors cause the epigenome to activate of inhibit specific sets of genes.
  • The environmental signals stimulates proteins to carry its message inside the cell form where it is passed by a series of other proteins into the nucleus.
  • Here the message passes to a specific protein which can be attached to a specific sequence of bases on the DNA.
  • Once attached the protein has two possible effects.
35
Q

What are the two effects that the protein which attaches to DNA during epigenetics can have

A

It can change:
1) Acetylation of histones leading to the activation or inhibition of a gene.
2) Methylation of DNA by attracting enzymes that can add or remove methyl groups.

36
Q

Describe how the DNA-histone complex (chromatin works)

A
  • Where the association of DNA with histones is weak, the DNA-histone complex is less condensed.
  • In this condition the DNA is accessible by transcription factors which can initiate the production of mRNA, ie. can switch the gene on.
  • Where the association of histones with DNA is strong, the DNA-histone complex is more condensed.
  • In this condition the DNA is not accessible by transcriptional factors, which therefore cannot initiate production of mRNA so the gene is switched off.
37
Q

What does the condensation of the DNA-histone complex do

A

Inhibits transcription

38
Q

What two things cause the condensation of the DNA-histone complex and thus inhibit transcription

A
  • Decreased acetylation of the histones
  • By methylation of DNA.
39
Q

Explain how the decreased acetylation of associated histones inhibits transcription

A
  • Acetylation is the process whereby an acetyl group is transferred to a molecule.
  • In this case the molecule donating the acetyl group is acetylcoenzymeA
  • Decreased acetylation increases the positive charge on histones and therefore increases their attraction to the phosphate groups of DNA.
  • The association between DNA and histones is stronger and the DNA is not accessible to transcription factors.
  • These transcription factors cannot initiate mRNA production from DNA, so the gene is switched off.
40
Q

What is deacetylation

A

The reverse of acetylation- where an acetyl group is removed from a molecule.

41
Q

Explain how the increased methylation of DNA inhibits transcription

A
  • Methylation is the addition of a methyl group (CH3) to a molecule.
  • In this case the methyl group is added to the cytosine bases of DNA.
  • Methylation normally inhibits the transcription of genes in two ways:
    1) preventing the binding of transcription factors to the DNA
    2) Attracting proteins that condense the DNA-histone complex (by inducing deacetlyation of the histones) making the DNA inaccessible to transcription factors.
42
Q

List the six factors which allow you to determine whether a gene is accessible or inaccessible

A
  1. Histones
  2. DNA
  3. DNA-histone complex
  4. Chromatin type
  5. Transcription factors
  6. Gene
43
Q

List what is happening to the 6 factors when DNA is inaccessible

A

1) Histones: Decreased acetylation
2) DNA: Increased methylation
3) DNA-histone complex: more condensed
4) Chromatin type: Heterochromatin
5) Transcription factors: No access
6) Gene: Inactive

44
Q

List what is happening to the 6 factors when DNA is accessible

A

1) Histones: Increased acetylation
2) DNA: Decreased methylation
3) DNA-histone complex: Less condensed
4) Chromatin type: Euchromatin
5) Transcription factors: Access
6) Gene: Active

45
Q

Does epigenetic inheritance take place

A

Yes

46
Q

Describe the experiment undertaken on rats which illustrated epigenetic inheritance

A
  • Female offspring who received good care when young respond better to stress in later life and themselves nurture their offspring better.
  • Female offspring receiving low-quality care nurture their offspring less well.
  • Good maternal behaviour in rats transmits epigenetic information onto their offsprings DNA without passing through an egg or sperm.
47
Q

What is a human example of something which proves epigenetic inheritance

A
  • When a mother has a condition known as gestational diabetes, the fetus is exposed to high concentrations of glucose.
  • These high glucose concentrations cause epigenetic changes in the daughter’s DNA, increasing the likelihood that she will develop gestational diabetes herself.
48
Q

How is it thought that epigenetic inheritance works

A
  • It is thought that in sperm and eggs during the earliest stages of development a specialised cellular mechanism searches the genome and erases its epigenetic tags in order to return the cells to a genetic ‘clean slate’.
  • However, a few epigenetic tags escape this process and pass unchanged from parent to offspring.
49
Q

Describe how epigenetic changes are linked to colorectal cancer

A
  • In 1983, researchers found that diseased tissue taken from patients with colorectal cancer had less DNA methylation than normal tissue from the same patients.
  • Increased DNA methylation inhibits transcription.
  • This means that these patients with less DNA methylation would have higher than normal gene activity- more genes are turned on.
50
Q

What epigenetic abnormality is linked to the early stages in the development of cancer

A
  • There are specific sections of DNA near promoter regions that that have no methylation in normal cells.
  • However, in cancer cells these regions become highly methylated, causing genes that should be active to switch off.
  • This abnormality happens in the early development of cancer.
51
Q

How are epigenetic changes linked to inherited cancers

A
  • Whilst epigenetic changes do not alter the sequence of DNA, they can increase the incidence of mutations.
  • Some active genes normally help repair DNA and so prevent cancers.
  • In people with various types of inherited cancer, it is found that increased methylation of these genes has led to these protective genes being switched off.
  • As a result, damaged base sequences in DNA are not repaired and so can lead to cancer.
52
Q

How can epigenetic drugs be used to treat cancer

A
  • Drugs are designed to inhibit enzymes involved in histone acetylation or DNA methylation.
  • For example, drugs that inhibit enzymes that cause DNA methylation can reactivate genes that have been silenced.
  • This can counteract the epigenetic changes that have caused the disease.
53
Q

Why is it important that epigenetic therapies (drugs) be targeted on cancer cells

A

If the drugs were to affect normal cells, they could activate gene transcription and make them cancerous (causing the thing they were designed to cure).

54
Q

Apart from cancer therapy, what is another use of epigenetics in disease treatments and why does it work

A
  • Epigenetics have helped to develop diagnostic tests that help to detect the early stages of diseases such as cancer, brain disorders and arthritis.
  • These tests can identify the level of DNA methylation and histone acetylation at early stages of the disease.
  • This allows those with these diseases to seek early treatment and so have a better chance at a cure.
55
Q

Summarise how RNA interference can affect gene expression

A
  • In eukaryotes and some prokaryotes the translation of mRNA produced by a gene can be inhibited by breaking mRNA down before its coded information can be translated into a polypeptide.
  • One type of small RNA molecule that may be involved is small interfering RNA (siRNA).
56
Q

Describe the mechanism by which small interfering RNA affects gene expression

A
  • An enzyme cuts large, double-stranded molecules of RNA into smaller sections called small interfering RNA (siRNA).
  • One of the two siRNA strands combines with an enzyme.
  • The siRNA molecule guides the enzyme to a messenger RNA molecule by pairing up its bases with the complementary ones on a section of the mRNA molecule.
  • Once in position, the enzyme cuts the mRNA into smaller sections.
  • The mRNA is no longer capable of being translated into a polypeptide.
  • This means that the gene has not been expressed- it has been blocked.

8- The control of gene expression (7 decks)

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