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AQA A level Biology > The control of gene expression > Flashcards

The control of gene expression Flashcards

1
Q

Genetic mutations
- Insertion

A
  • Causes a frameshift
  • When a nucleotide is randomly
    inserted into DNA sequence
  • Affects function of polypeptide

Cause Huntignton’s Disease

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2
Q

Genetic mutations
- Substitution

A
  • Where a DNA base/ nucleotide is swapped for a different one
  • Silent substitution alters amino acid sequence
  • Missense substitution alters single amino acid
  • Nonsense substitution creates a premature stop codon

Cause Sickle Cell Anemia

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3
Q

Genetic mutations
- Deletion

A
  • Causes a frameshift
  • A nucleotide is randomly deleted
  • Affects function of polypeptide

Cause Cystic Fibrosis

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4
Q

Genetic mutations
- Inversion

A
  • A single gene is cut into 2 pieces, inverted 180° and rejoined
  • Results in non functional protein

Cause Haemophilia A

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5
Q

Genetic mutations
- Duplication

A
  • One or more bases are duplicated in the DNA sequence
  • Original gene is not changed

Cause Charcot- Marte Tooth Disease

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6
Q

Genetic mutations
- Translocation

A
  • A section of a chromosome is added to another chromosome which is not its homologous partner
  • Philadelphia chromosome (22) found in Leukemia Cancer

Cause Cancer, Infertility and Down Syndrome

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7
Q

Mutagenic agents / Mutagens

A
  • High energy ionising radiation for example, short wavelength radiation such as X-rays and ultra violet light. These forms of radiation can disrupt the structure of DNA.
  • Chemicals such as nitrogen dioxide may directly alter the structure of DNA or interfere with transcription. Benzopyrene, a consitituent of tobacco smoke, is a powerful mutagen that inactivates a tumour-suppressor gene TP53 leading to a cancer.
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8
Q

Cell differentiation

A

When a cell becomes specialised through differential gene expression to carry out a particular function

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9
Q

What is pluripotency?

A

Pluripotent stem cells are found in embryos and can differentiate into almost any type of cell.
- Examples are embryonic stem cells and fetal stem cells.

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10
Q

What is unipotency?

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.

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11
Q

What is multipotency?

A

Multipotent stem cells are found in adults and can differentiate into a limited number of specialised cells.
- Examples of multipotent cells are adult stem cells and umbilical cord blood stem cells

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12
Q

What is totipotency?

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 it divides and matures, its cells develop into slightly more specialised cells called pluripotent stem cells.

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13
Q

Induced pluripotent stem cells (iPS cells)

A

Produced from adult somatic cells using appropriate protein transcription factors

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14
Q

Embryonic stem cells

A

Come from embryos in the early stages of development
- totipotent if taken in the first 3-4 days after fertilisation
- pluripotent if taken on day 5

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15
Q

Adult stem cells

A

Found in the body tissues of the fetus through to the adult.
They are specific to a particular tissue or organ within which they produce the cells to maintain and repair tissues throughout an organism’s life.
- multipotent
- unipotent

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16
Q

Umbilical cord blood stem cells

A

Derived from umbilical cord blood and are similar to adult stem cells

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17
Q

Placenta stem cells

A

Found in the placenta and develop into specific types of cells

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18
Q

What are stem cells?

A

Stem cells are undifferentiated dividing cells that occur in adult animal tissues and need to be constantly replaced. They therefore have the ability to divide to form an identical copy of themselves in a process called self-renewal.

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19
Q
A
  • 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 (i.e is switched off) 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.
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20
Q

What are transcriptional factors?

A

For transcription to begin the gene is switched on by specific molecules (transcriptional factors) that move from the cytoplasm into the nucleus.

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21
Q

Oestrogen

A

A steroid hormone involved in switching on a gene and thus starting transcription by combining with a receptor site on the transcriptional factor, this activates the DNA binding site by causing it to change shape.

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22
Q

The effect of oestrogen on gene transcription

A
  • Oestrogen is a lipid-soluble molecule and therefore diffuses easily through the phospholipid portion of cell-surface membranes
  • Once inside the cytoplasm of a cell, oestrogen binds with a site on a receptor molecule of the transcriptional factor. The shape of this site and the shape of the oestrogen molecule complement one another
  • By binding with the site, 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 chrough 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.
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22
Q

What are epigenetics?

A

A relatively new scientific field that provides explanations as to how environmental influences such as diet, stress, toxins, etc can subtly alter the genetic inheritance of an organism’s offspring

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23
Q

What chemicals cover DNA and histones

A

Known as tags

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24
Q

The epigenome

A

All of the chemical modifications to all histone proteins and DNA (except base changes) in an organism, determining the shape of the DNA-histone complex
- it is flexible as the chemical tags are influenced by the environment
- the accumulation of the signals it has received during its lifetime and it therefore acts like a cellular memory

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25
Q

How does the epigenome determine the DNA-histone complex shape?

A

It keeps genes that are inactive in a tightly packed arrangement and therefore ensures that they cannot be read, switches them off (epigenetic silencing)
It unwraps active genes so that the DNA is exposed and can easily be transcribed, switching them on

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26
Q

How do hormones influence the epigenome?

A

Activate or inhibit specific sets of genes.

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27
Q

How does the environment influence the epigenome?

A

Stimulates proteins to carry its message inside the cell from 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 or bases on the DNA. Once attached the protein has two possible effects. It can change:
- acetylation of histones leading to the activation or inhibition a gene
- methylation of DNA by attracting enzymes that can add or remove methyl groups.

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28
Q

Acetylation

A

A chemical reaction in which a small molecule called an acetyl group is added to other molecules

29
Q

Methylation

A

A chemical reaction in the body in which a small molecule, a methyl group gets added to DNA, proteins or other molecules

30
Q

Weak association of histones to DNA

A

The DNA-histone complex is less condensed (loosely packed). In this condition the DNA is accessible by transcription factors, which can initiate produtction of mRNA and can switch the gene on.

31
Q

Strong association of histones to DNA

A

The DNA- histone complex is more condensed (tightly packed). In this condition the DNA is not accessible by transcription factors, which therefore cannot initiate production of mRNA and so the gene is switched off

32
Q

Correlation of association between DNA and histones

A

Condensation of the DNA-histone complex therefore inhibits transcription, which can be brought about by decreased acetylation of the histones or by methylation of DNA.

33
Q

Deacetylation

A

The reverse reaction where an acetyl group is removed from a molecule.

34
Q

Decreased acetylation of associated histones

A

Decreased acetylation increases the positive charges 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, and so the gene is switched off.

35
Q

Increased methylation of DNA

A

Methyl group is added to the cytosine bases of DNA. Methylation normally inhibits the transcription of genes in two ways:
- preventing the binding of transcriptional factors to the DNA
- attacts proteins that condense the DNA-histone complex (by inducing deacetylation of the histones) making the DNA inaccessible to transcription factors

36
Q

Epigenetics and inheritance

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.

37
Q

Experiments on epigenetics and inheritance

A

Experiments on rats have shown that 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 offspring’s DNA without passing through an egg or sperm.

38
Q

Epigenetics and disease
- mutations

A
  • Epigenetic changes do not alter the sequence of bases in 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.
39
Q

Epigenetics and disease
- methylation

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. As we saw earlier, increased DNA methylation normally inhibits transcription (switches off genes). This means that these patients with less DNA methylation would have higher than normal gene activity - more genes were turned on.
40
Q

Epigenetics and disease
- methylation

A
  • It is known that there are specific sections or DNA (ones near regions called promoter regions) 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 early in the development or cancer.
41
Q

Treating diseases with epigenetic therapy

A

These treatments use drugs to inhibit certain enzymes involved in either histone acetylation or DNA methylation.
- Epigenetic therapy must be specifically targeted on cancer cells. If the drugs were to affect normal cells they could activate gene transcription and make them cancerous, so causing the very disorder they were designed to cure.

42
Q

Examples of treating diseases with epigenetic therapy

A
  • Drugs that inhibit enzymes that cause DNA methylation can reactivate genes that have been silenced.
  • Development of diagnostic tests that help to detect the early stages of diseases such as cancer, brain disorders and arthritis, which can identify the level of DNA methylation and histone acetylation at an early stage of disease allowing those with these diseases to seek early treatment and so have a better chance of cure.
43
Q

The effect of RNA interference on gene expression

A
  • An enzyme cuts large double-stranded molecules of RNA into smaller sections 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 inro smaller sections
  • The mRNA is no longer capable of being translated into a polypeptide.
  • This means that the gene has not been expressed but it has been blocked.
44
Q

Small interfering RNA (siRNA)

A

Small, double-stranded RNA molecules
- bind to mRNA that has been transcribed from target genes (the genes to be ‘silenced’) as their base sequence is complementary
- Each siRNA is attached to a protein complex which is able to breakdown the mRNA that has been transcribed from target genes

45
Q

What is cancer?

A

Cancer is a group of diseases caused by damage to the genes that regulate mitosis and the cell cycle. This leads to unrestrained growth of cells. As a consequence, a group of abnormal cells, called a tumour, develops and constantly expands in size.

46
Q

Malignant tumours

A

Cancerous tumours

47
Q

Benign tumours

A

Non- cancerous tumours

48
Q

Characteristics of benign tumours

A

Can grow to a large size
Grow very slowly
The cell nucleus has a relatively normal appearance
Cells are often well differentiated (specialised]
Cells produce adhesion molecules that make them stick together and so they remain within the tissue from which they arise = primary tumours
Tumours are surrounded by a capsule of dense tissue and so remain as a compact structure
Much less likely to be life-threatening but can disrupt functioning of a vital organ
Tend to have localised effects on the body Can usually be removed by surgery alone Rarely reoccur after treatment

48
Q

Characteristics of malignant tumours

A

Can also grow to a large size
Grow rapidly
The cell nucleus is often larger and appears darker due to an abundance of DNA
Cells become de-differentiated (unspecialised)
Cells do not produce adhesion molecules and so they tend to spread to other regions of the body, a process called metastasis, forming secondary tumours
Tumours are not surrounded by a capsule and so can grow finger-like projections into the surrounding tissue
More likely to be life-threatening, as abnormal tumour tissue replaces normal tissue
Often have systemic [whole body) effects such as weight loss and fatigue
Removal usually involves radiotherapy and/or chemotherapy as well as surgery More frequently reoccur after treatment

48
Q

The two main types of genes that play a role in cancer

A
  • Tumour suppressor genes
  • oncogenes
49
Q

Cancer and the genetic control of cell division
- generality of cancer origins

A

DNA analysis of tumours has shown that, in general, cancer cells arc derived from a single mutant cell. The initial mutation causes uncontrolled mitosis in this cell. Later, a further mutation in one of the descendant cells leads to other changes that cause subsequent cells to be difrerent from normal in growth and appearance

50
Q

Cancer and the genetic control of cell division
- Oncogenes

A

Most oncogenes are mutations of proto-oncogenes which stimulate a cell to divide when growth factors attach to a protein receptor on its cell-surrace membrane. This then activates genes that cause DNA to replicate and the cell to divide. If a proto-oncogene mutates into an on cogene it can become permanently activated for two reasons:
- The receptor protein on the cell-surface membrane can be permanently activated, so that cell division is switched on even in the absence of growth factors.
- The oncogene may code for a growth factor that is then produced in excessive amounts, again stimulating excessive cell division.

51
Q

Cancer and the genetic control of cell division
- Tumour suppressor genes

A

Slow down cell division, repair mistakes in DNA, and ‘tell’ cells when to die - a process called apoptosis (programmed cell death).
They therefore have the opposite role from proto-oncogenes. A normal tumour suppressor gene maintains normal rates of cell division and so prevents the formation of tumours. If a tumour suppressor gene becomes mutated it is inactivated (switched off). As a result, it stops inhibiting cell division and cells can grow out of control. The mutated cells that are formed are usually structurally and functionally different from normal cells. While most of these die, those that survive can make clones of themselves and form tumours.
- TP53, BRCA 1 and BRCA 2

52
Q

Cancer and the genetic control of cell division
- Difference between oncogenes and tumour supressor genes

A

While oncogenes cause cancer as a result of the activation of proto-oncogenes, tumour suppressor genes cause cancer when they are inactivated.

53
Q

Abnormal methylation of tumour suppressor genes

A

Abnormal DNA methylation is common in the development of a variety of tumours. The most common abnormality is hypermethylation:
- Hypermethylation occurs in a specific region (promoter region) of tumour suppressor genes.
- This leads to the tumour suppressor gene being inactivated.
- As a result, transcription of the promoter regions of tumour suppressor genes is inhibited.
- The inactivation leads to increased cell division and the formation of a tumour. Thought to occur in BRCA 1leading to the development of breast cancer.

Another form of abnormal merhylation is hypomethylation:
- This has been found to occur in oncogenes where it leads to their activation and hence the formation of tumours.

54
Q

Oestrogen concentrations and breast cancer

A
  • Oestrogens play a central role in regulating the menstrual cycle in women. It is known that after the menopause. a woman’s risk of developing breast cancer increases. This is thought to be due to increased oestrogen concentrations.
  • At first this seemed paradoxical because the produaion of oestrogens from the ovaries diminishes after the menopause. However, the fat cells of the breasts tend to produce more oestrogens after the menopause.
  • These locally produced oestrogens appear to trigger breast cancer in postmenopausal women. Once a tumour has developed, it further increases oestrogen concentration which therefore leads to increased development of the tumour.
  • It also appears that white blood cells that are drawn to the tumour increase oestrogen prodution. This leads to even greater development of the tumour.
55
Q

How can oestrogen cause a tumour to develop?

A

The mechanism by which oestrogen effectively activates a gene by binding to a gene which promotes transcription.
- If the gene that oestrogen acts on is one that controls cell division and growth, then it will be activated and its continued division could produce a tumour. It is known, for example, that oestrogen causes proto­ oncogenes of cells in breast tissue to develop into oncogenes. This leads to the development of a tumour (breast cancer).

56
Q

What is bioinformatics

A

The science of collecting and analysing complex biological data such as genetic codes. It uses computers to read, store, and organise biological data at a much faster rate than previously. It also utilises algorithms to analyse and interpret biological data.

57
Q

Possibilities created using bioinformatics

A

When you consider thar the human genome consists of over 3 billion base pairs organised into a round 20 000 genes, sequencing every one of those bases is a mammoth task and yet it took just 13 years to complete using bioinformatics

58
Q

How to determine the complete DNA base sequence

A

The technique of whole-genome shotgun (WGS) sequencing
- researchers cut the DNA into many small, easily sequenced sections and then using computer algorithms to align overlapping segments to assemble the entire genome

59
Q

What are single nucleotide polymorphisms?

A

SNPs are single-base variations in the genome that are associated with disease and other disorders.

59
Q

Medical advancements of DNA sequencing

A
  • Over 1.4 million single nucleotide polymorphisms (SNPs) have been found in the human genome.
  • Medical screening of individuals has allowed quick identification of potential medical problems and for early intervention to treat them
  • Possibility of establishing the evolutionary links between species.
60
Q

What is the proteome?

A

The full range of proteins that a cell produces/ DNA can code for in a given time, under specified conditions

61
Q

Determining the genome and proteome of simpler organisms

A

Used in bacteria because:
- the vast majority of prokaryotes have just one, circular piece of DNA that is not associated with histoncs
- there are none of the non-coding portions of DNA which are typical of eukaryotic cells.

62
Q

First bacterium genome fully sequenced

A

Haemophilus influenza in 1995.
H. influenza contains 1700 genes comprising 1.8 million bases.

63
Q

What is the Human Microbiome Project?

A

The sequencing of genomes of thousands of prokaryotic and single-celled eukaryotic organisms
- It is hoped that the information gained will help cure disease and provide knowledge of genes that can be usefully exploited

64
Q

Example of DNA sequencing

A

P/asmodium falciparum which causes malaria.
- All 5300 genes on Plasmodium ‘s 14 chromosomes have been sequenced giving us an insight into its metabolism and knowledge of the proteins it produces.
- All this will be invaluable in helping us to develop the elusive vaccine against this globally important disease.

65
Q

Determining the genome and proteome of complex organisms

A
  • The success in mapping the human genome in 2003 is a testimony to what can be achieved in mapping DNA sequences of complex organisms.
  • The problem in complex organisms is translating knowledge of the genome into the proteome
    This is because the genome of complex organisms contains many non -coding genes as well as others that have a role in regulating other genes.
    There is a human proteome project currently underway to identify all the proteins produced by humans
66
Q

How many genes are said to be in the human genome?

A

There are around 20,000 genes in the human genome although this number is constantly being revised down as our techniques for identifying genes improves

67
Q

Percentage of genes which code for proteins

A

it is thought that as few as l.5% of genes may code for proteins

68
Q
A

AQA A level Biology (8 decks)

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