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AQA Categories Archives: 3.8 The control of gene expression

Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the design of new industrial and medical processes – differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions

The control of gene expression (AQA A2 Biology) PART 7 of 8 TOPICS

 

 

TOPICS: Alternation of the sequence of bases in DNA can alter the structure of proteins  Gene expression is controlled by a number of features – most of the cell’s DNA is not translated  Gene expression is controlled by a number of features – regulation of transcription and translation  Gene expression is controlled by a number of features – gene expression and cancer  Using genome projects  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – recombinant DNA technology  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – genetic fingerprinting

 

 

 

 

Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the design of new industrial and medical processes – differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions:

Gel electrophoresis is where DNA fragments of desired genes are separated by size. This is how it’s done:

  • A gel with wells along one edge is placed in a tank full of buffer where each of the wells has the DNA fragments of the desired gene. These fragments have been cut out by restriction enzymes and have got a marker on them either carbon-13 or fluorescence e.t.c.
  • The buffer is used to dissolve the DNA fragments. NB: You do not need to know the buffer that is involved in this process.
  • The edge that has the wells full of the DNA fragments is towards the negative terminal making the other towards the positive terminal. This because DNA fragments are negatively charged because of the phosphate group.
  • The circuit is switched on.
  • The DNA fragments then move to the positive terminal at different speeds due to their size where the largest DNA fragments being more towards the negative terminal and the smallest ones being more towards the positive terminal.
  • UV-light is shone over the gel to allocate where the DNA fragments are on the gel if a marker for fluorescence has been used. Carbon-13 will go black in the presence of light.
  • The DNA fragments are then transferred on to a nylon membrane using a weight so that it can be stored forever as the gel can dry out. NB: The name of this process is called Southern transfer, which is named after Edwin Southern, does not need to be known for the exam.

DNA probes are used to identify which of the DNA fragments has the desired gene as they have complementary bases:

  • DNA hybridisation:
  • Gel electrophoresis takes place.
  • As DNA probes are single stranded, the DNA molecules have to be separated by breaking the hydrogen bonds. This is done by heating the DNA fragments to 95 degrees to break the hydrogen bonds. Other techniques can be used to break hydrogen bonds e.g. using DNA helicase or soaking the gel in alkaline solutions.
  • The DNA probes with a marker, e.g. fluorescence, are added and are washed away to get rid of any unbound DNA probes.
  • Any DNA probes that have bounded will glow under UV-light shows that the DNA fragments that are complementary to the DNA probes have the gene that you are looking for. This is a good technique to use for when you are looking for a mutation in a gene that causes disease.
  • DNA microarray:
  • This involves using a glass slide with different DNA probes on it.
  • A DNA sample is added to the slide where only one type of DNA probe is complementary to the desired gene on the fragment. The fragments have a marker on them e.g. fluorescence.
  • The slide is then washed to remove any unbound DNA fragments.
  • The parts of the slide that fluoresce under the presence of UV-light shows that the desired gene is present. This technique can screen for more than one type of gene at the same time.

Genetic counselling is the giving of advice and information about the risks of genetic disease and its outcomes. Counselling is a very challenging task. Counsellors must have adequate knowledge and understanding about the topic and need to be well trained in sympathetic counselling techniques. They must help clients come to terms of making their own decisions rather than imposing their own views on them. They should be made aware that the features of a certain genetic disease varies widely. It should also be made very clear of the sorts of support that are available to the child with the genetic disorder and to the parents/guardians as well as family.

 

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Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – genetic fingerprinting

The control of gene expression (AQA A2 Biology) PART 8 of 8 TOPICS

 

 

TOPICS: Alternation of the sequence of bases in DNA can alter the structure of proteins,  Gene expression is controlled by a number of features – most of the cell’s DNA is not translated,  Gene expression is controlled by a number of features – regulation of transcription and translation,  Gene expression is controlled by a number of features – gene expression and cancer,  Using genome projects,  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – recombinant DNA technology,  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions,  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – genetic fingerprinting

 

 

 

 

 

Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – genetic fingerprinting:

Genetic fingerprint is a banding pattern produced by DNA that is particularly unique to that individual. This is useful at a crime scene or the establishment of paternity. This is how it’s done:

  • Collection: The sample of DNA is collected which can come from the cheek cells, hair, semen, blood e.t.c.
  • Extraction: The DNA fragment of variable number tandem repeats (bases that are repeated in the introns sections of the DNA) is obtained. NB: The process of extraction does not need to be known for the exam. Variable number tandem repeats can be abbreviated into VNTRs. In some text books and online resources the word ‘minisatellites’ may be used. This is an old name for variable number tandem repeats which is not used now.
  • Digestion: Restriction enzymes cut the DNA close to but not in the VNTR regions allowing the DNA fragments to keep the lengths for the characteristics to show up unique to that individual.
  • Separation 1: Gel electrophoresis takes place and then the DNA double strands are broken by breaking the hydrogen bonds using alkaline solution or DNA helicase.
  • Separation 2: The DNA is then transferred onto a nylon membrane using a weight. NB: The name of this process, Southern transfer does not need to be known for the AQA exam.
  • Hybridization: Labelled DNA probes with markers on complementary to the desired gene are added to the DNA and then is washed to remove any of unbound DNA probes. Then the membrane is dried.
  • Development: An x-ray film is placed over the membrane to make the bands visible which are known as DNA fingerprints.

] That’s all that you need to know 🙂 [

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Using genome projects

The control of gene expression (AQA A2 Biology) PART 4 of 8 TOPICS

 

 

TOPICS: Alternation of the sequence of bases in DNA can alter the structure of proteins  Gene expression is controlled by a number of features – most of the cell’s DNA is not translated  Gene expression is controlled by a number of features – regulation of transcription and translation  Gene expression is controlled by a number of features – gene expression and cancer  Using genome projects  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – recombinant DNA technology  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – genetic fingerprinting

 

 

 

 

 

Using genome projects:

Sequencing projects have read the genomes of a wide range of organisms including humans. An example of this is The Human Genome Project which was completed in 2003 and mapped the entire sequence of the human genome for the first time.

Determining the genome of simpler organisms allows the sequences of the proteins that derive from the genetic code of the organism to be determined. This is because the simple organisms such as bacteria have not many non-coding DNA compared to larger much more complex organisms. This means the proteome (the proteins coded by the DNA) is easily determined and is useful in medicine where the antigens on the bacteria are identified and vaccines can be developed.

Sequencing methods are continuously updated and have become automated (computer-based). Automated sequencing means it is much faster (it sequences 400 million bases in a ten-hour period to be precise) and is cost-effective. NB: The speed at which automated sequencing occurs does not need to be known.

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Gene technologies allow the study of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – recombinant DNA technology

The control of gene expression AQA A2 Biology PART 6 of 6 TOPICS

 

 

TOPICS: Alternation of the sequence of bases in DNA can alter the structure of proteins  Gene expression is controlled by a number of features – most of the cell’s DNA is not translated  Gene expression is controlled by a number of features – regulation of transcription and translation  Gene expression is controlled by a number of features – gene expression and cancer  Using genome projects  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – recombinant DNA technology  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – genetic fingerprinting

 

 

 

 

 

Gene technologies allow the study of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – recombinant DNA technology:

Recombinant DNA technology involves the transfer of fragments of DNA from one organism or species to another. Since the genetic code is universal, as are transcription and translation mechanisms, the transferred DNA can be translated within cells of the recipient organism. A fragment of DNA would be made if we wanted it in vast quantities. An example would be for the insulin gene. This would be wanted in vast numbers because of diabetics who do not produce enough or do not produce any insulin at all.

Fragments of DNA can be produced by several methods:

  • Reverse transcription:
  • DNA fragments are produced from mRNA using the enzyme reverse transcriptase. This is done by identifying the cells that make the large amounts of proteins from the desired gene. These cells also contain a large amount of mRNA. Continuing with the example of the insulin gene, the mRNA would be found in the beta cells of the islets of Langerhans in the pancreas.
  • The mRNA is then isolated by centrifugation. NB: You do not need to know the step by step process of centrifuging.
  • Free DNA nucleotides are then added to the mRNA and the enzyme reverse transcriptase which makes a complementary/copy DNA from the mRNA. NB: Complementary/copy DNA can be abbreviated into cDNA but it is best if you use the full name.
  • The mRNA is then broken down by another enzyme leaving just the complementary/copy DNA. NB: The enzyme that is involved in this is called ribonuclease H but this name does not need to be known for the exam.
  • As the complementary/copy DNA is single stranded, DNA nucleotides are added and DNA polymerase to make the DNA double stranded with the desired gene so that it can be inserted into the host. The double stranded complementary/copy DNA does not have introns as the mRNA did not have any introns in the first place because the introns are removed leaving the exons to be spliced together.
  • Restriction enzymes: NB: The different types of these enzymes do not need to be known as well as the palindromic sequence that they cut into. This will be given to you in the exam in a question where a certain enzyme cuts in between two bases of a specific palindromic sequence.
  • These enzymes catalyse a hydrolysis reaction by breaking the phosphodiester bond between nucleotides at certain places. These enzymes make small DNA fragments.
  • The enzyme cuts between two bases in a palindromic sequence on both strands of DNA. This makes ‘sticky ends’ (ends which have exposed bases which can bind to complementary bases. This is known by the word anneal). An illustration of this bullet point is shown where it shows what a palindromic is and the process of cutting:

 

If the restriction enzyme EcoR1 cuts in between G and A in the sequence GAATTC (shown in italics) it creates sticky ends. It recognises the palindromic sequence from 5′ to 3′. Other enzymes also create sticky ends using different palindromic sequence:

 

The dotty line shows where EcoR1 cuts.

  • The same restriction enzyme should be used for the cutting of the DNA of the vector (the host that will carry the desired gene). This is so complementary bases can be made to attach to the sticky ends (shown by the bold and italic bases. DNA ligase then sticks the DNA fragment and the genome of the vector together called ligation:

 

  • Gene machine:
  • Technology has been developed so that fragments of DNA can be made from scratch without the need of a DNA template.
  • The desired sequence is made.
  • The first nucleotide is fixed to a support e.g. a bead.
  • Nucleotides are then added one at a time in a process where the protecting groups are added. Protecting groups prevent unwanted branching by making sure that the nucleotides are joined up correctly.
  • Short DNA sequences known as oligonucleotides (up to 20 nucleotides long) are obtained. Once these are made the support is removed as well as the protecting groups. The oligonucleotides are then joined together to make a longer DNA fragment.

There are two types of methods as to how the fragments of DNA with the desired gene are amplified. This means how to clone the DNA to obtain high numbers of the desired gene:

  • In-vivo amplification:
  • This involves using hosts that are cells e.g. bacteria or bacteriophages. Bacteria are always used as they can replicate quickly.
  • The DNA fragment is obtained and inserted into the host’s vector outside of the host by the restriction enzyme. If bacterial cells are used the vector would be the plasmid (contains the bacteria’s DNA) or bacteriophages (bacteria that can be infected by viruses).
  • The new combination of DNA in the plasmid is called recombinant DNA.
  • The vector has to be taken up by the host. If a bacteriophage is used the virus infects the bacteria with the recombinant DNA which is then incorporated into the bacteria’s DNA.
  • The cells have to be identified as to which ones have the recombinant DNA and which ones do not. Therefore a genetic marker has to be used. This is an extra gene coding for a special characteristic which is attached to the DNA fragment. So the genetic marker could be coding for a fluorescent protein. Colonies of bacteria can be grown on an agar plate where some of the colonies will glow and some will not under UV light. This shows that the colonies that glow have the recombinant DNA. This is so only the right type of bacteria with the recombinant DNA can be grown in the fermenter in the right conditions. NB: Students will not be required to recall specific marker genes in a written paper.
  • In order for the fragment of DNA to be expressed a promoter and terminator sequence must be added or is already part of the DNA to allow this to happen. Without these the desired gene will not be expressed properly.
  • In-vitro amplification:
  • A mixture is set which has the DNA sample, free nucleotides, primers and DNA polymerase.
  • The DNA is heated to 95 degrees to break the hydrogen bonds between the two strands.
  • The mixture is then cooled to 55 degrees so the primers can bind (anneal) to the strand. Primers are short DNA molecules that have complementary bases to the DNA of the desired fragment.
  • The mixture is then heated to 72 degrees for DNA polymerase to work. The enzyme lines up the free DNA nucleotides up and a new strand is formed from 3′ to 5′.
  • The process is repeated again so that more fragments are obtained.

Benefits to humans:

  • Agriculture:
  • Agricultural crops can be transformed to create higher yield or are more nutritious. This means that malnutrition and famine is brought under-control and kept at a minimum. Some crops have been genetically modified to withstand pests which keeps the use of pesticides to a minimum making the method cost-effective and is environmentally friendly. It also keeps bioaccumulation at its lowest.

EXAMPLE: The golden rice has a gene from a maize plant and another from soil bacterium which enables beta-carotene to be formed. Beta-carotene allows our bodies to absorb vitamin A and is of particular benefit to people in less developed or developing countries in parts of South-Asia and Africa where vitamin A deficiency is common.

  • Industry:
  • Biological catalysts are used in industries and can be made from transformed organisms. These enzymes are produced in large quantities for a low price being cost-effective

EXAMPLE: An enzyme is used in industry to make cheese from a transformed organism. Before this method was used an enzyme was used from a cow’s stomach to make cheese. Now, cows are not killed for the enzyme making some types of cheese vegetarian.

 

  • Medicine:
  • Many different drugs and vaccines are made by transformed organisms using recombinant DNA. They can be made quickly, cheaply and in large quantities.

EXAMPLE: Insulin.

There are problems with these aspects:

  • Agriculture:
  • One type of crop is only grown in the farm which means all the plants are prone to one type of disease.
  • There is a possibility of supersedes, weeds which are resistant to herbicides. This would only be the result of crop breeding with wild plants
  • Industry:
  • Some consumer markets, particularly within the EU, do not import GM foods and produce. This means there is a loss of profit.
  • Without proper labelling, customers will not have the choice between GM foods.
  • Medicine:
  • Money may be used for technologies to support genetic modification making less money being used for resources.
  • Currently it is illegal to design babies but people are still concerned that the technology will be used unethically.

 

 

 

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Gene expression is controlled by a number of features – gene and cancer

The control gene expression (AQA A2 Biology) PART 4 of 8 TOPICS

 

 

TOPICS: Alternation of the sequence of bases in DNA can alter the structure of proteins  Gene expression is controlled by a number of features – most of the cell’s DNA is not translated  Gene expression is controlled by a number of features – regulation of transcription and translation  Gene expression is controlled by a number of features – gene expression and cancer  Using genome projects  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – recombinant DNA technology  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – genetic fingerprinting

 

 

 

 

 

 

Gene expression is controlled by a number of features – gene and cancer:

There are two types of tumours that need to be known; each of them having characteristics to make them different from the other:

  • Malignant: These tumours are cancerous which grow rapidly and invade to destroy surrounding tissues. These tumours can break into cells which can travel in the blood stream (and lymphatic system) causing the tumour to spread to other parts of the body.
  • Benign: These tumours are not cancerous which grow slowly compared to malignant tumours. Benign tumours are often covered in fibrous tissue which stops cells invading surrounding tissue. The damage that these tumours can cause are blockages and can put pressure on organs. (Some benign tumours an also become malignant.

Although these tumours are different to one another, they look similar:

  • Shape: Tumours are irregular compared to normal cells
  • Nucleus: Their nuclei are dark and large compared to normal cells
  • Proteins: Not all the proteins are made to function properly
  • Antigens: These are different where the immune recognises these as non-self
  • Growth: They do not respond to growth regulating processes
  • Division: The divide more frequently by mitosis compared to normal cells

In order for any type of tumour to be developed a process should be followed. There are five ways in which a tumour can come about:

  • Tumour suppressor genes:
  • When functioning normally, tumour suppressor genes should slow cell division down by producing a protein that stops cells dividing or cause apoptosis (self-destruction of cells).
  • These genes are inactivated when there is a mutation in the DNA base sequence.
  • The mutation does not allow the protein to be produced. Therefore tumour suppressor genes will no longer be able to slow division down. The cells divide uncontrollably to produce a tumour.

NB: You do not need to know the name of the protein which slows cell division down for AQA.

  • Proto-oncogene:
  • When functioning normally, proto-oncogenes stimulate cell division by producing a protein which allows cells to divide.
  • These genes can become over-active when there is a mutation in the DNA base sequence.
  • The mutated proto-oncogenes, known as oncogenes, make more proteins than it should do therefore increasing the cell division to produce a tumour.

NB: You do not need to know the name of the protein which increases cell division for AQA.

  • Abnormal methylation of tumour suppressor genes:
  • When functioning normally, a methyl group is added (methylation) on to DNA which regulates gene expression, controlling whether a gene is transcribed or translated.
  • A mutation causes hypermethylation (too much methylation). Tumour suppressor genes are not transcribed therefore the protein needed to slow cell division is not made.
  • The cells continue to divide forming a tumour.
  • Abnormal methylation of proto-oncogenes:
  • When functioning normally, a methyl group is added (methylation) on to DNA which regulates gene expression, controlling whether a gene is transcribed of translated.
  • A mutation causes hypomethylation (too little methylation). Proto-oncogenes become mutated to be called oncogenes. There is an increase in production of proteins causing and increase in cell division creating a tumour.
  • Oestrogen:
  • The exact reasoning behind how oestrogen causes an increase in risk in getting some breast cancers is not fully understood however there are few theories.
  • Oestrogen can stimulate certain breast cells to divide and replicate. The more cell divisions there are the more likely a mutation may occur causing an increase in chance that the cells will be cancerous.
  • Oestrogens’ ability to stimulate cell division can assist with cancerous cells replicating even quicker causing a tumour.
  • Other research conducts that oestrogen can introduce new mutations to the DNA directly of certain breast cells which increase the chance of these cells becoming cancerous.

There two factors which increase the risk of cancer:

  • Genetic: If you inherit the allele that causes cancer you are more likely to get the cancer but it does not mean that you definitely will get the same type of cancer.
  • Environmental: These are carcinogens such as radiation and lifestyle choices such as smoking, alcohol consumption and a high-fatty diet.

There is no single cure for all cancers, but many can be prevented or treated successfully. Prevention includes minimising exposure to known carcinogens (cancer-causing agents in the environment like radiation with tar in cigarettes being a chemical carcinogen). Effective treatment also depends on early diagnosis of a cancer. Most cancers of the skin, colon, breast and cervix can be cured if there is an early diagnosis. Cancer research is a very active field and new knowledge is gained daily, enabling doctors to establish the causes of the diseases and to design effective treatments. However much remains unknown. Finding cures for cancers will continue to be one of our greatest challenges in the 21st century.

  • Prevention:
  • If a specific cancer-causing mutation is known, then it is possible to screen for the mutation (look for the mutation).

EXAMPLE: BRCA1 and BRCA2 are tumour suppressor genes. Mutation of these genes has been linked to hereditary breast and ovarian cancer.

  • Knowing about the increased risk means preventative steps can be taken in order to reduce it.

EXAMPLE: A woman can choose to have a mastectomy (the removal of or both the breasts) to reduce the risk of breast cancer developing. Other women may screen for the signs of breast cancer for an early diagnosis which increases the chance of recovery.

  • Knowing the specific mutations also means that more sensitive tests can be developed leading to an earlier more accurate diagnosis.

EXAMPLE: A mutation in the RAS proto-oncogene exists in around half of all bowel cancers. This cancer can be detected early looking for RAS mutations in the DNA of bowel cells.

  • Treatment and cure:
  • The treatment of cancer can vary for different mutations so knowing how specific mutations actually cause cancer can be useful in developing drugs to effectively target them.

EXAMPLE: A mutation of the HER2 proto-oncogene causes breast cancer and it can be treated by Herceptin. The drug binds specifically to the altered HER2 protein receptor and suppresses cell division and growth. If breast cancer was caused by a mutation of another gene, for instance BRCA1 or BRCA2, the drug will not work

  • Some cancer-causing mutations require more aggressive treatment compared to others, so understanding how these mutations causes cancer can give us the best treatment scheme.

EXAMPLE: If a mutation is known to cause an aggressive cancer (fast-growing), it may be treated by high doses or radiotherapy or by removing large areas of the tumour and surrounding tissue during surgery.

  • Gene therapy (faulty genes are replaced by ones that are working) may also be able to treat cancer caused by certain mutations.

EXAMPLE: If you know that the cancer was caused by an inactivated tumour suppressor gene it is hoped that gene therapy will work in the future to replace faulty alleles.

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Gene expression is controlled by a number of features – most of the cell’s DNA is not translated

 

The control of gene expression (AQA A2 Biology) PART 2 of 8 TOPICS

 

 

TOPICS: Alternation of the sequence of bases in DNA can alter the structure of proteins  Gene expression is controlled by a number of features – most of the cell’s DNA is not translated  Gene expression is controlled by a number of features – regulation of transcription and translation  Gene expression is controlled by a number of features – gene expression and cancer  Using genome projects  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – recombinant DNA technology  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – genetic fingerprinting

 

 

 

 

 

Gene expression is controlled by a number of features – most of the cell’s DNA is not translated:

Totipotent cells can divide into and produce any type of body cell. They are present in mammals in the first few stages of cell division.

During development, totipotent cells translate only part of their DNA resulting in a specialised cell:

  • Stem cells all contain the same genes but not all the genes are transcribed and translated. This happens under the right conditions.
  • mRNA made is shorter as only a few genes are transcribed.
  • The mRNA is then translated into proteins.
  • The proteins modify the cell as they determine the cell structure and processes that occur in the cell.
  • Changes made by the proteins cause the stem cell to be specialised. Once a cell is specialised it cannot be reversed into a stem cell.

After the first few stages of cell division, the cells become pluripotent. Pluripotent cells can specialise into any body cell apart from placental cells. Pluripotent cells are used in treating human disorders.

In mature mammals there are multipotent cells (are able to differentiate into a few types of body cells) and unipotent cells (are able to differentiate into one type of cell).

An example of unipotent cells is cardiomyocytes which are heart muscle cells. It was thought that these cells could not divide to replicate themselves. This can be a major problem as if the heart muscle gets damaged there would no replacement. Recent research has proven that cardiomyocytes can divide and replicate. Scientists are able to come up with an explanation that the cells are replaced by unipotent cells differentiating into cardiomyocytes. They also believe that this process could be occurring constantly but there are disagreements as to how quick this is happening. Some believe that it is a really slow process where a possibility of some cells never being replaced may occur. Others believe that it is a really quick process so every cardiomyocyte is replaced.

IPSC (Induced Pluripotent Stem Cells) are cells created in the lab by scientists. They follow a process called reprogramming so that specialised adult body cells become pluripotent cells. The adult cells are made to express a series of transcription factors that are normally associated with pluripotent cells. The transcription factors cause the adult body cells to express genes that are associated with pluripotency. One way in which these transcription factors could be introduced to adult body cells is by infecting them with a specially-modified virus. The virus has the genes coding for the transcription factors in its DNA. When the virus infects the adult body cell (by injecting its DNA into the cell or by entering into the cell by endocytosis), the adult cell will be able to produce transcription factors as the virus will pass on its DNA to the cell.

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Gene expression is controlled by a number of features – regulation of transcription and translation

The control of gene expression (AQA A2 Biology) PART 3 of 8 TOPICS

 

 

TOPICS: Alternation of the sequence of bases in DNA can alter the structure of proteins  Gene expression is controlled by a number of features – most of the cell’s DNA is not translated  Gene expression is controlled by a number of features – regulation of transcription and translation  Gene expression is controlled by a number of features – gene expression and cancer  Using genome projects  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – recombinant DNA technology  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – genetic fingerprinting

 

 

 

 

 

Gene expression is controlled by a number of features – regulation of transcription and translation:

In eukaryotes, transcription or target genes can be stimulated or inhibited when specific transcriptional factors move from the cytoplasm into the nucleus. As only target genes are transcribed, it means that specific proteins are made. Each type of body cell has different target cells so they give different characteristics i.e. a nerve cell is different to a red blood cell. Transcription factors can change the rate of transcription and the process is as follows:

  • The transcription factors move in by diffusion into the nucleus from the cytoplasm.
  • When in the nucleus they may bind to promoter sequence (the sequence which is the start of the target gene).
  • The transcription factors either increase or decrease the rate of transcription depending if they have bound onto the promoter sequence.

Some transcription factors are called activators where they increase the rate of transcription. This is done by the transcription factors helping the RNA polymerase to bind to the promoter sequence to activate transcription. Others are called repressors where they decrease the rate of transcription. This is done by the transcription factors binding to the promoter sequence preventing RNA polymerase from binding. This stops transcription.

Oestrogen can initiate the transcription of target genes. NB: Sometimes it can cause a transcription factor to be a repressor. You don’t need to know this for the AQA exam. A transcription factor may be bound to an inhibitor stopping it from binding to the promoter sequence. Oestrogen binds to the transcription factor making an oestrogen-oestrogen receptor complex and changes the site where the inhibitor is joined on (called DNA binding site). This means that the inhibitor is detached allowing the transcription factor to attach to the promoter sequence. NB: You don’t need to know the name of the inhibitor. Also the DNA binding site on the transcription factor stays changed whilst the oestrogen has bound to it.

In eukaryotes and some prokaryotes, translation of the mRNA produced from target genes can be inhibited by RNA interference known as RNAi. Short RNA molecules such as micro RNA, known as miRNA, and small interference RNA, known as siRNA, form an RNA Induced Silencing Complex, known as RISC, with proteins. NB: The small RNA molecules known to be double stranded in the revision guides or in textbooks; this is confusing so it is better to start the process as miRNA and siRNA being single stranded. RNA forms a complex with a protein which is an enzyme called RNA hydrolase. miRNA does not form a complex with RNA hydrolase but another protein. These RNA molecules can each make a RISC with more then one protein and the proteins involved do not need to be known for AQA. The complexes each attach to their target mRNA sequence and preventing translation in different ways. This is how it is done for each small RNA molecules:

  • siRNA/miRNA in plants:
  • The bases on the siRNA attach to the bases on the mRNA by complementary base pairing.
  • RNA hydrolase hydrolyses the mRNA strand into fragments preventing translation to occur as the whole polypeptide chain will not be made

NB: It is not necessary to know that the fragments are degraded in the processing body. If you want to learn this there is no harm.

  • miRNA in mammals:
  • The bases on the miRNA attach to the bases on the mRNA by complementary base pairing.
  • Ribosomes are prevented from attaching to the mRNA strand stopping translation from occurring.

NB: Again here, it is not necessary to know that mRNA is degraded or stored in the processing body.

Epigenetics involves heritable changes in gene function, without changes to the DNA base sequence. These changes are caused by changes in the environment (more exposure to pollution) that inhibit transcription by:

  • Increased methylation of DNA: A methyl group (known as an epigenetic mark) attaches to cytosine that has to be part of the nucleotide that is attached to guanine by a phosphodiester bond. NB: You may be confused right now but look at the diagram below of one strand of DNA and notice which of the cytosine nucleotides the methyl group joins on to. Notice that the nucleotide on the far right of the strand and the third one from the left does not have a methyl group as they are not next to a nucleotide with guanine as the base. The joining of the methyl group should not be confused by joining on to cytosine which is complementary to guanine on the other strand as this is wrong. Also the methyl group – CH3 – does not change the base sequence but the structure. As the structure has changed, it has become harder for enzymes to attach to the DNA stopping the expression of a gene. If the tumour suppressor gene is not transcribed it can cause cancer.

  • Decreased of associated histones: An acetyl group – COCH3 – is another epigenetic mark which attaches to histone proteins to make the chromatin (mixture of DNA wound around histone proteins) less condensed for easy genetic expression to occur. The problem originates when histone deacetylase breaks the bond between the histone protein and acetyl group. The DNA becomes highly condensed making hard for enzymes to carry out the gene expression. NB: Histone deacetylase can be abbreviated into HDAC but it is best that you stay with the full name.

Epigenetic changes to the DNA are fortunately reversible therefore they are good targets by drugs to stop the effects of epigenetic occurring. These drugs can either stop DNA methylation or can inhibit histone deacetylase allowing the acetyl groups to remain attached to the DNA.

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Alternation of the sequence of bases in DNA can alter the structure of proteins

The control of gene expression (AQA A2 Biology) PART 1 of 8 TOPICS

 

 

TOPICS: Alternation of the sequence of bases in DNA can alter the structure of proteins,  Gene expression is controlled by a number of features – most of the cell’s DNA is not translated,  Gene expression is controlled by a number of features – regulation of transcription and translation,  Gene expression is controlled by a number of features – gene expression and cancer,  Using genome projects,  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – recombinant DNA technology,  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions,  Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the new design of industrial and medical processes – genetic fingerprinting

 

 

 

 

 

Alternation of the sequence of bases in DNA can alter the structure of proteins:

Gene mutations might arise during DNA replication. The types of mutations include:

  • Addition: This is where a base is added in between two bases. So if a base sequence was ATCGGA, addition of a base may cause ATA This causes a frame shift to the right.
  • Deletion: This is where a base is deleted from the sequence. So if a base sequence was ATCGGA, deletion may cause ATGGA. This causes a frame shift to the left.
  • Substitution: This is where a base is swapped for another base. So if a base sequence was ATCGGA, substitution may cause ATCTGA.
  • Inversion: This is where the base sequence is inverted. So if a base sequence was ATCGGA, inversion may cause AGGCTA.
  • Duplication: This where one or more bases is repeated. So if a base sequence was ATCGGA, duplication may cause ATT This causes a frame shift to the right.
  • Translocation: This is where the base sequence is moved on to a different locus on the same chromosome or onto a whole new chromosome.

Gene mutations occur spontaneously but the rate of mutation is increased by mutagenic agents. These include x-rays and other ionising radiation, tar in cigarettes and radioactive isotopes of elements.

Mutations can result in a different amino acid sequence in the encoded polypeptide. The mutations that result in a different primary structure are addition and deletion. A change in the primary structure causes a change in the tertiary structure and could make a non-functional protein. Substitution is least likely to cause a mutation as the genetic code is degenerate i.e. more than one triplet can code for the same amino acid.

 

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