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Edexcel Categories Archives: Topic 2: Genes and Health

Genetic Counselling

GENETIC COUNSELLING 

  • Helps us to understand how disease is inherited and the chance so children being carriers or victims of the disease
  • Explain available tests
  • Explains possible courses of action to take place after test results/outcome
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Ethical Decisions

ETHICAL DECISIONS

1.RIGHTS + DUTIES

  • Human rights – right to life = fair trial
  • Guardians have duty to their ward
  • Rights occur from religion + social conventions

2.MAXIMISING THE AMOUNT OF GOOD IN THE WORLD

  • Utilitarianism believe that you should to the best thing for the majority
  • There are circumstances where something may be deemed wrong, but actually is the best outcome

3.MAKING OWN DECISIONS

  • Providing informed consent otherwise it is morally wrong to go ahead with the operation, trial etc.
  • Autonomy is desirable if benefits outweigh the costs
  • It is our right to act autonomously but it is also our duty to think of others

4.LEADING A VIRTUOUS LIFE

  • Consider different virtues and morals e.g. justice
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Genetic Testing #2

GENETIC TESTING   

What is genetic testing?

Genetic testing is the use of laboratory tests to determine the genetic status of individuals already suspected to be at high risk for a particular genetic disorder based on family history or a positive screening test.

What is genetic screening?

Genetic screening is tile use of simple diagnostic tests performed on a large number of individuals to identify those who are at a high risk of having or passing on a specific genetic disorder.

What is the difference between genetic testing and genetic screening?

 Genetic testing and screening are similar in that both involve the use of laboratory tests to reveal the presence of disease-causing genes. Although genetic screening and testing are essentially the same since they both involve the same medical procedures, the major difference between them can be explained in examining WHY an individual undergoes laboratory testing.

  • Genetic screening – If someone desires to be tested due to the possibility that he or she may have a disease gene because a large percentage of people in the same age group or ethnic group are at high risk for having the gene, this individual would need to undergo genetic screening. Screening is often called “population-based” screening since it is used to test those individuals in the population who are at a higher risk for having a disease-gene.
  • Genetic testing – If the individual suspects he or she may have a disease gene as a result of a family member having the gene, this person would need to undergo genetic testing. Genetic testing is used not to screen a population already at risk, but to “test” individuals for the presence of a specific gene.

 What is done in a gene test?

In gene tests, scientists scan a patient’s DNA sample for mutated sequences. A DNA sample can be obtained from any tissue, including blood. For some types of gene tests, researchers design short pieces of DNA called probes, whose sequences are complementary to the mutated sequences. These probes will seek their complement among the three billion base pairs of an individual’s genome. If the mutated sequence is present in the patient’s genome, the probe will bind to it and flag the mutation. Another type of DNA testing involves comparing the sequence of DNA bases in a patient’s gene to a normal version of the gene.

Who can benefit from genetic testing and screening?

  • Those who are concerned that they may have a genetic or chromosomal disorder because of a specific condition in their families
  • Couples who already have a child with a genetic disorder, unexplained mental retardation or a birth defect
  • Women who have had two or more miscarriages or whose baby dies in infancy or deliver a child after 35yrs
  • Couples who would like testing or information about genetic disorders that occur frequently in ethnic group
  • Pregnant women concerned about the effects of exposure to medication, chemicals or radiation
  • Couples who are first cousins or other blood relatives.

Key Points on Genetic Testing

  • Everyone may not need genetic testing, but those with a history of genetic disorders within the family should consider testing.
  • Genetic screening and testing has both positive and negative consequences.
  • It’s important to weigh the advantages and disadvantages when considering genetic screening or testing.
  • The value of genetic testing depends on several factors: the accuracy of the test, the reliability of the interpretation of the results, the ability to treat the condition and the quality of genetic counselling available.
  • Genetic tests can provide medical information that affects the entire family.
  • Testing has implications for future reproductive decisions, usually providing a probability of disease, rather than predicting future “diagnoses.”
  • Reliable genetic tests can predict the chance of having a disease gene with high accuracy. However, not everyone with a disease gene may go on to develop a disease or condition due to possible effects of other genes or environmental factors.
  • Informed consent, the communication of information between a patient and health care provider about a genetic test, can be an important component of genetic testing, yet there are no national standards to guide health care providers on how this information should be given.
  • Although physicians have not agreed on standards for the testing of children, some experts think that testing should be conducted only when there is clear benefit to the child. Others feel that testing should also be considered if it can benefit other family members.
  • The results of a genetic test can confirm or rule out a suspected genetic condition or help determine a person’s chance of developing or passing on a genetic disorder.

What screening tests are available?

  • Chronic villus sampling (CVS) – usually done at 10 to 12 weeks of pregnancy to obtain a sample of the placenta by passing a plastic tube through the vagina and into the uterus or by passing a needle through the abdomen and into the uterus. This allows doctors to diagnose many of the same conditions as amniocentesis, but earlier in the pregnancy.
  • Amniocentesis may be done at 13 to 18 weeks of pregnancy and is a widely-used procedure of obtaining amniotic fluid from the uterus by using a needle to pass through the abdomen.
  • Ultrasound – usually performed as early as possible in pregnancy, and is a non-invasive procedure that provides a visual image of the foetus.
  • Embryo biopsy can be used on an embryo conceived by in vitro fertilization to determine if the embryo is free of certain genetic disorders before it is implanted in the uterus. Increased risk for disease. Because genetic conditions often run in families, information about your genetic makeup might be useful to other family members. If family members are aware that a genetic condition runs in the family, it might prevent them from being misdiagnosed. This information might also be of use to them when they are planning children.

TYPES OF GENETIC TESTING

  • Newborn screening is used just after birth to identify genetic disorders that can be treated early in life. Newborn screening identifies biochemical or other inherited conditions in newborns that may result in mental retardation or other complications. Newborn screening is an effective measure in preventing mental retardation. However, only a handful of disorders are screened for since screening all newborns is expensive.
  • Diagnostic testing is used to identify or rule out a specific genetic or chromosomal condition. In many cases, genetic testing is used to confirm a diagnosis when a particular condition is suspected based on physical signs and symptoms. Diagnostic testing can be performed before birth or at any time during a person’s life, but is not available for all genes or all genetic conditions. The results of a diagnostic test can influence a person’s choices about health care and the management of the disorder.
  • Carrier testing is used to identify people who carry one copy of a gene mutation that, when present in two copies, causes a genetic disorder. This type of testing is offered to individuals who have a family history of a genetic disorder and to people in certain ethnic groups with an increased risk of specific genetic conditions. If both parents are tested, the test can provide information about a couple’s risk of having a child with a genetic condition. Carrier screening is often considered by couples who want to have children, but who are concerned that they may “carry” a gene for a certain disorder that has the potential to affect their children.
  • Prenatal testing is used to detect changes in a fetus’s genes or chromosomes before birth. This type of testing is offered during pregnancy if there is an increased risk that the baby will have a genetic or chromosomal disorder. In some cases, prenatal testing can lessen a couple’s uncertainty or help them make decisions about a pregnancy. It cannot identify all possible inherited disorders and birth defects, however. Prenatal screening is available to those individuals who have a higher risk for passing a disease gene on to the child. It is used to determine the genetic make-up of an unborn child.
  • Pre-implantation Genetic Diagnosis (PGD) is a specialized technique that can reduce the risk of having a child with a particular genetic or chromosomal disorder. It is used to detect genetic changes in embryos that were created using assisted reproductive techniques such as in-vitro fertilization. In-vitro fertilization involves removing egg cells from a woman’s ovaries and fertilizing them with sperm cells outside the body. To perform preimplantation testing, a small number of cells are taken from these embryos and tested for certain genetic changes. Only embryos without these changes are implanted in the uterus to initiate a pregnancy.
  • Predictive and presymptomatic types of testing are used to detect gene mutations associated with disorders that appear after birth, often later in life. These tests can be helpful to people who have a family member with a genetic disorder, but who have no features of the disorder themselves at the time of testing.
  • Predictive testing can identify mutations that increase a person’s risk of developing disorders with a genetic basis, such as certain types of cancer.
  • Presymptomatic testing is used for predicting adult-onset disorders. It can determine whether a person will develop a genetic disorder, before any signs or symptoms appear. It is also for estimating the risk of developing adult-onset genetic disorders (e.g. Huntington’s, Alzheimers)
  • Forensic testing uses DNA sequences to identify an individual for legal purposes. Unlike the tests described above, forensic testing is not used to detect gene mutations associated with disease. This type of testing can identify crime or catastrophe victims, rule out or implicate a crime suspect, or establish biological relationships between people (for example, paternity).

PROS & CONS of Genetic Testing

 

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Genetic Testing

GENETIC TESTING 

Ethical Issues: (Germ line therapy is illegal due to these reasons)

  • Might affect the development of a fetus in unexpected ways
  • Long-term side effects that are not yet known.
  • People who would be affected by germline gene therapy are not yet born, so they can’t choose whether to have the treatment

 

 

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Gene Therapy

GENE THERAPY

Gene therapy involves inserting copies of a normal allele into the chromosomes of an individual who carries a faulty allele. If a mutated gene causes a necessary protein to be faulty or missing, gene therapy may be able to introduce a normal copy of the gene to restore the function of the protein.

However, it is not always successful, and research is continuing.

It is illegal to do this to sex cells (Germ Line Therapy), because any (bad) changes would be inherited by the individual’s offspring. Instead, gene therapy is used on somatic (body) cells. This means that the individual could pass on their faulty allele to their children, even if they get better themselves L

A gene that is inserted directly into a cell usually does not function. Instead, a carrier called a vector is genetically engineered to deliver the gene. Certain viruses are often used as vectors because they can deliver the new gene by infecting the cell. The viruses are modified so they can’t cause disease when used in people. If the treatment is successful, the new gene delivered by the vector will make a functioning protein.

The basic process

  1. Doing research to find the gene involved in the genetic disorder.
  2. Cutting out the normal allele. Special enzymes are used to do this.
  3. Making many copies of the allele.
  4. Insert copies of the normal allele into the target cells of a person who has the genetic disorder, via liposome’s / plasmids or GM virus
  5. The normal gene is transcribed and translated
  6. A functioning protein is produced.

Researchers are testing several approaches to gene therapy, including:

  • Replacing a mutated gene that causes disease with a healthy copy of the gene = currently being tested
  • Inactivating, or “knocking out,” a mutated gene that is functioning improperly.
  • Introducing a new gene into the body to help fight a disease.

Problems in the process

  • the alleles may not go into every target cell
  • the alleles may join with the chromosomes in random places, so they do not work properly
  • treated cells may be replaced naturally by the patient’s own untreated cells

VIRAL VECTORS

PLASMIDS

Plasmid = extra loop of DNA that has been GM to contain the “correct” allele to produce the functioning protein

Liposome = spherical phospholipid bilayer that fuses with the cell membrane, whilst carrying in the GM plasmid

  1. The normal allele is cut out via restriction enzymes
  2. It is inserted into the plasmid via ligase – the plasmid has been genetically modified
  3. The (-ve)plasmid combines with the (+ve) liposome (the liposome on the outside) = Plasmid – liposome complex formed
  4. Liposome fuses with the cell membrane when carrying the plasmid into the cell
  5. The plasmid enters the nucleus, and the normal gene is inserted into the DNA and replaces the faulty gene
  6. The mRNA transcribes the correct sequence
  7. This sequence is translated by ribosome’s in the cytoplasm to create a functioning protein that can open the CFTR channel

PROS & CONS of Gene Therapy

The Pros:

  • There is only one way of curing the disease – replacing the defective gene with a healthy copy – and therefore gene therapy is the only hope of finding cures for such disorders
  • If gene therapy targets the reproductive cells of carriers of such genetic disorders as cystic fibrosis, Parkinson’s disease, or cancer, it is possible that any children the carrier goes on to have would be free of the defective gene and on a bigger scale the disease can be wiped out completely
  • Gene therapy, when successful, can have a number of advantages over drug therapy such as providing a cure rather than easing the symptoms.
  • Comprehensive federal laws, regulations, and guidelines help protect people who participate in research studies (called clinical trials).

The Cons:

  • The current lack of knowledge and understanding of the treatment means that its safety is unknown. The current scientific understanding is based on theory rather than solid fact. This, however, can be improved with further research and practice.
  • In clinical trials already carried out the effects of the treatment have only been short-lived. To achieve long term results much more research is needed.
  • Drug therapy, although not offering the possibility of a cure, is a tried and tested method and is therefore deemed safer
  • With current knowledge there is no guarantee that the vector carrying the healthy gene will end up in the specific place it is intended – there is a risk of causing even more damage to the genetic make-up that can result in severe consequences for the patient
  • Very serious side effects e.g. inflammatory response, headaches, fatigue, fever, increased heart rate

Ethical, religious and moral issues:

The intrusive nature of gene therapy means that we can discover information about our genetic make-up than we are meant to know. The knowledge could have a negative impact on their lives and if that knowledge was to influence any life decisions in a negative way then it is questionable whether genetic screening is morally correct.

Genetic screening can also be carried out on unborn babies – if this screening showed that a child was carrying a disease this may lead to the parents deciding to abort the child. This is clearly a very morally questionable act as many would argue that a person does not have the right to play God with another person’s life.

Similarly, a couple who are aware of their genetic make-up and know that they’re both carriers of a specific genetic disorder may decide against having children to avoid passing on the defective gene. Again, many would argue that this goes against the natural order.

Gene therapy has the potential to be misused – for instance the concept of “designer babies”, where specific genes are selected in order to create the perfect child.

So, the cons of gene therapy in terms of quantity very clearly outweigh the pros.

 

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Inheritance

INHERITANCE

Predicting genotypes 

Sex cells only contain one allele each, so you need to find out your chances of the baby having CF through punnett squares. You can trace family history through pedigree diagrams.

Diseases

Monohybrid inheritance – characteristics are controlled by one gene

However, many genes usually control a specific characteristic

Caused by recessive allele – thalassaemia, albinism, phenylketonuria, sickly cell anaemia

Caused by dominant allele – Huntington’s, achondroplasia

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Mutations

MUTATIONS 

There are many environmental factors (e.g. UV radiation) that could cause gene mutations but it can also be inherited.

When the DNA is being built an incorrect base may slip into place which could cause a mutation in the gene. If this mutation occurs in the reproductive organs, the DNA will be replicated in the sperm / egg cell and so the fertilized zygote will have the mutated gene in every cell.

Any mutations that occur in sections of DNA that don’t play a role in protein synthesis have no affect on the organism.

Sickle Cell Anaemia 

  1. Mutation occurs in gene coding for haemoglobin; the A replaces the T as a base
  2. When mRNA is produced, it forms a complimentary base U (instead of A)
  3. The codon now codes for the amino acid Valine which is non – polar and therefore less soluble in water
  4. The haemoglobin is now less soluble
  5. When oxygen levels are low the oxygen molecules form long sticky fibres inside the RBC as there is no soluble amino acids to prevent them from doing this
  6. The shape of the RBC becomes distorted into
  7. This sickle shape means that the haemoglobin is able to carry around less oxygen as well as increase the risk of atherosclerosis and thrombosis

Cystic Fibrosis  

Chromosome 7 carries the code to make the CFTR protein.

Why is it caused?

ATP energy is unable to bind to the ATP binding site thus the chloride ion channel cannot open and let the chloride through. The concentration gradient alters the electric gradient, which in term affects the net flow of water via osmosis. The water is drawn out from the mucus to maintain equilibrium, and so the mucus is more dry and sticky. It is able to trap more bacteria and dust hence cystic fibrosis is the result.

Most common way it’s caused (DF508):

  • 3 nucleotides are deleted so there is a loss on one amino acid
  • The CFTR protein shape is altered and it cannot fold into the correct shape
  • The protein reduces the movement of Cl ions through channels as the Cl cannot fit into the shapes properly
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DNA Replication

DNA REPLICATION

The Process:

  1. DNA strands run anti-parallel to one another
  2. Enzyme helicase unzips the double helix and the hydrogen bonds between bases break
  3. Free nucleotides line up with their complimentary bases and hydrogen bonds form
  4. DNA polymerase allows the free nucleotides to attach to their complimentary bases
  5. A complimentary strand has been formed for either template strand
  6. Any fragments in the double helix are joined by ligase
  7. There are now two identical DNA molecules formed – each with a daughter and parent strand

Meselson and Stahl Experiment – Density Centrifugation Gradient

 

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Protein Synthesis

PROTEIN SYNTHESIS

The genetic information stored in DNA is transcribed into mRNA and then transformed into protein.

Transcription

  1. Sigma factors enable RNA polymerase to bind to promoter DNA.
  2. TRNA polymerase allows the DNA to untwist then unzips, the hydrogen bonds between complementary DNA nucleotides break
  3. Free RNA nucleotides form complementary base pairs with one DNA strand (Template/ sense strand)

The unused strand is named the Antisense / coding strand – b/c it has the same coding as the mRNA

  1. Weak hydrogen bonds form between base pairs
  2. Sugar phosphate bonds form between RNA nucleotides
  3. mRNA strand is synthesized
  4. Hydrogen bonds of the untwisted RNA + DNA helix break, freeing the newly synthesized RNA strand.
  5. mRNA peels off the DNA and is small enough to move out of the nuclear pores into the cytoplasm

Translation

Translation takes place on the ribosome’s in the cytoplasm – the ribosome’s are the sites of protein synthesis

  1. The mRNA strand attaches to a ribosome and passes along it in short spurts of 3 nucleotides at a time so that it can be “read”.
  2. Each mRNA codon codes for a specific amino acid
  3. mRNA is decoded by a ribosome complex and signals are sent to induce the binding of tRNA to mRNA
  4. tRNA molecules transport specific amino acids to the ribosome that are chained together into a polypeptide
  5. The anti-codons in tRNA and codons in mRNA match up and form complementary base pairs
  6. Peptide bonds form between the adjacent amino acids to form the polypeptide (protein)
  7. A stop codon is finally met so the ribosomal complex falls apart and the protein is released into the cell
  8. tRNA is reused and collects another specific amino acid e.g. broken down into free nucleotides to be reused.

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