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DNA Hybridisation

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DNA Hybridisation

There are about 3 billion nucleotides in human DNA, making it about 1.8 meters (5 feet) long! Finding a genetic mutation or a specific allele would be like looking for a needle in a haystack. However, thanks to scientists, there are easy ways for detecting specific mutations in one’s DNA. This article explores DNA and the methodology and applications of DNA hybridisation.

DNA probes

DNA probes are short pieces of single-stranded DNA labelled to be easily identified. The probes can be designed to be complementary to a specific allele or mutation so that they can specifically bind to those sequences.

Two types of DNA probes are commonly used. They are:

  1. Radioactively labelled probes: These probes contain radioactive nucleotides with a radioactive isotope of phosphorous (32P). This property gives the probe radioactive features and allows it to be easily identifiable when it is exposed to an X-ray film.
  2. Fluorescently labelled probes: These have fluorescent properties that allow them to emit light and glow when placed in specific conditions.

DNA hybridisation A level Biology

DNA hybridisation describes the combination of a section of DNA with a single-stranded piece of DNA that contains complementary sequences.

DNA RNA hybridisation describes the combination of a section of RNA with a single-stranded piece of DNA that contains complementary sequences.

In the context of this article, the single-stranded piece of DNA is the DNA probe, and the section of DNA or RNA is a particular allele.

A variant of a gene is called an allele.

For the probe to bind to the complementary sequence of the specific allele, the double-stranded DNA needs to be separated. Heating the DNA sample denatures the DNA molecules and separates the two strands by breaking the hydrogen bonds between them.

Then the separated DNA strands would be mixed with the probes, and the temperature would be lowered, allowing for hydrogen bonds to be formed (anneal) between complementary DNA strands. Although most of the DNA strands would pair up with their original complementary strand, some strands would bind to the probes. This binding process is referred to as DNA hybridisation.

DNA hybridisation Technique

DNA probes and DNA hybridisation can be used to locate a specific allele. This process involves many steps:

  1. The sequence of the nucleotide base of the gene of interest needs to be determined. Sequence the DNA or refer to genetic libraries to obtain the sequence of the mutation of interest.
  2. Then a fragment of DNA with complementary sequences to the particular allele we are interested in is produced.
  3. Multiple copies of the DNA fragment are produced using polymerase chain reaction (PCR).
  4. The DNA fragments produced must be labelled by adding radionucleotides (nucleotides containing 32P) or fluorescent nucleotide triphosphates to the PCR mixture to create radioactive or fluorescent DNA probes, respectively.
  5. We need to obtain the DNA that we want to analyse by extracting the DNA from the cells obtained from the individual of interest. The extract is then heated to denature and separate the two DNA strands from each other.
  6. The DNA probes that we designed are then added to the extract, and the temperature is lowered, allowing for the complementary sequences to pair.
  7. If the extracted DNA contains the mutation, our DNA probe would likely bind to it since it has complementary base sequences.
  8. Any unattached DNA probe is removed from the mixture.
  9. The hybridised DNA now will be either radioactive or fluorescent depending on what type of DNA probe we used.
  10. We can use an X-ray film or use fluorescent microscopy to detect the presence of the hybridised DNA and confirm whether the suspected person had the mutation or not.

Application of DNA hybridisation in genetic screening

Thanks to the work of countless scientists around the world, we now know the location and type of mutations that cause many hereditary diseases. DNA hybridisation and DNA probes can be used to screen individuals for specific hereditary conditions.

Multiple genetic mutations can be simultaneously investigated by fixing multiple DNA probes in an array on a glass slide and then adding the donor DNA to the array. Any complementary DNA sequences, if present, would then bind to their corresponding probe.

Cancer screening

Genetic screening can detect oncogenes (genes that increase the likelihood of developing cancer). There are two types of oncogenes, tumour suppressor genes and proto-oncogenes.

Tumour suppressor genes code for proteins that act as breaks in the cell cycle and stop potentially cancerous cells from proliferating.

Proto-oncogenes code for proteins that accelerate the cell cycle and promote cell proliferation.

Therefore, any mutation that inactivates tumour suppressor genes or increases the activity of proto-oncogenes can result in uncontrolled cell division and lead to the development of cancer.

The presence of mutations in these proto-oncogenes can be detected by genetic screening. This would allow the individual with the mutation to make an informed decision about their lifestyle. For instance, they would be able to lower the risk of developing cancer by eating healthier and avoiding mutagenic substances such as cigarette smoke.

Mutagens are chemicals or anything that can create a mutation in the DNA.

People can also undergo regular check-ups to detect cancer early and improve the chances of successful treatment.

Personalised medicine

Another advantage of genetic screening is personalised medicine. It allows doctors to treat patients based on their genotype. For instance, drug A may be more effective in treating a condition in one patient but not another patient with a different genotype. Another drug can then be used for the other patient that provides maximum efficacy in their treatment.

Knowing the patients’ genotypes allows doctors and pharmacists to prescribe the exact dose of a specific medicine to produce the desired outcome on the patients’ treatment plan. This would save money and prevent any unnecessary harm and side effects that some medications may have.

The liver contains enzymes that metabolise and remove drugs such as painkillers. Different individuals express varying levels of these enzymes depending on their genotype. Therefore, some individuals with higher levels of liver enzymes may need more painkillers than others. Knowing one’s genotype allows for a safer and more effective prescription of painkillers.

Vitamin E is known to lower the risk of cardiovascular disease in diabetic individuals with a certain genotype. However, vitamin E increases the risk of cardiovascular diseases in diabetic individuals with a different genotype. Therefore, knowing one’s genotype can allow for a better informed and safer prescription of vitamin E supplements.

Application of DNA hybridisation in genetic counselling

Genetic counselling is a special form of social work. It provides advice and information to people, helping them make personal decisions about themselves and their children. Based on family history of hereditary conditions and genetic screening, genetic counselling can inform couples about their children’s chances of having a disease.

Here is some information to refresh your memory; alleles are usually either dominant or recessive. Suppose an individual is heterozygous for the dominant allele (i.e. has the dominant allele on one chromosome and the recessive allele on the other chromosome). In that case, they will show the dominant phenotype despite carrying the recessive allele. Only if an individual is homozygous for the recessive allele (i.e. has the recessive allele on both chromosomes) will they have the recessive phenotype.

Remember sickle cells anaemia? It is caused by a mutation in the gene coding for the beta-globin polypeptide, a subunit of haemoglobin. The mutated sickle cell allele is recessive to the wild type globin beta allele. Therefore, a heterozygous individual will not have symptoms of sickle cell anaemia.

However, if one heterozygous individual had offspring with another heterozygous individual, there would be a 25% chance that their child would be homozygous for the sickle cell allele and hence would have sickle cell anaemia. Genetic screening allows the identification of a mutated recessive allele in heterozygous individuals who have the dominant and healthy phenotype.

Counsellors would inform couples about their children’s chances of having a disease and provide further information about its emotional, psychological, medical, social and economic consequences. Based on the counsellor’s advice, couples would then choose whether or not to have children.

Ethical concerns of DNA hybridisation

Despite the many advantages of genetic screening, there are some major ethical considerations around genetic counselling. Genetic counsellors face a common ethical dilemma involving genetic screening on pregnant patients. We mentioned earlier that genetic screening could determine whether one or both parents are carriers of a medical problem. Other tests can determine whether the fetus has genetic mutations that will result in birth deformities, mental disability, or physical impediments. Counsellors may find it ethical challenging if they know that their pregnant patients may decide to terminate the pregnancy over carrying a fetus to term.

Inappropriate testing is another ethical challenge that surfaced when genetic testing became increasingly prevalent. This refers to couples who seek genetic counselling and testing in the hopes of having a baby with specific characteristics that they desire. Couples can undergo testing and abort pregnancies that do not result in ideal children, or even pregnancies that result in a child of the opposite sex to the one they desired.

Hybridisation DNA - Key takeaways

  • DNA probes are short pieces of single-stranded DNA that are labelled to be easily identifiable. The probes can be designed to be complementary to a specific allele or mutation so that they can specifically bind to those sequences.
  • The DNA probes are either labelled with radioactive phosphorous or are labelled with a fluorescent dye.
  • DNA hybridisation and DNA probes can screen individuals for specific hereditary conditions or the presence of mutant proto-oncogenes or tumour suppressor genes.
  • Knowing patients’ genotypes allows doctors to treat them according to their genotypes. For instance, drug A may be more effective in treating a condition in one patient but not another patient with a different genotype. Another drug can then be used for the other patient that provides maximum efficacy in their treatment.
  • Genetic counselling informs couples about their children’s chances of having a disease and provides further information about its emotional, psychological, medical, social and economic consequences. Based on the counsellor’s advice, couples would then choose whether or not to have children.

Frequently Asked Questions about DNA Hybridisation

DNA probes are short pieces of single stranded DNA that are labelled so that they are easily identifiable.

The probes can be designed to be complementary to a specific allele or mutation so that they would be able to specifically bind to those sequences

DNA hybridisation describes the combination of a section of DNA or RNA with a single stranded piece of DNA probe that contains complementary sequences and is labelled to ease its detection.

DNA hybridisation describes the combination of a section of DNA or RNA with a single stranded piece of DNA that contains complementary sequences

In order for the probe to be able to bind to the complementary sequence of the specific allele, the double stranded DNA needs to be separated. This is done by heating the DNA sample which denatures the DNA molecules and separates the two strands by breaking the hydrogen bonds between them.

Then the separated DNA strands would be mixed with the probes and the temperature would be lowered allowing for hydrogen bonds to be formed between complementary DNA strands (anneal). Although the majority of the DNA strands would pair up with their original complementary strand, some strands would bind to the probes. This binding process is referred to as DNA hybridisation.

Final DNA Hybridisation Quiz

Question

What is a DNA probe?

Show answer

Answer

DNA probes are short pieces of single stranded DNA that are labelled so that they are easily identifiable.

Show question

Question

What is DNA hybridisation?

Show answer

Answer

DNA hybridisation describes the combination of a section of DNA or RNA with a single stranded piece of DNA probe that contains complementary sequences and is labelled to ease its detection.

Show question

Question

Name two different types of DNA probe labelling?


Show answer

Answer

Radiolabelling with radioactive phosphorous or labelling with a fluorescent dye

Show question

Question

Why are donor DNA molecules heated before DNA hybridisation?


Show answer

Answer

To break the hydrogen bonds between strands and separate the DNA strands from each other.

Show question

Question

How is the sequenc of DNA probes designed?


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Answer

By either sequencing a piece of DNA that contains the mutation or referring to genetic libraries.

Show question

Question

How are DNA fragments amplified?


Show answer

Answer

By using PCR

Show question

Question

How are DNA probes labelled during their synthesis?


Show answer

Answer

By adding radionucleotides (nucleotides that contain 32P) or fluorescent nucleotide triphosphates to the PCR mixture to create radioactive or fluorescent DNA probes respectively.

Show question

Question

How are multiple genetic mutations investigated simultaneously?


Show answer

Answer

Multiple genetic mutations can be simultaneously investigated by fixing multiple DNA probes in an array on a glass slide and then adding the donor DNA to the array. Any complementary DNA sequences, if present, would then bind to their corresponding probe.

Show question

Question

What are tumour suppressor genes?


Show answer

Answer

Tumour suppressor genes code for proteins that act as breaks in the cell cycle and stop potentially cancerous cells from proliferating

Show question

Question

What are proto-oncogenes?


Show answer

Answer

Proto-oncogenes code for proteins that act as accelerators of the cell cycle and promote cell proliferation

Show question

Question

How can an individual with detected mutated proto-oncogenes make better decisions about their lifestyle?


Show answer

Answer

Eat healthier and avoiding mutagenic chemicals such as cigarette smoking

Show question

Question

How can knowing an individual’s genotype help in safer and more effective prescription of painkillers?


Show answer

Answer

The liver contains enzymes that metabolises and removes drugs such as painkillers. Different individuals express varying levels of these enzymes depending on their. Therefore, some individuals with higher levels of liver enzymes may need more painkillers than other. Knowing ones genotype allows for safer and more effective prescription of painkillers

Show question

Question

How can knowing an individual’s genotype help in safer and more effective prescription of vitamin E supplements?


Show answer

Answer

Vitamin E is known to lower the risk of cardiovascular disease in diabetic individuals with a certain genotype. However, vitamin E is known to increase the risk of cardiovascular diseases in diabetic individuals with a different genotype. Therefore, knowing ones genotype can allow for a better informed and safer prescription of vitamin E supplements.

Show question

Question

What is genetic counselling?


Show answer

Answer

Genetic counselling can be thought of a special form of social work. It provides advice and information to people helping them to make personal decisions about themselves and their children. Based on family history of hereditary conditions and genetic screening, genetic counselling can inform couples about the chances of their children having a disease.

Show question

Question

A man and a woman are both heterozygous for the sickle-cell allele. What are the chances of their offspring having sickle cell anaemia?


Show answer

Answer

There would be 25% chance that their child would be homozygous for the sickle cell allele and hence would have sickle cell anaemia

Show question

Question

What radioactive element is found in radioactive nucleotides?

Show answer

Answer

Phosphorous (32P)

Show question

Question

True or False: In DNA hybridization, a section of DNA is combined with a double-stranded piece of complementary DNA.

Show answer

Answer

False

Show question

Question

True or False: DNA fragments can only be hybridized with DNA.

Show answer

Answer

False

Show question

Question

A variant of a gene is called an _____.

Show answer

Answer

allele

Show question

Question

During DNA hybridization, the DNA molecule is denatured by ________ the DNA sample.

Show answer

Answer

heating

Show question

Question

What are the ways for identifying the sequence of a desired gene or piece of DNA?

Show answer

Answer

Going to genetic libraries

Show question

Question

How are DNA fragments amplified?

Show answer

Answer

Polymerase chain reaction

Show question

Question

True or False: Radioactive nucleotides are added to the DNA fragments after PCR.

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Answer

False

Show question

Question

What is used for detecting DNA that has been hybridized with radioactive probes?

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Answer

X-ray film

Show question

Question

What is used for detecting DNA that has been hybridized with fluorescent probes?

Show answer

Answer

X-ray film

Show question

Question

What genes encode proteins that stop cancerous cells from proliferating?

Show answer

Answer

Tumour suppressor genes

Show question

Question

What genes encode proteins that accelerate the cell cycle?

Show answer

Answer

Proto-oncogenes

Show question

Question

A mutation that __________ the activity of proto-oncogenes, increases the risks of developing cancer. 

Show answer

Answer

increases

Show question

Question

A mutation that __________ the activity of tumour suppressor genes, increases the risks of developing cancer. 


Show answer

Answer

decreases

Show question

Question

__________ are chemicals or anything that can create a mutation in the DNA.


Show answer

Answer

Mutagens

Show question

Question

True or False: Medications work the same for all individuals. 

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Answer

False

Show question

Question

Following a genetic test, it's been found that Amir expresses more than average liver enzymes that break down drugs. Should his medication dosage be increased or decreased?

Show answer

Answer

increased

Show question

Question

Vitamin E lowers the risk of cardiovascular disease in all diabetic individuals. 

Show answer

Answer

False

Show question

Question

Inheritance pattern of sickle cell anaemia is _________.

Show answer

Answer

Autosomal recessive

Show question

Question

What gene is mutated in sickle cell anaemia?

Show answer

Answer

Beta globin gene

Show question

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