Genetics & Evolution
Mutations & Biotechnology
5th Year · 6th Year (Leaving Cert)
- ✓By the end of this lesson students will be able to define mutation and classify different types, including gene and chromosome mutations.
- ✓By the end of this lesson students will be able to explain the principles and applications of genetic engineering, including the production of Genetically Modified Organisms (GMOs).
- ✓By the end of this lesson students will be able to describe the process, components, and applications of Polymerase Chain Reaction (PCR) (HL).
- ✓By the end of this lesson students will be able to explain the principles, procedure, and applications of gel electrophoresis (HL).
- ✓By the end of this lesson students will be able to discuss the ethical implications associated with genetic engineering and biotechnology.
Key concepts
A mutation is a sudden, random, and heritable change in the genetic material (DNA or RNA) of an organism. Mutations can occur spontaneously during DNA replication or be induced by external factors called mutagens. They are the ultimate source of genetic variation.
Mutations are broadly classified into gene mutations and chromosome mutations. **Gene (Point) Mutations**: Involve changes to one or a few nucleotide bases within a single gene. * **Substitution**: One nucleotide base is replaced by another. This can lead to: * **Silent mutation**: No change in the amino acid sequence due to the degeneracy of the genetic code. * **Missense mutation**: A change in one amino acid in the protein sequence. * **Nonsense mutation**: A change that results in a premature stop codon, leading to a truncated protein. * **Insertion**: One or more nucleotide bases are added into the DNA sequence. This causes a **frameshift mutation**. * **Deletion**: One or more nucleotide bases are removed from the DNA sequence. This also causes a **frameshift mutation**. **Chromosome Mutations**: Involve changes in the number or structure of chromosomes. * **Changes in Chromosome Number**: * **Aneuploidy**: An abnormal number of chromosomes (e.g., Trisomy 21 in Down syndrome, where there is an extra copy of chromosome 21). * **Polyploidy**: The presence of more than two complete sets of chromosomes (common in plants). * **Changes in Chromosome Structure**: * **Deletion**: A segment of a chromosome is lost. * **Duplication**: A segment of a chromosome is repeated. * **Inversion**: A segment of a chromosome is reversed end-to-end. * **Translocation**: A segment of one chromosome moves to a non-homologous chromosome.
Genetic engineering is the artificial manipulation or alteration of genes in an organism. It involves the transfer of genes from one organism to another, often across species, to introduce new traits or modify existing ones. **Key Steps in Genetic Engineering**: 1. **Isolation**: The desired gene (from a donor organism) and a plasmid (a small, circular DNA molecule from a bacterium, acting as a vector) are isolated. 2. **Cutting**: The desired gene and the plasmid are cut using the *same restriction enzyme* (also called restriction endonuclease). Restriction enzymes recognise specific DNA sequences and cut the DNA, often creating 'sticky ends' (short, single-stranded overhangs). 3. **Ligation/Insertion**: The isolated gene is mixed with the cut plasmid. The complementary 'sticky ends' base-pair, and the enzyme **DNA ligase** forms phosphodiester bonds, joining the gene into the plasmid to create a **recombinant plasmid**. 4. **Transformation**: The recombinant plasmid is introduced into a host cell (usually a bacterium or yeast). The host cells are treated to make their cell membranes permeable, allowing the plasmid to enter. 5. **Expression/Cloning**: The transformed host cells are cultured. As they multiply, they replicate the recombinant plasmid, making many copies of the gene (cloning). The host cells then express the inserted gene, producing the desired protein product. Selection markers (e.g., antibiotic resistance genes) on the plasmid help identify successfully transformed cells.
Genetically Modified Organisms (GMOs) are organisms whose genetic material has been altered using genetic engineering techniques. This typically involves inserting a gene from another species to confer a new trait. **Examples**: Bt corn (contains a gene from *Bacillus thuringiensis* that produces an insecticide), Golden Rice (engineered to produce beta-carotene, a precursor to Vitamin A). **Advantages**: Increased crop yield, enhanced nutritional value, pest and herbicide resistance, drought tolerance, production of pharmaceuticals. **Disadvantages/Concerns**: Potential for 'superweeds' or 'superpests', impact on biodiversity, unknown long-term health effects, ethical concerns regarding tampering with nature, potential for allergic reactions.
PCR is a laboratory technique used to amplify (make millions of copies of) a specific segment of DNA from a very small initial sample. It is an essential tool in forensics, medical diagnostics, and research. **Components**: DNA template, two primers (short, single-stranded DNA sequences complementary to the ends of the target DNA), Taq polymerase (a heat-stable DNA polymerase), deoxyribonucleotides (dNTPs - the building blocks of DNA), and a buffer solution. **Steps (Cycles)**: 1. **Denaturation (approx. 90-95°C)**: The DNA sample is heated to separate the double-stranded DNA into two single strands by breaking hydrogen bonds. 2. **Annealing (approx. 50-65°C)**: The temperature is lowered, allowing the primers to bind (anneal) to their complementary sequences on the single-stranded DNA templates. 3. **Extension (approx. 70-75°C)**: The temperature is raised slightly, and Taq polymerase synthesises new complementary DNA strands by adding dNTPs, starting from the primers and extending along the template strands.
Gel electrophoresis is a technique used to separate DNA fragments (or proteins) based on their size and electrical charge. It is commonly used after PCR or restriction enzyme digestion for DNA profiling and analysis. **Principle**: DNA molecules are negatively charged due to their phosphate groups. When placed in an electric field within a gel matrix (e.g., agarose gel), DNA fragments migrate towards the positive electrode. Smaller fragments move more easily and therefore travel faster and further through the gel pores than larger fragments. **Procedure**: DNA samples are loaded into wells at one end of an agarose gel. An electric current is applied. DNA fragments separate based on size. A 'DNA ladder' (fragments of known sizes) is run alongside the samples for comparison. After separation, the DNA is stained (e.g., with ethidium bromide) and visualised under UV light, appearing as bands. **Applications**: DNA profiling (forensics, paternity testing), genetic disease diagnosis, gene mapping, analysis of PCR products.
Key facts to remember
- 1A mutation is a sudden, heritable change in the genetic material of an organism.
- 2Gene mutations include substitution, insertion, and deletion, which can lead to frameshifts.
- 3Chromosome mutations involve changes in chromosome number (e.g., aneuploidy) or structure (e.g., deletion, duplication, inversion, translocation).
- 4Genetic engineering involves the isolation, cutting, ligation, transformation, and expression of genes.
- 5Restriction enzymes cut DNA at specific recognition sites, often creating 'sticky ends'. DNA ligase joins DNA fragments.
- 6PCR (Polymerase Chain Reaction) amplifies specific DNA sequences through repeated cycles of denaturation, annealing, and extension.
- 7Gel electrophoresis separates DNA fragments based on their size and charge; smaller fragments travel further through the gel.
- 8GMOs (Genetically Modified Organisms) have had their genetic material altered using genetic engineering techniques to introduce new traits.
Worked examples
Example 1
A segment of a DNA coding strand has the sequence 5'-ATG GGC CTA-3'. A mutation occurs, changing the sequence to 5'-ATG GTC CTA-3'. (a) Identify the type of gene mutation that has occurred. (b) Using the mRNA codon table (assume standard genetic code), determine the amino acid sequence coded by the original DNA segment and the mutated DNA segment. (c) State the effect of this mutation on the protein produced.
Answer
(a) Original DNA coding strand: 5'-ATG GGC CTA-3' Mutated DNA coding strand: 5'-ATG GTC CTA-3' The base 'G' at the second position of the second codon (GGC) has been replaced by 'T' (GTC). This is a **substitution** mutation. (b) Original DNA coding strand: ATG GGC CTA Original mRNA codons: AUG GGC CUA Original amino acid sequence: Methionine (Met) - Glycine (Gly) - Leucine (Leu) Mutated DNA coding strand: ATG GTC CTA Mutated mRNA codons: AUG GUC CUA Mutated amino acid sequence: Methionine (Met) - Valine (Val) - Leucine (Leu) (c) The mutation has resulted in a change of one amino acid (Glycine to Valine) in the protein sequence. This is a **missense mutation**.
Remember that the coding strand has the same sequence as the mRNA (with T instead of U). If given the template strand, you would first transcribe it to mRNA (complementary and T->U).
Example 2
Describe, in sequence, the main steps involved in genetically engineering a bacterium to produce a human protein, such as insulin.
Answer
The main steps involved in genetically engineering a bacterium to produce a human protein like insulin are: 1. **Isolation**: The desired human gene (e.g., for insulin production) is isolated from human cells. Simultaneously, a plasmid, which is a small, circular piece of DNA, is isolated from a bacterium. Plasmids serve as vectors to carry the foreign gene. 2. **Cutting**: The isolated human gene and the bacterial plasmid are cut using the *same restriction enzyme* (also known as a restriction endonuclease). This enzyme recognises a specific DNA sequence and cuts the DNA, often creating 'sticky ends' (short, single-stranded overhangs) that are complementary. 3. **Ligation/Insertion**: The isolated human gene is mixed with the cut plasmid. The complementary 'sticky ends' of the human gene and the plasmid base-pair. The enzyme **DNA ligase** then forms phosphodiester bonds, permanently joining the human gene into the plasmid to create a **recombinant plasmid**. 4. **Transformation**: The recombinant plasmid is introduced into a host bacterium (e.g., *E. coli*). The bacteria are typically treated (e.g., with calcium chloride and heat shock) to make their cell membranes permeable, allowing the recombinant plasmid to enter. 5. **Expression/Cloning and Selection**: The transformed bacteria are grown in a suitable culture medium. As the bacteria multiply, they replicate the recombinant plasmid, thereby making many copies of the human gene (cloning). The bacteria then express the inserted human gene, producing the desired human protein (e.g., insulin), which can then be harvested. Selection markers (e.g., antibiotic resistance genes) on the plasmid help identify and select bacteria that have successfully taken up the recombinant plasmid.
A clear diagram illustrating these steps would also be beneficial in an exam.
Example 3
A forensic scientist receives a tiny blood sample from a crime scene. To generate enough DNA for analysis, and then identify a suspect, they use PCR followed by gel electrophoresis. (a) Explain the purpose of PCR in this scenario. (b) Briefly describe the three main temperature-dependent steps of a single PCR cycle. (c) After PCR, the amplified DNA from the crime scene and three suspects (X, Y, Z) is run on an agarose gel. The results show that the crime scene DNA has bands at 500 bp, 300 bp, and 100 bp. Suspect X has bands at 500 bp and 100 bp. Suspect Y has bands at 500 bp, 300 bp, and 100 bp. Suspect Z has bands at 300 bp and 100 bp. Which suspect's DNA matches the crime scene sample? Justify your answer.
Answer
(a) The purpose of PCR (Polymerase Chain Reaction) in this scenario is to **amplify** (make millions of copies of) the tiny amount of DNA from the crime scene blood sample. This is crucial because the initial sample is often too small to be analysed directly by techniques like gel electrophoresis, which require a sufficient quantity of DNA. (b) The three main temperature-dependent steps of a single PCR cycle are: 1. **Denaturation (approx. 90-95°C)**: The DNA sample is heated to a high temperature to break the hydrogen bonds between the complementary base pairs, thereby separating the double-stranded DNA into two single strands. 2. **Annealing (approx. 50-65°C)**: The temperature is lowered, allowing short, single-stranded DNA primers to bind (anneal) to specific complementary sequences on each of the separated single-stranded DNA templates. 3. **Extension (approx. 70-75°C)**: The temperature is raised slightly, and a heat-stable DNA polymerase (Taq polymerase) synthesises new complementary DNA strands by adding free deoxyribonucleotides (dNTPs), starting from the primers and extending along the template strands. (c) Crime scene DNA: Bands at 500 bp, 300 bp, 100 bp Suspect X DNA: Bands at 500 bp, 100 bp Suspect Y DNA: Bands at 500 bp, 300 bp, 100 bp Suspect Z DNA: Bands at 300 bp, 100 bp **Suspect Y's DNA matches the crime scene sample.** **Justification**: The banding pattern of Suspect Y (bands at 500 bp, 300 bp, and 100 bp) is identical in both the number and the specific sizes of the DNA fragments to the crime scene DNA sample. This identical DNA profile indicates a very high probability that the DNA originated from Suspect Y.
Remember that in gel electrophoresis, smaller DNA fragments travel further down the gel towards the positive electrode.
Common mistakes
- ✗Confusing the roles of restriction enzymes (cut DNA) and DNA ligase (join DNA fragments).
- ✗Incorrectly stating that larger DNA fragments travel further in gel electrophoresis (it's the opposite; smaller fragments travel further).
- ✗Forgetting that Taq polymerase is heat-stable, which is crucial for its function in PCR at high temperatures.
- ✗Not understanding the purpose of primers in PCR (to define the region of DNA to be amplified).
- ✗Failing to discuss the ethical considerations and potential risks associated with genetic engineering and GMOs.
Exam tips
- ★Learn the precise definitions of all key terms (e.g., mutation, restriction enzyme, plasmid, PCR, gel electrophoresis).
- ★Be able to draw and label clear diagrams illustrating the processes of genetic engineering, PCR, and gel electrophoresis.
- ★Practise sequencing the steps for each biotechnological process accurately and explaining the function of each component.
- ★Prepare to discuss the advantages, disadvantages, and ethical implications of genetic engineering and GMOs, providing specific examples where possible.
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