Topic 7: Modern Genetics — Edexcel A-Level Biology B
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Topic 7: Modern Genetics explained
Topic 7: Modern Genetics covers 5 specification points, including ● to predict the amino acid sequence of… and Factors affecting gene expression i Know that….
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Open a point below for its explanation and revision guidance.
7.1 Using gene sequencing i Understand what is meant by the term genome. ii Understand how PCR can be used to amplify DNA samples, and how these samples can be used: ● to predict the amino acid sequence of proteins and possible links to genetically determined conditions, using gene sequencing. ● in forensic science, to identify criminals and to test paternity, using DNA profiling.
The genome is the complete set of genetic material in a cell or organism: all its DNA, including coding and non-coding regions. PCR amplifies DNA samples by thermal cycling: denaturation separates strands, primers anneal, and a thermostable DNA polymerase extends new strands, doubling DNA each cycle. Amplified samples can be sequenced to read base order, so the amino acid sequence of a protein can be predicted using the genetic code and triplet codons. Changes in sequence may link to genetically determined conditions. In forensic science, DNA profiling compares amplified variable regions between samples to identify criminals, and the same approach can test paternity by matching inherited profiles between a child and alleged parent.
7.2 Factors affecting gene expression i Know that transcription factors are proteins that bind to DNA. ii Understand the role of transcription factors in regulating gene expression. iii Understand how post–transcription modification of mRNA in eukaryotic cells (RNA splicing) can result in different products from a single gene. iv Understand that gene expression can be changed by epigenetic modification, including non-coding RNA, histone modification and DNA methylation. v Know that epigenetic modification is important in ensuring cell differentiation.
Transcription factors are proteins that bind to DNA and regulate gene expression, often by helping or blocking RNA polymerase at a promoter. In eukaryotic cells, mRNA is modified after transcription; RNA splicing removes introns and joins exons, and alternative splicing can produce different mature mRNA molecules and therefore different products from a single gene. Gene expression can also be changed by epigenetic modification, including non-coding RNA, histone modification and DNA methylation. These changes do not alter the base sequence but affect how accessible DNA is for transcription. Epigenetic modification is important in ensuring cell differentiation, because different patterns of gene expression give cells their specialised structures and functions.
7.3 Stem cells i Understand what is meant by a stem cell, including the differences between totipotent, pluripotent and multipotent stem cells. ii Understand that pluripotent stem cells from embryos provide opportunities to develop new medical advances although there are ethical considerations. iii Understand how epigenetic modifications can result in totipotent stem cells in the embryo developing into pluripotent cells in the blastocyst and finally into fully differentiated somatic cells.
A stem cell is an undifferentiated cell that can divide repeatedly and give rise to specialised cell types. Totipotent cells, such as the zygote and early cleavage cells, can form every cell type including extra-embryonic tissues like placenta. Pluripotent cells, found in the inner mass of the blastocyst, can form all three germ layers but not extra-embryonic tissues. Multipotent cells, such as bone marrow stem cells, give rise to a limited range of related cell types, for example blood cells. Pluripotent embryonic stem cells offer potential medical advances, including tissue repair and disease modelling, but raise ethical considerations about embryo use. Epigenetic modifications, such as DNA methylation and histone modification, silence or activate genes so totipotent cells progressively become pluripotent and then fully differentiated somatic cells.
7.3 Stem cells continued iv Understand how differentiated fibroblasts can be reprogrammed to form induced pluripotent stem cells (iPS cells) by the artificial introduction of named genes. v Understand why the use of iPS stem cells may be less problematic than the use of embryonic stem cells.
Differentiated fibroblasts can be reprogrammed into induced pluripotent stem cells (iPS cells) by artificially introducing named genes, such as Oct4, Sox2, Klf4 and c-Myc, often using viral vectors. These genes encode transcription factors that reactivate the pluripotency network, causing the fibroblast to revert to a pluripotent state. iPS cells may be less problematic than embryonic stem cells because they avoid the destruction of embryos, reducing ethical objections. They can also be derived from the patient's own cells, so the resulting cells are genetically matched, lowering the risk of immune rejection. However, iPS cells may still carry risks such as tumour formation from the introduced genes.
7.4 Gene technology i Understand how recombinant DNA can be produced, including the role of restriction endonucleases and DNA ligase. ii Understand how recombinant DNA can be inserted into other cells, and the use of various vectors such as viruses and gene guns. iii Understand how antibiotic resistance marker genes and replica plating are used to identify recombinant cells. iv Understand how ‘knockout’ mice can be used as a valuable animal model to investigate gene function. v Understand the process of genetic modification of soya beans and how it has been used to improve production, including altering the balance of fatty acids to prevent oxidation of soya products. vi Understand why the widespread use of genetic modification of major commercial crops and other transgenic processes have caused public debate of their advantages and disadvantages.
Recombinant DNA is formed when restriction endonucleases cut DNA at specific recognition sequences, leaving sticky ends, and DNA ligase seals the gene into a vector. Vectors like viruses or gene guns transfer the recombinant DNA into host cells. Marker genes confer antibiotic resistance to select transformed cells. Replica plating identifies recombinant cells by transferring colonies between plates with different antibiotics; recombinant plasmids with insertional inactivation of the second resistance marker fail to grow on that plate. Knockout mice have a disrupted gene and serve as models for gene function. Soya beans are genetically modified to improve fatty acid balance and reduce oxidation. Widespread GM crops and transgenic methods raise debated advantages and concerns.
Your focus
- Define genome accurately, including non-coding DNA.
- Describe the stages and requirements of PCR.
- Explain how sequencing predicts amino acid sequence and how profiling is applied in forensic and paternity testing.
Show all 18 objectives
- State that transcription factors are DNA-binding proteins and describe their regulatory role.
- Explain how RNA splicing can produce different products from one gene.
- Describe epigenetic modifications and explain their importance in cell differentiation.
- Define a stem cell and distinguish between totipotent, pluripotent and multipotent stem cells.
- Explain how pluripotent embryonic stem cells offer opportunities for medical advances and identify the associated ethical considerations.
- Describe how epigenetic modifications drive the progression from totipotent cells in the embryo to pluripotent cells in the blastocyst and then to fully differentiated somatic cells.
- Describe how differentiated fibroblasts can be reprogrammed to form iPS cells by the artificial introduction of named genes.
- Explain why the use of iPS cells may be less problematic than the use of embryonic stem cells, including ethical and immune considerations.
- Evaluate the potential risks associated with iPS cell therapy, such as tumour formation.
- Describe how restriction endonucleases and DNA ligase produce recombinant DNA.
- Explain how vectors such as viruses and gene guns insert recombinant DNA into cells.
- Explain how antibiotic resistance marker genes and replica plating identify recombinant cells.
- Describe how knockout mice are used to investigate gene function.
- Describe genetic modification of soya beans and its effect on fatty acid balance and oxidation.
- Discuss advantages and disadvantages that cause public debate about widespread GM crops and transgenic processes.
Topic 7: Modern Genetics exam tips
Marking Points
- The genome is the complete genetic material of a cell or organism, including coding and non-coding DNA.
- PCR amplifies DNA using cycles of denaturation, primer annealing and extension by a thermostable DNA polymerase.
- Gene sequencing determines the base sequence of amplified DNA, allowing prediction of amino acid sequence via triplet codons.
- Sequence differences can be linked to genetically determined conditions.
- DNA profiling of amplified variable regions is used in forensic science to identify criminals and to test paternity.
- Transcription factors are proteins that bind to DNA.
- They regulate gene expression by influencing transcription, for example by assisting or blocking RNA polymerase.
- Post-transcription modification of mRNA in eukaryotic cells includes RNA splicing, which removes introns and joins exons.
- Alternative splicing can result in different products from a single gene.
- Epigenetic modification includes non-coding RNA, histone modification and DNA methylation, and changes gene expression without changing the DNA base sequence.
- Epigenetic modification is important in ensuring cell differentiation.
- Defines a stem cell as an undifferentiated cell capable of repeated division and differentiation into specialised cell types.
- Distinguishes totipotent cells (form all cell types including extra-embryonic tissues) from pluripotent cells (form all three germ layers but not extra-embryonic tissues) and multipotent cells (form a limited range of related cell types).
- Explains that pluripotent embryonic stem cells offer opportunities for medical advances such as tissue repair and disease modelling.
- Recognises ethical considerations surrounding the use of embryos as a source of pluripotent stem cells.
- Describes how epigenetic modifications such as DNA methylation and histone modification regulate gene expression during development.
- Explains the developmental sequence: totipotent cells in the embryo become pluripotent cells in the blastocyst, which then become fully differentiated somatic cells.
- States that differentiated fibroblasts can be reprogrammed to form induced pluripotent stem cells (iPS cells).
- Identifies named genes used in reprogramming, such as Oct4, Sox2, Klf4 and c-Myc.
- Explains that the artificial introduction of these genes reactivates pluripotency in the differentiated cell.
- Explains that iPS cells may be less problematic than embryonic stem cells because they avoid the destruction of embryos, reducing ethical objections.
- Explains that iPS cells can be derived from the patient's own cells, reducing the risk of immune rejection.
- Recognises that iPS cells may still carry risks, such as tumour formation from the introduced genes.
- Restriction endonucleases cut DNA at specific recognition sequences, producing sticky ends that allow complementary base pairing.
- DNA ligase catalyses the formation of phosphodiester bonds, sealing the gene into the vector to form recombinant DNA.
- Vectors such as viruses or gene guns deliver recombinant DNA into host cells.
- Antibiotic resistance marker genes allow transformed cells to be selected, and replica plating identifies recombinant cells by comparing growth on plates containing different antibiotics to detect insertional inactivation of the second resistance gene.
- Knockout mice have a specific gene inactivated and are used as an animal model to investigate gene function.
- Genetic modification of soya beans improves production, including altering the balance of fatty acids to prevent oxidation of soya products.
- Widespread genetic modification of major commercial crops and other transgenic processes cause public debate about advantages and disadvantages.
Examiner Tips
- 💡Learn the three PCR stages in order and state the role of each component.
- 💡Use the genetic code table to convert a DNA triplet into an amino acid when asked to predict sequence.
- 💡Distinguish clearly between sequencing (reading bases) and profiling (comparing patterns).
- 💡Link each application to its purpose: predicting protein sequence, identifying individuals or testing paternity.
- 💡Use precise terms: bind to DNA, promoter, RNA polymerase, introns, exons, methylation, histones.
- 💡When explaining alternative splicing, state that different exon combinations produce different mRNA and protein products.
- 💡For epigenetic modification, always note that the DNA sequence itself is unchanged.
- 💡Link epigenetic modification to cell differentiation by explaining how gene expression patterns become specialised.
- 💡Learn a clear definition of a stem cell and a one-line distinction for each potency type so you can eliminate wrong options quickly.
- 💡When a question describes a cell forming placenta as well as embryo, select totipotent; if it forms all three germ layers only, select pluripotent.
- 💡Link epigenetic modifications to gene expression control, not to mutation, when answering questions about differentiation.
- 💡Read the stem of each MCQ carefully: words such as 'not', 'except' or 'least' change which option is correct.
- 💡Memorise at least two named genes used in iPS cell production, such as Oct4 and Sox2, to answer recall questions confidently.
- 💡When comparing iPS cells with embryonic stem cells, structure your answer around ethical considerations and immune rejection.
- 💡Use the phrase 'induced pluripotent stem cells' in full at least once, then abbreviate to iPS cells, to show precise terminology.
- 💡Check whether a question asks for advantages, disadvantages or both before selecting your answer.
- 💡Link each enzyme to its exact job: restriction endonuclease cuts, DNA ligase joins.
- 💡When describing replica plating, state that colonies are compared on antibiotic-containing and control plates to find recombinant cells.
- 💡For soya beans, name the fatty acid balance change and link it to preventing oxidation of soya products.
- 💡For public debate, give one advantage and one disadvantage of widespread GM crops rather than only listing opinions.
Common Mistakes
- Thinking the genome contains only protein-coding genes; correction: the genome includes all DNA, coding and non-coding.
- Believing PCR can amplify any sample without primers or a polymerase; correction: PCR requires primers, nucleotides, a thermostable polymerase and controlled temperature cycles.
- Confusing DNA profiling with whole-genome sequencing; correction: profiling compares variable repeat regions, while sequencing reads base order.
- Assuming a DNA profile proves guilt absolutely; correction: it provides evidence of a match that must be interpreted with other evidence.
- Thinking transcription factors are enzymes that synthesise DNA; correction: they are proteins that bind to DNA and regulate transcription.
- Believing splicing occurs in prokaryotes as it does in eukaryotes; correction: the statement specifies eukaryotic cells, where introns are removed and exons joined.
- Confusing epigenetic modification with mutation; correction: epigenetic changes alter gene expression without changing the base sequence.
- Assuming all cells express the same genes; correction: differential gene expression, influenced by epigenetic modification, ensures cell differentiation.
- Confusing pluripotent with totipotent: totipotent cells can form extra-embryonic tissues such as placenta, whereas pluripotent cells cannot. Correction: totipotent = all cell types including extra-embryonic; pluripotent = all three germ layers only.
- Assuming multipotent cells can form any cell type: multipotent cells are limited to a related family, such as blood cells from bone marrow. Correction: multipotent means a restricted range of related cell types.
- Treating epigenetic modification as a change to the DNA base sequence: epigenetics alters gene expression without changing the base sequence. Correction: methylation and histone modification affect whether genes are transcribed, not the genetic code itself.
- Believing embryonic stem cell use has no ethical dimension: the destruction of embryos raises ethical considerations that must be acknowledged. Correction: ethical considerations are a recognised part of the debate about embryonic stem cell research.
- Thinking iPS cells are the same as embryonic stem cells: iPS cells are reprogrammed from differentiated cells, not taken from embryos. Correction: iPS cells are artificially induced from somatic cells such as fibroblasts.
- Believing iPS cells are completely risk-free: introduced genes can cause tumour formation. Correction: iPS cells reduce ethical and immune concerns but may still carry risks such as tumour formation.
- Confusing the role of the introduced genes: they encode transcription factors that reactivate pluripotency, not genes that cause differentiation. Correction: the named genes reactivate the pluripotency network.
- Assuming iPS cells are always genetically identical to the patient: they are derived from the patient's own cells, so they are genetically matched, but this depends on the source of the cells. Correction: iPS cells derived from the patient's own cells are genetically matched, reducing immune rejection.
- Thinking DNA ligase cuts DNA: it joins DNA by forming phosphodiester bonds, while restriction endonucleases cut at recognition sites.
- Believing any vector can carry any gene without matching sticky ends: the gene and vector must be cut with the same restriction endonuclease so their sticky ends are complementary.
- Confusing marker genes with the gene of interest: antibiotic resistance marker genes are used to identify transformed cells, not to provide the desired trait.
- Assuming knockout mice have no genes at all: a knockout mouse has one specific gene inactivated so its function can be studied.