Sciences · Topic 2.1
Biology — DNA and Cell Division
DNA is the molecular blueprint of life. At Year 4 Advanced, you must not only describe DNA structure and cell division but evaluate these processes, justify their significance, and discuss strengths and limitations of scientific models and evidence.
What You'll Learn
- Describe the double helix structure of DNA and base pairing rules (A-T, C-G)
- Explain how genes code for proteins and why the sequence of bases matters
- Compare mitosis and meiosis: purpose, products, chromosome number, and genetic variation
- Explain the stages of mitosis (PMAT) with justification for each stage's function
- Evaluate natural selection as a mechanism of evolution using specific evidence
- Discuss the limitations of the Watson-Crick DNA model as a simplification
IB Assessment Focus
Criterion A: Explain WHY meiosis must reduce chromosome number; apply understanding to novel genetic scenarios.
Criterion B: Design an investigation to test a hypothesis about cell division or mutation rates.
Criterion C: Use precise biological terminology; communicate findings with appropriate data representation.
Criterion D: Evaluate strengths and limitations of natural selection as an explanation for evolution; discuss ethical implications of genetic technology.
Key Vocabulary
| Term | Definition |
|---|---|
| DNA | Deoxyribonucleic acid — a double helix of nucleotides with complementary base pairs (A-T, C-G) |
| Gene | A section of DNA coding for a specific protein via a sequence of codons |
| Chromosome | Coiled DNA and protein structure; humans have 46 (23 pairs) |
| Mitosis | Cell division producing 2 genetically identical diploid daughter cells |
| Meiosis | Cell division producing 4 genetically unique haploid gametes |
| Diploid (2n) | A cell with two sets of chromosomes (46 in humans) |
| Haploid (n) | A cell with one set of chromosomes (23 in humans); gametes |
| Mutation | A change in the DNA base sequence; may alter protein function |
| Natural selection | Differential survival and reproduction based on heritable variation and environmental pressures |
Sciences · Topic 2.1
DNA Structure and Function
DNA's double helix structure, first described by Watson and Crick in 1953 (based on Franklin's X-ray data), enables both accurate replication and protein synthesis. Understanding structure-function relationships is fundamental to Year 4 Biology.
Structure of the Double Helix
- Backbone: Two strands of alternating deoxyribose (sugar) and phosphate groups.
- Rungs: Nitrogenous base pairs joined by hydrogen bonds: A pairs with T (2 hydrogen bonds), C pairs with G (3 hydrogen bonds).
- Antiparallel strands: The two strands run in opposite directions (3' to 5' and 5' to 3').
- Coiling: The helix winds around itself, and further coils into chromosomes for efficient storage.
Base Pairing Rules
| Base (DNA strand 1) | Pairs with | Number of H-bonds |
|---|---|---|
| Adenine (A) | Thymine (T) | 2 |
| Cytosine (C) | Guanine (G) | 3 |
Why C-G bonds are stronger: Three hydrogen bonds (C-G) are stronger than two (A-T). This means regions rich in C-G have higher melting temperatures and are more stable. This is biologically significant: highly conserved gene regions often have higher C-G content.
From DNA to Protein
- Transcription: DNA is copied into mRNA (messenger RNA) in the nucleus. Each codon (3 bases) codes for one amino acid.
- Translation: Ribosomes in the cytoplasm read the mRNA and assemble the corresponding amino acids into a polypeptide chain.
- Protein folding: The polypeptide folds into a functional 3D protein, determining its specific function.
Critical Rule: A single base change (point mutation) in a gene may change one amino acid, potentially altering the protein's entire 3D shape and function. However, the genetic code is degenerate (multiple codons can code for the same amino acid), meaning not all mutations change the protein — these are called silent mutations.
Sciences · Topic 2.1
Mitosis
Mitosis is cell division for growth, repair, and asexual reproduction. It produces two genetically identical diploid daughter cells. Understanding the purpose of each stage justifies why the process occurs in the specific order it does.
Stages of Mitosis (PMAT)
| Stage | Key events | Why this must happen |
|---|---|---|
| Prophase | Chromosomes condense; nuclear envelope breaks down; spindle fibres form | Condensation makes chromosomes compact enough to move without tangling; spindle fibres are needed for separation |
| Metaphase | Chromosomes align at the cell equator (metaphase plate) | Alignment ensures each pole gets exactly one copy of every chromosome |
| Anaphase | Sister chromatids are pulled to opposite poles by spindle fibres | Separation ensures each daughter cell gets a complete set of DNA |
| Telophase | Nuclear envelopes reform; chromosomes decondense; cell divides (cytokinesis) | Restores functional nuclei; cytokinesis produces two separate cells |
Before mitosis — Interphase: Before division begins, the cell must copy all its DNA. During the S phase of interphase, each chromosome is replicated to produce two identical sister chromatids joined at the centromere. This DNA replication is the prerequisite for mitosis.
Common Mistake: Mitosis is NOT the same as the cell cycle. Mitosis is just the nuclear division phase (M phase). The cell cycle also includes interphase (G1, S, G2), during which the cell grows and replicates its DNA.
Sciences · Topic 2.1
Meiosis and Genetic Variation
Meiosis produces four haploid gametes, each genetically unique. The two key functions — chromosome number reduction and generating genetic variation — are both critical for sexual reproduction. You must be able to justify why both functions are necessary.
Mitosis vs Meiosis — Comparison
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Sexual reproduction (gamete production) |
| Divisions | 1 | 2 (Meiosis I and Meiosis II) |
| Cells produced | 2 | 4 |
| Chromosome number | 2n → 2n (diploid) | 2n → n (haploid) |
| Genetic variation | None — genetically identical | High — crossing over & independent assortment |
| Location | All body (somatic) cells | Gonads (ovaries and testes) |
Sources of Genetic Variation in Meiosis
- Crossing over (Prophase I): Homologous chromosomes exchange segments of DNA, creating new combinations of alleles (recombinant chromosomes).
- Independent assortment (Metaphase I): Homologous chromosome pairs line up randomly. Each pair can face either pole independently, generating 2²³ (>8 million) possible combinations in humans.
- Random fertilisation: Any sperm can fertilise any egg, multiplying genetic combinations further.
Why must meiosis halve chromosome number?
Sexual reproduction requires the fusion of two gametes (sperm + egg) to form a zygote. If gametes were produced by mitosis (diploid), each would have 46 chromosomes. The resulting zygote would have 92 chromosomes — and this would double every generation, producing non-viable offspring. Meiosis prevents this by producing haploid gametes (23 chromosomes), so fertilisation restores the diploid number (46).
Sexual reproduction requires the fusion of two gametes (sperm + egg) to form a zygote. If gametes were produced by mitosis (diploid), each would have 46 chromosomes. The resulting zygote would have 92 chromosomes — and this would double every generation, producing non-viable offspring. Meiosis prevents this by producing haploid gametes (23 chromosomes), so fertilisation restores the diploid number (46).
Critical Rule: Meiosis is NOT just "cell division in sex organs." It is specifically the process that both reduces chromosome number AND introduces genetic variation. These two functions must be stated separately in any exam answer asking you to explain the role of meiosis.
Sciences · Topic 2.1
Natural Selection and Evolution
Natural selection is the mechanism by which heritable variation leads to differential reproductive success, driving evolution over generations. At Year 4, you must evaluate both the evidence for natural selection AND its limitations as a complete explanation of evolution.
Darwin's Four Conditions for Natural Selection
- Variation: Individuals within a population differ in their heritable traits.
- Heredity: Traits are passed from parent to offspring through DNA.
- Selection pressure: Environmental factors (predators, disease, food availability) affect survival.
- Differential reproduction: Individuals with advantageous traits survive longer and reproduce more, passing on those traits.
Evidence for Natural Selection
| Evidence | Explanation | Limitation |
|---|---|---|
| Antibiotic resistance | Bacteria with random mutations conferring resistance survive antibiotic treatment and reproduce; resistant populations grow | Only demonstrates microevolution; does not directly prove macroevolution |
| Peppered moth (Biston betularia) | Dark moths survived better in industrial England (soot-blackened trees); light moths returned after clean-air legislation | Later research questioned some methodology (e.g., moth positioning); though the overall conclusion is well-supported |
| Fossil record | Gradual changes in species morphology over time visible in fossil sequences | Incomplete record; gaps in the fossil record limit certainty about transitional forms |
| Homologous structures | Similar bone structures (human arm, bat wing, whale flipper) suggest common ancestry with modification | Similar structures can arise through convergent evolution (not common ancestry) |
Evaluating Natural Selection as a Complete Explanation:
Strengths: Supported by multiple independent lines of evidence (molecular, anatomical, fossil, experimental); explains observed changes in real-time (antibiotic resistance). Testable, falsifiable hypothesis.
Limitations: Does not fully explain the origin of life; does not account for genetic drift (random allele frequency changes in small populations); epigenetics introduces non-DNA-based inheritance that selection theory alone does not explain.
Strengths: Supported by multiple independent lines of evidence (molecular, anatomical, fossil, experimental); explains observed changes in real-time (antibiotic resistance). Testable, falsifiable hypothesis.
Limitations: Does not fully explain the origin of life; does not account for genetic drift (random allele frequency changes in small populations); epigenetics introduces non-DNA-based inheritance that selection theory alone does not explain.
Sciences · Topic 2.1
Worked Examples
Extended responses showing the reasoning required at Year 4 Advanced.
EXAMPLE 1Explain why meiosis, not mitosis, is used to produce gametes.
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Full Solution
Sexual reproduction requires two gametes to fuse in fertilisation. If gametes were produced by mitosis, they would each have the full diploid number of chromosomes (46 in humans). Fusion would produce a zygote with 92 chromosomes — double the normal number. This would be lethal in most species, and chromosome numbers would double every generation.
Meiosis halves the chromosome number, producing haploid gametes (23 chromosomes). When two haploid gametes fuse, the correct diploid number (46) is restored. This is the chromosome number reduction function of meiosis.
Additionally, meiosis introduces genetic variation through crossing over and independent assortment, ensuring offspring are genetically unique. Mitosis produces genetically identical cells — no variation would be introduced into the gene pool, reducing the adaptability of the population.
Meiosis halves the chromosome number, producing haploid gametes (23 chromosomes). When two haploid gametes fuse, the correct diploid number (46) is restored. This is the chromosome number reduction function of meiosis.
Additionally, meiosis introduces genetic variation through crossing over and independent assortment, ensuring offspring are genetically unique. Mitosis produces genetically identical cells — no variation would be introduced into the gene pool, reducing the adaptability of the population.
EXAMPLE 2A DNA strand reads: 3'-TACGCAATT-5'. Write the complementary strand and the mRNA sequence.
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Full Solution
Complementary DNA strand (antiparallel, A-T and C-G pairing):
Template: 3'-T-A-C-G-C-A-A-T-T-5'
DNA copy: 5'-A-T-G-C-G-T-T-A-A-3'
mRNA strand (same sequence as DNA copy but with U replacing T):
mRNA: 5'-A-U-G-C-G-U-U-A-A-3'
Note: The mRNA codon AUG is the start codon — transcription typically begins here, coding for methionine as the first amino acid.
Template: 3'-T-A-C-G-C-A-A-T-T-5'
DNA copy: 5'-A-T-G-C-G-T-T-A-A-3'
mRNA strand (same sequence as DNA copy but with U replacing T):
mRNA: 5'-A-U-G-C-G-U-U-A-A-3'
Note: The mRNA codon AUG is the start codon — transcription typically begins here, coding for methionine as the first amino acid.
EXAMPLE 3Evaluate the peppered moth case study as evidence for natural selection.
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Full Solution
Evidence FOR natural selection: Before industrialisation, light-coloured moths were better camouflaged on lichen-covered tree bark and survived predation better. After pollution darkened trees with soot, dark (melanic) moths were better camouflaged and their frequency increased dramatically. After clean-air laws reduced pollution, light moths recovered. This matches the prediction of natural selection: organisms with better camouflage (a heritable trait) survive and reproduce more.
Strengths of this evidence: The change was observed over a relatively short time; the trait (wing colour) is heritable; the selection pressure (predation) is identifiable; the reversal of selection pressure led to a reversal of allele frequency — a powerful test.
Limitations: Later research by Michael Majerus (and critiques of original Kettlewell data) questioned whether moths normally rest on tree trunks during the day — which was assumed in the original explanation. Majerus's own later work (2012) largely vindicated the selection explanation, though the mechanism is more complex. Additionally, this is an example of microevolution (change within a species); it does not directly demonstrate speciation or macroevolution.
Conclusion: Despite methodological criticisms, the peppered moth remains strong evidence for natural selection in action, particularly when combined with other lines of evidence (genetic analysis, experimental predation studies).
Strengths of this evidence: The change was observed over a relatively short time; the trait (wing colour) is heritable; the selection pressure (predation) is identifiable; the reversal of selection pressure led to a reversal of allele frequency — a powerful test.
Limitations: Later research by Michael Majerus (and critiques of original Kettlewell data) questioned whether moths normally rest on tree trunks during the day — which was assumed in the original explanation. Majerus's own later work (2012) largely vindicated the selection explanation, though the mechanism is more complex. Additionally, this is an example of microevolution (change within a species); it does not directly demonstrate speciation or macroevolution.
Conclusion: Despite methodological criticisms, the peppered moth remains strong evidence for natural selection in action, particularly when combined with other lines of evidence (genetic analysis, experimental predation studies).
EXAMPLE 4A body cell with 8 chromosomes (2n=8) undergoes meiosis. Describe the chromosome number at each key stage.
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Full Solution
Starting cell: 8 chromosomes (2n = 8), i.e., 4 pairs of homologous chromosomes.
After DNA replication (before meiosis begins): 8 chromosomes, each consisting of 2 sister chromatids (16 chromatids total). Chromosome number is still 8.
After Meiosis I: 4 chromosomes each (n = 4). Homologous pairs have been separated into two cells. Each cell has 4 chromosomes (each still consisting of 2 sister chromatids).
After Meiosis II: 4 cells, each with 4 chromosomes (n = 4). Sister chromatids have separated. Each cell contains one chromosome from each homologous pair.
Result: 4 haploid gametes, each with 4 chromosomes. Chromosome number has been halved from 8 to 4.
After DNA replication (before meiosis begins): 8 chromosomes, each consisting of 2 sister chromatids (16 chromatids total). Chromosome number is still 8.
After Meiosis I: 4 chromosomes each (n = 4). Homologous pairs have been separated into two cells. Each cell has 4 chromosomes (each still consisting of 2 sister chromatids).
After Meiosis II: 4 cells, each with 4 chromosomes (n = 4). Sister chromatids have separated. Each cell contains one chromosome from each homologous pair.
Result: 4 haploid gametes, each with 4 chromosomes. Chromosome number has been halved from 8 to 4.
EXAMPLE 5Compare the genetic consequences of a mutation in a body (somatic) cell vs a gamete.
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Full Solution
Somatic cell mutation: The mutation affects only the cells that develop from the original mutated cell (via mitosis). It is not inherited by offspring. It may cause localised problems (e.g., cancer if the mutation affects cell cycle control genes) but does not affect the individual's gametes.
Gamete mutation: The mutation is present in the sex cell itself. If the gamete participates in fertilisation, every cell in the resulting offspring will carry the mutation (it will be inherited). This can be passed to future generations. If the mutation affects a critical gene, it may cause a heritable genetic disorder. Beneficial mutations in gametes are the source of new variation for natural selection to act on.
Significance: Only germline (gamete) mutations have evolutionary consequences because only they can be inherited. Somatic mutations matter clinically (disease) but are evolutionarily "dead ends."
Gamete mutation: The mutation is present in the sex cell itself. If the gamete participates in fertilisation, every cell in the resulting offspring will carry the mutation (it will be inherited). This can be passed to future generations. If the mutation affects a critical gene, it may cause a heritable genetic disorder. Beneficial mutations in gametes are the source of new variation for natural selection to act on.
Significance: Only germline (gamete) mutations have evolutionary consequences because only they can be inherited. Somatic mutations matter clinically (disease) but are evolutionarily "dead ends."
EXAMPLE 6Explain why the stages of mitosis must occur in the order PMAT.
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Full Solution
Each stage creates the necessary conditions for the next:
Prophase must come first: Chromosomes must condense to become compact and manageable before they can be moved. The nuclear envelope must break down to allow spindle fibres to attach to chromosomes. Without these preparations, chromosomes cannot be aligned or separated accurately.
Metaphase must follow: After spindle fibres form (Prophase), chromosomes are aligned at the equatorial plate. This alignment is essential to ensure each pole receives exactly one copy of every chromosome. Skipping this step would result in unequal distribution.
Anaphase must follow: Once chromosomes are aligned, spindle fibres can shorten and pull sister chromatids to opposite poles. This separation can only happen after correct alignment ensures each chromatid goes to the correct pole.
Telophase must come last: Only after chromosomes have fully separated can new nuclear envelopes form around each set and cells divide (cytokinesis), producing two genetically identical daughter cells. The order is therefore causally necessary, not arbitrary.
Prophase must come first: Chromosomes must condense to become compact and manageable before they can be moved. The nuclear envelope must break down to allow spindle fibres to attach to chromosomes. Without these preparations, chromosomes cannot be aligned or separated accurately.
Metaphase must follow: After spindle fibres form (Prophase), chromosomes are aligned at the equatorial plate. This alignment is essential to ensure each pole receives exactly one copy of every chromosome. Skipping this step would result in unequal distribution.
Anaphase must follow: Once chromosomes are aligned, spindle fibres can shorten and pull sister chromatids to opposite poles. This separation can only happen after correct alignment ensures each chromatid goes to the correct pole.
Telophase must come last: Only after chromosomes have fully separated can new nuclear envelopes form around each set and cells divide (cytokinesis), producing two genetically identical daughter cells. The order is therefore causally necessary, not arbitrary.
Sciences · Topic 2.1
Practice Q&A
Attempt each question before revealing the model answer. Use precise biological terminology in every response.
DESCRIBEDescribe the structure of DNA.
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Model Answer
DNA is a double-stranded helix. Each strand consists of nucleotides containing a deoxyribose sugar, a phosphate group, and a nitrogenous base (A, T, C, or G). The two strands are held together by hydrogen bonds between complementary base pairs: A pairs with T (2 H-bonds) and C pairs with G (3 H-bonds). The strands are antiparallel (run in opposite directions: 3' to 5' and 5' to 3'). The helix is further coiled into chromosomes within the nucleus.
COMPAREState three differences between mitosis and meiosis.
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Model Answer
1. Mitosis produces 2 daughter cells; meiosis produces 4.
2. Mitosis produces diploid cells (2n); meiosis produces haploid cells (n).
3. Mitosis produces genetically identical cells; meiosis produces genetically varied cells due to crossing over and independent assortment.
2. Mitosis produces diploid cells (2n); meiosis produces haploid cells (n).
3. Mitosis produces genetically identical cells; meiosis produces genetically varied cells due to crossing over and independent assortment.
EXPLAINExplain how a mutation can be silent (have no effect on the organism).
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Model Answer
The genetic code is degenerate — most amino acids are coded for by more than one codon (e.g., UCU, UCC, UCA, UCG all code for serine). If a point mutation changes a codon to a different codon that codes for the same amino acid, the protein sequence is unchanged, so the protein's structure and function remain unaffected. This is called a synonymous (silent) mutation.
EVALUATEEvaluate the claim: "Natural selection explains all evolutionary change."
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Model Answer
In favour: Natural selection is well-supported by multiple lines of evidence and explains observed evolution in bacteria (antibiotic resistance), insects (pesticide resistance), and vertebrates (peppered moth). It is the primary mechanism of adaptive evolution.
Against: Natural selection cannot fully explain: (1) Genetic drift — random changes in allele frequency in small populations due to chance, not selection; (2) Neutral evolution — many DNA changes are neither beneficial nor harmful and accumulate without selection acting; (3) Sexual selection — traits may evolve due to mate preference, not survival advantage (peacock tail); (4) Epigenetics — heritable changes in gene expression that do not involve DNA sequence change.
Conclusion: Natural selection is the most powerful mechanism of evolutionary change but does not explain all evolutionary phenomena. Modern evolutionary theory (the Extended Modern Synthesis) incorporates additional mechanisms beyond Darwin's original natural selection framework.
Against: Natural selection cannot fully explain: (1) Genetic drift — random changes in allele frequency in small populations due to chance, not selection; (2) Neutral evolution — many DNA changes are neither beneficial nor harmful and accumulate without selection acting; (3) Sexual selection — traits may evolve due to mate preference, not survival advantage (peacock tail); (4) Epigenetics — heritable changes in gene expression that do not involve DNA sequence change.
Conclusion: Natural selection is the most powerful mechanism of evolutionary change but does not explain all evolutionary phenomena. Modern evolutionary theory (the Extended Modern Synthesis) incorporates additional mechanisms beyond Darwin's original natural selection framework.
APPLYA human cell has 46 chromosomes. After meiosis I, how many chromosomes are in each cell? Justify.
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Model Answer
After Meiosis I: 23 chromosomes per cell. During Meiosis I, homologous chromosome pairs are separated — one chromosome from each pair goes to each new cell. The chromosome number is halved from 46 to 23. Importantly, each chromosome at this stage still consists of two sister chromatids joined at the centromere — these are separated in Meiosis II.
DESIGNDescribe how you would investigate whether UV light causes mutations in bacteria.
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Model Answer
Hypothesis: UV light increases the mutation rate in bacteria compared to no UV exposure.
Method: (1) Prepare identical plates of bacteria (same strain, same concentration) on nutrient agar. (2) Experimental group: expose plates to UV light for different durations (0, 5, 10, 20 seconds). Control group: no UV exposure. (3) Incubate all plates at the same temperature for 24 hours. (4) Count colony numbers (fewer colonies = more lethal mutations; altered-morphology colonies = surviving mutants). (5) Repeat 3 times per UV duration to improve reliability.
Variables: Independent = UV exposure duration. Dependent = colony count and morphology. Controlled = temperature, agar type, bacterial strain, incubation time.
Limitation: Colony count reflects lethal mutations; non-lethal mutations that don't affect colony appearance would not be detected.
Method: (1) Prepare identical plates of bacteria (same strain, same concentration) on nutrient agar. (2) Experimental group: expose plates to UV light for different durations (0, 5, 10, 20 seconds). Control group: no UV exposure. (3) Incubate all plates at the same temperature for 24 hours. (4) Count colony numbers (fewer colonies = more lethal mutations; altered-morphology colonies = surviving mutants). (5) Repeat 3 times per UV duration to improve reliability.
Variables: Independent = UV exposure duration. Dependent = colony count and morphology. Controlled = temperature, agar type, bacterial strain, incubation time.
Limitation: Colony count reflects lethal mutations; non-lethal mutations that don't affect colony appearance would not be detected.
DISCUSSDiscuss ethical considerations related to genetic engineering.
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Model Answer
Benefits: Genetic engineering has produced life-saving medicines (insulin via recombinant DNA), disease-resistant crops reducing pesticide use, and potential cures for genetic disorders (gene therapy for conditions like cystic fibrosis).
Concerns: (1) Germline editing (e.g., CRISPR in human embryos) permanently alters the genome of all future descendants without their consent — raises issues of autonomy and intergenerational justice. (2) Biodiversity risks: Genetically modified organisms released into the environment may outcompete native species. (3) Equity: Access to genetic technologies may be limited to wealthy nations, widening global health inequalities. (4) "Designer babies": Non-medical enhancements (selecting for intelligence, appearance) raise concerns about eugenics and social discrimination.
Conclusion: Genetic engineering requires robust regulatory frameworks balancing scientific progress with ethical safeguards. The science itself is not inherently good or bad; ethical evaluation depends on the specific application and its social context.
Concerns: (1) Germline editing (e.g., CRISPR in human embryos) permanently alters the genome of all future descendants without their consent — raises issues of autonomy and intergenerational justice. (2) Biodiversity risks: Genetically modified organisms released into the environment may outcompete native species. (3) Equity: Access to genetic technologies may be limited to wealthy nations, widening global health inequalities. (4) "Designer babies": Non-medical enhancements (selecting for intelligence, appearance) raise concerns about eugenics and social discrimination.
Conclusion: Genetic engineering requires robust regulatory frameworks balancing scientific progress with ethical safeguards. The science itself is not inherently good or bad; ethical evaluation depends on the specific application and its social context.
JUSTIFYWhy do C-G base pairs contribute to greater DNA stability than A-T pairs?
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Model Answer
C-G base pairs are held together by three hydrogen bonds, compared to only two hydrogen bonds for A-T pairs. More hydrogen bonds require more energy to break, making C-G pairs more resistant to denaturation (strand separation). DNA strands with a higher proportion of C-G pairs have a higher melting temperature and are more thermally stable. This is why organisms living in extreme heat environments (thermophilic bacteria) tend to have DNA with higher G-C content — natural selection has favoured greater molecular stability.
Sciences · Review
Flashcard Review
Tap each card to reveal the answer. Try to answer from memory first.
What are the base pairing rules in DNA?
A pairs with T (2 hydrogen bonds).
C pairs with G (3 hydrogen bonds).
The strands are antiparallel.
C pairs with G (3 hydrogen bonds).
The strands are antiparallel.
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What is the difference between diploid and haploid?
Diploid (2n): two sets of chromosomes (e.g., 46 in humans). Haploid (n): one set of chromosomes (e.g., 23 in humans — gametes).
Tap to reveal
How many cells does mitosis produce? What type?
2 genetically identical diploid cells. Used for growth, repair, and asexual reproduction.
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How many cells does meiosis produce? What type?
4 genetically unique haploid gametes. Used for sexual reproduction.
Tap to reveal
What are the stages of mitosis?
PMAT: Prophase, Metaphase, Anaphase, Telophase. (Preceded by Interphase where DNA replication occurs.)
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What is crossing over and when does it occur?
Exchange of DNA segments between homologous chromosomes during Prophase I of Meiosis. Creates new allele combinations (recombinant chromosomes).
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Why is meiosis essential for sexual reproduction?
It reduces the chromosome number from diploid to haploid, so when two gametes fuse at fertilisation, the correct diploid number is restored (not doubled each generation).
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State Darwin's four conditions for natural selection.
1. Variation — individuals differ in heritable traits.
2. Heredity — traits are passed to offspring.
3. Selection pressure — environmental factors affect survival.
4. Differential reproduction — fitter individuals reproduce more.
2. Heredity — traits are passed to offspring.
3. Selection pressure — environmental factors affect survival.
4. Differential reproduction — fitter individuals reproduce more.
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What is a mutation?
A change in the DNA base sequence. Can be caused by errors in DNA replication, radiation (UV, X-rays), or chemical mutagens. Can be silent, harmful, or rarely beneficial.
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What is independent assortment?
During Metaphase I of meiosis, homologous chromosome pairs align randomly. Each pair can face either pole independently, generating 2²³ (>8 million) possible chromosome combinations in humans.
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What is the central dogma of molecular biology?
DNA → RNA → Protein. Genetic information flows from DNA (transcription to mRNA) to protein (translation at ribosomes).
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Why are C-G base pairs stronger than A-T pairs?
C-G pairs form 3 hydrogen bonds; A-T pairs form only 2. More hydrogen bonds = more energy required to separate the strands = greater thermal stability.
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What is the difference between a somatic and germline mutation?
Somatic: affects body cells only; not inherited by offspring. Germline: occurs in gametes; CAN be inherited by offspring and passed to future generations.
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What is natural selection? (Key definition)
The process by which individuals with heritable traits better suited to their environment survive longer and reproduce more, passing advantageous traits to offspring. Over generations, this changes allele frequencies in a population.
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Give one strength and one limitation of the fossil record as evidence for evolution.
Strength: shows gradual morphological change over time; provides direct evidence of extinct species.
Limitation: incomplete — soft tissue rarely fossilises; gaps in the record make some transitional forms uncertain.
Limitation: incomplete — soft tissue rarely fossilises; gaps in the record make some transitional forms uncertain.
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Sciences · Practice Test
Practice Test — 20 Questions
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