Key Takeaways for Reproduction

UK-Style English | Detailed Revision Notes

1. Sexual vs. Asexual Reproduction

Definitions & Examples

  • Asexual Reproduction:
    • One parent → Genetically identical offspring (clones).
    • Examples:
      • Bacteria (binary fission: 1 parent→2 identical daughter cells1 parent→2 identical daughter cells).
      • Plants (runners in strawberries, tubers in potatoes).
    • Cell division: Mitosis only.
  • Sexual Reproduction:
    • Two parents → Genetically varied offspring.
    • Examples: Humans (sperm + ovum), flowering plants (pollen + ovum).
    • Cell division: Meiosis (gametes) + fertilisation (fusion).

Advantages

AsexualSexual
Fast, no mate needed (e.g., bacteria doubling every 20 mins).Genetic variation → evolution + adaptation.

TIP: Use Venn diagrams to compare/contrast.

2. Meiosis

Key Process

  • Produces 4 haploid gametes (23 chromosomes in humans) from 1 diploid cell (46 chromosomes).
  • Stages:
    1. DNA replicates → homologous chromosomes pair up.
    2. Crossing over (exchange of DNA segments → genetic variation).
    3. Two divisions → 4 non-identical cells.

Mitosis vs. Meiosis

MitosisMeiosis
2 diploid cells4 haploid cells
Growth/repairGamete formation
No variationGenetic variation

TIP: Memorise: “PMAT x2” (Prophase, Metaphase, Anaphase, Telophase, repeated twice).

3. DNA & Genome

Structure

  • Double helix (sugar-phosphate backbone + base pairs):
    • A-T (Apples on Trees), C-G (Cars in Garages).
  • Nucleotide = sugar + phosphate + base.

Genome: Entire genetic material (≈3 billion base pairs in humans).

  • Genes: Sections of DNA coding for proteins (e.g., eye colour).
  • Non-coding DNA (98%): Used in fingerprinting (unique patterns).

Human Genome Project (HGP)

  • Mapped all human DNA.
  • Ethical issues: Privacy (employers/insurers), discrimination.

TIP: Draw labelled DNA diagrams to visualise base pairing.

4. Protein Synthesis

Two Stages

  1. Transcription (nucleus):
    • DNA → mRNA (messenger RNA).
    • DNA→enzymemRNADNAenzyme​mRNA.
  2. Translation (ribosome):
    • mRNA → protein (via tRNA bringing amino acids).
    • Triplet code: 3 bases = 1 amino acid (e.g., GGT = glycine).

Mutations

  • Substitution: 1 base changed → 1 amino acid altered (e.g., sickle cell anaemia).
  • Insertion/Deletion: Frameshift → multiple amino acids altered.

TIP: Use flashcards for transcription/translation steps.

5. Genetic Inheritance

Key Terms

  • Alleles: Different versions of a gene (e.g., B = brown eyes, b = blue).
  • Genotype: Genetic makeup (e.g., BB, Bb, bb).
  • Phenotype: Physical trait (e.g., brown eyes).

Punnett Squares

  • Example: Heterozygous parents (Bb × Bb):
    • .

Inherited Disorders

  • Cystic Fibrosis (recessive):
    • Genotype: cc (both parents must be carriers, Cc).
  • Polydactyly (dominant):
    • Genotype: Pp or PP (only 1 dominant allele needed).

TIP: Practice genetic crosses using family trees.

6. Sex Determination

  • XX = female, XY = male.
  • Sperm determines sex:
    • 50% X (girl), 50% Y (boy).

Equation:
P(male)=12,P(female)=12P(male)=21​,P(female)=21​

TIP: Link to probability in maths (e.g., coin toss analogy).

Revision Tips

  1. Flashcards: Key terms (e.g., mitosis vs. meiosis).
  2. Diagrams: Draw processes (e.g., protein synthesis).
  3. Past Papers: Practice genetic cross questions.
  4. Mnemonics: E.g., “All Tigers Can Growl” (A-T, C-G).

Common Mistakes

  • Confusing mitosis/meiosis → use comparison tables.
  • Forgetting non-coding DNA’s role in fingerprinting.

Example Exam Question:
“Explain how meiosis increases genetic variation.”
Answer: Crossing over (exchange of DNA) + independent assortment (random chromosome alignment).

50 GCSE Biology Questions on Reproduction, DNA, and Genetics

Section 1: Sexual vs. Asexual Reproduction

  1. Define asexual reproduction and provide two examples.
  2. State two advantages of asexual reproduction.
  3. Explain why bacteria reproduce asexually via binary fission.
  4. Compare genetic variation in offspring from sexual vs. asexual reproduction.
  5. Name the process by which strawberry plants reproduce asexually.

Section 2: Meiosis and Mitosis

  1. Describe the outcome of meiosis in humans.
  2. How does crossing over during meiosis increase genetic variation?
  3. State three differences between mitosis and meiosis.
  4. Calculate the number of chromosomes in a human gamete.
  5. Why are gametes haploid?

Section 3: DNA Structure and the Genome

  1. What is a nucleotide? Label its components.
  2. Explain the base-pairing rule in DNA (e.g., A-T, C-G).
  3. Define the term “genome”.
  4. What percentage of human DNA is non-coding?
  5. How is DNA fingerprinting used in forensics?

Section 4: Protein Synthesis

  1. Outline the two stages of protein synthesis.
  2. What is the role of mRNA in transcription?
  3. Explain the term “triplet code”.
  4. How does a substitution mutation affect protein structure?
  5. Why is the shape of a protein critical to its function?

Section 5: Genetic Inheritance

  1. Define “allele” and provide an example.
  2. Distinguish between genotype and phenotype.
  3. Using a Punnett square, show the possible offspring of two heterozygous parents (Bb) for brown eyes.
  4. Why can two parents without cystic fibrosis have a child with the disorder?
  5. Explain why polydactyly is a dominant genetic disorder.

Section 6: Sex Determination

  1. How is sex determined in humans? Use XX and XY in your answer.
  2. Calculate the probability of a couple having a daughter.
  3. Why are sperm cells genetically different from each other?
  4. What is the difference between sex and gender?
  5. Explain why the global population is roughly 50% male and 50% female.

Section 7: Human Genome Project (HGP)

  1. What was the main achievement of the HGP?
  2. Discuss one ethical concern related to the HGP.
  3. How has the HGP contributed to medical research?

Section 8: Mutations

  1. Define “mutation” and list two causes.
  2. How does a frameshift mutation differ from a substitution?
  3. Explain why not all mutations are harmful.

Section 9: Practical Applications

  1. Describe the steps to extract DNA from an onion.
  2. Why is chilled ethanol used in DNA extraction?
  3. What is tissue culture, and how is it used in plant cloning?

Section 10: Extended Application

  1. Explain how natural selection benefits from genetic variation.
  2. Why might a species switch between sexual and asexual reproduction?
  3. Describe how enzymes are involved in DNA replication.
  4. Interpret a family tree to identify carriers of cystic fibrosis.
  5. Analyse the statement: “All mutations lead to evolution.”
  6. Why do identical twins not always look identical?

Section 11: Calculations

  1. If a plant has 20 chromosomes in its diploid cells, how many chromosomes are in its gametes?
  2. Calculate the probability of two heterozygous parents (Tt) having a child who cannot roll their tongue.
  3. Using P(recessive)=14P(recessive)=41​, explain genetic probabilities in a monohybrid cross.

Section 12: Ethics and Society

  1. Should employers have access to an individual’s genome? Justify your answer.
  2. Evaluate the benefits and risks of embryo screening for genetic disorders.

Detailed Answers

  1. Asexual reproduction involves one parent producing genetically identical offspring (clones). Examples: Binary fission in bacteria, tubers in potatoes.
  2. Advantages of asexual reproduction: Rapid reproduction (e.g., bacteria doubling every 20 mins), no energy spent finding a mate.
  3. Bacteria use binary fission (1 parent→2 identical cells1 parent→2 identical cells) for efficiency in stable environments.
  4. Sexual reproduction creates genetic variation (crossing over, independent assortment), while asexual produces clones.
  5. Strawberry plants reproduce via runners (stolons) that form new plantlets.
  6. Meiosis outcome: 4 haploid gametes (23 chromosomes each in humans).
  7. Crossing over exchanges DNA segments between homologous chromosomes, creating new allele combinations.
  8. Mitosis vs. Meiosis:
    • Mitosis: 2 diploid cells, no variation.
    • Meiosis: 4 haploid cells, genetic variation.
  9. Human gametes have 23 chromosomes23 chromosomes (haploid).
  10. Gametes are haploid to restore diploid number (46) after fertilisation.
  11. Nucleotide: Sugar + phosphate + base (A, T, C, G).
  12. Base pairing: A-T (Apples on Trees), C-G (Cars in Garages).
  13. Genome: Entire genetic material of an organism (≈3 billion base pairs in humans).
  14. Non-coding DNA: 98% of human DNA (used in fingerprinting).
  15. DNA fingerprinting identifies individuals by analysing unique non-coding regions.
  16. Protein synthesis stages:
  • Transcription: DNA → mRNA in nucleus.
  • Translation: mRNA → protein in ribosome.
  1. mRNA carries the genetic code from DNA to ribosomes.
  2. Triplet code: 3 bases (e.g., GGT) code for 1 amino acid.
  3. Substitution mutation alters one amino acid (e.g., sickle cell anaemia).
  4. Protein shape determines function (e.g., enzyme active sites).
  5. Allele: Variant of a gene (e.g., B for brown eyes, b for blue).
  6. Genotype: Genetic makeup (e.g., Bb). Phenotype: Physical trait (e.g., brown eyes).
  7. Punnett square (Bb × Bb):
  • Offspring: 25% BB (brown), 50% Bb (brown), 25% bb (blue).
  1. Cystic fibrosis: Both parents must be carriers (Cc) to have a 25% chance of cc offspring.
  2. Polydactyly is dominant; only one allele (P) is needed to express the trait.
  3. Sex determination:
  • XX = female, XY = male (sperm determines sex).
  1. Probability of daughter: P(XX)=12P(XX)=21​.
  2. Sperm variation: Crossing over and independent assortment during meiosis.
  3. Sex is biological (XX/XY); gender is social/cultural identity.
  4. 50:50 sex ratio: Sperm are 50% X and 50% Y.
  5. HGP achievement: Mapped all 3 billion base pairs in human DNA.
  6. Ethical concern: Privacy issues (e.g., genetic discrimination by insurers).
  7. Medical contribution: Identified genes linked to diseases like breast cancer.
  8. Mutation: Permanent DNA change (causes: UV radiation, chemicals).
  9. Frameshift alters all downstream codons; substitution affects one codon.
  10. Neutral mutations (e.g., non-coding DNA) have no effect; some are beneficial.
  11. DNA extraction steps: Blend onion + salt → filter → add detergent → ethanol precipitation.
  12. Ethanol precipitates DNA by dehydrating it.
  13. Tissue culture: Cloning plants from small tissue samples in sterile conditions.
  14. Natural selection favours advantageous traits in varied populations.
  15. Reproductive switch: Asexual for rapid colonisation, sexual for genetic diversity under stress.
  16. Enzymes (e.g., DNA polymerase) replicate DNA during mitosis/meiosis.
  17. Family tree analysis: Carriers (Cc) have one recessive allele but no symptoms.
  18. Mutation and evolution: Only beneficial mutations drive adaptation.
  19. Identical twins: Environmental factors (diet, lifestyle) cause differences.
  20. Gamete chromosomes: 202=10 chromosomes220​=10 chromosomes.
  21. Tt × Tt: 25% tt (non-rollers).
  22. Probability: P(recessive)=14P(recessive)=41​ in monohybrid crosses.
  23. Employer access: Risks genetic discrimination; benefits workplace safety.
  24. Embryo screening: Pros (prevent disorders), Cons (ethical concerns about selection).

Need more practice? Try past papers and annotate diagrams! 📚🔍