Fiche de révision : Genetics and Cell Biology Fundamentals

📋 Course Outline

1. DNA structure and base pairing
2. Genes, chromosomes and karyotypes
3. Mitosis, meiosis and fertilisation
4. Inheritance and Punnett squares
5. Mutations and genetic disorders
6. Cell theory and cell types
7. Cell organelles and functions
8. Surface area and cell size
9. Scientific investigations and variables
10. Bioethics and genetic engineering
11. Cloning and DNA editing

📖 1. DNA structure and base pairing

🔑 Key Concepts & Definitions

• Deoxyribonucleic acid : Deoxyribonucleic acid is a nucleic acid that carries genetic code and is located in the nucleus of cells.
• Nucleotide : A nucleotide is a DNA subunit made of deoxyribose sugar, a phosphate group, and a nitrogenous base.
• Chargaff’s rule : Chargaff’s rule states that in DNA adenine pairs with thymine and cytosine pairs with guanine.
• DNA double helix : A DNA double helix is a double-stranded, helical structure where the strands are twisted around each other.
• Hydrogen bonding : Hydrogen bonding is the attraction between DNA bases that holds complementary pairs together.

📝 Essential Points

• DNA nucleotides contain deoxyribose, a phosphate group, and one nitrogenous base from A, T, G, or C.
• Chargaff’s rule gives base pairing: A binds to T and C binds to G in double-stranded DNA.
• Hydrogen bonding differs by pair: C–G uses 3 hydrogen bonds, while A–T uses 2 hydrogen bonds.
• DNA is double stranded, helical, and anti-parallel, meaning the two strands run in opposite directions.
• DNA is universal: the same four nucleotides (A, T, G, C) code for DNA in all living organisms.
• Key dates: DNA was discovered in 1869, nucleotides were identified in 1929, base pairing was established in 1950, and Watson and Crick solved the double helix in 1953.

💡 Memory Hook

A–T has 2 hydrogen bonds and C–G has 3, so the ladder rungs “count” like 2 and 3.

📖 2. Genes, chromosomes and karyotypes

🔑 Key Concepts & Definitions

• Gene : A gene is a DNA segment that carries instructions for making a particular polypeptide (protein).
• Chromosome : A chromosome is a DNA-containing structure in the nucleus that packages genetic material into “packages” during cell division.
• Locus : A locus is the specific location of a gene on a particular chromosome.
• Karyotype : A karyotype is an arranged set of stained, photographed chromosomes grouped into matching pairs by size and shape.

📝 Essential Points

• In humans, somatic cells contain 46 chromosomes: 44 autosomes matched into 22 homologous pairs, plus 2 sex chromosomes.
• Sex chromosomes are X and Y: females have XX and males have XY, which determines an individual’s sex.
• Chromosomes are only visible during cell division because DNA coils are unwound and spread through the nucleus in non-dividing cells.
• The centromere is the region where sister chromatids attach, and the telomere is the protective cap at each chromosome’s end.
• Karyotyping involves staining cells about to divide, photographing chromosomes, then cutting and rearranging them into pairs from largest to smallest to reveal abnormalities.
• Down syndrome is a trisomy involving three chromosome 21 copies instead of two, with incidence 1/700 live births. and Turner syndrome is a missing/incomplete sex chromosome in females with a birth occurrence between 1/2,000 and 1/5,000.

💡 Memory Hook

Locus tells where on the chromosome, and karyotype tells what the chromosome pair looks like—size, shape, and banding.

📖 3. Mitosis, meiosis and fertilisation

🔑 Key Concepts & Definitions

• Mitosis : Mitosis is nuclear division that produces genetically identical cells for growth, repair and replacement.
• Meiosis : Meiosis is nuclear division that halves chromosome number to form haploid gametes for sexual reproduction.
• Fertilisation : Fertilisation is the fusion of haploid sperm and ovum to form a diploid zygote.

📝 Essential Points

• In mitosis, daughter cells are genetically identical to each other and to the original parent cell, so they act like clones.
• Mitosis has prophase, metaphase, anaphase and telophase, followed by cytokinesis that splits the cytoplasm into two daughter cells.
• In meiosis, chromosome number is reduced: diploid 2n parent cells produce haploid n gametes after meiosis I and meiosis II.
• Crossing over can occur in prophase I when homologous chromosomes exchange DNA, increasing variation in gametes.
• Fertilisation combines sperm (n) and ovum (n) to form a zygote (2n), and the embryo’s sex is determined by the sperm cell.

💡 Memory Hook

Meiosis halves in two steps: meiosis I splits homologous pairs, meiosis II splits sister chromatids.

📖 4. Inheritance and Punnett squares

🔑 Key Concepts & Definitions

• Crossing over : Crossing over is the exchange of genetic material between homologous chromosomes during meiosis I.
• Punnett square : A Punnett square is a diagram that predicts the possible offspring genotypes from a genetic cross.
• Gametes : Gametes are sex cells formed by meiosis that carry alleles for traits.

📝 Essential Points

• Crossing over occurs in meiosis I when homologous chromosomes exchange DNA segments, increasing genetic variation.
• A Punnett square combines parental alleles by placing each parent’s alleles in the outer squares and pairing them to fill the inner offspring genotypes.
• When two heterozygous parents are crossed for a trait (e.g., Ee × Ee), the genotypic ratio is 1 EE:2 Ee:1 ee.
• For Ee × Ee, the phenotypic ratio is 3 showing the dominant phenotype:1 showing the recessive phenotype.
• For ABO blood types, alleles are $I_A$, $I_B$, and $i$ with $I_A$ and $I_B$ codominant to each other and both dominant over $i$.
• X-linked recessive traits are expressed more often in males because males are hemizygous ($X_BY$ or $X_bY$) and need only one recessive allele to show the trait.

💡 Memory Hook

Punnett square = “Outside alleles go in, inner boxes give offspring genotypes.”

📖 5. Mutations and genetic disorders

🔑 Key Concepts & Definitions

• Mutation : A mutation is a change in DNA, a gene, or chromosomes that can cause biological effects.
• Germline mutation : A germline mutation is a DNA change in gametes that can be inherited by offspring.
• Somatic mutation : A somatic mutation is a DNA change in body cells that is not passed to the next generation.
• Mutagenic agent : A mutagenic agent is a factor that triggers mutations in cells.
• Point mutation : A point mutation is a mutation that affects a single nucleotide in DNA.

📝 Essential Points

• Spontaneous mutations occur when the mutation’s cause is not identifiable, while induced mutations have a known trigger.
• Mutations can be caused by radiation, chemicals, or infectious agents such as HPV.
• Deletion or insertion mutations can cause a frameshift because they change how triplets are read during translation.
• Substitution point mutations can be missense (amino acid changes), silent (amino acid unchanged), or nonsense (amino acid changed to a STOP codon).
• Chromosomal mutations can involve addition/deletion of entire chromosomes or rearrangements of chromosome segments.
• Not all mutations are harmful because they can add variation, such as antibiotic resistance in bacteria and malaria resistance in sickle-cell carriers.

💡 Memory Hook

Frameshift = “frame” breaks because one nucleotide is added/deleted, so every following triplet is read differently.

📖 6. Cell theory and cell types

🔑 Key Concepts & Definitions

• Cell theory : Cell theory is the biological model stating that all living things are made of cells, and that cells originate only from pre-existing cells.
• Prokaryotic cell : A prokaryotic cell is a cell type that lacks a nucleus and membrane-bound organelles and contains DNA mainly as a single circular loop.
• Eukaryotic cell : A eukaryotic cell is a cell type with a nucleus and membrane-bound organelles, and whose DNA is typically arranged as multiple linear chromosomes.
• Virion : A virion is an independent viral particle outside host cells that contains genetic material protected by a protein coat, and sometimes a lipid envelope.

📝 Essential Points

• By the early 1800s, Schwann and Schleiden proposed cells as the basic building blocks of living things, and Virchow later added that new cells arise only from pre-existing cells.
• Cell theory states: all living things are made of cells, cells are the smallest and most basic units of life, and all cells come from pre-existing cells.
• Most animal cells are about 10–40 μm, human red blood cells are about 6–8 μm, plant cells are about 10–100 μm, and bacterial cells are about 0.4–2.0 μm in diameter.
• Compound light microscopes can magnify up to about 1500×, while electron microscopes can magnify up to about 10 million× and require dead specimens.
• Prokaryotic cells lack a nucleus and typically have a single loop of circular DNA plus possible plasmids, and they replicate by binary fission.
• Eukaryotic somatic cells usually replicate by mitosis and germline cells by meiosis that produces four gametes, whereas viruses replicate only inside host cells as virions.

💡 Memory Hook

Prokaryotes: no nucleus + circular DNA + binary fission; Eukaryotes: nucleus + linear chromosomes + mitosis/meiosis.

📖 7. Cell organelles and functions

🔑 Key Concepts & Definitions

• Mitochondrion : A mitochondrion is a membrane-bound organelle that produces ATP during aerobic cellular respiration using oxygen.
• Chloroplast : A chloroplast is a membrane-bound organelle in plants and algae that captures sunlight energy to carry out photosynthesis.
• Lysosome : A lysosome is a membrane-bound organelle that digests waste materials and helps break down unwanted cell contents.
• Endosymbiotic theory : Endosymbiotic theory proposes that mitochondria and chloroplasts evolved from once free-living bacteria engulfed by an early cell.

📝 Essential Points

• Mitochondria are the main site of ATP production, generating more than 95% of the total aerobic yield.
• Cellular respiration occurs in the cytosol and mitochondria, but mitochondria only support the aerobic pathway, not anaerobic stages.
• In chloroplasts, thylakoid membranes in the granum contain chlorophyll, which drives the light-dependent reactions.
• Chloroplasts use light, carbon dioxide, and water to produce glucose and oxygen during photosynthesis.
• Evidence for endosymbiosis includes double membranes, inner membranes like bacteria, circular DNA with bacterial-sized ribosomes, and independent replication by binary fission.

💡 Memory Hook

Mito = “more than 95% ATP” (aerobic power), Chloro = “chlorophyll in thylakoids” (sun to glucose).

📖 8. Surface area and cell size

🔑 Key Concepts & Definitions

• Surface area to volume ratio : Surface area to volume ratio compares how much surface area an object has for each unit of its volume.
• High SA:V : High SA:V means relatively large surface area with relatively small volume, making exchange with the environment more efficient.
• Volume : Volume is the three-dimensional space within an object, found from length × width × height for a cuboid.
• Surface area : Surface area is the total area covering the faces of a three-dimensional object.
• Surface area and exchange : Surface area and exchange describes how a cell’s outer area affects how effectively it imports nutrients and removes wastes.

📝 Essential Points

• By having many small cells rather than one large cell, nutrient transport and waste removal become more efficient in the body.
• Cells are kept small because this reduces the distance molecules must travel to move into, around, and out of the cell.
• A high surface-area-to-volume ratio improves transport into and out of cells and helps temperature regulation.
• Volume of a cuboid is length × width × height, and for a cube the surface area is the area of one face × number of faces (6).
• For a rectangular cuboid with different edge lengths, calculate each face area (length×width, length×height, width×height), double each, then add them all.
• Example sizes: red blood cells are about 7 μm across and ova about 0.1 mm across, and small cells can squeeze through tiny capillaries for oxygen delivery.

💡 Memory Hook

Small cells = shorter travel + more contact: high SA:V makes exchange fast, like trading on the most “frontage” per “space inside”.

📖 9. Scientific investigations and variables

📖 10. Bioethics and genetic engineering

🔑 Key Concepts & Definitions

• Non-maleficence : Bioethical principle focused on preventing harm to others when making decisions.
• Genetic privacy and consent : Bioethical concern about who can access someone’s genetic information and whether that access is freely agreed to.
• Compulsory DNA profiling : Ethical issue involving requiring DNA testing or collection, which can threaten civil liberties and personal data control.

📝 Essential Points

• Sana’s broken-glass situation is best handled using non-maleficence by prioritising immediate protection from injury.
• The artificial photosynthetic cell makes its own chemical energy from light using ATP synthase and bacteriorhodopsin working together.
• In those artificial cells, DNA is used to make mRNA, and mRNA is then used to produce proteins, mimicking how real cells build components.
• A major bioethical risk linked to genome data is misuse that enables discrimination by insurance or employers based on genetic predisposition.
• Genome-project-related consent concerns include uncertainty over who should access sensitive genetic information and whether testing is truly voluntary.
• The bioethical debate on regulation includes whether people with disease should be told or subjected to actions such as sterilisation or genetic alteration.

💡 Memory Hook

Non-maleficence = “First stop harm” (Sana reports broken slides).

📖 11. Cloning and DNA editing

🔑 Key Concepts & Definitions

• Embryo splitting cloning : Embryo splitting cloning is a method where an early embryo is divided into two embryo masses that develop into identical individuals.
• Somatic cell nuclear transfer : Somatic cell nuclear transfer is cloning that uses a donor body cell nucleus placed into an enucleated egg to create a genetically identical embryo.
• Dolly the sheep : Dolly the sheep is a well-known cloned mammal created using somatic cell nuclear transfer from a single adult cell donor.
• CRISPR-Cas9 : CRISPR-Cas9 is a DNA-editing tool used to target specific DNA sections for modification.

📝 Essential Points

• Embryo splitting starts with IVF and splits an early embryo into two smaller embryos using a fine glass needle.
• Each embryo from embryo splitting is implanted into a surrogate uterus, producing genetically identical offspring.
• Dolly was made from a donor’s adult cell by removing the egg’s nucleus, fusing the donor DNA in, and using electrical impulses.
• Dolly is a clone because her genetic information matches the donor ewe (the source of the nucleus), not the egg donor.
• In the cloning method, the surrogate (foster) female provides the uterus for development rather than the nucleus DNA source.
• CRISPR-Cas9 is listed as a tool for separating DNA fragments by size in DNA tool-matching content.

📅 Key Dates

📊 Synthesis Tables

DNA vs RNA

⚠️ Common Pitfalls & Confusions

1. Mixing up which base pairs use 2 vs 3 hydrogen bonds (A–T uses 2; C–G uses 3).
2. Thinking Chargaff’s rule says A pairs with G (it says A with T, and C with G).
3. Confusing locus with karyotype: locus is a specific gene location; a karyotype is arranged, photographed chromosome pairs.
4. Saying meiosis produces genetically identical cells (meiosis increases variation via crossing over and reshuffling).
5. Saying translation is when mRNA is copied from DNA (that is transcription; translation is protein-building at ribosomes).
6. Assuming DNA passes through nuclear pores (DNA cannot fit; mRNA moves out).
7. For X-linked recessive traits, forgetting that males are hemizygous (need only one recessive X to show the trait).

✅ Exam Checklist

1. Define DNA, nucleotide, Chargaff’s rule, hydrogen bonding, double helix, and anti-parallel in DNA.
2. Use Chargaff’s rule to predict base percentages (e.g., if adenine is 23%, thymine must match).
3. Describe the roles of Rosalind Franklin’s X-ray evidence and Watson/Crick in solving DNA’s structure.
4. Explain genes, chromosomes, homologous pairs, centromere, telomere, and what a karyotype reveals.
5. State sex chromosome combinations (XX female, XY male) and how karyotypes determine gender.
6. Distinguish mitosis vs meiosis: purpose, ploidy change (2n→n in meiosis), and key stage roles (prophase/metaphase/anaphase/telophase plus cytokinesis).
7. Explain fertilisation as fusion of sperm (n) and ovum (n) to form a zygote (2n) and that embryo sex is determined by the sperm cell.
8. For protein synthesis, outline transcription (DNA→mRNA using RNA polymerase; no T in RNA, U replaces T) and translation (start at AUG; codons→amino acids; STOP ends).
9. Distinguish genotype vs phenotype and identify alleles, dominance/recessiveness, carriers (heterozygous) and degrees of dominance (complete, incomplete, codominance).
10. Construct and interpret Punnett squares for typical outcomes (e.g., Ee×Ee gives 1 EE:2 Ee:1 ee; dominant phenotype ratio 3:1).
11. Classify mutation types (point: substitution→missense/silent/nonsense; frameshift from insertion/deletion) and explain germline vs somatic mutations.
12. Apply bioethical principles: identify non-maleficence (first stop harm) and describe how integrity/justice/respect relate to given scenarios; also identify the key science-skills variables (IV/DV/controls) and evidence terms (qualitative vs quantitative, primary vs secondary).