IB Biology HL: Mutations, Gene Editing, Reproduction & Water Potential

Complete Study Guide

Topics Covered

1. Mutations and Gene Editing (D1.1, D1.2)
2. Water Potential and Osmosis (D2.1)
3. Reproduction and Development (D3.1, D3.2)
4. Gene Expression Regulation HL (D4.1, D4.2)
5. Mixed Practice — Exam Style
6. May 2026 Prediction Questions

Aligned to IB Biology HL 2025 syllabus — Theme D: Continuity and Change

---

Jump to section: Mutations & Gene Editing · Water Potential · Reproduction · Gene Expression HL · Mixed Practice · Predictions

---

Section 1: Mutations and Gene Editing (D1.1, D1.2)

A mutation is a heritable change in the nucleotide sequence of DNA. Mutations are the ultimate source of all genetic variation. Without mutation, evolution could not occur — but mutations can also cause disease.

1.1 Types of Gene Mutations (Point Mutations)

Gene mutations (also called point mutations) affect individual nucleotides within a gene. There are two main categories:

Substitution mutations replace one nucleotide with another. Because the genetic code is degenerate (multiple codons encode the same amino acid), a substitution may or may not change the amino acid sequence:

Insertion and deletion mutations add or remove one or more nucleotide bases. Because codons are read as triplets, adding or removing a base shifts the reading frame of all downstream codons — these are called frameshift mutations.

1.2 Chromosomal Mutations

Chromosomal mutations (also called chromosomal aberrations) affect the number or structure of entire chromosomes, not just single nucleotides.

1.3 Mutagens

A mutagen is any agent that increases the rate of mutation above the spontaneous background rate.

The link between mutations and cancer: proto-oncogenes (genes that promote cell division) can become oncogenes through mutation, driving uncontrolled proliferation. Tumour suppressor genes (e.g. p53, BRCA1) normally halt damaged cells; loss-of-function mutations remove this brake.

1.4 CRISPR-Cas9 Gene Editing

CRISPR-Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats) is a precise gene-editing technology derived from a bacterial immune system.

Mechanism:

1. A guide RNA (gRNA) is designed to be complementary to the target DNA sequence (up to ~20 nucleotides).
2. The gRNA binds to the Cas9 endonuclease protein, forming a complex.
3. The gRNA-Cas9 complex scans the genome for the matching sequence and binds to it.
4. Cas9 cuts both strands of the DNA at the target site (double-strand break).
5. The cell’s repair machinery then acts:
   • Non-homologous end joining (NHEJ) — error-prone; often introduces small insertions/deletions (indels) that disrupt the gene (knockout)
   • Homology-directed repair (HDR) — if a template DNA is supplied, the correct sequence is inserted (knock-in)

1.5 Gene Therapy

Gene therapy aims to treat or cure genetic diseases by introducing, altering, or replacing genetic material in a patient’s cells.

Delivery methods (vectors):

• Viral vectors — modified retroviruses or adeno-associated viruses (AAV) that cannot replicate but insert therapeutic genes. Risk: insertional mutagenesis if the virus integrates near a proto-oncogene.
• Non-viral methods — liposomes (lipid nanoparticles), electroporation, or direct injection. Generally safer but less efficient.

---

Section 2: Water Potential and Osmosis (D2.1)

Osmosis is the passive movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential.

2.1 Water Potential (Ψ)

Water potential ($\Psi$, psi) measures the tendency of water to move from one location to another. It is measured in kilopascals (kPa) and always has a value less than or equal to zero for biological solutions (pure water at atmospheric pressure = $\Psi = 0$).

The total water potential of a cell or solution is the sum of two components:

$\Psi = \Psi_s + \Psi_p$

Direction of osmosis: Water always moves from the region of higher (less negative) $\Psi$ to the region of lower (more negative) $\Psi$.

2.2 Solute Potential ($\Psi_s$)

The solute potential depends on the concentration of dissolved solutes. As solute concentration increases, $\Psi_s$ becomes more negative, which lowers the overall water potential and makes water less likely to leave (or more likely to enter).

For a simple ideal solution:

$\Psi_s = -iCRT$

where $i$ is the ionisation factor (number of particles per formula unit), $C$ is molar concentration ($\text{mol dm}^{-3}$), $R$ is the ideal gas constant ($0.00831 \text{ dm}^3\text{ MPa mol}^{-1}\text{ K}^{-1}$), and $T$ is temperature in Kelvin.

2.3 Pressure Potential ($\Psi_p$) and Turgor Pressure

In plant cells, the rigid cellulose cell wall resists expansion when water enters. As water enters by osmosis, the cell swells and the cell wall pushes back with a wall pressure. This inward pressure from the wall equals the turgor pressure pushing outward from within the cell.

• Turgor pressure is the positive $\Psi_p$ that builds up inside a plant cell when it absorbs water.
• A fully turgid cell has maximum $\Psi_p$, and because $\Psi_p$ is positive it partially offsets the negative $\Psi_s$, raising the cell’s overall $\Psi$ until eventually $\Psi_{\text{cell}} = \Psi_{\text{surroundings}}$ and net movement stops.
• A flaccid cell (lost water) has $\Psi_p = 0$; the cell’s $\Psi$ equals its $\Psi_s$ (very negative).
• Plasmolysis occurs when a plant cell is placed in a hypertonic solution: water leaves so rapidly that the plasma membrane pulls away from the cell wall. The cell is said to be plasmolysed.

2.4 Osmosis in Animal Cells

Animal cells have no cell wall, so they cannot develop turgor pressure. This makes them more sensitive to osmotic changes:

• Hypotonic solution (lower solute concentration than the cell): water enters → cell swells → may lyse (burst)
• Isotonic solution (same solute concentration as the cell): no net water movement → cell maintains normal volume
• Hypertonic solution (higher solute concentration than the cell): water leaves → cell crenates (shrinks and shrivels)

2.5 Experimental Determination of Water Potential

The water potential of plant tissue can be estimated by measuring changes in mass or length when tissue samples are placed in solutions of known concentration.

Procedure:

1. Cut uniform pieces of plant tissue (e.g. potato cylinders) and measure initial mass/length.
2. Place each piece in a series of solutions of different sucrose concentrations.
3. After a set time (e.g. 30 minutes), remove, blot dry, and measure final mass/length.
4. Calculate percentage change in mass for each concentration.
5. Plot % change in mass vs sucrose concentration.
6. The point where the graph crosses 0% change is the concentration at which $\Psi_{\text{solution}} = \Psi_{\text{tissue}}$.

---

Section 3: Reproduction and Development (D3.1, D3.2)

Reproduction ensures the continuity of genetic information across generations. Sexual reproduction introduces genetic variation through meiosis and random fertilisation; asexual reproduction produces genetically identical offspring rapidly.

3.1 Sexual vs Asexual Reproduction

3.2 Gametogenesis

Gametogenesis is the production of gametes (sex cells) through a series of mitotic and meiotic divisions. The meiosis-genetics guide covers the mechanics of meiosis itself; this section focuses on the biology of gametogenesis in animals.

3.2.1 Spermatogenesis (male gamete production)

Spermatogenesis occurs in the seminiferous tubules of the testes:

1. Spermatogonia (2n, stem cells in the germinal epithelium) divide by mitosis to maintain the stem cell population and produce primary spermatocytes.
2. Primary spermatocytes (2n) undergo meiosis I → two secondary spermatocytes (n).
3. Secondary spermatocytes (n) undergo meiosis II → four spermatids (n).
4. Spermatids undergo spermiogenesis (differentiation without cell division) → mature spermatozoa.

Each primary spermatocyte ultimately yields four functional spermatozoa.

Structure of a spermatozoon:

• Head: contains the haploid nucleus and is covered by the acrosome (a vesicle containing hydrolytic enzymes that digest the zona pellucida of the egg during fertilisation)
• Midpiece: packed with mitochondria to supply ATP for flagellar movement
• Tail (flagellum): propels the sperm

3.2.2 Oogenesis (female gamete production)

Oogenesis occurs in the ovaries and is distinctive because it is largely complete before birth:

1. Oogonia (2n) divide by mitosis and differentiate into primary oocytes during fetal development.
2. Primary oocytes begin meiosis I but arrest in prophase I. At birth, the ovaries contain all the primary oocytes a female will ever have (~1–2 million).
3. At puberty, each month one primary oocyte resumes meiosis I → secondary oocyte (n) + first polar body (discarded).
4. The secondary oocyte then begins meiosis II but arrests in metaphase II. This is the cell that is ovulated.
5. Meiosis II is completed only if the secondary oocyte is fertilised → mature ovum (n) + second polar body (discarded).

3.3 Fertilisation and Early Embryonic Development

Fertilisation is the fusion of a haploid spermatozoon and a haploid secondary oocyte to form a diploid zygote.

Stages of fertilisation in humans:

1. The sperm penetrates the corona radiata (surrounding granulosa cells) and contacts the zona pellucida (glycoprotein coat of the egg).
2. The acrosome reaction occurs: the acrosome fuses with the sperm plasma membrane, releasing hydrolytic enzymes (acrosin, hyaluronidase) that digest the zona pellucida.
3. The sperm plasma membrane fuses with the oocyte plasma membrane. The sperm nucleus enters the oocyte.
4. Immediately, the cortical reaction occurs: cortical granules beneath the oocyte membrane fuse with it and release enzymes that harden the zona pellucida → zona reaction → polyspermy block (prevents more than one sperm fertilising the egg).
5. Penetration of the sperm triggers completion of meiosis II in the secondary oocyte.
6. The haploid nuclei of egg and sperm (pronuclei) fuse → syngamy → diploid zygote.

Early development:

Stage	Events
Cleavage	Rapid mitotic divisions of the zygote; cells (blastomeres) get smaller; no growth between divisions
Morula	Solid ball of ~16 cells
Blastocyst	Hollow ball; inner cell mass (ICM, embryoblast) will form the embryo; trophoblast will form the placenta
Implantation	Blastocyst embeds into the endometrium (~6–10 days after fertilisation)
Gastrulation	ICM reorganises into three germ layers: ectoderm, mesoderm, endoderm

3.4 In Vitro Fertilisation (IVF) HL

IVF (in vitro fertilisation) is an assisted reproductive technology in which eggs are fertilised outside the body.

Key steps:

1. Ovarian stimulation: The patient receives gonadotrophin injections (FSH and LH) to stimulate the development of multiple follicles simultaneously (superovulation).
2. Egg retrieval: Mature oocytes are aspirated from follicles using an ultrasound-guided needle.
3. Fertilisation: Sperm (from partner or donor) are incubated with the eggs in a culture dish. In ICSI (intracytoplasmic sperm injection), a single sperm is injected directly into an oocyte.
4. Embryo culture: Fertilised eggs are cultured for 3–5 days (to blastocyst stage) in controlled conditions.
5. Embryo transfer: One or two viable embryos are transferred to the uterus. Remaining embryos may be frozen for future use.
6. Luteal support: Progesterone is given to support the endometrium for implantation.

---

Section 4: Gene Expression Regulation HL (D4.1, D4.2)

The DNA sequence (genotype) is not destiny — cells in the same organism share identical DNA yet have radically different structures and functions (a neuron vs a muscle cell vs a liver cell). This is because gene expression is tightly regulated. Epigenetics is the study of heritable changes in gene expression that do NOT involve changes to the DNA sequence itself.

4.1 Chromatin Structure and Gene Accessibility

DNA in eukaryotes is packaged around histone proteins to form chromatin. The fundamental unit of chromatin is the nucleosome: approximately 147 base pairs of DNA wound around an octamer of histone proteins (2 copies each of H2A, H2B, H3, and H4).

• Euchromatin: loosely packed chromatin. Genes in euchromatin are accessible to transcription factors and RNA polymerase → transcriptionally active.
• Heterochromatin: tightly packed chromatin. Genes are inaccessible → silenced.

The packaging state of chromatin is regulated by covalent modifications to histone tails — the flexible N-terminal extensions of histone proteins that protrude from the nucleosome.

4.2 Histone Modification

4.3 DNA Methylation

DNA methylation is the addition of a methyl group ($\text{–CH}_3$) to a cytosine base, usually in the context of a CpG dinucleotide (cytosine followed by guanine in the 5’→3’ direction). The enzyme responsible is DNA methyltransferase (DNMT).

Effect on gene expression:

• Methylation of CpG sites in gene promoter regions generally silences gene expression.
• Heavily methylated promoters are associated with heterochromatin and reduced transcription factor binding.
• Conversely, hypomethylation of CpG islands (clusters of CpG sites) in promoters is associated with active transcription.

Inheritance of methylation patterns: DNA methylation patterns can be copied to the daughter strand after DNA replication by maintenance methyltransferases, which is why they are described as epigenetically heritable — the pattern is transmitted to daughter cells.

4.4 miRNA Regulation of Gene Expression

MicroRNAs (miRNAs) are small (approximately 21–23 nucleotide) single-stranded RNA molecules that regulate gene expression post-transcriptionally — after the mRNA has been produced but before or during translation.

Mechanism:

1. miRNA genes are transcribed from the genome to produce pre-miRNA (a hairpin-loop precursor).
2. The pre-miRNA is processed in the nucleus and exported to the cytoplasm, where the enzyme Dicer cleaves it into a mature miRNA duplex.
3. One strand of the duplex is incorporated into the RNA-induced silencing complex (RISC).
4. The miRNA within RISC base-pairs with the 3’ UTR (untranslated region) of its target mRNA.
   • Perfect complementarity → mRNA is cleaved and degraded.
   • Partial complementarity → translation is blocked (mRNA is not degraded but cannot be translated).
5. Result: the protein encoded by the target mRNA is not produced (or produced at reduced levels).

4.5 Cell Differentiation and Epigenetic Reprogramming

All cells in a multicellular organism derive from a single zygote and have (with minor exceptions) the same genome. Cell differentiation is the process by which cells acquire specialised structures and functions through differential gene expression — different genes are expressed in different cell types.

How differentiation is established:

• During development, signalling molecules (morphogens, cell-surface signals, paracrine signals) activate transcription factors specific to each cell lineage.
• Once a cell fate is set, the epigenetic landscape (methylation patterns, histone modifications) is adjusted to stably silence genes not needed in that cell type and maintain expression of lineage-specific genes.
• This epigenetic state is inherited through subsequent cell divisions, creating stable cell lineages.

Epigenetic reprogramming can reverse differentiation:

• Induced pluripotent stem cells (iPSCs): Somatic cells can be reprogrammed back to a pluripotent state by introducing transcription factors (Oct4, Sox2, Klf4, c-Myc — the Yamanaka factors). This erases the cell-type-specific epigenetic marks and re-establishes a broadly permissive epigenome.

---

Mixed Practice — Exam Style

10 MCQs covering all sections (IB Paper 1 style)

1. [Mutations] A single nucleotide insertion in the middle of a coding sequence most likely results in:

   A. A change in exactly one amino acid in the polypeptide

   B. A frameshift affecting all codons downstream of the insertion

   C. No change in the amino acid sequence because the code is degenerate

   D. Premature termination at the insertion site

2. [Mutations] Non-disjunction in meiosis I produces gametes with:

   A. The correct number of chromosomes

   B. Either two copies of one chromosome or no copies, in all resulting gametes

   C. One extra nucleotide in the DNA sequence

   D. A chromosomal translocation

3. [CRISPR] In CRISPR-Cas9 gene editing, what is the role of the guide RNA?

   A. It cleaves both strands of the target DNA

   B. It transcribes the replacement gene into mRNA

   C. It directs Cas9 to the correct DNA sequence by complementary base pairing

   D. It repairs the double-strand break by homology-directed repair

4. [Water potential] A cell has $\Psi_s = -900 \text{ kPa}$ and $\Psi_p = +300 \text{ kPa}$. Its water potential is:

   A. $-1200 \text{ kPa}$

   B. $+600 \text{ kPa}$

   C. $-600 \text{ kPa}$

   D. $-300 \text{ kPa}$

5. [Water potential] A plant cell is placed in a hypertonic solution. Which sequence correctly describes the changes?

   A. Water enters → cell swells → turgor pressure increases

   B. Water leaves → turgor pressure decreases → possible plasmolysis

   C. Solutes enter → cell shrinks → lysis

   D. No net movement occurs because the cell wall prevents osmosis

6. [Reproduction] The secondary oocyte is ovulated and remains arrested until:

   A. The LH surge at ovulation

   B. Penetration by a sperm triggers completion of meiosis II

   C. The oocyte reaches the uterus

   D. Progesterone levels fall after menstruation

7. [Reproduction] In spermatogenesis, one primary spermatocyte produces:

   A. One functional spermatozoon

   B. Two spermatozoa and two polar bodies

   C. Four functional spermatozoa

   D. Two secondary spermatocytes that cannot divide further

8. [Reproduction] HL During IVF, multiple eggs are collected because:

   A. Each egg requires a different sperm donor

   B. Ovarian stimulation forces the ovary to release all stored primary oocytes

   C. FSH and LH injections cause superovulation, maturing multiple follicles simultaneously

   D. Each egg takes several weeks to mature in culture

9. [Gene expression] HL Histone acetylation promotes gene transcription primarily because:

   A. Acetyl groups directly activate RNA polymerase

   B. Acetylation reduces the positive charge on histones, weakening DNA–histone interactions and opening chromatin

   C. Acetylation methylates CpG islands in the promoter

   D. Acetyl groups bind to the TATA box and stabilise the transcription complex

10. [Gene expression] HL A miRNA with partial complementarity to a target mRNA will most likely:

    A. Cleave the mRNA at the base-pairing site

    B. Methylate the target gene’s promoter

    C. Block translation by sterically hindering ribosome progression

    D. Increase transcription of the target gene

Show Answers

1. B — A single nucleotide insertion shifts the reading frame for every codon downstream, producing a completely different amino acid sequence from the insertion point onwards. Option A describes a missense substitution. Option C describes a silent substitution. Option D is possible eventually (a frameshift often introduces a premature stop codon), but the primary, immediate effect is the frameshift itself.

2. B — Non-disjunction in meiosis I means both homologues move to the same pole. After two rounds of meiosis, two gametes will contain both homologues (n+1 = disomic) and two will contain neither (n-1 = nullisomic). This is distinct from meiosis II non-disjunction, which affects only two of the four resulting gametes.

3. C — The guide RNA contains a sequence complementary to the target DNA. It is the “address label” that brings Cas9 to the right location. Cas9 (not the guide RNA) performs the cutting. HDR requires a supplied DNA template, not the guide RNA.

4. C — $\Psi = \Psi_s + \Psi_p = -900 + 300 = -600 \text{ kPa}$

5. B — A hypertonic solution has a lower (more negative) water potential than the cell. Water moves out of the cell down the water potential gradient. As water leaves, turgor pressure ($\Psi_p$) decreases. If the solution is very hypertonic, water loss is sufficient to cause plasmolysis (the plasma membrane pulls away from the cell wall).

6. B — The secondary oocyte is arrested in metaphase II. Penetration by a sperm triggers a calcium ion wave that activates biochemical pathways completing meiosis II and expelling the second polar body. The LH surge triggers ovulation (completion of meiosis I), not meiosis II.

7. C — Each primary spermatocyte (2n) undergoes meiosis I to give two secondary spermatocytes (n), each of which undergoes meiosis II to give two spermatids. All four spermatids differentiate into functional spermatozoa. This differs from oogenesis where only one functional gamete forms.

8. C — Injecting FSH (and sometimes LH) stimulates multiple follicles to mature simultaneously (superovulation), yielding multiple oocytes per cycle for collection. This maximises the number of embryos available for transfer and freezing, improving overall IVF success rates.

9. B — Histones are positively charged (rich in lysine and arginine). Acetylation of lysine residues neutralises the positive charge. This weakens the electrostatic attraction to the negatively charged DNA backbone, causing nucleosomes to loosen and chromatin to become euchromatic — accessible to transcription machinery.

10. C — Partial complementarity (imperfect base pairing) between the miRNA and the 3’ UTR of the target mRNA results in translational repression: the RISC complex stalls or prevents ribosome movement. Perfect complementarity triggers mRNA cleavage and degradation (option A describes this). miRNAs act post-transcriptionally and do not directly methylate DNA.

---

May 2026 Prediction Questions

---

---

---