How cells divide and control their genes shapes development. This HL lesson covers D2.1 Cell & nuclear division, D2.2 Gene expression (HL) and D2.3 Water potential.
You will compare mitosis and meiosis, study epigenetic regulation, and use water-potential calculations.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you are ready.
D2.1 Cell division
Mitosis and meiosis
Mitosis: one division giving two genetically identical diploid cells for growth and repair.
Meiosis: two divisions giving four haploid gametes. Variation arises from crossing over and independent assortment. Non-disjunction (failure to separate) can cause conditions such as trisomy 21.
Calculate
Your turn — calculate
1A plant cell has a solute potential Ψs of −600 kPa and a pressure potential Ψp of +250 kPa. Using Ψ = Ψs + Ψp, calculate its water potential Ψ.
kPa
Hint: Ψ = −600 + 250.
D2.2 Gene expression
Gene expression and epigenetics (HL)
All body cells share the same genome, yet differ because of differential gene expression — only some genes are switched on.
Regulation involves transcription factors and epigenetic marks:
DNA methylation tends to silence genes;
histone modification (e.g. acetylation) opens or closes chromatin.
Epigenetic changes alter phenotype without changing the DNA base sequence, and the environment can influence them.
Calculate
Your turn — calculate
2A diploid human cell has 46 chromosomes. State the number of chromosomes in each cell immediately after meiosis I (a haploid number, each chromosome still having two chromatids).
chromosomes
Hint: meiosis I separates homologous pairs, halving 46 to give a haploid number.
D2.3 Water potential
Water potential and osmosis
Water potential (Ψ) predicts the direction of osmosis: water moves from higher to lower water potential.
Ψ = Ψs + ΨpΨs (solute) is negative; Ψp (pressure) is usually positive
In a hypotonic solution a plant cell becomes turgid; in a hypertonic solution it loses water and may plasmolyse. Animal cells lack a wall, so they can swell and burst or shrink (crenate).
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Mitosis
Meiosis
Epigenetic mechanism
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Question
?Epigenetic changes such as DNA methylation alter the phenotype by...
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Question
?Genetic variation in gametes arises from crossing over and...
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Question
?A cell placed in a solution with a more negative water potential than the cell will...
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Match each term to its correct description.
Item
Partner
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Question
?Compared with mitosis, meiosis produces cells that are...
Recap
The big ideas to take away
D2.1: mitosis (identical diploid) vs meiosis (varied haploid); non-disjunction
D2.2 (HL): differential gene expression; methylation and histone modification
D2.3: Ψ = Ψs + Ψp; osmosis toward lower water potential; turgor and plasmolysis
Skills: water-potential arithmetic; halving of chromosome number in meiosis
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