Pearson Edexcel International A Level Biology: Stem Cells, Gene Expression and Phenotype (YBI11) – Revision Notes

Quick recall for specification statements 3.17 to 3.21 of the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2. These statements are in Topic 3, which is part of the International AS (IAS) content. For full explanations and longer worked examples use the study guide, then test yourself with the practice questions.

Course pages: Edexcel A Level Biology hub, printable checklist, free 10-minute diagnostics and the IAL Biology exam preparation page.

Key terms at a glance (3.17 i)

Term	Learn this
Stem cell	Unspecialised cell that divides by mitosis, renews itself and can differentiate
Totipotent	Can form every cell type plus extra-embryonic tissue (placenta); whole organism possible
Pluripotent	Can form all body cell types; cannot form extra-embryonic tissue, so not a whole organism
Morula	Solid ball of cells from early cleavage
Blastocyst	Hollow ball: trophoblast (outer, to placenta) and inner cell mass (to embryo) around a cavity

Order of development: zygote, cleavage divisions, morula, blastocyst. Embryonic stem cells are taken from the inner cell mass.

Cell number reminder. Cells after n doublings = starting number × 2ⁿ. A culture of 500 stem cells doubling every 8 hours for 48 hours has 6 doublings: 500 × 2⁶ = 32 000 cells.

Using stem cells: the discussion (3.17 ii)

Build every answer from four strands:

• Benefit: treat disease or injury, for example replacing damaged blood cells.
• Risk: tumour formation, rejection, unknown long-term effects.
• Ethics: embryo destroyed for embryonic cells; consent for donors; fairness of access and cost.
• Who decides: scientists supply evidence, while society, through laws, regulators and public debate, decides what is acceptable. Rules differ between countries.

Compare sources: embryonic (pluripotent, wide range, embryo destroyed), adult (no embryo destroyed, narrower range), cord blood (harmless to collect, limited range).

Differential gene expression (3.18)

Method in steps, for any “explain how a cell became specialised” question:

1. All cells from one zygote hold the same genes.
2. A signal or transcription factor activates certain genes in that cell.
3. Those genes are transcribed to give active mRNA.
4. mRNA is translated to proteins.
5. Proteins control processes (enzymes) or determine structure (for example keratin, haemoglobin).
6. Other genes stay unexpressed.

Say “expressed”, never “has different genes”. The idea applies to animals and plants: a xylem vessel element expresses genes for wall-thickening and lignin, while a palisade cell expresses genes for chlorophyll and photosynthetic enzymes.

Worked reminder. Relative mRNA: gene P is 0 in cell X and 85 in cell Y; gene Q is 60 in both. Q is expressed in both cell types; P is switched on only in Y. The cells have the same genes but different expression.

Alternative splicing (3.19)

• Pre-mRNA has exons and introns. Introns are removed and exons joined to form mature mRNA.
• Alternative splicing: different exons are kept or removed, giving different mRNAs and so different polypeptides from one gene.
• Also add: 5’ cap and poly-A tail, which protect mRNA from breakdown.
• Result: more proteins than genes (about 20 000 protein-coding genes in humans).

Worked reminder. A gene has four exons: exon 1 = 54, exon 2 = 39, exon 3 = 81, exon 4 = 45 nucleotides (stop codon in exon 4). Exon 2 is optional.

• Without exon 2: 54 + 81 + 45 = 180 nucleotides = 60 codons = 59 amino acids.
• With exon 2: 219 nucleotides = 73 codons = 72 amino acids.

Divide by 3, then subtract 1 for the stop codon, which codes for no amino acid.

Genotype, environment and epigenetics (3.20)

Idea	Key points
Phenotype	Genotype plus environment, interacting
Genetically identical organisms differ	Difference is caused by environment
DNA methylation	Methyl groups added to cytosine, often at promoters; gene expression falls
Histone modification	Acetylation loosens chromatin and raises expression; deacetylation tightens it and lowers expression
Epigenetic marks after division	Methylation pattern on the parent strand is copied onto the new strand, so daughter cells keep it

Epigenetic change alters activity, not the base sequence. It is how a specialised cell lineage keeps its identity through many mitoses.

Worked reminder. A promoter has 30 CpG sites and 12 are methylated: 12 ÷ 30 × 100 = 40% methylated. If methylation rose, expect less mRNA from that gene.

Multiple alleles and polygenic inheritance (3.21)

• Multiple alleles: three or more alleles of one gene exist in the population, but a diploid carries two. n alleles give n(n + 1) ÷ 2 genotypes, so 4 alleles give 10 and 5 alleles give 15.
• ABO: IA and IB codominant, IO recessive. Always give both parents’ genotypes before predicting the children.
• Polygenic: several genes, additive effects, plus environment, giving continuous variation and a bell-shaped distribution.

Worked reminder. Cross a group A parent (IA IO) with an AB parent (IA IB). Offspring: IA IA, IA IB, IA IO, IB IO. Group A = 2/4 = 1/2, AB = 1/4, B = 1/4, O = 0.

Three-gene reminder. Cross AaBbCc × AaBbCc with six alleles. Capital allele totals 0 to 6 give 7 classes with frequencies 1, 6, 15, 20, 15, 6, 1 (total 64). Either extreme has probability 1/64, and the middle class has 20/64 = 5/16.

Reading data questions

Most data questions in this topic give a table of expression, methylation or phenotype values. Work in this order:

1. Name what is measured on each axis or column, with units.
2. Quote values, including a calculation such as a percentage change or a ratio, for example (new - old) ÷ old × 100.
3. State the pattern in words: higher methylation goes with lower mRNA, or a larger difference between sites.
4. Link the pattern to the biology: methylation at the promoter reduces transcription factor binding.
5. Add a limit: a correlation alone does not show cause, and small samples limit confidence.

Worked reminder. CpG sites methylated at a promoter: tissue F 3 of 30, tissue G 24 of 30. Tissue F is 10% methylated and tissue G is 80%. G has 8 times the methylation of F (80 ÷ 10). If mRNA from this gene is 120 units in F and 15 units in G, then G has 105 fewer units, which is a fall of 105 ÷ 120 × 100 = 87.5% compared with F. The pattern fits methylation switching the gene down, but the data alone do not prove it.

Must-know distinctions

• Totipotent versus pluripotent: placenta or not.
• Morula versus blastocyst: solid versus hollow.
• Gene versus gene expression: holding versus using.
• Exon versus intron: kept versus removed.
• Multiple alleles versus polygenic: one gene with many alleles versus many genes.
• Continuous versus discontinuous variation: measured on a scale versus distinct categories.

Quick self-test

1. Define pluripotent.
2. Name the two parts of a blastocyst.
3. A culture of 200 stem cells doubles every 6 hours. How many cells after 24 hours?
4. Why can a muscle cell and a liver cell contain the same genes but make different proteins?
5. A gene has two optional exons that are included independently. How many different mRNAs are possible?
6. State two ways epigenetic marks can reduce gene activity.
7. How does a methylation pattern survive DNA replication?
8. Two genetically identical plants differ in height. What causes this?
9. How many genotypes are possible with 6 alleles of a gene?
10. A group O woman and an AB man have a child. List the possible blood groups and their probabilities.
11. What is the probability of an extreme phenotype in a trait controlled by two additive genes (cross AaBb × AaBb)?
12. Give one reason why a trait with polygenic inheritance shows continuous variation.

Answers

1. Can differentiate into any body cell type but cannot form extra-embryonic tissue.
2. Trophoblast (outer layer) and inner cell mass.
3. 24 ÷ 6 = 4 doublings, 200 × 2⁴ = 3200 cells.
4. Different genes are expressed in each, so different mRNA and proteins are made.
5. Each optional exon is in or out: 2 × 2 = 4 mRNAs.
6. DNA methylation at promoters, and histone changes that pack chromatin tightly (deacetylation).
7. Enzymes copy the pattern from the methylated parent strand onto the new strand.
8. The environment, since the genotype is the same.
9. 6 × 7 ÷ 2 = 21.
10. Mother IO IO, father IA IB. Children IA IO (group A) 1/2 and IB IO (group B) 1/2. Group AB and O have probability 0.
11. 1/16 for each extreme (all four capitals, or none).
12. Many genes each add a small effect, so there are many phenotype classes that overlap, and the environment smooths the distribution further.

Where marks are usually lost

• Writing “different genes” instead of “different genes expressed”.
• Calling blastocyst cells totipotent. Inner cell mass cells are pluripotent.
• Giving only one side of a stem cell discussion, or stating an opinion with no evidence or reasoning.
• Forgetting to subtract the stop codon when converting nucleotides to amino acids.
• Describing alternative splicing as a change to the DNA.
• Saying epigenetic marks change the base sequence.
• Not giving both parental genotypes in a multiple alleles cross.
• Calling every continuous trait “genetic only” and forgetting environment.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology, Specification, Issue 2 (February 2021), Pearson Education Limited: Topic 3, statements 3.17 to 3.21.