Stem cells, gene expression and phenotype — Edexcel International AS Level Biology
Stem cells in medicine, differential gene expression, changes to mRNA after transcription, epigenetics, multiple alleles and polygenic inheritance.
Revision notes
6 short notes, in the order of the specification. Each one in short:
A stem cell is an undifferentiated cell that can divide by mitosis and differentiate into specialised cells. Totipotent cells can differentiate into any cell type, including the cells of the placenta, and form a whole organism. Pluripotent cells can form any cell type in the body but not extra-embryonic tissues. The morula is a solid ball of cells and the blastocyst is a hollow ball.
Stem cells can be grown into specialised cells to replace those damaged by disease or injury. Using embryonic stem cells raises ethical concerns because it involves destroying embryos. Society makes decisions by weighing scientific evidence about benefits and risks against ethical, religious and economic views, and by setting laws and regulations that control the research and its uses.
All the cells of an organism contain the same genes, but each type of cell expresses only some of them. The genes that are expressed are transcribed to give active mRNA, which is translated into specific proteins. These proteins either control cell processes, as enzymes, or determine cell structure, so cells become specialised in animals and in plants.
In eukaryotes the first RNA made from a gene is pre-mRNA, which contains exons (coding) and introns (non-coding). The introns are removed and the exons joined in splicing. If the exons are joined in different combinations, different mature mRNA molecules form from the same gene, so one gene can give rise to more than one protein.
Phenotype is the result of the interaction between genotype and the environment. Epigenetic modifications change gene activity without changing the DNA base sequence. DNA methylation (adding methyl groups to DNA) and histone modification (such as acetylation) switch genes on or off, and the pattern can be passed on to daughter cells after cell division.
Some genes have more than two alleles in a population, such as the three alleles of the ABO blood group gene. Polygenic characteristics are controlled by many genes, each with a small additive effect. With the influence of the environment, polygenic inheritance gives continuous variation, in which phenotypes form a range, such as human height.
Exam questions
10 exam-style questions (34 marks), each with its mark scheme.
Answer the questionsFlashcards
16 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of cell structure, reproduction and development on one page, so you can see where this subtopic fits.
Open the mind mapPrint it
Free PDFs to print or save.
Quick check questions
What is the difference between a totipotent cell and a pluripotent cell?
Show answer
A totipotent cell can form any cell type including placental cells; a pluripotent cell can form any body cell but not extra-embryonic tissue.
What is an exon?
Show answer
A coding section of a gene, which is kept in the mature mRNA.
How does DNA methylation affect gene expression?
Show answer
It usually switches the gene off by preventing transcription.
Name the three alleles of the ABO blood group gene.
Show answer
Iᴬ, Iᴮ and Iᴼ.
Why do cells with the same genes become different cell types?
Show answer
Different genes are expressed in each cell type, giving different active mRNA and proteins.
Frequently asked questions
What is the difference between pluripotent and totipotent stem cells?
Totipotent cells, such as those in the early morula, can become any type of cell including the placenta, so they can form a whole organism. Pluripotent cells, such as the inner cell mass of the blastocyst, can become any body cell but not the extra-embryonic tissues.
How does a cell become specialised?
A cell becomes specialised through differential gene expression. Only some of its genes are transcribed to active mRNA, which is translated into specific proteins. These proteins either control cell processes as enzymes or form part of the cell's structure, so the cell takes on a particular shape and function.
How can one gene code for more than one protein?
After a eukaryotic gene is transcribed to pre-mRNA, introns are removed and exons are joined together. By joining the exons in different combinations (alternative splicing), different mRNAs are made from the same gene and are translated into different proteins.
What are epigenetic changes?
Epigenetic changes are chemical modifications, such as DNA methylation and histone modification, that switch genes on or off without changing the DNA base sequence. They can be influenced by the environment, and they are passed on to daughter cells when the cell divides.
What is the difference between continuous and discontinuous variation?
Continuous variation shows a range of values with no clear categories, such as height. It is polygenic and affected by the environment. Discontinuous variation has distinct categories with no intermediates, such as ABO blood groups, and is controlled by one or a few genes.
Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026