Stem cells and differentiationEdexcel International A Level Biology: Revision notes
Section 1
Stem cells and potency
A stem cell is an undifferentiated cell that can divide by mitosis and differentiate into specialised cell types. How many types it can become is called its potency.
- Totipotent cells can differentiate into any cell type, including the extra-embryonic cells that form the placenta. They can form a whole organism.
- Pluripotent cells can differentiate into any cell of the body but not placental cells.
Differentiation is the process by which an unspecialised cell becomes a specialised cell.
Pluripotent cells cannot form the placenta. Only totipotent cells can form every cell type, including placental cells.
Section 2
The early human embryo
After fertilisation the zygote divides repeatedly (cleavage) to form a solid ball of cells, the morula. The cells of the early morula are totipotent.
The morula develops into a hollow ball, the blastocyst. It has an outer layer of cells that will form the placenta, a fluid-filled cavity, and a cluster of cells called the inner cell mass. The inner cell mass contains pluripotent embryonic stem cells that can form all the tissues of the body.
Section 3
Stem cell therapies and society
Stem cells could be used to replace damaged cells, for example in treating type 1 diabetes, spinal injury or blood disorders.
Science can show what is possible, but society decides whether it should be done. Benefits: possible treatments for serious disease; surplus IVF embryos would otherwise be discarded. Concerns: obtaining embryonic stem cells destroys an embryo, which some people see as a human life; transplanted cells may be rejected, or form tumours.
Because of this, stem cell research is regulated by law and licensing, and each person weighs benefit against harm. In an evaluation, finish with a justified judgement.
Section 4
Differential gene expression
Every body cell of an organism contains the same genes. Cells become specialised because different genes are expressed in different cell types. This is differential gene expression.
Signals cause transcription factors to bind to DNA near particular genes and switch their transcription on or off. Only active genes are transcribed to mRNA and translated, so each cell type makes its own proteins, for example insulin in β cells and myosin in muscle cells. These proteins give the cell its structure and function.
A specialised cell still contains the genes it does not use. Say 'not transcribed' or 'switched off', never 'not present'.
Section 5
One gene, more than one protein
A gene is transcribed to a primary mRNA transcript containing exons (coding regions) and introns (non-coding regions). Before leaving the nucleus the introns are removed and the exons are spliced together.
In alternative splicing, different exons are kept or removed in different tissues, so one gene gives different mature mRNAs. These have different codon sequences, so they give different amino acid sequences, different tertiary structures and different proteins. Example: a gene with exons 1 and 4 always kept and exons 2 and 3 optional can give 2 × 2 = 4 different mRNAs.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Stem cells and differentiation
- An embryologist at a fertility clinic is observing the early development of a human embryo created by in vitro fertilisation. Three days after fertilisation the embryo is a solid ball of cells. By day five it has developed into a hollow structure with a cluster of cells on one side.Explain why a single cell from the morula could develop into a complete organism, whereas a single cell from the inner cell mass of the blastocyst could not.2 marks
- A pancreatic β cell and a skeletal muscle cell from the same person both contain the gene for insulin and the gene for myosin. The β cell makes large amounts of insulin and no myosin, whereas the muscle cell makes large amounts of myosin and no insulin.Explain why the muscle cell does not produce insulin even though its nucleus contains the insulin gene.2 marks
- A single human gene contains four exons separated by introns. When the gene is transcribed, the primary mRNA transcript is processed before it leaves the nucleus. In different tissues the same gene is used to make different proteins.Describe how the primary mRNA transcript is processed to give mature mRNA, and how this can produce different mature mRNAs from the same gene.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).