Differentiation and Variation — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

A stem cell is an undifferentiated cell that can divide by mitosis an unlimited number of times.

Adult stem cells are found in tissues such as bone marrow (used to produce blood cell types) and the brain (neural and glial cells). Small numbers of these remain to produce cells for growth, cell replacement and tissue repair.

Because stem cells can differentiate, they are very valuable in research and treatment, as they can replace damaged tissues and cells.

The use of stem cells collected from embryos created by IVF is ethically questionable, because it destroys a viable embryo (one that could become a foetus if implanted in a uterus).

Stem cells become specialised through differential gene expression: only certain genes in the DNA of the stem cell are activated and expressed.

Differential gene expression is controlled in eukaryotes by transcription factors: proteins that control the transcription of genes by binding to a specific region of DNA, usually near the gene's promoter.

In prokaryotes gene expression is often controlled using operons.

The lac operon controls the use of lactose. The components in order are the regulatory gene (coding for the repressor protein), the promoter, the operator, and the structural genes lacZ (coding for lactase, beta-galactosidase), lacY (coding for lactose permease, which moves lactose into the cell) and lacA.

An organism's internal or external environment can influence gene expression. The levels of regulatory proteins or transcription factors can change in response to stimuli such as light, and to chemicals including drugs and hormones. For example, enzymes are activated by ultraviolet radiation and increase expression of melanin-producing genes, so more melanin is made and the skin becomes pigmented (tanning).

Epigenetics is the study of heritable changes in gene function that do not involve changes in the DNA base sequence.

The amino acid lysine has a positively charged R group, which forms ionic bonds with the negatively charged phosphate backbone of DNA.

Evidence shows that modifications to the epigenome in one generation can be passed on to the next generation at the cellular or whole-organism level.

The observable characteristics of an organism are its phenotype, and phenotypic variation is the difference in phenotypes between organisms of the same species.

The environment can also affect the phenotype.

Exam questions may ask why variation due to genetics is inherited but variation due to the environment is not. Genetic variation directly affects the DNA of the gametes; variation caused by the environment does not.

Variation in the phenotypes of organisms can be due to qualitative or quantitative differences. Discontinuous variation involves qualitative differences: individuals fall into distinct categories with no intermediates, for example human blood groups (A, B, AB, O).

Some phenotypes are affected by many genes (polygenic inheritance), each with multiple alleles at different loci, as well as by the environment. This often gives rise to continuous variation, because the many combinations of alleles give many different phenotypes.

Where the alleles are additive, each dominant (or contributing) allele adds the same amount to the phenotype.

Be careful

• it is not a given that each gene has the same effect on the phenotype as in the example.
• Check the information in the question.

Continuous data in a sample of a population usually give a normal distribution: a symmetrical bell-shaped curve in which most individuals are near the mean and few are at the extremes.

Example of an additive model with three genes: if each upper-case allele adds 2 cm, the genotype AABBCC adds 12 cm, AaBbCc adds 6 cm and aabbcc adds 0 cm, so there are seven phenotype classes, which form a bell-shaped distribution when plotted.