3.12, 3.13 - Differential gene expression and organisation
A muscle cell and an insulin-secreting cell can contain the same genes yet carry out very different jobs. In 3.12, you will trace how selective gene expression produces specialised cells, and how the bacterial lac operon regulates protein production. In 3.13, you will connect specialised cells to the tissues, organs and systems of animals and plants.
Same genes, different expression
Most nucleated body cells of one animal, or one plant, inherit essentially the same set of genes by mitosis, but they do not use every gene equally.
Differential gene expression
The expression of different genes, or different levels of expression of the same genes, in different cells or conditions.
For a protein-coding gene, gene expression involves transcription to produce mRNA and translation of that mRNA to make a polypeptide. In a eukaryotic cell, the initial RNA is processed into mature, functional mRNA that can be translated by ribosomes. This is what is meant here by producing active mRNA: an available message for protein synthesis, rather than a new kind of gene or an enzyme that makes the protein itself.
During differentiation, an unspecialised cell develops features suited to a particular role. Signals can alter the activity of regulatory proteins, including transcription factors, which affect whether particular genes are transcribed. Different available mRNAs lead to different proteins, or different amounts of those proteins. Some proteins form cell structures; enzymes and regulatory proteins control cell processes. These differences produce the cell's specialised structure and function.
For example, increased expression of genes for particular contractile proteins allows a developing muscle cell to build abundant contractile machinery. The genes for many other specialist functions remain in its DNA even when they are not being transcribed. Differentiation normally changes the pattern of expression, rather than deleting all the genes the cell does not currently need. Cells also share expression of many genes needed for common activities, such as protein synthesis and respiration.
Distinguish three things: a gene can be present in DNA, its mRNA can be produced, and its protein product can be made and function. Evidence for one is not automatically evidence for all three.
The same-genome statement has limits: mature mammalian red blood cells, for example, have lost their nucleus. It describes the usual basis of differentiation in nucleated cells, not every specialised cell at every stage.
Protein patterns and specialisation
The consequences of differential expression become clearer when the protein product is linked to what the cell actually does.
In an animal, a muscle cell produces large amounts of specialised actin and myosin proteins, assembled into contractile structures. Their interaction produces force, so the cell can contribute to movement or pumping. A pancreatic beta cell expresses the insulin gene and translates its mRNA into the precursor from which insulin is produced. Its secretory machinery processes and releases the hormone. The two cell types have different specialist outputs, despite usually carrying the genes for both outputs. A protein can therefore help determine structure, act in a cell process, or do both.
The same principle operates in plants. A leaf palisade mesophyll cell expresses genes needed for chloroplast development, chlorophyll production and photosynthetic proteins. Its many chloroplasts and photosynthetic enzymes support light absorption and carbon fixation. A root hair cell instead expresses a pattern of genes supporting its long outgrowth and the membrane transport proteins used in mineral-ion uptake. Its extension increases the area in contact with the soil; transport proteins help move particular ions across its membrane. These are linked outcomes of development, not evidence that the root cell has discarded all genes associated with photosynthesis.
Do not interpret specialisation as “one cell expresses one gene”. Many genes act together to produce a functional cell, and expression can change with development or the environment. Nor does the presence of mRNA guarantee an exactly proportional amount of working protein: translation, protein processing and protein breakdown also affect the final outcome.
When interpreting evidence, ask what was measured. Detecting a gene in a DNA sample establishes its presence. Detecting its mRNA supports expression at the RNA level. Measuring its protein or activity tests a later step. For instance, finding the same gene in leaf and root DNA does not show that both make equal amounts of its protein. An mRNA difference supports differential expression, but a claim about enzyme activity needs an activity measurement or other suitable evidence.
The lac operon and its control regions
The bacterium Escherichia coli can regulate genes for using lactose according to its surroundings, providing a relatively simple example of control at transcription.
Operon
A group of genes transcribed together from a shared promoter, with associated DNA control regions that regulate their transcription.
The lac operon contains three structural genes. Here, structural gene means a gene coding for a functional protein product, rather than a DNA control site. The name does not mean that every product is a structural component of a cell.
| Component | Role |
|---|---|
| Promoter | DNA region where RNA polymerase binds to initiate transcription. |
| Operator | Regulatory DNA sequence to which the lac repressor binds. It is close to, and partly overlaps, the promoter region in the real control arrangement. |
| lacZ | Codes for beta-galactosidase, which hydrolyses lactose to glucose and galactose and also forms the inducer allolactose from lactose. |
| lacY | Codes for lactose permease, a membrane protein that promotes lactose entry. |
| lacA | Codes for a transacetylase. Do not confuse this with the enzyme that hydrolyses lactose. |
The three structural genes are transcribed into one mRNA containing the coding information for three proteins; ribosomes translate its separate coding regions. A separate regulatory gene, lacI, has its own promoter and codes for the repressor protein. The repressor is not made by the operator: the operator is DNA, whereas the repressor is a protein that can bind to it.
When lactose is absent, very little allolactose is available. The repressor binds the operator and interferes with productive transcription by RNA polymerase. Very little lac mRNA is made, so production of the lactose-use proteins stays low. This avoids making large amounts of proteins for an unavailable substrate. Repression is not an absolutely sealed switch: low basal expression leaves small amounts of permease and beta-galactosidase available, allowing an initial response when lactose becomes available.
This operon is a bacterial example of selective expression. Animal and plant differentiation also involves gene regulation, but their specialised cells are not explained by copying the lac operon's three-genes/one-mRNA arrangement into every eukaryotic cell. Eukaryotic protein-coding genes are generally regulated and transcribed individually through more complex control systems.
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