How should we define DNA (or in some cases, the nucleus) in secondary school? Since beginning teaching biology I have seen such things as:
The DNA/nucleus is the brain of the cell
The DNA/nucleus is the control centre of the cell
The DNA/genes are the blueprint for the body
The DNA/genes are the instructions for the body
And later in secondary education:
A gene codes for a trait / characteristic
All of which are poor metaphors leading to misconceptions. If we consider the value of these metaphors in terms of match (how well they describe the real thing), and range (how far they can take student understanding in education before it needs to be modified), then all rate poorly.
Let's look at them in more detail.
The DNA/nucleus is the brain of the cell
The DNA/nucleus is the control centre of the cell
These two metaphors invite the student to believe that:
a single cell can think,
that there is an innate feeling of purpose and direction, and
that a cell somehow consciously knows about its parts, and can exert some sort of direct control over them.
All these points are major obstacles for understanding how cells differ to inanimate matter. Cells are dynamic systems that emerge from countless interactions of molecules that encounter each other, often due to simple Brownian motion. The control and homeostasis of the cell lies not so clearly in the genetic material, but in feedback loops involving the whole cell and it's context.
In this case, the genetic material plays a role in these interactions, but not the role, in the cell's state.
How about these?
The DNA/genes are the blueprint for the body
The DNA/genes are the instructions for the body
These two anthropomorphic metaphors invite the student to believe in a design element to biology. Both blueprints and instructions are artifacts, which are designed to be followed for a purpose. The reading of blueprints and instructions invites a student to believe that a cell somehow has the ability of interpretation and conscious knowledge of the bigger picture.
The design of blueprints and instructions can lead a student towards believing in Intelligent Design rather than forming an understanding of natural selection.
Furthermore, they liken the organism to a machine despite them differing in many aspects. Biological systems reproduce themselves and are flexibly robust with the ability to repair or compensate, through other processes, for any problems a system may incur.
A gene codes for a trait / characteristic
This final example metaphor, as with the previous two, encourage the student
to think with a genetically deterministic philosophy. Such erroneous outlooks on biology often shore up the equally fallacious social arguments of racists. Genes do not code for a trait, but for a protein or an RNA. It is the complex interactions of gene products and gene expression with the environment that produce the phenotype.
Evo-devo, and phenotypic plasticity both show the crucial importance of environment on phenotype such that it is not a case of the old nature vs nurture dichotomy, but phenotype resulting from many interactions between genes and their products, and the environment. Dominant and recessive alleles, especially taught in the form of hereditary diseases, represent rare examples of simplicity compared to the totality phenotype causes.
So how should we define DNA to our secondary students, and what metaphors may be useful? Our youngest secondary students should be introduced to the big idea of DNA & information early in secondary biology education. Here I want to build a sound foundation for us to build upon over several years, but to also avoid unnecessary detail that may too abstract and confusing at this point in their biology education.
Ingo Brigandt suggests we should avoid all metaphors of machines and information in biology, but personally I like the word information as it has good match and range. If there are problems with it building misconceptions with a design principle, then we can attend to it, just as we may discuss problems with scientific models.
The next problem is to say what the information is that DNA contains. To evade forming ideas of genetic determinism we need to avoid anything that insinuates that the information pertains to the individual, as if we could read DNA and 'know' how that person will be.
Here's how I am currently teaching it to my Year 7 students.
During our first biology lesson I introduce unicellular organisms, using fantastic videos to see how they move and interact. It's not the DNA that is controlling the movements, but the living cell. The living cell has the goals and purpose, but where is the information for building the parts of the cell? In the DNA.
The definition I give at this stage is:
DNA provides information to build the parts of a cell.
The next step is to understand how a multicellular organism can form if DNA only provides information for cells. To show this, I have made the following image:
Here, again, the wording is important, I have chosen the word interacting carefully, and I have tried to make clear the influence of the environment.
In summary I see this as representing three things that I can show my Year 7 students:
DNA provides information to build the cell, but the organism is ultimately formed through
Interactions between cells
Interactions between the cells and the environment.
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