Phenomenon
In our families we saw that proteins were causing problems yet DNA—not protein—is passed from parent to offspring.
Question
What is the connection between DNA and protein?
What is the connection between DNA and traits?
(There are also ideas in our model that more directly address related questions that are really more about inheritance such as, "How can the cell make an exact copy of the DNA sequence when it divides?")
Model Ideas
Note: The Teacher Notes included in the PowerPoint (available in the compressed Downloadable Resources files at the bottom of the page) present a couple of different versions of the model in terms of wording. The key ideas, however, are the same. Be sure to read both before you start and decide how you will honor student ideas and student language when building the model together.
- DNA is the hereditary material and it codes for the protein.
- DNA is in a code and it is read by the base pairing rules: A pairs with T and C pairs with G. This base pairing allows the cell to make an exact copy of DNA to pass along to daughter cells.
- Because DNA is in the cell’s nucleus and ribosomes (where proteins are made) are found in the cytoplasm, a messenger RNA molecule transports the code from the nucleus to the ribosomes where translation of the code occurs.
- The messenger RNA is read in groups of 3 bases, that corresponds to an amino acid, the ribosome then strings the amino acids together to make a protein.
- Depending on the function of the protein, you will see different patterns of phenotypes (dominance versus codominance) in heterozygotes.
- When multiple genes are involved in determining a trait, patterns of inheritance are more complex.
- Errors in the processing of DNA can lead to changes in the genetic material and are called mutations.
- Mutations are a source of genetic variation.
- Not all DNA codes for a protein. (The reason behind this is left open for the moment and is picked up in other models.)
Overview
Transition in: We've been tracking family traits across generations in order to make sense of the patterns of inheritance. Now we dive into an examination of the molecule of inheritance, DNA.
We have figured out that each parent contributes genes to their offspring and that those genes interact to cause the offspring to look or behave a certain way. Basically, we inherit traits from our parents—a concept we’ve had a handle on for a while. But now we begin to wonder what the link might be between the genes we inherit and the traits that make us who we are. We recognize through examination that a number of the traits we’ve studied thus far can be linked back to a protein, spurring us to ask the more immediate question: how is DNA linked to protein?
We do a bit of review about Chromosomes, genes, and DNA , we track the historical data that led to Watson and Crick’s model for DNA. With this we can reason first about how the cell makes an exact copy of the DNA every time it divides (in mitosis or meiosis) and then how it might be a code for assembling proteins. After working through some of those details, we spend some time thinking about the role mutations play in generating variation and how that variation may or may not affect the protein structure and function. We pull together our ideas about the links between DNA, proteins and traits by trying to understand more completely what is happening at the molecular level in achondroplasia and PKU, two heritable diseases we explored in Classical Genetics.
Finally we wonder about the link between mutation, variation in traits, and evolution by natural selection. The unit ends with a bit of a new mystery: it turns out that most of our DNA (over 98% in humans) doesn’t directly code for protein at all. So what is it all for!?! We take up this phenomenon in Growth and Development, though not right away.
Transition out: Now that we have a broad understanding of how the instructions encoded in DNA are a recipe for building us trait by trait, we wonder how that process unfolds during growth and development.
Advanced Planning
Check "Advanced Planning" for Growth and Development.
Though the materials for this unit require little preparation (unless you choose to engage in Optional Learning Segment B - see details), you will need to start generating the phenomenon for the next unit, Growth and Development. The wet labs require you order animals and start observations in the unit ahead of G&D. For most classrooms, the unit leading to G&D will be this one.
1. We return to our families from Classical Genetics and reason that proteins are causing the different diseases. Yet we think that DNA is the molecule that is passed down across generations. So we formulate a driving question: How does that work? What is the link between DNA and protein?
We noticed that proteins cause the diseases but the protein is not passed from parent to offspring. DNA is. So we are ready to explore some new connections.
2. We review the relationships between chromosomes, genes and DNA and use this a motivation to list what we already know and what we want to know about DNA.
We confirmed the relationship between chromosomes, DNA, and genes. We have a list of what we know, or think we know about DNA, and we have even more questions.
3. We look at data and evidence from previous research studies to uncover the structure of DNA.
DNA structure has a pattern in which there are base pairs: In DNA, A pairs with T and C pairs with G. We may have also figured out that the structure of DNA pairs bases via weak hydrogen bonds. We’ll leverage these model ideas to make sense of replication next.
4. We return to our question about transmission of DNA, which includes the idea of replication. We leverage the ideas we just pieced together about DNA’s structure in order to generate an explanation for how the cell makes an exact copy of the DNA prior to mitosis or meiosis.
We used our structural model to figure out how cells make an exact copy of DNA before they divide.
5. We dig a little deeper into the history of the discovery of the structure of DNA to help develop our model.
We’ve reinforced the model ideas we’ve generated so far by reading Francis Crick’s letter to his son.
6. We return to wondering how DNA and proteins are connected. Since we know about the structure of DNA, we look at what makes up a protein and try to make some connections between the two molecules.
*extra time may be needed if your class has yet to consider protein structure
At the end of this segment, we have a conversation about the state of our model before moving on.
DNA is the hereditary material that codes for the protein. Every 3 nucleotide is a codon that codes for 1 amino acid in a protein and this code has a redundancy. These ideas begin to directly address our Driving Question.
7. We further problematize an aspect of the DNA to protein process that we just figured out: how does DNA isolated in the nucleus inform protein synthesis in the cytoplasm? With this new phenomenon of spatial separation, we need to add to our model in order to generate a sensible explanation.
We figured out that messenger RNA (mRNA) provides the link between nuclear-bound DNA and the protein-making machinery in the cytoplasm. Our ideas about the link from gene to protein are fairly well fleshed-out.
8. We return to the family pedigrees to figure out how changes (mutations) in the DNA sequence can lead to the observed health conditions and begin to reinforce some of our understanding of the link between DNA and traits.
We tracked variation in traits to the source: mutation.
Mutations are a source of genetic variation that may lead to changes in protein sequences. But we are perhaps still left with some questions about the connection between the protein changes and the variants in the traits.
9. We now more completely discuss proteins for two of the family pedigrees to examine what is happening with the patterns of inheritance and their explanations at the molecular level.
Depending on the function of the protein, you will see different patterns of phenotypes in the heterozygote (dominance/recessiveness versus codominance) resulting from the protein-protein interactions of the two alleles.
10. We apply our model back to an old phenomenon: variation in traits in a population. This allows us to reflect on the complex nature of some traits and also to recognize a key concept: the variation we see in nature arises from mutation.
We recognized that not all mutations are bad. We’ve connected our ideas about inheritance and how genes code for traits to our evolutionary models through our ideas about mutations. They are the original source of variation that is carried along in complex ways in our mapping of DNA to traits. Changes in traits not only depend on changes in proteins, but also on the interactions of those proteins whether stemming from one or multiple genes.
11. We briefly examine some data that makes us recognize that much of our DNA “doesn’t code for anything”. So, what is it for? We table this for now, assuring students we’ll come back to it later.
We’ve discovered that only a small fraction of our DNA is made up of genes that code for protein. This leaves us wondering: what is the rest of it for?
Optional Learning Segment A: How do we know DNA is the “molecule of inheritance”?
We review the historical experiments that helped 20th century scientists develop a body of evidence for DNA as the carrier of hereditary information. Best positioned as part of Learning Segment 01. (See presenter notes in PowerPoint.)
We reviewed the historical experiments that provided evidence for DNA as the hereditary molecule (thus confirming the role of chromosomes in transmission of hereditary information) and reinforced our ideas of science as a process, not only a body of knowledge.
Optional Learning Segment B: The Details of Protein Synthesis.
This Optional Learning Segment can serve as a substitute for LS06 in order to more deeply track the processes of transcription and translation. Note: NGSS no longer calls us to explore the details of these process (e.g. no requirement to cover modifications to mRNA or the role of tRNA).
Note: The materials for this Learning Segment are not officially a part of the MBER-bio resources at this time. We include them, however, because many teachers will continue to want an option for teaching the details of protein synthesis. The materials are open source and the activities engaging. They simply go beyond the scope of ideas covered in this Model Triangle. (And therefore are included in the Downloadable Resources as a separate compressed or ".zip" file.)
We figured out how DNA codes for a protein (in detail).