Phenomenon
We observe and review how different organisms grow, develop and regenerate.
Question
How do organisms grow, develop, and regenerate? (There are a number of sub-questions generated in this unit, but this captures the gist of them all succinctly.)
Model Ideas
- Growth mostly occurs because of cell division. Individual cells can grow too (and this is important for single-celled life especially!), but there are limits to how big they can grow.
- When a cell divides to form two new cells, it gives each daughter cell identical DNA. (Their DNA is also identical to that of the original cell.
- [From our model for DNA: DNA is the hereditary material in all cells and it codes for proteins.]
- Different cells, however, may contain different proteins or different amounts of proteins. This allows cells to look different (form) and to behave differently (function) from one another.
- The differences in cell “phenotype” are due to differences in how the genes in the cell are turned on or off (expressed) so that they do or do not make the protein they code for.
- The signal to turn genes on and off comes from the environment.
- Once cells begin to “differentiate”, they usually cannot go back. They have committed to a “fate”.
- Cell division also allows organisms to regenerate tissue (but most cells cannot divide to become any other kind of cell). Special cells, called stem cells can become any cell type. (And some cells are kind of like stem cells and can become lots of different kinds of cells, but not every kind of cell—e.g. bone marrow cells.)
Overview
Transition in: We understand many things about the inheritance of traits. We also know that DNA codes for traits and that it does this through proteins But how does it all work? We are ready to wonder through our examinations of growth and development.
Students know that multicellular organisms are composed of cells with identical DNA but different forms and functions (heart cells, skin cells, muscle cells etc.). Now that they know more about DNA and how it determines traits, we ask them to ponder that phenomenon, hoping it creates some dissonance and leads them to wonder “How this is possible?”. How do cells with the same set of “directions” turn out to be different kinds of cells that do different things? We address the topic by looking at mitosis so students understand why all cells of an organism have the same DNA. Next, we turn to development - one of the most stunning of all biological phenomena. Ideally students have the experience of observing it directly in a Planaria and/or Medaka lab, or even by observing fertilized fish or frog eggs obtained from Carolina. However, if time or other constraints make this impractical, there are videos that can motivate this model. Once students see the phenomenon, we ask them to brainstorm possible explanations for what they observed. We ask them what ideas they have about how cells with the same set of instructions [DNA] become many different kinds of cells and organize to form organs etc. The list generated usually includes the explanation we are looking for (different genes are active in different cells) but also includes misconceptions such as “mutation”. As a class we discuss and revise the list until we reach a consensus around our model: cells with the same DNA differentiate because different genes are active. We also touch back on DNA function to make the connection between genes activated, proteins produced, and the form and function of a cell. Information on why and how different genes get “turned on (or off)” at specific times in development is included in the materials provided but do not expect students to figure this out on their own. This provides a perfect opportunity to discuss stem cells and recent advancement in stem cell research. Current and relevant topics intrigue students and keep them invested in their learning.
Transition out to Population Variation: We've spent a lot of time understanding how traits are passed along and expressed in individual organisms, how does this help us to understand frequencies of traits in populations? We return to both a population and evolutionary perspective to apply what we've learned in the past few units.
Transition out to Chemical Reactions: In our last activity, we used our model to try to make sense of how a caterpillar can metamorphose, rearranging its body to energy as a butterfly with a whole different set of traits. One question that emerges is: how does the organism use its matter and find the energy to accomplish this?
Advanced Planning
Generation of the phenomenon/phenomena for this model is ideally carried out through one or two hands-on labs working with live animals and therefore requires a considerable amount of advanced planning. ALSO, these labs must be completed (or nearly completed) by the commencement of the unit. They require one day for students to set up and then regular (perhaps daily) short observations over the course of two weeks. Therefore, you must start the work with students during the prior triangle (DNA for most classrooms).
Please read the teacher guides for the following recommended phenomena carefully and order materials ahead of the labs as needed. If you do not have time for both labs (or either of them), there are teacher notes in the first learning segment that point to alternative ways to start this unit.
Planaria Lab (see Teacher Notes for amounts and details):
- Order planaria from Niles or Carolina
- Full regeneration takes 10-14 days from when they are cut so plan accordingly. Students need to see this phenomenon before they can develop the model.
Medaka / Fish Development or Frog Development Lab (see Teacher Notes for amounts and details):
- Order zebra fish, Medaka, or frog eggs from Carolina to arrive the day before you plan to have students begin their observations. Fish eggs take about two weeks to hatch, so start about two weeks prior to having students develop the model.
- If you are more adventurous, order Medaka or zebra fish, mate them in your classroom aquarium and collect your own eggs.
Download Resources
| Attachment | Size |
|---|---|
| GD Growth and Development vApril 2021.zip | 28.92 MB |