To introduce students to the genetic information stored in DNA within the human cell nucleus.
The goal of this lesson is to introduce students to the human cell and its DNA as the genetic information that governs how the cell will function. It is recommended to teach this lesson before delving into the more technical and biochemical process of replication, transcription, and translation.
When learning about cells, students often jump immediately into structures and functions. This compartmentalized approach lends itself to misunderstanding the relationship between cells and the living organisms that they compose. Cells are described by biology textbooks as the building blocks of life, yet students often cannot explain how something so small can help form a human being, a tree, or bacteria. Students often think that cells are floating within structures, as blood cells are depicted moving through arteries and veins. Even the use of certain language can promote this type of faulty thinking; the phrase “cells in the heart” may be understood to mean that there are cells that are within the heart, but not actually making the heart itself. Research indicates that it may be easier for students to understand that the cell is the basic unit of structure (which they can observe) than that the cell is the basic unit of function (which has to be inferred from experiments) (Dreyfus & Jungwirth, 1989). In this lesson, students will begin to understand cells as a basic unit of structure and begin to explore how DNA informs the function of the cell.
Research also shows that high-school students may hold various misconceptions about cells after traditional instruction (Dreyfus & Jungwirth, 1988). While students learn the details of mitosis, cellular respiration, and photosynthesis, the larger understanding of how these detailed chemical and biological processes relate to life and growth is often not addressed. Furthermore, students are usually taught the form and function of “typical” cells, often in isolation to other cells. Differences between prokaryotic and eukaryotic cells are highlighted, as well as differences between plant and animal cells. Conceptually, this encourages students to believe that all plant cells are physically and morphologically the same, and likewise for all animal cells. Since cells are often studied in isolation—particularly through drawings or models of single animal and plant cell prototypes—students also may not be able to describe how cells in a single organism communicate with and relate to one another.
Finally, students are often unable to reconcile two concepts that are taught during a cell biology unit. On the one hand, they learn that all the genetic information that codes for a living organism is encoded within the cell’s DNA and that this genetic code is the same in each cell of the living organism. Yet on the other hand, students learn that cells differentiate and specialize. They may become cells of the liver, xylem, thyroid, etc. How can two cells of a living organism have exactly the same genetic information, yet become different types of cells with specialized functions and morphology?
This lesson is for the high-school level and assumes that students have some previous information about cells and DNA. The Motivation section and the beginning of the Development in this lesson will help you determine student preconceptions.
A few days before the lesson, ask students, “What are living things made of?” Have students write down no more than three responses on a blank piece of paper or on an index card. They do not need to write their names on their responses, but ensure that all students participate in this short activity. Collect the responses and organize them into categories outside of classroom time. The activity can be done verbally as well by asking students to share responses and writing them on the board. However, past experience with this activity has shown that: 1) not all students contribute to the discussion with their thoughts, preventing you from ascertaining what all students think, and 2) students are sometimes shy to share the first thoughts or terms that come into their mind, such as “sperm.” Collecting responses anonymously ensures that you know the preconceptions of all the students.
Write down the final list of words and phrases on the board or flipchart paper. Related terms should be clumped together, such as “carbon,” “water,” and “atoms.” The term “cell” will usually be contributed by students. You also may want to include the terms “tissue” and “organ.”
Ask students to arrange the terms in order of their relationship to one another. Facilitate the discussion through guiding questions, such as:
- What is the relationship between carbon and atoms?
- How can we arrange these terms according to the size of the item?
- Are there other things inside of cells besides DNA?
- What are some examples of cells? (This is where terms like “sperm” or “red blood cell” can be incorporated.)
The outcome may look more like a web than a list. Tell students that the class will begin a unit on cell biology, where they will learn about different types of cells, their functions, and their various compartments. Point out that cells are often described as the building blocks of life. However, cells themselves are made up of different parts, each with a specific function.
In this part of the lesson, student will examine cells more closely by exploring some online animations and interactives. Begin by asking students:
- You may have learned that half of the genetic information of a human being comes from the mother and half from the father. What is this genetic information?
- (It is DNA.)
- How does half the DNA come from the mother?
- (It comes from the egg.)
- How does the other half come from the father?
- (It comes from the sperm.)
Discuss with students that when the egg and sperm unite, they form one embryonic cell that contains a complete set of DNA.
- How does that first one embryonic cell become two or three or four and so on?
- (It begins to divide.)
- Does the DNA also divide or does it get divided between all the new cells?
- (The DNA duplicates and divides with each cell division.)
Share with students that each cell has the same copy of the genetic information that was in the very first embryonic cell. Show students the Animal Cell Mitosis animation and describe how the two cells that are produced from cell division are exactly the same as the first cell. If this keeps happening over and over again, as it does for embryos, then each cell is a replica of the first cell with the same DNA that was in that first cell. Based on this, ask students to consider where the liver comes from, or the heart, the skin, the brain, bones, etc.
- What is each organ made of?
- (Each organ is made of cells.)
- Are the cells identical to one another in their DNA?
- Are the cells identical to one another in their function?
Show students Figure 1 from Animal Cells and Tissues. Alternatively, you can print out the picture and provide it to students as a sheet. Ask students:
- In this diagram, how many different types of stomach cells are shown?
- (There are five.)
Describe how each type of cell of the stomach has a different function. For example, the cells of the smooth muscle tissue contract and expand allowing food and nutrients to move along the digestive tract. The columnar epithelium cells line the inside of the stomach, absorbing nutrients into the bloodstream. The red blood cells provide oxygen to all the other cells of the stomach. Cells that make up the nervous tissue transmit nerve messages to and from the brain. Cells that make up the connective tissue support and protect the stomach. You also can review how the cells morphologically differ from one another. Remind students that all of these cells are the result of cell division from a previous cell. Ultimately, the first cell of that organism was the embryonic cell that contained DNA from both the egg and the sperm. This means that cell division results in cells that are genetically identical to one another. All five types of cells that the students reviewed in the diagram have the exact same genetic information. In other words, even though the cells differ in morphology and function, they are genetically identical.
Have students use the Introducing the Human Cell and DNA student esheet to access the From Cell to DNA online animation. Students should go through the animation and answer the corresponding questions. They can write their answers on the From Cell to DNA student sheet.
Go over the questions with the students. Explain any new vocabulary introduced by the animation, such as bacterium, eukaryote, micrometers, nanometers, histone proteins, and polymeric molecule. Review with students that a unique feature of eukaryotic cells is the presence of a nucleus, where the DNA is stored.
- The animation mentions that humans have 46 chromosomes. How many chromosomes come from the mother? From the father?
- (23 chromosomes come from each parent.)
- Every time a cell divides, what do you think happens to the chromosomes?
- (They replicate and each new cell gets a copy of the same 46 chromosomes.)
- The DNA of chromosomes is described as a double helix. What does that mean?
- (There are two strands that wrap around each other in a helical fashion, held together by bases.)
If available, show students a model of DNA. Point out how attraction between the bases holds the helical structure together. Show students images of the molecular structure of the bases, pointing out the different elements such as carbon and hydrogen. Remind students of their brainstorming activity during the Motivation and point out that the smallest unit is an atom.
Discuss with students that each cell in the human body has the same 46 chromosomes as every other cell. That is, the genetic information stored in the nucleus of every human cell is identical to every other cell in that organism. However, as the students saw with the image of the stomach, cells of the human body differ from one another in terms of function and morphology. Why do some cells become heart cells while others become liver cells? How do cells know what function they should be performing?
To explore this question, students should use their esheet to go to Tour of the Basics on the Learn Genetics site and answer the corresponding questions on the esheet. They can record their answers on the What Is DNA? student sheet. You can find answers to the questions on the What Is DNA? teacher sheet.
After completing the activity, review the questions with students. Discuss with students that all the genetic information in a cell is stored in the pattern of four bases, which are adenine, guanine, cytosine, and thymine. These are respectively abbreviated A, G, C, and T. The entire genetic code of a living organism—including how each cell will grow, function, and look—is stored in the pattern of these four letters. The pattern of these letters make up “sentences” called genes. A gene is a stretch of DNA that codes for a protein. The DNA never leaves the nucleus so it can’t actually do the function of the cell. Instead, the DNA is like a blueprint made up of genes. The genes are read by the cell’s nuclear machinery and produce specific proteins. Each gene codes for a specific protein. There are genes for proteins that have to do with hearing, heart function, immune defense, nutrient absorption, and so on.
- In your estimation, do you think there are a few or many genes?
- (There are many genes in DNA.)
- Do you think a single cell produces every single type of protein coded in the DNA?
- Why do different cells produce different proteins?
- (The function of the cell is related to the types of proteins produced. Cells of the heart, for example, produce proteins that are specific to heart function. Cells of the eye, on the other hand, will produce proteins specific to eye function.)
- Why does a cell in the inner ear look and function differently than a cell in the stomach based on our understanding of genes?
- (The two cells are producing different proteins based on different genes of the DNA being read by the cell’s machinery. The cell of the inner ear produces proteins that have to do with its function, and the cell of the stomach does likewise. This is why they look and function differently.)
- Do the cells of the inner ear and stomach have the exact same DNA?
- How do we know that these two types of cells—and all the cells of an individual human being—have the exact same DNA as each other?
- (They are produced from mitosis, which is cell division in which the DNA replicates. All cells have the same DNA as the first embryonic cell.)
- How many chromosomes does each of the cells have?
- (Each cell has 46 chromosomes.)
- Given that the DNA in these two types of cell—the inner ear and the stomach—have the exact same DNA, can we say that they also have the exact same genes?
- Why do we know that they have the exact same genes?
- (Genes are segments of DNA. If the DNA is the same in both cells, the genes are also the same because they are a part of the whole.)
- Are the two cells reading the same genes as each other?
- As a result, are the cells producing different proteins?
Ask students to consider the following: Every student in the classroom started as a single cell, produced from an egg and a sperm. That first cell had 46 chromosomes, 23 contributed from each parent. How did that single cell become the fully functioning human being that they are today? Encourage students to think about cell division and that cells specialize based on the genes that become active within the DNA.
Extracting DNA is a Science NetLinks lesson that provides students an opportunity to extract DNA.
After understanding the concept of genes, students can explore the human genome and the importance of uncoding all the genes within the human DNA. Use the Science NetLinks lesson Cracking the Genetic Code.
Students can explore the function and morphology of different cells in the human body by looking at prepared slides. An online Histology Laboratory provides numerous images of prepared human slides that can be magnified. Reinforce that while the cells within a human body differ, they contain the same genetic information: 46 chromosomes, where half comes from the mother and half from the father.
Have students watch the rest of the Tour of the Basics online interactive to get a more complete understanding of how genes code for proteins that determine the actual function and morphology of cells. In addition, students can learn more about DNA and compare it to the genetic information in a mouse and a flower through an online interactive game DNA The Double Helix. The Dolan DNA Learning Center also provides lesson plans and online interactives about the discovery of DNA, the molecule, genetic technology, and future applications.
For a laboratory activity, have students observe their own cheek cells and extract DNA. A lesson for this experiment can be found at Hands-on Activities for Teaching Biology to High School or Middle School Students.
Educational resources and information about the Human Genome Project can be found at the National Human Genome Research Institute’s website.