Pose the following question to the class, “If every television on the planet were suddenly frozen, would we expect that more or less than half of them were showing commercials?” Have students discuss with their partners, then share their answers. (Answer is less than half.) Ask students to explain why less than half would be on commercials. (During an hour-long show, less than half of the time is spent on commercials.)
Tell students, “A similar type of experiment can be done with cells to determine how much time they spend in each phase of mitosis. The phases we examined yesterday don’t all take the same amount of time to complete. In order to find out how long each phase takes, images of the cells must be taken, and then they must be sorted into each phase. If most of them are in metaphase, then metaphase must take longer than the other phases.”
Explain that students will be doing this type of sorting using an online simulation. Distribute copies of the Online Onion Root Tips―Mitosis worksheet (S-7-4-3_Online Onion Root Tips-Mitosis and KEY.doc) and have students begin by following the directions on the handout. Monitor students’ progress to ensure that they are successful with the activity.
When students are finished, discuss the results from the table at the end of the activity by fielding student questions and posing the following questions listed below. Collect the handout when finished.
- “In what stage do cells spend the most time?” (interphase)
- “Is interphase during mitosis or before mitosis?” (before mitosis; mitosis begins with prophase)
- “What part of the plant were these cells from, and why was that part chosen?” (the root, it divides relatively quickly)
- “If we wanted to find cells going through mitosis in our bodies right now, where would we want to look and why?” (any place where cells are dividing quickly; lining of digestive system, bone marrow producing blood cells, skin, cuts, bruises, etc.)
- “How do the two cells resulting from mitosis compare to the original cell?” (They are identical.)
Pose the following question to students, “Where do you get your own DNA from?” (from both parents) “Who has more DNA in your cells, you or your mother?” (The correct answer is the same amount.) If a student answers correctly, play devil’s advocate and point out that s/he should have more since s/he is a combination of both his/her mother’s and father’s DNA. If the student initially answers that s/he has more or later agrees that s/he must have more, ask,
- “How much more would you have than your mother?” (double the amount)
- “Does this seem right, that each generation has double the DNA of the previous, what difficulties would this lead to?” (Many students may be familiar with how doubling anything leads to some very extreme growth; as a result a cell would very quickly be bursting at the seams with DNA.)
Assess if any students see a solution to the paradox. If not, guide the discussion by describing that a mother and father must not give all of their DNA to their children. Explain that there must be a way to make a new cell that has only half of the adult amount of DNA. Tell students that the name of this process is meiosis and that they will now draw it and compare it with mitosis.
Have students take out their Mitosis and Meiosis Drawing Packet (S-7-4-2_Mitosis and Meiosis Drawing Packet and KEY.docx). Redistribute the same colored markers used in the previous lesson. Use the Drawing Meiosis PowerPoint presentation or draw each phase onto the board (S-7-4-3_Drawing Meiosis Presentation.pptx). Have students copy the pictures into their packet. At the onset, point out that two of the chromosomes should be large and two should be small, so that they can be distinguished. If desired, additional concepts and vocabulary can be added to the Notes section on the right side. Note: One key difference from mitosis to stress is the pairing of similarly sized chromosomes during prophase of meiosis I and the resulting paired lineup during metaphase I. Also, compare the outcomes of mitosis (two diploid cells) and meiosis (four haploid cells).
Have students make two columns on a page in their notes. Have them label one column mitosis and the other meiosis. Tell students that they now need to look through their mitosis and meiosis drawings and think about the things they have learned about each to list similarities and differences between the two. Start them off with “starts with one cell” written in both columns. Give them 5 minutes to come up with as many similarities and differences as possible with another student in pairs. Afterward, have students share the similarities and differences they thought of and compile them on the board, making sure to tell students that any new ideas that are shared should also be written in their notes. Answers will vary, but may include:
Mitosis Meiosis
starts with one cell starts with one cell
produces two cells produces four cells
each phase happens one time each phase happens two times
chromosomes don’t pair chromosomes pair in prophase 1
chromosome # stays same chromosome # is halved
has phases named IPMAT has phases named IPMAT
sorts chromosomes into piles sorts chromosomes into piles
Tell students that they will now take what they have learned about the differences and similarities between mitosis and meiosis and apply those ideas to the life cycles of humans and daphnia that they have studied. Hand out the Advanced Life Cycles document (S-7-4-3_Advanced Life Cycles and KEY.docx). Have students read the directions frame on the handout and complete the handout with the help of their mitosis/meiosis comparison table. After students have completed the handout, work together as a class and add more to the mitosis/meiosis table that the life cycle diagram has brought forth. Answers include:
Mitosis Meiosis
makes somatic (body) cells makes sex cells (sperm/egg)
how offspring grow how sperm/egg are formed
keeps human chromosomes at 46 cuts human chromosomes down to 23
keeps daphnia chromosomes at 20 cuts daphnia chromosomes down to 10
As a concluding assessment, have students work individually on a written summary of the human and Daphnia life cycles. Tell students that they need to summarize the two life cycles on a separate piece of paper that will be collected and graded. The summary should consist of two paragraphs: one for the human life cycle, and one for the Daphnia life cycle. The summary should have enough detail so that someone could read the summary and recreate the important ideas of the life cycles diagram. Other criteria are:
- Start with offspring, end with offspring.
- Use all words in frames of the life cycle diagram.
- Paraphrase connecting words between frames.
- Mention changes in chromosome numbers.
Collect and grade the written summaries. Below is an example of a completed summary with each word from a frame of the life cycle diagram in bold.
Summary Example:
Human offspring grow into adults through mitosis. Cells in the male adults go through meiosis to make sperm and cells in the female adults go through meiosis to make eggs. The eggs and sperm have 23, or half, the number of chromosomes as the other adult cells. The sperm and egg combine through sexual reproduction to make a new offspring that has 23 plus 23 or 46 chromosomes.
Daphnia offspring grow into adults through mitosis. Some females go through mitosis during asexual reproduction to make offspring with the exact same chromosomes as the mother. Cells in the male adults go through meiosis to make sperm and cells in the female adults go through meiosis to make eggs. The eggs and sperm have 10, or half, the number of chromosomes as the other adult cells. The sperm and egg combine through sexual reproduction to make a new offspring that has 10 plus 10 or 20 chromosomes.
Extension:
- For enrichment, students can brainstorm or research into the causes that determine if a female organism will go through meiosis to make an egg or mitosis to make a clone of herself. In other words, when would a female daphnia go through asexual reproduction or when would it go through meiosis? It may be helpful to have students first determine what the benefits are of each option. The females are more likely to go through asexual reproduction when conditions are ideal. More clones are fine since things are going well. If conditions become difficult they are more likely to go through sexual reproduction since it will generate variability/ mixing of DNA and the possibility of a new combination of DNA that will do better under the unfavorable conditions.
Students who are challenged in completing the final written summary of the two life cycles may benefit from being given a completed written summary of the human cycle and being instructed to write a written summary of the Daphnia’s cycle (or vice versa) so that they can see how a finished paragraph should look.
