ZERO-IN

Italicized font represents information to be shared orally or physically completed with the students at this time.

The non-italicized font represents additional information included to support the teacher’s understanding of the content being introduced within the CELL.

ANALYZE IT

Note: Questions marked with a triangle (∆) are included to enrich students’ understanding. These questions do not appear in students’ SDRs but should be used as additional discussion points during the PostLab.

Review the process of mitosis.

• ∆ Each body cell has two copies of each chromosome. These two copies are called homologous chromosomes. Are homologous chromosomes identical? Explain. No. Homologous chromosomes are not identical. Homologous chromosomes contain the same genes. However, they usually have different alleles of the genes. Homologous chromosomes are not identical because one chromosome originally came from the mother and one chromosome originally came from the father.

• ∆ Before mitosis begins, the cell is in interphase. What happens during this part of the cell cycle? Cells grow larger and all of the chromosomes are copied (duplicated). The two identical copies of each homologous chromosome are called sister chromatids and are joined together at the centromere.

• ∆ Mitosis is divided into four phases. What are the phases of mitosis in order? The four phases are: prophase, metaphase, anaphase, and telophase.

• ∆ What happens in prophase? In prophase, the chromosomes condense into rodlike structures that can be seen under the microscope. The nuclear membrane dissolves.

• ∆ How was prophase modeled in this Investigation? Prophase was modeled by removing the rubber band which represented the nuclear membrane.

• ∆ What happens in metaphase? In metaphase, duplicated chromosomes line up with the centromeres on spindle fibers in the center of the cells. How was metaphase modeled in this Investigation? Spindle fibers were modeled with string. One set of sister chromatids was placed on each spindle fiber with homologous chromosomes on adjacent fibers.

• ∆ What happens in anaphase? In anaphase, the sister chromatids separate and move down the spindle fibers to opposite ends of the cell.

• ∆ How was anaphase modeled in this Investigation? The clay representing the centromere was pinched in half and the gram cubes representing the sister chromatids were separated.

• ∆ What happens in telophase? The chromatids have moved to the ends of the spindle fibers. The spindle fibers disappear and new nuclear membranes form around each set of chromosomes.

• ∆ How was telophase modeled in this Investigation? The gram cubes representing chromatids were moved to the end of the strings. The strings representing the spindle fibers were removed. Rubber bands were placed around both sets of chromosomes to represent nuclear membranes.

• ∆ What would happen next in the cell cycle? The cell will divide (cytokinesis) and each new cell will have identical sets of chromosomes in its nucleus.

• ∆ What would happen if the DNA in a single cell had a mutation? Student answers may vary. In most cases, the mutation would not be passed on. Cells have complex proofreading and repair mechanisms to recognize and fix mutations as they occur. However, if a mutation was not repaired by the cell, this mutation would be passed on to daughter cells through mitosis and cell division.

GET FOCUSED

Instruct students to complete the Focus Question in their SDRs then discuss it as a class. Use the suggested response below to guide students’ answers.

• Why can mutations in the DNA of a single cell affect the functions of an entire organism? A mutation of the DNA in a gene in a single body cell will get copied when the chromosomes duplicate. All of the new body cells formed through mitosis will then have the mutation. If the mutation causes a change in a protein, all cells will produce the mutant protein. If the cells in an organ are producing mutant proteins, the organ may not function properly. Thus the organism may not function properly.

COMPREHENSION CHECK

The Comprehension Check is designed to summarize the Core Experience Learning Lab and provide the teacher with an informal way to assess students’ understanding of the big ideas.

If time permits, ask your students to answer each Focus Question below. Use the suggested responses below to guide students’ answers.

• How does DNA control the functions of an organism? DNA controls the functions of an organism by coding for the proteins that carry out each individual function of the organism. This genetic information is carried from the DNA of a gene by mRNA that is then translated into a protein amino acid sequence. This was modeled in Investigation One by illustrating that the DNA and the RNA sequences determine a protein’s sequence. Changes in the protein sequence were shown to alter the function of the protein.

• Can mutations in DNA cause changes in an organism? In Investigation One, students demonstrated that mutations in a DNA sequence result in changes in a protein and therefore may lead to changes in an organism. However mutations may not always result in changes in a protein. If a particular mutation in a DNA sequence does not cause a change in amino acid sequence, then no change in the function of the protein will be observed. Some mutations to the water binding site of the protein, while changing the amino acid sequence resulted in no change in water transfer. Some mutations resulted in a greater rate of transfer while others resulted in a lower rate. Based on the results of Investigation Two, mutations can cause changes in protein structure and function which would lead to changes in an organism. These changes could be harmful or beneficial. However, as also illustrated in Investigation Two, not all changes in DNA sequence result in changes in an organism. In Investigation Two, students learned that mutations do not always lead to changes in amino acid sequence and therefore no change in protein structure or function. In Investigation Three, students discovered that changes in amino acid sequence do not always lead to changes in protein structure. It was also demonstrated that changes in protein structure do not always cause changes in protein function.

• Why can mutations in the DNA of a single cell affect the functions of an entire organism? In Investigation Three, students modeled how cells copy their DNA and divide so that each new daughter cell has the exact same DNA (chromosomes) as the original cell. If a mutation occurs in a cell’s DNA, it can be copied when the chromosomes duplicates and passed on to each new generation of cells. If students have difficulty with this concept, ask them to refer back to the diagrams they made as they modeled mitosis. Ask students to think of the orange gram cube as a mutation and follow what happens to the mutation as mitosis progresses. If a mutation causes a change in protein function, then this mutation will change protein function in each new daughter cell and could eventually affect the functioning of the organ and the entire organism.