Content Benchmark L.12.A.3
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Students know all body cells in an organism develop from a single cell and contain essentially identical genetic instructions. E/S

One idea from the Cell Theory is that all cells must arise from pre-existing cells through a process of cellular division (mitosis or meiosis). In a one-celled organism, a mitotic cell division results in the creation of another individual of that species with identical DNA. In multicellular organisms, mitotic cell division results in new body cells for growth and repair. Some rare multicellular organisms reproduce asexually through mitosis to produce a complete, independent offspring with identical DNA. However, most multicellular organisms reproduce sexually which involves the production of cells with half the compliment of DNA (a haploid cell) that recombine with another haploid cell, usually from another organism, to produce a genetically unique cell with a full compliment of DNA that can now grow and develop into a new individual.

Cell Cycle
All cells progress through the cell cycle at some part of their lives. The cell cycle starts right after the successful division of two daughter cells. Cells that cease cell division, like nerve cells, are not considered to be in the cell cycle. The cell cycle consists of four distinct stages, G1, S, G2 and M (Figure 1). G1, S, G2 are collectively called Interphase. Interphase is the period between cell division where the cell will grow, duplicate its DNA, and fulfill the role of the cell. The time that a cell remains in Interphase depends on the role of the cell. For example, human skin cells divide about once a day. Therefore, the cells will remain in Interphase approximately 22 hours. Conversely, a liver cell may go years without dividing.

Figure 1. The cell cycle. http://www.biology.arizona.edu/Cell_bio /tutorials/cell_cycle/cells2.html	Figure 2. The cell in Interphase. http://staff.jccc.net/pdecell/celldivision /mitosis1.html

The G1 (Gap 1) phase is the first part of Interphase. Most of the cell’s growth takes place in this phase. The cell will increase in size and synthesize new organelles. A cell may progress quickly to the S-phase or remain in the G1 phase near indefinitely.

The S (synthesis) phase follows the G1. Chromosomes are replicated in this phase. At the beginning of the S phase, each chromosome consists of a single double-helix strand of DNA, called a chromatid. At the end of the S phase, a chromosome consists of two sister chromatids.

Lastly, the cell enters the G2 (Gap 2) phase of Interphase. The cell will make final preparations for mitosis. This phase can be seen as a safety check point, where the DNA can be checked for errors.

Mitosis

After Interphase, the cell is ready to divide. Mitosis is divided into 4 major phases: Prophase, Metaphase, Anaphase, and Telophase.

1. Prophase
Prophase is the first phase of mitosis. Several processes take place during this phase. First, the chromatic material condenses into visible chromosomes and the sister chromatids are joined together at the centromere. The nuclear membrane disappears. In the cytoplasm, a pair of centrioles begins to separate and migrate to opposite poles of the cell. A network of microtubules begins to form between the centrioles called the spindle. The spindle fibers will be instrumental in guiding the chromatids to opposite ends of the cell.

Figure 3. This figure shows the relationship between DNA, chromatin, chromatids, and chromosomes.	Figure 4. a.) The photograph shows two cells in Prophase. (http://www.bio.txstate.edu/) b.)The diagram shows the important steps in Prophase. (http://staff.jccc.net/pdecell/celldivision /mitosis1.html)

2. Metaphase
During Metaphase, the chromosomes line up along the middle of the cell and are attached to the spindle fibers by the centromeres.

Figure 5. a.) The diagram shows the important steps in Metaphase. (http://staff.jccc.net/pdecell/celldivision/mitosis1.html)	b.) The photograph shows a cell in Metaphase. (http://www.bio.txstate.edu/)

3. Anaphase
Anaphase begins when the centromeres holding the sister chromatids together split and the spindle fibers shorten, pulling one chromatid toward each end of the cell.

Figure 6. a.) The photograph shows a cell in Anaphase. (http://www.bio.txstate.edu/)	b.)The diagram shows the important steps in Anaphase. (http://staff.jccc.net/pdecell/celldivision/mitosis1.html)

4. Telophase
Telophase is essentially the reverse of Prophase. The spindle fibers disappear, the chromosomes unravel back to chromatin, and two new nuclear membranes form.

Figure 7. a.)The diagram shows the important steps in Telophase. http://staff.jccc.net/pdecell/celldivision/mitosis1.html)	b.) The photograph shows a cell in Telophase. (http://www.bio.txstate.edu/)

5. Cytokinesis
Occurring concurrently with Telophase is cytokinesis. Cytokinesis is the division of the cytoplasm. Cytokinesis is different in animal and plant cells. In plant cells, a cell plate forms a new cell wall divider between the two nuclei and grows outward until it fully separates the two daughter cells. In animal cells, the cell membrane is pulled inward by a ring of filaments. This make the cell appear to “pinch in” from the sides. This process continues until two new daughter cells result.

Figure 8. A comparison of cytokinesis in plant and animal cells. From (http://www.trentu.ca/biology/101/4.html)
Figure 9. Cytokinesis in a plant cell. http://bio.txstate.edu	Figure 10. Cytokinesis in a animal cell. http://www.biologymad.com/ CellDivision/CellDivision.htm

Meiosis
Meiosis is a second type of cellular division. In meiosis, the result is 4 cells with half the complement of DNA instead of two identical cells. Therefore, the purpose of meiosis is to convert a diploid cell to a haploid gamete that would be involved in sexual reproduction to increase diversity in the offspring. The cells going through meiosis split twice, but the chromosome material is replicated only once. These two divisions are denoted as Meiosis I and Meiosis II.

1. Meiosis I

In Meiosis I, the number of chromosome sets is reduced by half (2n to n). The separation of each homologous chromosome pair is a random event which results in gametes containing a random combination of chromosomes. The phases of meiosis I are Prophase I, Metaphase I, and Anaphase I.

Prophase I – The chromosomes enter Prophase I already replicated forming a pair of sister chromatids connected at their centromeres. The homologous chromosomes do not move independently as they did in mitosis. The homologous chromosomes pair up forming a tetrad (maternal and paternal homologous chromosomes each made up of two sister chromatids). Crossing over (or transfer) of genetic material may occur between homologous chromosomes, thereby increasing genetic variability. Other events of Prophase I are very much like mitosis’ Prophase. The chromatin material coils up into chromosomes becoming visible, the nuclear membrane disappears, spindle fibers form, and centromeres separate.

Metaphase I – Just as in mitosis, the chromosomes line up on the middle of the cell. However, in meiosis the chromosomes line up with their homologous partner and attach themselves to the spindle fibers.

Figure 12. Anaphase I. http://www.biologycorner.com/worksheets/meiosis.html

Anaphase I – Again Anaphase I looks very similar to Anaphase in mitosis. The chromosomes separate and travel toward the poles. The difference is that it is not the sister chromatids that are separating but the homologous chromosomes that separate resulting in half the number of chromosomes at each pole but each chromosome is double stranded. The separation of homologous chromosomes is called disjunction.

Telophase I – Telophase I ends with the first meiotic division and cytokinesis. Some cells deconsolidate the chromosomes and form a simple nuclear membrane others do not and proceed directly into Prophase II. The result of Meiosis I is two haploid daughter cells.

Figure 13. An overview of Meiosis I ending with Telophase I. http://homepages.ius.edu/GKIRCHNE/Mitosis.htm

Telophase I – Telophase I ends with the first meiotic division and cytokinesis. Some cells deconsolidate the chromosomes and form a simple nuclear membrane others do not and proceed directly into Prophase II. The result of Meiosis I is two haploid daughter cells.

2. Meiosis II

Meiosis II is simply the mitotic division of the two haploid cells resulting from meiosis I.

Prophase II – A new set of spindle fibers form.

Metaphase II – The chromosomes line up on the middle of each cell and attach to the
spindle fibers.

Anaphase II – The sister chromatids separate and travel towards the pole.

Telophase II and cytokinesis – the chromatids unravel into chromatin, nuclear
membranes reform and the cell physically divided into two.

Figure 14. An overview of Meiosis http://www.nature.com/ng/journal/v37/n7/fig_tab/ng0705-662_F1.html

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Content Benchmark L.12.A.3

Students know all body cells in an organism develop from a single cell and contain essentially identical genetic instructions. E/S

Common misconceptions associated with this benchmark:

1. Students see the familiar X-shaped structure seen in a light microscope is a “basic” single (unreplicated) chromosome.

The X-shaped structures seen in a light microscope are condensed, replicated chromosomes containing two identical DNA double helices.

2. A chromosome is a chromosome - there is little differentiation between replicated and unreplicated states.

In late anaphase and G1 of interphase, a chromosome is unreplicated and consists of a single DNA double helix.

3. The X-shaped chromosomes are homologous chromosome pairs.

The X-shaped structures are unpaired, replicated chromosomes. Pairing of homologous chromosomes does not occur during mitosis.

4. Unreplicated chromosomes seen in anaphase are unpaired chromosomes.

These are simply unreplicated chromosomes, and this is the only time they are condensed and therefore visible.

5. The two non-identical homologous chromosomes in a parent cell go to separate daughter cells.

In anaphase, the identical chromatids of a replicated chromosome go to separate daughter cells. Each daughter cell gets a complete copy of the chromosomes in the parent cell.

For more information on common misconceptions associated with this benchmark, go to
http://www.biologylessons.sdsu.edu/classes/lab8/altern.html

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Content Benchmark L.12.A.3

Students know all body cells in an organism develop from a single cell and contain essentially identical genetic instructions.  E/S

Sample Test Questions

1st Item Specification:  Explain how organisms grow through the processes of cellular division and differentiation.

Depth of Knowledge Level 1

 1. Use the following diagram to answer the next question.

           
            Which diagram correctly represents mitosis?

A. Diagram 1

B. Diagram 2

C. Diagram 3

D. Diagram 4

2.  Normal mitotic division results in

A. two daughter cells with half the number of chromosomes as the parent cell.

B. two daughter cells with the same number of chromosomes as the parent cell.

C. four daughter cells with half the number of chromosomes as the parent cell.

D. four daughter cells with the same number of chromosomes as the parent cell.

3. If the diploid number of chromosomes is 20, what would be the chromosome number in the egg cells of this species?

A. 5                                 

B. 10                               

C. 20                               

D. 40

Depth of Knowledge Level 2

4. The following list describes some of the events associated with normal cell division.

               I. A Nuclear membrane forms around each of set of new of chromosomes
              II. Alignment of chromosomes on the equatorial plate
             III. Replication of each chromosome
             IV. Movement of single-stranded chromosomes toward opposite ends of cell.

During normal cell division, which series of events is chronologically correct?

A. III, II, IV, I                       

B. I, II, III, IV                       

C. III, IV, II, I                       

D. IV, III, I, II

5. Use the diagram below to answer the following question.

The two cells below are undergoing cytokinesis. 
 

 (Diagram adapted from Examgen 4.3 diagrams)

Which statement best describes these cells?

A. Division 1 could be in a plant and Division 2 could be in an animal.

B. Both cell divisions could be occurring in animal cells.

C. Division 1 could be in an animal and Division 2 could be in a plant.

D. Both cell divisions could be occurring in plants cells.

6. Use this diagram to answer the following question.

What two processes are represented by A and B?

A. Mitosis and Fertilization

B. Meiosis and Fertilization

C. Mitosis and Pollination

D. Meiosis and Pollination

2nd Item Specification:  Recognize that all body cells in an organism have fundamentally the same DNA.

Depth of Knowledge Level 1

7. An organism is described as 2n=30.  How many chromosomes do all somatic cells of this organism contain?

A. 15

B. 20

C. 25

D. 30

8. The cells in a cat are described as 2n=38.  Which of the following types of cells does NOT contain 38 chromosomes in the cat?

A. Muscle cell

B. Brain cell

C. Egg cell

D. Skin cell

Depth of Knowledge Level

9.  Use the following diagram to answer the question below.

Diagram adapted from Examgen 4.3 diagrams

What processes are represented by A, B, and C?

A. Meiosis, Mitosis, Differentiation

B. Fertilization, Meiosis, Differentiation

C. Fertilization, Mitosis, Differentiation

D. Meiosis, Fertilization, Differentiation

10. Use the following diagram to answer the question below.

Location X represents the resulting cells in a developing organism.  These cells

A. have different numbers of chromosomes because each cell will become a different body part.

B. have different numbers of chromosomes because each cell will only contain the DNA necessary to become different cell types.

C. have the same number of chromosomes because each cell received a complete set of DNA.

D. may or may not have the same number of chromosomes because each organism is unique.

Content Benchmark L.12.A.3

Students know all body cells in an organism develop from a single cell and contain essentially identical genetic instructions.  E/S

Answers to Sample Test Questions

1. A, DOK Level 1
2. B, DOK Level 1
3. B, DOK Level 1
4. A, DOK Level 2
5. C, DOK Level 2
6. B, DOK Level 2
7. D, DOK Level 1
8. C, DOK Level 1
9. C, DOK Level 2
10. C, DOK Level 2

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Content Benchmark L.12.A.3

Students know all body cells in an organism develop from a single cell and contain essentially identical genetic instructions. E/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. Cells Alive – Animal Cell Mitosis
This website has many well done animations about mitosis, meiosis, and cell
cycle. It is a good resource for visual learners. It also has many links for students under “homework links”.

To access this simulation, go to http://www.cellsalive.com/mitosis.htm

2. The Biology Project - The Cell Cycle & Mitosis Tutorial
This exercise is designed to introduce you to the events that occur in the cell cycle and the process of mitosis that divides the duplicated genetic material creating two identical daughter cells.” This website activity has several pages of reading about the cell cycle and the phases of mitosis. It even has another animation of mitosis. It ends with a 11 question self quiz about what was read. Once done with the quiz, choose the “Online Onion Root Tips” activity to conduct a virtual lab to “Determining time spent in different phases of the cell cycle”. Students will be presented with 36 pictures of cells and have to identify the phase of each and then calculate what percentage of time spent in each phase of the cell cycle.

To access this exercise, go to
http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html

3. The Biology Project – Meiosis Tutorial
This exercise is designed to help you understand the events that occur in process
of meiosis, which takes place to produce our gametes”. This is the partner site to the one above on mitosis.

To access this exercise, go to
http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/main.html

4. Web-based Inquiry - Using real world evidence
You will have to open an account for this project as will your students. It is also recommended that the teacher walk through each project before presenting to students. “WISE (Web-Based Inquiry Science Environment) is a simple yet powerful learning environment where students examine real world evidence and analyze current scientific controversies. Our curriculum projects are designed to meet standards and complement your current science curriculum, and your grade 5-12 students will find them exciting and engaging. A web browser is all they need to take notes, discuss theories, and organize their arguments... they can even work from home! Our Teacher Area lets you explore new projects and grade your students' work on the Web. Best of all, everything in WISE is completely free.” There is a WISE project entitled TELS: Mitosis and Meiosis that examines the “two processes that cells use when they reproduce-mitosis and meiosis. They will learn when and where these kinds of cell reproduction happen in their body, and also what happens when something goes wrong.” There is another WISE project entitled Mitosis & Cell Processes that helps “students understand the stages of mitosis and associated cell structures within the context of learning about cancer. The students actively explore mitosis by investigating three hypothetical plant-based medicines. Each plant interferes with mitosis in a different way. The students will recommend a plant for further research based on what they discover through their inquiry.

To access this project, go to http://wise.berkeley.edu/welcome.php

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