11 biology 6-10-08 6e mitosis-meiosis

TEKS 7.9 A

TEKS 6 EMeiosis and Mitosis

TAKS Objective 2 – The student will demonstrate an understanding of living systems and the environment.

TEKS Science Concepts 6 E

The student knows the structures and functions of nucleic acids in the mechanisms of genetics. The student is expected to

(A) compare the processes of mitosis and meiosis and their significance to sexual and asexual reproduction

TAKS Objective 2 page 1

TEKS 7.9 A

For Teacher’s Eyes Only

Mitosis and Meiosis

Student Prior Knowledge

Students should be familiar with the components associated with body systems TEKS 6.10 (C) identify how structure complements function at different levels of organization including organs, organ systems, organisms, and populations and the functions of these systems.


TEKS 7.9 A

5 E’s

ENGAGE

Mitosis/Meiosis Dance

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EXPLORE

Explore 1

Mitosis, Meiosis and Fertilization Teacher Prep Notes

By Dr. Scott Poethig, Ingrid Waldron and Jennifer Doherty, Department of Biology, University of Pennsylvania, 20071

Equipment and Supplies:

Sockosomes (1 per student or 4 per group -- see chart on page 2)

Optional: string to represent spindle fibers

Sockosome supplies:

Small or medium children’s socks (no more than half of any one color; even number of pairs of each color sock; preferably half small and half medium or otherwise all of one size)

Fiber fill

Small squares or circles of Velcro

Needle and thread or fabric glue

1 These teacher preparation notes and the related student handout are available at

http://serendip.brynmawr.edu/sci_edu/waldron.


Meiosis

TEKS 7.9 A

Masking tape and Sharpies

Teacher Preparations:

To produce sockosomes

1. Sew (or glue) one-half of a piece of Velcro (the fuzzy half) to the heel of one sock, and sew the other half (the part with hooks) to the heel of the other sock.

2. Fill each sock with fiber fill, and sew the end of each sock closed (sewing works much better than gluing for this step).

3. Stick the socks together at the heels. You now have a chromosome with two chromatids, where each sock represents a chromatid.

4. Pairs of homologous chromosomes will be represented by two sockosomes of the same color, one with a stripe marked along the length of each sock with a permanent marker (representing the different alleles on the two homologous chromosomes). Add a ring of tape around each sock in each sockosome to represent a gene that is inherited by simple Mendelian transmission. For each pair of homologous sockosomes, label the tape on each sockosome with a different allele of the same gene. For example:

Albinism (a)

(Albino)

Albinism (A)

(Pigmented skin)

Obviously the allele labeled on both socks in a single sockosome should be the same.

Use the chart on the next page to guide you, as you label half the pairs of homologous sockosomes of each color with the alleles for skin pigmentation (A for pigmented skin and a, the albino allele) and label the other half of the pairs of homologous sockosomes of each color with the alleles for thumb bending (H and h). If half of your socks are small and half medium, then all of the small socks should be labeled with an A or a, and all of the medium socks should be labeled with an H or h.


TEKS 7.9 A

Sockosomes Needed for Two Groups of Four Students Each

Mitosis & Meoisis Activities -- Group 1

a sockosome in solid color 1

A sockosome in the solid color 1 but with a stripe

h sockosome in solid color 2

H sockosome in solid color 2 but with a stripe

Mitosis & Meoisis Activities -- Group 2

h sockosome in solid color 1

H sockosome in the solid color 1 but with a stripe

a sockosome in solid color 2

A sockosome in solid color 2 but with a stripe

The same sockosomes can be used for these two groups of students for the final activity which models meiosis followed by fertilization, but for this activity one group should have all the a and A sockosomes, and the other group should have all the h and H sockosomes. The pair of sockosomes in one color will represent the mother's chromosomes, and the pair of sockosomes in the other color will represent the father's chromosomes. The different colors for the mother’s and father's sockosomes represent the fact that, although the labeled alleles are the same for the mother’s and father’s chromosomes, there are many genes on each chromosome and the mother’s and father’s chromosomes will have different alleles for many of these genes.

Additional Possible Activities

Sockosomes made with larger socks can be modified so they can be used to model crossing over and recombination. Using a larger pair of socks, cut off a portion of the top of the sock to be stuffed and sewed close separately. The top portion can then be reattached with Velcro, allowing it to be removed and swapped with the top portion of another sock. This can be particularly useful for lecture demonstrations.

We have found the video "Cell Division: Mitosis and Cytokinesis" to be an excellent overview of the subject. It is available for purchase from http://www.cytographics.com/ for AUD$89. We recommend showing it after the mitosis portion of the Mitosis, Meiosis and Fertilization activity as a review of the subject.


http://www.cytographics.com/

TEKS 7.9 A

Explore 2

Mitosis/Meiosis Manipulative

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Students will use a hand-on manipulative to take a cell from the beginning of meiosis to the end of meiosis. They will also do the same for mitosis and compare the two events.

EXPLAIN REWRITE

Complete the Meiosis PowerPoint presentation with your student with discussion and the completion of the following questions.

ELABORATE

Elaboration 1

Student will complete a Flow Chart comparing Mitosis and Meiosis.

Elaboration 2

Students will complete a chromosome number worksheet to compare mitosis and meiosis, as well as, solidify the haploid/diploid concepts of the two types of nuclear division.

EVALUATE


Section I.01 Mitosis, Meiosis, and Fertilizationby Dr. R. Scott Poethig, Dr. Ingrid Waldron, and Jennifer Doherty

Department of Biology, University of Pennsylvania, © 20072

Mitosis -- How Your Body Makes New CellsHow many cells do you think your body has?

Why does your body need to have lots of cells?

Each of us began as a single cell, so one important question is:

How did that single cell develop into a body with more than a trillion cells?

The production of such a large number of body cells is accomplished by many, many repeats of a cycle of cell division in which one cell gives rise to two cells, each of which in turn gives rise to two cells, etc. Thus, cell division is needed for growth.

Even in a fully grown adult, cells still undergo cell division. Why is this useful? Think about your skin, for example.

The two cells that come from the division of one cell are called daughter cells. (It may seem odd, but the cells produced by cell division are called daughter cells, even in boys and men.) Each of the daughter cells needs to have a complete set of chromosomes. What are chromosomes? Why does each cell need a complete set of chromosomes?

How do you think each daughter cell gets a complete set of chromosomes?

In each cycle of cell division, the cell first makes a copy of all of the DNA in each of the chromosomes, as shown in the figure below.

2 Teachers are encouraged to copy this student handout for classroom use. A Word file, which can be used to prepare a modified version if desired, and Teacher Preparation Notes are available at http://serendip.brynmawr.edu/sci_edu/waldron/.

8

http://serendip.brynmawr.edu/sci_edu/waldron/

(Adapted from Figure 9.9 in Biology by Johnson and Raven)

After the DNA in each chromosome has been copied, the cell undergoes a type of cell division called mitosis, which carefully separates the two copies of each chromosome to opposite ends of the dividing cell, so each daughter cell ends up with a complete set of chromosomes.

Mitosis -- The Basics

Once the DNA of a chromosome has been copied, the two copies of the DNA form two chromatids which are attached to each other at the centromere. These chromatids are often called sister chromatids because they are identical. (The two attached chromatids are called sister chromatids, even in the cells of boys and men.) During mitosis the two chromatids of a chromosome separate and become independent chromosomes; one of these chromosomes goes to each daughter cell.

9

10

Cell

Chromatids

Chromosome

Cell 1 Cell 2

ChromosomesCell

To keep things simple, we will begin by discussing mitosis in a cell which has only two chromosomes. These two chromosomes are a pair of homologous chromosomes. Both homologous chromosomes contain genes which control the same traits (e.g. eye color and skin color). For each gene on a pair of homologous chromosomes, there may be two different versions or alleles of the gene on the two different homologous chromosomes (e.g. an allele for brown eyes on one chromosome and an allele for blue eyes on the other chromosome). In contrast, the two sister chromatids of a chromosome have identical alleles of each gene, because the process of copying DNA results in exact copies of the original alleles.

You will model mitosis using a pair of sockosomes to represent the pair of homologous chromosomes. Each sockosome will have two socks joined at the heel to represent the sister chromatids in a chromosome after the DNA has been replicated. The pair of sockosomes will look like the chromosomes in the diagram on page 2.

1. Inside a cell, each chromatid consists of a single long molecule of _____________.

Are the alleles in the sister chromatids in a chromosome identical or different?

Are the alleles in the two homologous chromosomes identical or different?

2. Both sockosomes are the same color, to indicate that you have a pair of homologous chromosomes which both have the same genes. As shown in the figure on page 2, one of your sockosomes has a stripe on both socks and the other sockosome has no stripes; this indicates that the two homologous chromosomes have different alleles, even though they have the same genes.

What is wrong with the diagram shown below? Explain why sister chromatids could nothave different alleles.

Cell

Chromatids

Chromosome

11

3. Together with your partner, use your sockosomes to demonstrate how the two chromosomes line up at the beginning of mitosis. Then demonstrate how the sister chromatids of each chromosome separate during mitosis and become separate chromosomes, one of which goes to each daughter cell.

4. Before the cells produced by mitosis can divide again, what has to happen to the chromosomes in each cell?

Draw what the chromosomes in each cell will look like when the cell is ready to undergo the next round of mitosis. Label the chromosomes and chromatids.

12

Cell

The next section describes how mitosis is accomplished in a real live human cell and how this process ensures that each daughter cell receives a normal, complete set of chromosomes.

Mechanics of mitosis

In a living cell, when the cell is carrying out its normal activities, the DNA molecule of each chromosome is a long tangled thread. Each human cell has 46 chromosomes (23 pairs of homologous chromosomes). Obviously, it would be difficult to reliably separate the two copies of each of 46 long tangled DNA molecules. Therefore, in preparation for mitosis, the DNA is condensed into compact chromosomes, like those shown in the diagram on page 2.

The basic steps of mitosis, which ensure that each daughter cell receives a complete set of chromosomes, are as follows.

1. DNA is copied (called replication).

2. DNA is condensed into compact chromosomes (each with two sister chromatids); these are easier to move than the long tangled DNA. Spindle fibers which will move the chromosomes begin to form.

3. Spindle fibers line the chromosomes up in the middle of the cell.

4. Spindle fibers shorten to pull the sister chromatids apart toward opposite ends of the cell.

5. The cell begins to pinch in half, with one set of chromosomes in each half.

6. Two daughter cells are formed.

For each of the figures below, give the number of the corresponding stage described above. Draw arrows to indicate the sequence of events during mitosis. (For simplicity, the figures show cells that have only 4 chromosomes (2 pairs of homologous chromosomes), but the basic process is the same as in human cells which have 46 chromosomes.)

Sister chromatids are shown in two of these drawings; label each pair of sister chromatids (SC).Pairs of homologous chromosomes are shown in four of these drawings; circle and label each pair of homologous chromosomes (HC).

separating chromosomes

Chromosomes and Genes in Human Cells

The figure on the left shows a karyotype, which is a photograph of a magnified view of the chromosomes from a human cell that was ready to begin mitosis. Each chromosome has condensed double copies of its DNA, contained in a pair of sister chromatids linked by a centromere.

In a karyotype, the complete set of chromosomes is organized in homologous pairs and numbered. Each numbered pair of homologous chromosomes carries a specific set of genes. For example, both copies of human chromosome 11 have a gene for the production of the pigment melanin (a molecule that contributes to our skin and hair color), but one may have the A allele for normal melanin production and skin color, while the other may have the little a allele. If both chromosomes have the little a allele, the body's cells do not produce melanin, which results in albinism (the white skin and hair color shown in the figure above).

Use your sockosomes to model mitosis in a cell which has two pairs of homologous chromosomes. All of your sockosomes have labeled masking tape genes. Find two sockosomes in your group that have the

Adapted from Concepts of Genetics 8e by Klug, Cummings, and Spencer

two different alleles for the gene for albinism (A for normal melanin production and skin color and a for albinism). Next, find two sockosomes in your group that have the two different alleles for the gene for thumb bending (H for straight thumb and h for the hitchhiker’s thumb; you have a hitchhiker’s thumb if you can bend the top part of your thumb backwards more than 45º). Put these four sockosomes in a pile which will represent the two pairs of homologous chromosomes, each with the DNA copied, so the cell is ready to undergo mitosis. Model the steps in mitosis. Begin by arranging the sockosomes in the pattern observed for chromosomes in a real cell at the beginning of mitosis (see diagram on previous page). Use your arms or string to represent the spindle fibers. Describe your results by completing the following chart.

AA or Aa or aa?

HH or Hh or hh?

Which alleles were present in the original cell?Which alleles are present in each daughter cell produced by mitosis?

Questions on Mitosis1. Are the chromosomes and genes in the daughter cells produced by

mitosis the same as or different from the chromosomes and genes in the original cell? Explain why.

2. What would happen if a cell did not make a copy of its DNA (its chromosomes) before it divided?

3. Why is it important for the chromosomes to line up in the middle of the cell during mitosis?

4. In a cell which is ready for mitosis, why are the two chromatids of each chromosome genetically identical?

5. Are the two homologous chromosomes genetically identical?

Meiosis -- How Your Body Makes Sperm or Eggs

Mitosis gives rise to almost all the cells in the body. A different type of cell division called meiosis gives rise to sperm and eggs.

During fertilization the sperm and egg unite to form a single cell called the zygote which contains chromosomes from both the sperm and egg.

The zygote undergoes mitosis to begin development of the human embryo which eventually becomes a baby.

Why can't your body use mitosis to make sperm or eggs?

Suppose human sperm and eggs were produced by mitosis. How many chromosomes would each sperm or egg have? ____

If a sperm of this type fertilized an egg of this type, and both the sperm and egg contributed all of their chromosomes to a zygote, how many chromosomes would the resulting zygote have?

_____

In humans, how many chromosomes should a zygote have, so the baby's body cells will each have a normal set of chromosomes? _____

Obviously, if the body used mitosis to make sperm and eggs, the resultant zygote would have too many chromosomes to produce a normal baby. To produce a normal zygote, how many chromosomes should each sperm and egg have? _____

To produce the needed number of chromosomes in sperm and eggs, meiosis reduces the number of chromosomes by half. For example, in humans each sperm and each egg produced by meiosis has only 23 chromosomes, including one chromosome from each pair of homologous chromosomes. Therefore, after an egg and sperm are united during fertilization, the resulting zygote has 23 pairs of homologous chromosomes, one in each pair from the egg and one from the sperm. When the zygote undergoes mitosis to begin to form an embryo, each cell will have the normal number of 46 chromosomes.

Cells that have two copies of each chromosome (i.e. cells that have pairs of homologous chromosomes) are called diploid cells. Most of the cells in our bodies are diploid cells. Cells that only have one copy of every chromosome are called haploid cells. Which types of cells in our bodies are haploid?

Before meiosis, the cell makes a copy of the DNA in each chromosome. Then, during meiosis there are two cell divisions, meiosis I and meiosis II. This reduces the chromosome number by half and produces four haploid daughter cells.

Meiosis IMeiosis I is different from mitosis because homologous chromosomes line up next to each of other and then separate, as shown below. This produces daughter cells with half as many chromosomes as the parent cell, i.e. haploid cells. Notice that each of the daughter cells has a different chromosome from the homologous pair of chromosomes. This means that the alleles in each daughter cell are different.

Cell Cell

Cell

Meiosis IIMeiosis II is like mitosis. The sister chromatids of each chromosome are separated, so each daughter cell gets one copy of each chromosome in the mother cell.

In the diagram above, label the cells which would be the sperm or eggs produced by meiosis.

Using one pair of sockosomes, go through each step of meiosis until you are confident that you understand the difference between Meiosis I and Mitosis and the difference between Meiosis I and Meiosis II. For example, what is the difference in the way the pair of homologous chromosomes is lined up in a cell at the beginning of Meiosis I vs. at the beginning of Mitosis?

Now, use your group’s sockosomes to model meiosis in a cell which has two pairs of homologous chromosomes. Find two sockosomes that have the two different alleles for the gene for albinism (A for pigmented skin and a for albinism). Next, find two sockosomes that have the two different alleles for the gene for thumb bending (H for straight thumb and h for the hitchhiker’s thumb). Put these four sockosomes in a pile to represent the two pairs of homologous chromosomes, each with the DNA copied so the cell is ready to undergo meiosis. The genetic makeup of this cell is AaHh. Now, use these sockosomes to model the steps in meiosis. Begin by lining up the sockosomes the way real chromosomes line up at the

CellCellCellCell

Cell Cell

beginning of Meiosis 1. Notice that there is more than one possible way for the sockosomes to line up at the beginning of Meiosis 1. As a result, you can get different combinations of alleles in individual sperm or eggs. List all of the different possible combinations of alleles in the sperm or eggs that can be produced by meiosis.

Questions1. Describe the differences between the mother cell that undergoes meiosis and the daughter cells produced by meiosis.

2. Describe the differences between daughter cells produced by meiosis and daughter cells produced by mitosis.

3. The following diagram provides an overview of the information covered thus far. Review the diagram, and fill in the correct number of chromosomes per human cell in each blank.

Mother _____ Father _____

Meiosis ↓ Meiosis ↓ egg _____ sperm _____

Fertilization

zygote _____

Mitosis ↓ Embryo _____

Mitosis ↓ baby _____

Analyzing Meiosis and Fertilization to Understand Genetics

In this section you will investigate how events during meiosis and fertilization determine the genetic makeup of the zygote, which in turn determines the genetic makeup of the baby that develops from the zygote.

You already know that sisters or brothers can have different characteristics, even when they have the same parents. One major reason for these different characteristics is that the processes of meiosis and fertilization result in a different combination of alleles in each child.

To begin to understand this genetic variability, you will model meiosis and fertilization for a very simplified case where there is only one pair of homologous chromosomes per cell, and the two homologous chromosomes carry different alleles of the same genes. One person in your group will be the mother and another will be the father, with sockosomes as shown below. (Alternatively, you may have four sockosomes similar to those shown, but labeled h and H).

In this simple example, how many different types of eggs will be produced by meiosis? _____

How many different types of sperm will be produced by meiosis? _____

The different types of sperm can fertilize the different types of egg to result in zygotes with different combinations of chromosomes from the mother and the father. Fertilization can be demonstrated by having the mother and father each contribute one chromatid from one of their sockosomes to form a zygote. Thus, the zygote will have a pair of homologous chromosomes including one chromosome from the egg and one chromosome from the sperm. Try to produce as many different types of zygotes as you can by pairing each type of sperm with each type of egg. To demonstrate fertilization, it works best to lay the chromosomes out on the table, so you can more easily see the multiple different possible combinations.

a a A

FatherMother

A

How many different types of zygotes can be produced by fertilization in this simple case?

What different combinations of the labeled alleles can be observed in the zygotes?

A pair of human parents could produce a great many more different genetic combinations than observed in this simplified example. For example, humans have 23 pairs of homologous chromosomes, so many, many different combinations of chromosomes can be found in the eggs or sperm produced by one person, and the different combinations of eggs from one mother and sperm from one father could produce zygotes with approximately 70 trillion different combinations of chromosomes! You can see why no two people are genetically alike, except for identical twins who are derived from the same zygote.

Questions1. How many chromosomes are there in a human skin cell produced

by mitosis? ________

How many chromosomes are there in a human sperm cell produced by meiosis?

_______

2. Describe the differences between mitosis and meiosis.

3. What are the similarities between mitosis and meiosis?

Down Syndrome

Sometimes, meiosis does not happen perfectly, so the chromosomes are not divided completely equally between the daughter cells produced by meiosis. For example, an egg or a sperm may receive two copies of the same chromosome.

If a human egg receives an extra copy of a chromosome, and this egg is fertilized by a normal sperm, how many copies of this chromosome would there be in the resulting in zygote?

How many copies of this chromosome would there be in each cell in the resulting embryo?

When a cell has three copies of a chromosome, the extra copies of the genes on this chromosome result in abnormal cell function and abnormal embryonic development. Therefore, in most cases, a zygote which has an extra chromosome will die early in embryonic development, resulting in a miscarriage.

However, some babies are born with an extra copy of a small chromosome (chromosome 21), and this results in the condition known as Down Syndrome. A karyotype of a boy with Down Syndrome is shown below.3 Multiple abnormalities result from the extra copy of chromosome 21 in each cell, including mental retardation, a broad flat face, a big tongue, short height, and congenital heart disease.

3 In this karyotype it is difficult to see the sister chromatids in each chromosome, since they are very close to each other.

Mitosis/MeiosisDirections: Complete the concept map comparing mitosis and meiosis. Use these words or phrases one or more times: diploid cell, cell division, four haploid cells, original cell, two cell divisions, body cells, same, chromosomes, gamete-producing cells, half, two diploid cells.

Mitosis Meiosis

begins with a begins with a

occurs in occurs in

consists of consists of

forming forming

having the having

number of the number of

as the as the

original cell

Mitosis/MeiosisDirections: Complete the concept map comparing mitosis and meiosis. Use these words or phrases one or more times: diploid cell, cell division, four haploid cells, original cell, two cell divisions, body cells, same, chromosomes, gamete-producing cells, half, two diploid cells.

Mitosis Meiosis

begins with a begins with a

occurs in occurs in

consists of consists of

forming forming

having the having

number of the number of

as the as the

Diploid Diploid

Body Cell (Somatic Cell)

Gamete Producing Cells

One Cell Division

Two Cell Diviosns

2 diploid cells 4 haploid cells

SAME HALF

Chromosomes Chromosomes

Original Cell Original cell

Chromosome Number Worksheet

An organism has body cells with 42 chromosomes. Use this information to answer #1-7

1. A body cell prepares for cell division. How many chromosomes does it have at the beginning of prophase?__________

2. After the membrane pinches in half, how many chromosomes does each daughter cell have?___________

3. How many chromosomes did each of the sex cells have that formed this individual organism?____________

4. how many chromosomes does the egg cell of this organism have?__________5. How many chromosomes does the body cells have during interphase?

_________6. What is the diploid number of this organism?__________7. What is the haploid number of for this organism?___________

8. An organism has 12 chromosomes in its sperm cells. How many chromosomes does it have in its body cells?

9. An organism has 29 chromosomes in its egg cells. How many chromosomes does it have in its body cells?

10. An organism has 22 chromosomes in its body cells. How many chromosomes does it have in its sperm cells?

An organism has body cells with 22 chromosomes. Use this information to answer #11-17

11. A body cell prepares for cell division. How many chromosomes does it have at the beginning of prophase?__________

12. After the membrane pinches in half, how many chromosomes does each daughter cell have?__________

13. How many chromosomes did each of the sex cells have that formed this individual organism?__________

14. How many chromosomes does the egg cell of this organism have?_______15. How many chromosomes does the body cells have during interphase?_______16. What is the diploid number of this organism?_________17. What is the haploid number for this organism?__________

An organism has body cells with 78 chromosomes. Use this information to answer questions #18-2418. A body cell prepares for cell division. How many chromosomes does it have

at the beginning of prophase?____________19. After the membrane pinches in half, how many chromosomes does each

daughter cell have?________20. How many chromosomes did each of the sex cells have that formed this

individual organisms?___________________21. How many chromosomes does the egg cell or this organism have?_________22. How many chromosomes does the body cells have during interphase?

_________23. What is the diploid number of this organism?________24. What is the haploid number for this organism?_______

An organism has body cells with 46 chromosomes. Use this information to answer #25-3125. A body cell prepares for cell division. How many chromosomes does it have

at the beginning of prophase?________26. After the membrane pinches in half, how many chromosomes does each

daughter cell have?___________27. How many chromosomes did each of the sex cells have that formed this

individual organism?_________28. How many chromosomes does the egg cell or this organism have?_________29. How many chromosomes does the body cells have during interphase?


An organism has body cells with 8 chromosomes. Use this information to answer #32-3832. A body cell prepares for cell division. How many chromosomes does it have

at the beginning of prophase?________33. After the membrane pinches in half, how many chromosomes does each

daughter cell have?___________34. How many chromosomes did each of the sex cells have that formed this

individual organism?_________35. How many chromosomes does the egg cell or this organism have?_________36. How many chromosomes does the body cells have during interphase?


11 biology 6-10-08 6e mitosis-meiosis

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