Introduction to the MEIOSIS Cell CycleBackground: Mitosis is used for almost all of your body’s cell division needs. It adds new cells during development, repairs injury, and replaced old and worn out cells throughout your life. The goal of mitosis is to produce daughter cells that are genetically identical to their mothers, with not a single chromosome more or less.

Meiosis, on the other hand, is used for just one purpose in the human body: to produce the gametes, or sex cells, needed for sexual reproduction. Its goal is to make genetically varied daughter cells with exactly half as many chromosomes and the starting parent cell. To put it another way, meiosis in humans is a division process that takes us from a diploid cell (a cell with two sets of chromosomes) to haploid cells (ones with a single set of chromosomes). In humans, the haploid gamete cells made in meiosis are sperm and eggs. When a sperm and an egg join in fertilization, the two haploid sets of chromosomes are restored to a complete diploid set!

Mitosis vs Meiosis:

In many ways, meiosis is a lot like mitosis. The cell goes through similar stages and uses similar strategies to organize and separate chromosomes. Both processes result in the production of new cells. In meiosis, however, the cell has a more complex task and must result in genetically varied cells with half the number of chromosomes.

MITOSIS MEIOSIS Asexual reproduction

of cells

Produces somatic cells

Starts with one diploid parent cell

Ends with two, identical diploid

Necessary for sexual reproduction

Produces gamete cells

Starts with one diploid parent cell

Ends with four, genetically varied

daughter cells

Takes place all over the body

haploid daughter cells

Takes place in reproductive organs

Meiosis Cell Cycle:

The goals of meiosis to produce genetically varied, haploid daughter cells are accomplished through a two-step division process. Prior to cell division in the meiosis cell cycle, DNA is duplicated a single time in Interphase. Since DNA is duplicated only once, but cell division occurs twice during meiosis, one diploid starting cell produces four haploid gametes (egg or sperm).

The overall meiosis cell cycle consists of the following:

One unique quality of the meiosis cell cycle is that it results in cells that are genetically varied. Genetic diversity plays an important role in the survival and adaptability of a species. When a population’s habitat changes, the population may have to adapt to survive; the ability of the population to adapt to the changing environment will determine their ability to cope with an environmental challenge. In other words, a strong species is one that is genetically diverse because they are able to withstand many different conditions.

Crossing over is a critical process that only takes place in the meiosis cell cycle and results in genetic variation. Keep in mind, an individual receives a set of chromosomes from their mother and a set of chromosomes from their father. In humans, you have 23 pairs of chromosomes. Each pair of chromosomes is known as a homologous pair. Homologous chromosomes contain the same set of genes, but do not necessarily code for the same trait.

During prophase I of the meiosis cell cycle, homologous chromosomes pair up and crossing over occurs. Crossing over is the exchange genetic information (as seen in the diagram). This shuffling of genes helps create diversity!

Interphase Meiosis I Meiosis II Cytokinesis

CROSSING

Errors in Meiosis:

Because mitosis is used to produce somatic cells, normal body cells, an error during mitosis only affects the individual in which it takes place and cannot be passed on. Meiosis, however, produces gametes (sex cells). If an error takes place in meiosis and the egg or sperm containing the error is fertilized, the mistake is passed on to the next generation!

One such mistake which can take place during meiosis is nondisjunction. Nondisjunction refers to the chromosomes failing to separate correctly, resulting in gametes with too many or too few chromosomes.

In the diagram, the fertilized eggs A and B contain the correct number of chromosomes, while C and D contain chromosomal abnormalities. The fertilized egg C has too many chromosomes, and the fertilized egg D has too few.

You are probably familiar with the nondisjunction known as Trisomy 21, or Down syndrome. An individual with Trisomy 21 has three 21st chromosomes, instead of the usual pair.

So what takes place to ensure that a cell divides correctly?

INTERPHASE

- Longest stage of the cell cycle- Cell grows and carries out normal cell functions

- DNA is replicated

PROPHASE I

- Nucleus breaks down- Chromatin condenses into chromosomes

- ***Homologous chromosomes pair up and crossing over takes place***- Centrioles begin to form spindle fibers

METAPHASE I

- Spindle fibers attach to the chromosome centromeres- Chromosomes line up in PAIRS in the middle of the cell

ANAPHASE I

A B C D

NONDISJUNCTION

- Spindle fibers shorten- Chromosome PAIRS are pulled apart

- Chromosomes move to opposite poles in the cell

TELOPHASE I

- Chromosomes reach poles- Two nuclei reform

- Chromosomes uncoil back into chromatin

PROPHASE II

- Nucleus breaks down- Chromatin condenses into chromosomes

- Centrioles begin to form spindle fibers

METAPHASE II

- Spindle fibers attach to the chromosome centromeres- Chromosomes line up SINGLE FILE in the middle of the cell

ANAPHASE II

- Spindle fibers shorten- Chromosomes are pulled apart

- Chromatids move to opposite poles in the cell

TELOPHASE II

- Chromatids reach poles- Four nuclei reform

- Chromosomes uncoil back into chromatin

CYTOKINESIS

- The cutting of the cytoplasm- Four genetically varied cells are formed

- Results in haploid gametes (egg or sperm)

The back of this paper contains a series of illustrations which show the “dance of the chromosomes” that takes place during the Meiosis cell cycle. To help you

remember the steps of meiosis, remember the following… I PMAT PMAT C (eye p-mat p-mat see)

The meiosis Cell Cycle – Illustrations

INTERPHASE

PROPHASE I

METAPHASE I

ANAPHASE I

TELOPHASE I

CYTOKINESIS

Homologous chromosomes pair up and cross over

Homologous chromosomes line up in PAIRS in middle of cell

Chromosome pairs separate

Full CHROMOSOMES are pulled to poles

PROPHASE IIChromosomes condense,

but do not pair up

Chromosomes unwind

Chromosomes line up SINGLE FILE in middle of cell

METAPHASE II

Chromosomes separate, CHROMATIDS are pulled to poles

ANAPHASE II

TELOPHASE II

Four nuclei reform with haploid chromosomes

Four gametes - egg or sperm

Name ________________________________________________ Date ____________________ Period ___________

Introduction to the Meiosis Cell CycleInstructions: Read the provided information on the meiosis cell cycle and answer the following questions.

1. Summarize the purpose of the meiosis cell cycle.

2. The meiosis cell cycle is necessary for ( asexual / sexual ) reproduction.

3. The meiosis cell cycle produces gametes that are genetically ( identical to / different from ) the parent cell.

4. Define the following terms: Diploid -

Haploid -

5. Following meiosis, the chromosome number is restored when sperm and egg join during ___________________.

6. Provide at least TWO ways in which mitosis and meiosis are similar.

7. Complete the t-Chart to the right contrasting the mitosis and meiosis cell cycles.

8. Complete the diagram to the left by filling in the STARTING and ENDING number of chromosomes following mitosis and meiosis in humans.

9. True or False: In the meiosis cell cycle, DNA duplicates one time and the cell divides twice. If false, please explain.

MITOSIS MEIOSIS

10. How many haploid daughter cells are produced as a result of the meiosis cell cycle? _____________

11. What are the four major stages of the meiosis cell cycle?

______________________ ______________________ ______________________ _____________________

12. Provide an explanation for why genetic diversity is beneficial to a species.

13. The picture to the right shows the exchange of genetic information between homologous chromosomes. This is known as…

14. True or False: Crossing over takes place in both mitosis and meiosis. If false, please explain.

15. Describe homologous chromosomes.

16. Crossing over take place during the ___________________________________ stage of the meiosis cell cycle.

17. Errors in mitosis cannot be passed on to future generations, while errors in meiosis can. Defend this statement with an explanation.

18. Which term is used to refer to chromosomes failing to separate correctly, resulting in gametes with too many or too few chromosomes?

19. Place a RED circle around the fertilized egg(s) with the correct number of chromosomes. Place a BLUE circle around the fertilized egg with too many chromosomes. Place a GREEN circle around the fertilized egg with too few chromosomes.

Homologous Chromosomes

Gametes, or sex cells, are produced as a result of the meiosis cell cycle. In each box, draw the events listed, label the significant structures, and describe what is occurring in that particular phase of the cell cycle.

CYTOKINESIS

MEIOSIS Cell Cycle

Interphase Description: Prophase I Description:

Metaphase I Description:

Anaphase I Description:

Anaphase II Description:

Cytokinesis Description:

INTERPHASE PROPHASE I

METAPHASE I

ANAPHASE I

TELOPHASE IPROPHASE IIMETAPHASE II

ANAPHASE II

TELOPHASE II

Telophase I Description:Prophase II Description:Metaphase II Description:

Telophase II Description: