Module 5 Cell Division

Reflect and Connect

Meiosis is the part of our life cycle that gives rise to variety. Through the recombination of chromosomes, random assortment during separation, and finally random fertilization, it is clear that no two people will look alike. That is true as long as the production of that organism is a result of meiosis and fertilization. However, in the case of identical twins, the two separate organisms are actually the result of an anomaly in mitosis, not meiosis. The fertilization event has already occurred before the single mass separates into two new cell groups. From this point on, each cellular division occurs by mitosis. Mitosis is all about keeping the genetics the same, so identical twins result.

Self Check

Complete the questions 1-4 and 7-9 on p 572 in your textbook. You may wish to discuss your work with your instructor.

Try This

Discuss

In 1996 a sheep named Dolly made history as the first animal to be cloned from an existing adult. In many ways it was like making an identical twin, only much later in life. This was an amazing scientific development, and all kinds of groups began discussing how we could use this technology. Some advocated cloning people to help infertile couples. Others wanted to clone all kinds of livestock to make vast herds of perfect producers. However, some problems with this procedure became evident about six years after Dolly was born. She developed arthritis and lung cancer and was put down. Dolly’s breed of sheep normally lives 12 to 15 years. The cell she was created from was about six years old. Some speculate her chromosomes were already 6 years old and therefore really died of old age.

Retrieve your copy of Module 5: Lesson 4 Assignment that you saved to your computer earlier in this lesson. Complete Part 2. Save your completed assignment in your course folder. You will receive instructions later in this lesson on when to submit your assignment to your teacher.

Lesson Summary

During this lesson you were to concentrate on the following focusing questions:

• How does meiosis contribute to genetic variation?
• What differences exist between fraternal and identical twins?

Meiosis is the orderly separation of homologous chromosomes into haploid gametes. Through crossover events in Prophase 1, and through independent assortment in Metaphase 1, meiosis ensures the creation of unique gametes. When fertilization later occurs, the new offspring will have a genetic combination never before seen.

Fraternal twins result from the fertilization of two individual and different egg gametes by two individual sperm cells. Since meiosis ensures that each gamete is unique, they will each be genetically different from each other. Identical twins start out from the same sperm and egg. Early on in development, the cell mass splits into two, and from there onward, each mass grows by mitosis into a full person. Since mitosis does not create variation, these twins are genetically the same, or identical.