Meiosis and Stuff!

1. How does meiosis differ from mitosis? What is the ploidy number of the products in both processes?

Mitosis is a process of asexual reproduction in which the cell divides in two producing a replica, with an equal number of chromosomes in each resulting diploid cell. It basically creates all cells other than sex cells.

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Meiosis is a type of cellular reproduction in which the number of chromosomes are reduced by half through the separation of homologous chromosomes, producing two haploid cells. It produces sex cells.

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Crossing over of chromosomes only occurs in meiosis. 

I found a super great resource that describes Meiosis really well, here is a great summary of the process:

“To get ready for meiosis, a diploid germ cell must first undergo DNA replication. This process duplicates every strand of DNA so that there are two copies of every chromosome. The two copies are called sister chromatids and are attached to each other. Once DNA replication is complete, two rounds of chromosomal divisions take place, one each in meiosis I and meiosis II. During meiosis I, homologous chromosomes pair up, line up, and then are split apart.

Homologous chromosomes are those that are the same size and carry the same genes. You get one from your mom and one from your dad. In human diploid cells, there are 23 pairs of homologous chromosomes. After meiosis I, two cells are present, each with 23 different chromosomes that are still in duplicate as sister chromatids.

Both the cells produced in meiosis I undergo meiosis II. During this round, chromosomes line up and sister chromatids are pulled apart into two sets. Now, a total of four haploid cells are present. The pulling apart of either homologous chromosomes or sister chromatids is called disjunction and occurs during anaphase of meiosis I or meiosis II, respectively.” More at

2. What is non disjunction? Give an example.

If chromosomes don’t separate properly during meiosis, it could lead to non disjunction. It results in gametes with either too many or too few chromosomes. It happens either during Meiosis one or Meiosis two. If one of these abnormal gametes undergoes fertilization, then a baby with an abnormal number of chromosomes in its cells could be born. It could lead to Down Syndrome, for example. This occurs when there is an error in meiosis I that produces an extra copy of chromosome 21. The person ends up with three copies of the chromosome which leads to developmental problems.

3. Compare contrast asexual vs sexual reproduction. Describe pros and cons of each.

Asexual:  involves a single parent. It results in offspring that are genetically identical to each other and to the parent. All prokaryotes and some eukaryotes reproduce this way. Asexual reproduction can be very rapid, and this is an advantage for many organisms. It allows them to crowd out other organisms that reproduce more slowly. However, their reproduction is kept in check by limited resources, predators, and their own wastes. Also, in order to change, a mutation must occur.

Sexual: involves two parents and the offspring have unique DNA. It’s easier for creatures created through sexual reproduction to survive and adapt to their environment because the advantageous traits survive.

4. Describe what you learned in class on Thursday either about Molly or about chimeras.

Molly was born with a type of genetic anemia and needed bone marrow transplant to survive. She had received two recessive gene copies from her parents, which caused the anemia to be expressed. Molly was also born with bum copies of a gene on chromosome 6 that ensured proper red blood cell formation, and these bum copies created mutated genes which meant she lacked normal red blood cells. The red blood cells were shaped improperly, so her body couldn’t carry oxygen efficiently. Her parents struggled to find any sort of help because they couldn’t find a donor with no expression of the disease and good copies of the necessary genes. Oddly enough, they decided to create one, aka a GMO designer baby. This baby was successful in the creation and provided Molly with bone marrow that her body didn’t reject, didn’t express the disease and also provided the correct copy of the disease. Wow!





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