A. Blog

This chapter talked about the processes of division and the abnormalities that can occur. Cell division requires precision and accuracy. Cells are able to sense when the conditions are right for division, and will not hesitate to stop if these conditions are not present. For instance, if DNA has not be copied correctly, the cycle will pause until the error is fixed. This is the role of checkpoint proteins, cyclin and cyclin-dependent kinases. The cdk binds to cyclin to create a cyclin/cdk complex that phosphorylates proteins needed in the cycle. There are three checkpoints, G₁, G₂, and M (metaphase). In adults, cell division generally lasts 24 hours, with G₁ phase being the longest and M phase being the shortest.

Mitosis is when a cell replicates to produce two daughter cells, each identical. There are many structures involved in division. The mitotic spindle and centromeres act by pulling apart the sister chromatids during metaphase, and separating the poles so a cleavage furrow can form. Meiosis is the same process except instead of starting with sister chromatids, bivalents/tetrads are formed. Even though the starting points are different, both cycles end with haploid cells.

There are several processes that separate meiosis from mitosis. Crossing over is when chromosomes exchange segments. This causes genetic variation during the process. Alignment during metaphase is random, so there are countless possibilities. There is no pause between meiosis I and II. Meiosis II is comparable to mitosis. 

There are many mutations that can occur during meiosis. Deletion, duplication, inversion, simple and reciprocal translocation can occur. Also, organisms can be polyploid. This means that they have three or more sets. This is usually lethal to mammals, but not as much so in amphibians or reptiles. It is in fact common in plants. Aneuploidy is when the total number of chromosomes is not an exact multiple of the set. For example, a person can have three copies of one chromosome instead of two. This usually occurs because of nondisjunction, when chromosomes do not separate properly during division. Different syndromes such as Down and Triple X are examples of these.

B. Useful Materials

This clip gives a brief overview of mitosis in an animal cell. However, it only goes over the M phase and the formation of the cleavage furrow; G₁, S and G₂ are not included in the presentation. 

This video is a basic summary of meiosis. Meiosis is similar to mitosis except for the number of daughter cells it yields and other traits, for example: crossing over. It briefly covers that process and explains how it can lead to a variety of traits in the organism.

This article talks about how Down Syndrome is caused by a third copy of chromosome 21. It is a prime example of aneuploidy. There are also physical traits common to those with Down Syndrome. The study talks about how scientists can use several aids to identify what sections of the chromosome are responsible for this abnormality.

Down syndrome phenotypes: the consequences of chromosomal imbalance