Cell Cycle Control

Appreciate the consequences of nondisjunction and gene balance

Nondisjunction occurs when too many or too few chromosomes make it into the daughter cells following meiosis. Nondisjunction occurs due to incorrect separation of chromosomes during either anaphase I or II: sometimes both chromosomes of a pair, or both chromatids of a chromosome, might make it into one of the daughter cells.

Nondisjunction is problematic because of gene balance, in which you need the right number of genes to function normally. This is due to the gene-dosage effect, in which more copies of a gene result in more transcripts and more proteins made. If too many or too few proteins are made, there may be deleterious results. For instance, nearly all monosomic disorders (disorders where one of the chromosomes is missing from one of the pairs) are fatal. The only monosomic disorder that is not fatal is Turner's syndrome, in which the only sex chromosome present is the X chromosome. There are more viable trisomic disorders (disorders with an extra chromosome in one of the pairs), each with an identifiable phenotype. The most well-known trisomic disorder is Down Syndrome, which is usually caused by an extra chromosome 21.

Usually? you might be asking. Well, there are actually ways in which someone can get Down Syndrome without an extra chromosome 21. Someone can also get Down Syndrome if a part of chromosome 21 translocates onto part of another chromosome, and the new fusion chromosome that results is passed on. One translocation that can cause Down Syndrome is the Robertsonian translocation, in which part of chromosome 21 is translocated onto part of chromosome 14. If one parent passed on the Robertsonian chromosome 14 as well as a regular chromosome 21, and the other parent passed on a normal chromosome 21 and a normal chromosome 14, the child would then have Down syndrome as they would then have three copies of many of the genes on chromosome 21: two copies from the two normal chromosome 21s, and another copy from the Robertsonian chromosome.

Translocations such as the Robertsonian chromosome, where two different chromosomes exchange parts of their DNA, are also known as reciprocal translocations.

Understand the basics of the process of apoptosis
Understand the cell cycle phases & checkpoints

• Cell Cycle and Apoptosis

Understand how Cdk-cyclin complexes help regulate cell cycle progression, that cyclin levels oscillate and how Cdk activity is controlled

• Cell Cycle Control

Have a basic understanding of the need for communication between cells to control cell cycling and apoptosis.

Cells need to communicate between each other to figure out when they should and shouldn't divide. For instance, if a cell is packed in by other cells, there would be no need for it to divide and produce yet more cells. Cells send survival, proliferation, death, and growth-inhibiting cues to each other. If cells lose their sensitivity to these cues, they may grow out of control.