Scientists use a process called Flourescent In-Situ Hybridization, or FISH, to study genetic disorders in humans. FISH is a technique that uses spectrographic analysis to determine the presence or absence, as well as the relative abundance, of genetic material in human cells. 

To use FISH, scientists apply fluorescently-labeled bits of DNA of a known color, called probes, to samples of test DNA. These probes anneal to the sample DNA, and scientists can read the colors that result using laboratory equipment. One common use of FISH is to determine the presence of extra DNA in conditions of aneuploidy, a state in which a human cell has an abnormal number of chromosomes. Chromosomes are collections of DNA, the totality of which makes up a cell’s genome. Another typical use is in the study of cancer cells, where scientists use FISH labels to ascertain if genes have moved inappropriately in a cell’s genome.

Using red fluorescent tags, scientists label probe DNA for a gene known to be expressed more heavily in cancer cells than normal cells. They then label a probe for an immediately adjacent DNA sequence with a green fluorescent tag. Both probes are then added to three dishes, shown below.  In dish 1 human bladder cells are incubated with the probes, in dish 2 human epithelial cells are incubated, and in dish 3 known non-cancerous cells are used. The relative luminescence observed in regions of interest in all dishes is shown below.

In cancer cells, karyotype analysis shows the specific arrangement of chromosomes. Often, as cancer cells divide uncontrollably, the number of chromosomes becomes deranged. In which of the following stages of mitosis is a non-disjunction most likely to occur?

Combrestatin is a drug that prevents the polymerization of microtubules. Which of the following processes would be most affected by combrestatin?

Which of the following lists the correct sequence of mitotic events?

During which mitotic stage do the spindle fibers begin to form from microtubules?

A mouse has a mutation in which its sister chromatids are unable to separate during cell division. What phase of mitosis should researchers target, in order to alleviate the condition using drug therapy?

Which answer choice incorrectly matches the type of chromatin with the phase of mitosis?

Something goes wrong during meiosis in a male and the four daughter sperm cells that are produced all have the wrong amount of chromosomes. Two of the sperm have one extra chromosome and the other two have one missing chromosome. At what step did the problem most likely occur, and what was it?

When does genetic crossover occur during meiosis? 

The concept of genomic imprinting is important in human genetics. In genomic imprinting, a certain region of DNA is only expressed by one of the two chromosomes that make up a typical homologous pair. In healthy individuals, genomic imprinting results in the silencing of genes in a certain section of the maternal chromosome 15. The DNA in this part of the chromosome is "turned off" by the addition of methyl groups to the DNA molecule. Healthy people will thus only have expression of this section of chromosome 15 from paternally-derived DNA.

The two classic human diseases that illustrate defects in genomic imprinting are Prader-Willi and Angelman Syndromes. In Prader-Willi Syndrome, the section of paternal chromosome 15 that is usually expressed is disrupted, such as by a chromosomal deletion. In Angelman Syndrome, maternal genes in this section are deleted, while paternal genes are silenced. Prader-Willi Syndrome is thus closely linked to paternal inheritance, while Angelman Syndrome is linked to maternal inheritance.

Figure 1 shows the chromosome 15 homologous pair for a child with Prader-Willi Syndrome. The parental chromosomes are also shown. The genes on the mother’s chromosomes are silenced normally, as represented by the black boxes. At once, there is also a chromosomal deletion on one of the paternal chromosomes. The result is that the child does not have any genes expressed that are normally found on that region of this chromosome.

A scientist is investigating a cell undergoing division, and notes that chromosome 15 is aligned with the other chromosomes in the center of the cell, as if along a line. Each of the 23 pairs of chromosomes is arranged as a tetrad along this plate. Which phase of cell division is this cell most likely undergoing?

Which of the following processes occur in meiosis but do not occur in mitosis?

I. Spindle formation

II. Separation of homologous chromosomes

III. Separation of sister chromatids

IV. Recombination