Question

1. With reference to a pair of homologues, describe the exchange in the two participating nonsister chromatids following crossing-over.

2. What would be the appearance of a cell that completes mitosis but does not undergo cytokinesis? Be sure to mention chromosome number.

3. Give the answers for blanks (a) and (b) in the sentence below.

During anaphase I (a) _______________ separate; during anaphase II (b) ___________________ separate.

4. Account for why each of your body cells contains two of each kind of chromosome.

Answer 1

1. Chromosomal crossover, or crossing over, is the exchange of genetic material between two homologous chromosomes non-sister chromatids that results in recombinant chromosomes during sexual reproduction. It is one of the final phases of genetic recombination, which occurs in the pachytene stage of prophase I of meiosis during a process called synapsis. Synapsis begins before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome.

2.The result of mitosis without cytokinesis will be a cell with more than one nucleus. Such a cell is called a multinucleated cell. This can be a normal process. For example, humans have certain multinucleated bone cells (osteoclasts) that are formed this way.A mitotic cell cycle in which mitosis is completed but cytokinesis does not occur, resulting in a cell containing multiple nuclei each with a chromosomal complement of the original ploidy (usually 2N).

3. a) homologous chromosomes

b)sister chromatids

4. When a body cell of an organism has the full amount of chromosomes, with all of its homologous pairs, such as a human cell containing 46 chromosomes, it is said to have the diploid number of chromosomes. This is because di- means two and these cells have two of each type of chromosome present.