Mitosis and Meiosis

Put the following phases of mitosis in order: anaphase, telophase, prophase, and metaphase, and describe in one sentence the most characteristic event of each.

Four animal cells are observed in different phases of mitosis, numbered 1 to 4:

• Cell 1: condensed chromosomes scattered and breakdown of the nuclear envelope.

• Cell 2: chromosomes aligned at the equatorial plane.

• Cell 3: separated sister chromatids moving toward opposite poles.

• Cell 4: chromosomes at opposite cell poles and formation of a cleavage furrow.

Identify which phase (prophase, metaphase, anaphase, or telophase) corresponds to each numbered cell and justify your answer by describing an observable feature in each one.

Explain the biological function of mitosis in multicellular organisms and give two examples of processes in which it plays an essential role.

How many daughter cells are obtained after mitosis and how many after complete meiosis? State also the chromosome complement of these daughter cells compared to the parent cell in each process.

What is fertilisation? Reason why meiosis is essential for keeping the chromosome number constant from one generation to the next in sexually reproducing species.

During prophase I of meiosis, crossing over occurs. Explain what this phenomenon consists of and its consequence for the genetic variation of the gametes.

In the gonads of a diploid organism with $2n = 14$, four cells in different stages of cell division (labelled A, B, C, and D) are isolated, and their chromosomal structures are quantified:

• Cell A: Shows 7 bivalents (tetrads) aligned at the equatorial plate, with a total of 28 chromatids.

• Cell B: Shows 14 chromosomes individually aligned at the cell equator, with 28 double-stranded DNA molecules in total.

• Cell C: Shows 7 chromosomes of 2 chromatids each, arranged at the equatorial plate.

• Cell D: Shows 7 chromosomes consisting of a single unduplicated chromatid.

1. a) Reasonably identify which specific phase of mitosis or meiosis each of cells A, B, C, and D corresponds to.
2. b) If during the anaphase following the stage observed in Cell C non-disjunction of the sister chromatids of one chromosome occurs, predict the exact chromosome complement of the two daughter cells directly derived from that division.

Using fluorescent in situ hybridisation (FISH) techniques, the centromere of paternal chromosome 3 is labelled with a green probe and the centromere of maternal chromosome 3 with a red probe in cells of a diploid individual. Three cells arrested in metaphase are observed under the microscope:

• Cell 1: Shows a double green signal and a double red signal closely associated, forming pairs at the equatorial plate.

• Cell 2: Shows the double green signal and the double red signal aligned independently along the same equatorial plate.

• Cell 3: Shows only a double red signal aligned at the equatorial plate, with no green signal present anywhere in the cell.

1. a) Reasonably deduce which metaphase (mitotic, meiotic I, or meiotic II) each of the three observed cells corresponds to, based on the presence, absence, and orientation of the centromeres of both homologues.
2. b) Predict which fluorescent signals and how many chromatids of chromosome 3 each daughter cell will receive after anaphase in Cell 1 compared to anaphase in Cell 2.

A couple has a child with Down syndrome (standard trisomy 21). To determine the origin of the anomaly, the alleles of a microsatellite marker located very close to the centromere of chromosome 21, where the crossing-over rate is negligible, are analysed:

• Father: homozygous with allele $A_1$.

• Mother: heterozygous with alleles $A_2$ and $A_3$.

• Child with Down syndrome: genotype $A_1 A_2 A_3$.

1. a) Reasonably deduce in which parent the meiotic error occurred and demonstrate unequivocally whether the non-disjunction took place during anaphase I or during anaphase II.
2. b) If the non-disjunction error had occurred in the other meiotic division of the same parent, predict what alternative genotypes the child could have inherited regarding maternal alleles and what percentage of the four products of that meiosis would have been completely euploid ($n = 23$).