Mitosis

Below are four moments in the division of an animal cell, listed out of order and indicated with numbers:

1. Sister chromatids separate and migrate toward opposite poles of the cell.

2. Chromosomes decondense and nuclear envelopes reform.

3. Chromatin condenses into visible chromosomes and the nuclear envelope begins to break down.

4. Chromosomes line up at the equatorial plate.

Order the numbers from 1 to 4 as they occur in mitosis and name the phase corresponding to each one.

Explain what a chromosome is and what the two chromatids forming it are when it is duplicated. At which point are the chromatids held together until division?

Describe what happens to chromosomes during metaphase: where they align and which structure is responsible for moving them.

In a cell culture laboratory, a cell line from an animal species with $2n = 12$ chromosomes is analyzed. The nuclear DNA mass per cell is measured, knowing that in the G1 phase each cell contains exactly 3.0 picograms (pg) of DNA. During the study, three cells are isolated at different points in time:

• Cell 1: Contains 6.0 pg of DNA, and under the microscope 12 duplicated chromosomes aligned at the center of the cell are observed.

• Cell 2: Contains 6.0 pg of DNA, and 24 individual chromatids moving toward opposite poles are observed.

• Cell 3: Possesses 3.0 pg of DNA in a nucleus with decondensed chromatin after cytoplasmic division is completed.

1. a) Justify which exact phase of the cell cycle or mitosis each of the three cells is in.
2. b) If a drug prevented the degradation of cohesins in anaphase but cytokinesis occurred asymmetrically, predict how many chromosomes and what amount of DNA (in pg) each daughter cell would receive if all genetic material were pulled to a single pole.

Some chemotherapy drugs block the formation of the mitotic spindle. Explain at which phase the treated cells will stop, and reason why these drugs can be useful against tumours, as well as why they also damage healthy tissues such as hair or the intestinal mucosa.

State two differences between mitosis in an animal cell and in a plant cell, one referring to the spindle and another to the way the cytoplasm is divided.

State three biological functions of mitosis in a multicellular organism like yours and give an example of each.

A cell with $8$ chromosomes enters mitosis. How many chromosomes will each daughter cell have at the end? And how many chromatids does each chromosome have during metaphase?

A student states in an exam: "For mitosis to keep genetic information constant, the only essential requirement is that DNA replicates in the S phase; then, it is enough for the cell to pinch in half during cytokinesis and the distribution will have been successfully completed."

1. a) Refute the student's statement: demonstrate what would happen to the genetic endowment of the daughter cells if the mechanisms operating in metaphase and anaphase were dispensed with.
2. b) Design a reasoned prediction: what would you observe under the microscope and how would the chromosome distribution turn out if a mutation prevented kinetochores from attaching to the microtubules of the mitotic spindle?