Mitosis
Picture a cross-country runner at a Texas high school meet who scrapes a knee on the course. Within days, new skin cells fill in the scrape. Where do they come from? By the end of the G2 phase, the cell has all of its DNA replicated and checked. The mitotic (M) phase then begins. It has two separate processes: mitosis, which shares the duplicated chromosomes between two nuclei, and cytokinesis, which divides the cytoplasm. Cytokinesis is not a phase of mitosis; it usually overlaps with late anaphase and telophase and finishes the division (a cleavage furrow in animal cells, a cell plate in plant cells). The end result is two daughter cells that are genetically identical to each other and to the original cell.
What mitosis achieves
Chromosomes and chromatids
Before a cell divides, it copies its DNA in the S phase. Each chromosome then consists of two identical sister chromatids, held together at the centromere. A duplicated chromosome looks like an X. Chromatids stay joined until anaphase. After they separate, each one is counted as its own chromosome.
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…
Below are four moments in the division of an animal cell, listed out of order and indicated with numbers:
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Sister chromatids separate and migrate toward opposite poles of the cell.
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Chromosomes decondense and nuclear envelopes reform.
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Chromatin condenses into visible chromosomes and the nuclear envelope begins to break down.
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Chromosomes line up at the metaphase 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?
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?
The four phases in detail
Describe what happens to chromosomes during metaphase: where they align and which structure is responsible for moving them.
Describe what happens to chromosomes during metaphase: where they align and which structure is responsible for moving them.
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…
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?
In a cell culture lab, 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…
In a cell culture lab, 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:
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Cell 1: Contains 6.0 pg of DNA, and under the microscope 12 duplicated chromosomes aligned at the center of the cell are observed.
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Cell 2: Contains 6.0 pg of DNA, and 24 individual chromatids moving toward opposite poles are observed.
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Cell 3: Possesses 3.0 pg of DNA in a nucleus with decondensed chromatin after cytoplasmic division is completed.
- a) Justify which exact phase of the cell cycle or mitosis each of the three cells is in.
- 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…
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 tumors, as well as why they also damage healthy tissues such as hair or the intestinal mucosa.
Mitosis in animal and plant cells
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 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.
What mitosis is for
Vuelve al principio: the runner's scrape heals because cells at the edge of the wound divide by mitosis, making identical new skin cells.
State three biological functions of mitosis in a multicellular organism like yours and give an example of each.
State three biological functions of mitosis in a multicellular organism like yours and give an example of each.
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…
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."
- 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.
- 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?