Question

Starting with a cell (2n=4), illustrate the phases of mitosis with emphasis on the division of chromosomes.

Answer 1

Mitosis is a type of cell division in which one cell (the mother) undergo division to produce two new cells (the daughters) that are genetically identical to parent. In the context of the cell cycle, mitosis is the part of the division process in which the DNA of the cell's nucleus is split into two equal sets of chromosomes.

Interphase

It is also called the preparatory phase, in which the cell prepare itself to divide. It has 3 stages Growth 1 phase (G1), Synthesis phase (S), and Growth 2 (G2).The DNA replication occurs in the S phase of interphase. The chromosomes are duplicated in order to make sure that each daughter cell will receive one copy of every chromosome

Phases of mitosis

Mitosis consists of four basic phases: prophase, metaphase, anaphase, and telophase.

In early prophase, the cell starts to break down some structures and build others up, setting the stage for division of the chromosomes.

The two sister chromatid formed during the S phase of interphase starts to condens. The condensation of chromatid indicate the beginning of Prophase.

·         The mitotic spindle begins to form. it grows between the centrosomes as they move apart.

·         The nucleolus starts to disappear.

·         In late prophase, the mitotic spindle begins to capture and organize the chromosomes.

·         The chromosomes finish condensing, so they are very compact.

·         The nuclear membrane breaks down, releasing the chromosomes.

·         The mitotic spindle grows more, and some of the microtubules start to “capture” chromosomes.

·         Microtubules can bind to chromosomes at the kinetochore, a patch of protein found on the centromere of each sister chromatid. (Centromeres are the regions of DNA where the sister chromatids are most tightly connected.)

·         Microtubules that bind a chromosome are called kinetochore microtubules. Microtubules which are not binding to kinetochores aid in stabilizing the spindle. More microtubules extend from each centrosome towards the edge of the cell, forming a structure called the aster.

In metaphase, the spindle has captured all the chromosomes and lined them up at the middle of the cell.

All the chromosomes align at the metaphase plate.

·         At this stage, the two kinetochores of each chromosome should be attached to microtubules from opposite spindle poles.

Before proceeding to anaphase, the cell will check to make sure that all the chromosomes are at the metaphase plate with their kinetochores correctly attached to microtubules. This is called the spindle checkpoint and helps ensure that the sister chromatids will split evenly between the two daughter cells when they separate in the anaphase. If a chromosome is not properly aligned or attached, the cell will halt division until the problem is fixed.

In anaphase, the sister chromatids separate from each other and are pulled towards opposite ends of the cell.

·         The protein “glue” that holds the sister chromatids together is broken down, allowing them to separate. Each is now its own chromosome. The chromosomes of each pair are pulled towards opposite ends of the cell.

·         Microtubules that are not attached to chromosomes elongate and push apart, separating the poles and making the cell longer.

All of these processes are driven by motor proteins.

In telophase, the cell is nearly done dividing, and it starts to re-establish its normal structures as cytokinesis.

·         The mitotic spindle is broken down into its building blocks.

·         Two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli reappear.

·         The chromosomes begin to decondense.

·         Cytokinesis, the division of the cytoplasm to form two new cells, overlaps with the final stages of mitosis. It may start in either anaphase or telophase, depending on the cell, and finishes shortly after telophase.

·         In animal cells, cytokinesis is contractile, pinching the cell in two like a coin purse with a drawstring. The “drawstring” is a band of filaments made of a protein called actin, and the pinch crease is known as the cleavage furrow. Plant cells can’t be divided in this manner because they have a cell wall and are too stiff. Instead, a structure called the cell plate forms down the middle of the cell, splitting it into two daughter cells separated by a new wall.

When cytokinesis finishes, the cell end up with two new cells, each with a complete set of chromosomes identical to those of the mother cell. The daughter cells can now begin their own cellular “lives,” and – depending on what they decide to be when they grow up – may undergo mitosis themselves, repeating the cycle.