Question

What are the steps of mitosis? explain the steps

Answer 1

Prophase (From Greek words meaning “before” and “stage”):

Chromatin condenses into 2 visible rod structures, chromosomes, in a process called chromatin condensation (inspired name, right?).

Due to the DNA replication in interphase, there are two identical copies of each chromosome, referred to as sister chromatids, attached by a centromere.

At the end of prophase the nucleolus dissolves.

Technically this next section is classed as a separate stage but for all intents and purposes it is usually bundled up in the end of Prophase and the beginning of Metaphase. It’s called… *drum roll*… Prometaphase.

What happens here is the nuclear membrane breaks apart and chromosomes form structures known as kinetochores. For A-level you don't really need to know about these so I'll stop myself there.

Metaphase (from Greek meaning “adjacent” “stage”):

The centromeres of all chromosomes line up along the equator of the cell. Spindle fibres form between the poles of the cell and the centromeres.

Anaphase (more Greek! Means “up” “stage”):

The centromeres are split and the spindle fibres contract, pulling the sister chromatids to opposite poles of the cell. The chromatids form a V/Y shape as they are pulled backwards.

The cell also stretches into an oval (this movement of the cell is due to non-kintochore spindle fibres pushing against each other, but this is far above A-level).

Telophase (“end” “stage”):

The effects of prophase and prometaphase are effectively reversed here.

Two nuclei form in the cell, at both ends of the cell. Nuclear envelopes are reformed from components of the parent cell’s envelope.

There are 2 theories about how this happen:

Vesicle fusion: fragments of the initial nuclear membrane fuse to rebuild the nuclear membrane

Reshaping of the endoplasmic reticulum

The nucleoli also re-form, any remaining spindle fibres depolymerise and the chromosomes begin to unwind and expand into the chromatin that is seen in interphase.

Cytokenises begins, in which a Myosin II and actin filament ring contract to cleave the cell in two, resulting in two genetically identical daughter cells.

After that, they're both straight back to interphase.

Step-by-step explanation:

Answer 2

Today, mitosis is understood to involve five phases, based on the physical state of the chromosomes and spindle. These phases are prophase, prometaphase, metaphase, anaphase, and telophase.

Prophase

Mitosis begins with prophase, during which chromosomes recruit condensin and begin to undergo a condensation process that will continue until metaphase. In most species, cohesin is largely removed from the arms of the sister chromatids during prophase, allowing the individual sister chromatids to be resolved. Cohesin is retained, however, at the most constricted part of the chromosome, the centromere (Figure 9). During prophase, the spindle also begins to form as the two pairs of centrioles move to opposite poles and microtubules begin to polymerize from the duplicated centrosomes.

Prometaphase

Prometaphase begins with the abrupt fragmentation of the nuclear envelope into many small vesicles that will eventually be divided between the future daughter cells. The breakdown of the nuclear membrane is an essential step for spindle assembly. Because the centrosomes are located outside the nucleus in animal cells, the microtubules of the developing spindle do not have access to the chromosomes until the nuclear membrane breaks apart.

Prometaphase is an extremely dynamic part of the cell cycle. Microtubules rapidly assemble and disassemble as they grow out of the centrosomes, seeking out attachment sites at chromosome kinetochores, which are complex platelike structures that assemble during prometaphase on one face of each sister chromatid at its centromere. As prometaphase ensues, chromosomes are pulled and tugged in opposite directions by microtubules growing out from both poles of the spindle, until the pole-directed forces are finally balanced. Sister chromatids do not break apart during this tug-of-war because they are firmly attached to each other by the cohesin remaining at their centromeres. At the end of prometaphase, chromosomes have a bi-orientation, meaning that the kinetochores on sister chromatids are connected by microtubules to opposite poles of the spindle.

Metaphase

Next, chromosomes assume their most compacted state during metaphase, when the centromeres of all the cell's chromosomes line up at the equator of the spindle. Metaphase is particularly useful in cytogenetics, because chromosomes can be most easily visualized at this stage. Furthermore, cells can be experimentally arrested at metaphase with mitotic poisons such as colchicine. Video microscopy shows that chromosomes temporarily stop moving during metaphase. A complex checkpoint mechanism determines whether the spindle is properly assembled, and for the most part, only cells with correctly assembled spindles enter anaphase.

Anaphase, is the stage of mitosis after the process of metaphase, when replicated chromosomes are split and the newly-copied chromosomes are moved to opposite poles of the cell

Telophase and Cytokinesis

Mitosis ends with telophase, or the stage at which the chromosomes reach the poles. The nuclear membrane then reforms, and the chromosomes begin to decondense into their interphase conformations. Telophase is followed by cytokinesis, or the division of the cytoplasm into two daughter cells. The daughter cells that result from this process have identical genetic compositions.