Meiosis

Meiosis and Mitosis

What is meiosis?

Meiosis is a special type of cell division necessary for sexual reproduction.

Meiosis is divided into meiosis I and Meiosis II, each with 4 steps.

At the end of Meiosis I there are two daughter cells and at the end of Meiosis II those two daughter cells divide and now, at the end of the entire process, there are four daughter cells.

Steps of meiosis

Interphase

Before a cell can enter cell division, it needs to prepare itself by replicating its genetic information and all of the organelles. All of the preparations are done during the interphase

Prophase I

During Prophase I DNA is exchanged between homologous chromosomes. Sometimes this results in a cross-over of DNA wich can result in genetic variations and also beneficial new combinations of alleles

Metaphase I

During Metaphase the tetrads are arranged across the center by the movements of the kinetochores with the two centromeres opposite each other.

Anaphase I

During Anaphase I the microtubles shorten pulling the homologous chromosomes apart from eachother. Whole chromosomes are pulled toward opposing poles, forming two haploid sets, each chromosome still contains a pair of sister chromatids. The microtubes lengthen and the cell prepares for division down the middle.

Telophase I

The last Meiotic division. When the parent cell splits in half and the chromosomes arrive at the poles. Each daughter cell has now half the number of chromosomes theparents cell started with.

Meiosis II

Meiosis II is the second part of the meiotic process. Mechanically, the process is similar to mitosis. The result is four haploid cells. The cells do not go through interphase before meiosis II starts.

Prophase II

In prophase II the nuclear envelope is again dissolved and the spindle is set up again. Prophase II is identical to prophase of mitosis except that there is half the amount of chromosomes.

In both daughter cells, the chromosomes move into the center and line up. Now there are two chromosomes, instead of two tetrads, so that the chromatids will split off this time.

Anaphase II

In both daughter cells, the kinetochores move towards the poles, splitting up the sister chromatids.

Telophase II

In telophase II the chromatids concentrate in the poles and the two daughter cells split in half again. The cells divide for the last time, leaving a total of four haploid cells, which have half the chromosomes of a diploid cell. Unlike the daughter cells from mitosis, the daughter cells produced here cannot immediately cycle back to interphase.

STEPS OF MITOSIS

Interphase

Before a cell can enter cell division, it needs to prepare itself by replicating its genetic information and all of the organelles. All of the preparations are done during the interphase

Prophase

During prophase the chromosomes are identical chromatids connected at the center by a centromere, forming a X-shaped object. The distinguishing feature of prophase is the setup of the mitotic spindle, which is used to maneuver the chromosomes about the cell. The spindle is formed by excess parts from the dismantled cytoskeleton. The spindle is initially setup outside the nucleus.

Metaphase

The prometaphase provides a transition from prophase to metaphase. In prometaphase the nuclear envelope, which surrounds the nucleus, breaks up. The spindle now can move into the center of the cell. Kinetochores develop, which are attached to kinetochore fibers, which are linked to the chromosomes. The kinetochores then control the movements of the chromosomes. During this period the kinetochores are wildly oscillating as they try to attach themselves to one of the polar fibers. When they manage to do so the chromosome settles down.

In anaphase two events occur. First the kinetochores begin to move towards the poles. Then the polar fibers elongate, spreading the poles farther apart from each other.

Telophase

By telophase there are two separate groups of chromosomes at each pole. A nuclear envelope begins to form around each set of chromosomes to form two nuclei, that are temporarily in one cell. After the envelope reassembles RNA synthesis begins to break down the chromosomes, causing the nucleolus to reappear.

Cytokinesis

Now there are two separate nuclei, but they are in the same cell. The cell now needs to be split in half. Cytokenesis takes place in anaphase and continues through telophase. When the cell begins to pucker in is when cytokinesis begins. At the end of Cytokinesis the cell is split in half and Mitosis is finished.

Metaphase II

Anaphase

How does meiosis work?