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MITOSIS AND MEIOSIS

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MITOSIS 

Interphase

G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. Note that the G in G1 represents gap and the 1 represents first, so the G1 phase is the first gap phase.

S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized. Note that the S represents synthesis.

G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins and continues to increase in size. Note that the G in G2 represents gap and the 2 represents second, so the G2 phase is the second gap phase.

In the latter part of interphase, the cell still has nucleoli present.

The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin.

In animal cells, two pair of centrioles formed from the replication of one pair are located outside of the nucleus.

Prophase

In prophase, the chromatin condenses into discrete chromosomes. The nuclear envelope breaks down and spindles form at opposite "poles" of the cell. Many consider prophase (versus interphase) to be the first true step of the mitotic process.

Chromatin fibers become coiled into chromosomes with each chromosome having two chromatids joined at a centromere.

The mitotic spindle, composed of microtubules and proteins, forms in the cytoplasm.

In animal cells, the mitotic spindle initially appears as structures called asters which surround each centriole pair.

The two pair of centrioles (formed from the replication of one pair in Interphase) move away from one another toward opposite ends of the cell due to the lengthening of the microtubules that form between them.

In late prophase:

The nuclear envelope breaks up.

Polar fibers, which are microtubules that make up the spindle fibers, reach from each cell pole to the cell's equator.

Kinetochores, which are specialized regions in the centromeres of chromosomes, attach to a type of microtubule called kinetochore fibers.

The kinetochore fibers "interact" with the spindle polar fibers connecting the kinetochores to the polar fibers.

The chromosomes begin to migrate toward the cell center.

Metaphase

In metaphase, the spindle fully develops and the chromosomes align at the metaphase plate    (a plane that is equally distant from the two spindle poles).

The nuclear membrane disappears completely.

In animal cells, the two pair of centrioles align at opposite poles of the cell.

Polar fibers (microtubules that make up the spindle fibers) continue to extend from the poles to the center of the cell.

Chromosomes move randomly until they attach (at their kinetochores) to polar fibers from both sides of their centromeres.

Chromosomes align at the metaphase plate at right angles to the spindle poles.

Chromosomes are held at the metaphase plate by the equal forces of the polar fibers pushing on the centromeres of the chromosomes.

Anaphase

In anaphase, the paired chromosomes (sister chromatids) separate and begin moving to opposite ends (poles) of the cell. Spindle fibers not connected to chromatids lengthen and elongate the cell. At the end of anaphase, each pole contains a complete compilation of chromosomes.

The sister chromatids begin to move apart.

Once the paired sister chromatids separate from one another, each is considered a "full" chromosome, eventhough they are referred to as daughter chromosomes.

Through the spindle apparatus, the daughter (sister) chromosomes move to the poles at opposite ends of the cell.

As the daughter chromosomes migrate toward the opposite poles the kinetochore fibers become shorter as the chromosomes near a pole. (at this time it is not clear what the moving mechanism is).

In preparation for telophase, the two cell poles also move further apart during the course of anaphase. (at this time it is hypothesized that microtubles not attached to kinetochores push the spindle and cell further apart increasing the lenght of the cell for cellular separation) . At the end of anaphase, each pole contains a complete compilation of chromosomes.

Telophase

In telophase, the chromosomes are cordoned off into distinct new nuclei in the emerging daughter cells.

The polar fibers continue to lengthen.

Cytokinesis

Cytokinesis, the division of the original cell's cytoplasm, begins prior to the end of mitosis
and completes shortly after telophase/mitosis. At the end of cytokinesis, there are two distinct (identical) daughter cells.

MEIOSIS 

Interphase

G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. Note that the G in G1 represents gap and the 1 represents first, so the G1 phase is the first gap phase.

S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized. Note that the S represents synthesis.

G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins and continues to increase in size. Note that the G in G2 represents gap and the 2 represents second, so the G2 phase is the second gap phase.

In the latter part of interphase, the cell still has nucleoli present.

The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin.

In animal cells, two pair of centrioles formed from the replication of one pair are located outside of the nucleus.

 

Prophase I (sub-phases)

Leptotene: Chromosomes condense and attach to the nuclear envelope. Homologous pairs begin to search for each other
Zygotene: Synapsis occurs (a pair of homologous chromosomes lines up closely together) and a tetrad is formed. Each tetrad is composed of four chromatids.
Pachytene: Crossing over may occur.
Diplotene: Chromosomes thicken and detach from the nuclear envelope. Dyads begin to move apart revealing the chiasma.
Diakinesis: chiasmata terminalization is completed. Similar to mitosis, the centrioles migrate away from one another and both the nuclear envelope and nucleoli break down. Chromosomes begin their migration to the metaphase plate.

Metaphase I

Tetrads align at the metaphase plate.

homologous chromosomes are oriented toward the opposite cell poles.

 

Anaphase I

Chromosomes move to the opposite cell poles. Similar to mitosis, the microtubules and the kinetochore fibers interact to cause the movement.

Unlike in mitosis, the homologous chromosomes move to opposite poles yet the sister chromatids remain together.

 

Telophase I

The spindles continue to move the homologous chromosomes to the poles.

Once movement is complete, each pole has a haploid number of chromosomes.

In most cases, cytokinesis occurs at the same time as telophase I.

 

At the end of telophase I and cytokinesis, two daughter cells are produced, each with one half the number of chromosomes of the original parent cell.

 

Depending on the kind of cell, various processes occur in preparation for meiosis II. There is however a constant: The genetic material does not replicate again.

 

Interkinesis

Interkinesis or interphase II is a period of rest that cells enter during meiosis. It is the abbreviated interphase that occurs between meiosis I and II. No DNA replication occurs during this stage of Meiosis. In many organisms, particularly plants, the two daughter cells produced by meiosis I begin immediately to prepare for the second meiotic division. In others, however, nuclear envelopes temporarily appear and enclose each of the two separated sets of chromosomes between telophase I and prophase II, and there is a period of rest. This period, during which envelopes are again visible, is called interkinesis. Each chromosome is still composed of two sister chromatids. The period between meiosis I and meiosis II is sometimes called "interphase," this is confusing because it is not a true interphase such as that seen between rounds of mitosis because no synthesis occurs (all chromosomes have two chromatids throughout this stage).

 

Prophase II

 

The nuclear membrane and nuclei break up while the spindle network appears.

Chromosomes do not replicate any further in this phase of meiosis.

The chromosomes begin migrating to the metaphase II plate (at the cell's equator).

 

Metaphase II

 

The chromosomes line up at the metaphase II plate at the cell's center.

The kinetochores of the sister chromatids point toward opposite poles.

 

Anaphase II

The sister chromatids separate and move toward the opposite cell poles.

 

Telophase II

 

Distinct nuclei form at the opposite poles and cytokinesis occurs.

At the end of meiosis II, there are four daughter cells each with one half the number of chromosomes of the original parent cell.


GAMETOGENESIS: How Female Meiosis differs from Male Meiosis

gamete - sex cell
Spermatogenesis - male
Oogenesis - female