Cell DivisionMITOSIS & MEIOSIS

Interphase• Stages of 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

Changes that occur in a cell during prophase:

• 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•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

• 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•The paired centromeres in each distinct chromosome begin to move apart.

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

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

• The daughter chromosomes migrate centromere first and the kinetochore fibers become shorter as the chromosomes near a pole.

• In preparation for telophase, the two cell poles also move further apart during the course of anaphase. At the end of anaphase, each pole contains a complete compilation of chromosomes.

Telophase

• In telophase, the chromosomes are cordoned off in distinct new nuclei in the emerging 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.

Interphase

Prophase I:

•Chromosomes condense and attach to the nuclear envelope.

•Synapsis occurs (a pair of homologous chromosomes lines up closely together) and a tetrad is formed. Each tetrad is composed of four chromatids.Crossing over may occur.

• Chromosomes thicken and detach from the nuclear envelope.

• Similar to mitosis, the centrioles migrate away from one another and both the nuclear envelope and nucleoli break down.

• Likewise, the chromosomes begin their migration to the metaphase plate.

Metaphase I:•Tetrads align at the metaphase plate.

•Note that the centromeres of 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.

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.

CELL DIVISION:MITOSIS & MEIOSIS

Barro, Kevin Winge B.

Casas, Gregorio Jr. A.