Interphase• Stages of Interphase to increase in size. Note that the G G1 phase: The period prior to the in G2 represents gap and the 2 synthesis of DNA. In this phase, the represents second, so the G2 cell increases in mass in phase is the second gap phase. preparation for cell division. Note • In the latter part of interphase, the that the G in G1 represents gap and cell still has nucleoli present. the 1 represents first, so the G1 • The nucleus is bounded by a phase is the first gap phase. nuclear envelope and the cells• S phase: The period during which chromosomes have duplicated but DNA is synthesized. In most cells, are in the form of chromatin. there is a narrow window of time • In animal cells, two pair during which DNA is synthesized. of centrioles formed from the Note that the S represents replication of one pair are located synthesis. outside of the nucleus.• G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins and continues
Changes that occur in a cell during prophase:• Chromatin fibers become • In animal cells, the mitotic coiled into chromosomes with spindle initially appears as each chromosome having two structures called asters which chromatids joined at surround each centriole pair. a centromere.• The mitotic spindle, composed • The two pair of microtubules and proteins, f of centrioles (formed from the orms in the cytoplasm. 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 • The kinetochore fibers "interact" up. with the spindle polar fibers connecting the kinetochores to• Polar fibers, which are the polar fibers. microtubules that make up the spindle fibers, reach from each • The chromosomes begin to cell pole to the cells equator. migrate toward the cell center.• Kinetochores, which are specialized regions in the centromeres of chromosomes, attach to a type of microtubule called kinetochore fibers.
Metaphase•The nuclear membranedisappears completely.In animal cells, the twopair of centrioles align atopposite poles of thecell. •Chromosomes move randomly until•Polar fibers they attach (at their kinetochores) to(microtubules that make polar fibers from both sides ofup the spindle fibers) their centromeres. Chromosomes align at thecontinue to extend from metaphase plate at right angles to thethe poles 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 ineach distinct chromosomebegin to move apart.•Once the paired sisterchromatids separate fromone another, each isconsidered a "full"chromosome. They are •The daughter chromosomes migratereferred to as daughter centromere first andchromosomes. the kinetochore fibers become shorter as the chromosomes near a pole.•Through the spindleapparatus, the daughter •In preparation for telophase, the twochromosomes move to the cell poles also move further apart duringpoles at opposite ends of the course of anaphase. At the end ofthe cell. 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.
Interphase• G1 phase: The period prior to the to increase in size. Note that the G synthesis of DNA. In this phase, the in G2 represents gap and the 2 cell increases in mass in represents second, so the G2 preparation for cell division. Note phase is the second gap phase. that the G in G1 represents gap and the 1 represents first, so the G1 • In the latter part of interphase, the phase is the first gap phase. cell still has nucleoli present.• S phase: The period during which • The nucleus is bounded by a DNA is synthesized. In most cells, nuclear envelope and the cells there is a narrow window of time chromosomes have duplicated but during which DNA is synthesized. are in the form of chromatin. Note that the S represents synthesis. • In animal cells, two pair of centrioles formed from the• G2 phase: The period after DNA replication of one pair are located synthesis has occurred but prior to outside of the nucleus. the start of prophase. The cell synthesizes proteins and continues
Prophase I: •Chromosomes thicken and detach from the nuclear envelope. •Similar to mitosis,•Chromosomes condense the centrioles migrate awayand attach to the nuclear from one another and bothenvelope. the nuclear envelope and nucleoli break down.•Synapsis occurs (a pair of •Likewise, thehomologous chromosomes chromosomes begin theirlines up closely together) migration to the metaphaseand a tetrad is formed. plate.Each tetrad is composed offour chromatids.Crossing over may occur.
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 tomove the homologouschromosomes to the poles.Once movement iscomplete, each pole has ahaploid number ofchromosomes. •At the end of telophase I and cytokinesis, two daughter cells are•In most cases, cytokinesis produced, each with one half theoccurs at the same time as number of chromosomes of the originaltelophase I. 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 spindlenetwork appears.•Chromosomes do not replicate any further in this phase ofmeiosis.•The chromosomes begin migrating to the metaphase II plate(at the cells equator).
Metaphase II:•The chromosomes line up atthe metaphase II plate at thecells center.•The kinetochores of thesister chromatids pointtoward 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.
Barro, Kevin Winge B.Casas, Gregorio Jr. A.CELL DIVISION:MITOSIS & MEIOSIS