MEIOSIS. Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). Two successive nuclear divisions occur, Meiosis I

MEIOSISMEIOSIS

Two successive nuclear divisions occur, Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II Meiosis I (Reduction) and Meiosis II (Division). (Division).

Meiosis produces 4 haploid cells. Meiosis produces 4 haploid cells. ***Mitosis produces 2 diploid cells. ***Mitosis produces 2 diploid cells. Meiosis I reduces 2n to n (reduction) Meiosis I reduces 2n to n (reduction)

while Meiosis II divides the remaining while Meiosis II divides the remaining set of chromosomes in a mitosis-like set of chromosomes in a mitosis-like process (division). process (division).

Most of the differences between the Most of the differences between the processes occur during Meiosis I. processes occur during Meiosis I.

Meiosis is a series of two nuclear Meiosis is a series of two nuclear divisions called meiosis I and divisions called meiosis I and meiosis II. meiosis II.

These two divisions are each These two divisions are each divided into further phases: divided into further phases:

-Prophase -Prophase MetaphaseMetaphase Anaphase Anaphase TelophaseTelophase

Meiosis IMeiosis I

Meiosis I encompasses four Meiosis I encompasses four stages:stages: Prophase IProphase I

Metaphase IMetaphase I

Anaphase IAnaphase I

Telophase ITelophase I

The stages are similar to the The stages are similar to the stages in mitosis but the largest stages in mitosis but the largest differences occur in prophase I. differences occur in prophase I.

In most cases, at the end of In most cases, at the end of meiosis I, two daughter cells are meiosis I, two daughter cells are produced. produced.

Meiosis IIMeiosis II

Meiosis II also encompasses four Meiosis II also encompasses four stages: stages: Prophase IIProphase II

Metaphase IIMetaphase II

Anaphase IIAnaphase II

Telophase IITelophase II

At the end of meiosis II, four At the end of meiosis II, four daughter cells are produced. daughter cells are produced.

Each of these resulting daughter Each of these resulting daughter cells is haploid. (n)cells is haploid. (n)

InterphaseInterphase Before a dividing cell enters Before a dividing cell enters

meiosis, it undergoes a period of meiosis, it undergoes a period of growth called Interphase.growth called Interphase.

G1 (first gap) phase: The cell G1 (first gap) phase: The cell increases in mass in preparation increases in mass in preparation for cell div.for cell div.

S phase: during which DNA is S phase: during which DNA is synthesized.synthesized.

G2 (second gap) phase: the cell G2 (second gap) phase: the cell synthesizes proteins & continues synthesizes proteins & continues to increase in size.to increase in size.

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

Nucleus is bound by nuclear Nucleus is bound by nuclear envelope and the cell’s envelope and the cell’s chromosome have duplicated but chromosome have duplicated but are in the form of Chromatin.are in the form of Chromatin.

2 pairs of centrioles formed from 2 pairs of centrioles formed from the replication.the replication.

MEIOSIS-1MEIOSIS-1 the largest differences between the largest differences between

mitosis and meiosis occur in mitosis and meiosis occur in prophase I. prophase I.

Prophase I is usually longer in Prophase I is usually longer in duration usually much more duration usually much more complex.complex.

It can take days for prophase I to It can take days for prophase I to complete. complete.

It is estimated that prophase I It is estimated that prophase I accounts for some 85 - 95 percent of accounts for some 85 - 95 percent of the total time for meiosis. the total time for meiosis.

PROPHASE-1PROPHASE-1 Chromosomes condense and attach to the Chromosomes condense and attach to the

nuclear envelope.nuclear envelope.

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

Synapsis- the pairing of homologous Synapsis- the pairing of homologous chromosomes during prophase I of meiosis.chromosomes during prophase I of meiosis.

Crossing over may occur.Crossing over may occur.

Chromosomes thicken and detach from the Chromosomes thicken and detach from the nuclear envelope.nuclear envelope.

SynapsisSynapsis

PROPHASE-1PROPHASE-1 Similar to mitosis, the centrioles migrate Similar to mitosis, the centrioles migrate

away from one another and both the away from one another and both the nuclear envelope and nucleoli break nuclear envelope and nucleoli break down.down.

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

Chiasma-The places where pairs of Chiasma-The places where pairs of homologous chromatids remain in homologous chromatids remain in contact during late prophase to anaphase contact during late prophase to anaphase of the first meiotic division.of the first meiotic division.


Metaphase I is of much shorter Metaphase I is of much shorter duration and complexity when duration and complexity when compared to prophase I. compared to prophase I.

The following changes occur: The following changes occur: Tetrads align at the metaphase plate.Tetrads align at the metaphase plate.

Note that the centromeres of Note that the centromeres of homologous chromosomes are oriented homologous chromosomes are oriented toward the opposite cell poles.toward the opposite cell poles.


Metaphase I


Anaphase I is very similar to anaphase Anaphase I is very similar to anaphase in mitosis. in mitosis.

The following changes occur: The following changes occur:

Chromosomes move to the opposite Chromosomes move to the opposite cell poles. cell poles.

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


Anaphase I


The following changes occur:The following changes occur: The spindles continue to move the The spindles continue to move the

homologous chromosomes to the poles.homologous chromosomes to the poles.

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

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

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


Telophase I

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MEIOSIS. Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). Two successive nuclear divisions occur, Meiosis I