Understanding Stages of Meiosis - High School Biology

A diploid cell will have two copies of each chromosome, known as a homologous pair. A haploid cell will only have one copy of each chromosome, though the chromosome may consist of two sister chromatids.

Diploid cell: (XX)

Haploid cell: (X)

During meiosis I, the cell is diploid because the homologous chromosomes are still located within the same cell membrane. Only after the first cytokinesis, when the daughter cells of meiosis I are fully separated, are the cells considered haploid. Following this first division, the cell begins meiosis II with prophase II, making this the first haploid meiotic stage.

The S phase occurs between the G1 and G2 phases and is the stage during which DNA is replicated, and then checked for defects. Depending on the level of nutrients and energy available, the cell will either enter the G0 phase or the M phase.

During the G1 phase, the cell replicates organelles and grows in size. During the G2 phase, DNA is checked for damage and the cell prepares to divide. The M phase refers to mitosis, while the G0 phase refers to quiescence—a period during which the cell is not preparing for division.

In meiosis I, the homologous chromosomes have already been duplicated in S phase of interphase. The sister chromatids are identical at this stage. Homologous chromosomes pair in prophase I, forming tetrads. The tetrads then cross over, exchanging genetic material. Then, the genetically-mixed tetrads line up on the metaphase plate and are separated in anaphase I. Note that after the first meiotic division, the two daughter cells are nonidentical and are haploid.

Mitosis is also known as "karyokinesis." "Karyo-" refers to the nucleus. (Remember that eu-KARY-ote means true ("eu-") nucleus, and pro-KARY-ote means before ("pro-") nucleus.) The "-kinesis" part of "karyokinesis" comes from the same roots as "kinetic" and refers to movement. Thus, mitosis is the movement of the nucleus. Packing of the DNA occurs in prophase of mitosis so that it's easier to move rather than having to move the loose chromatin. Think of moving forty-six strands of hundreds of yards of yarn—we would want it to be tightly coiled to make it manageable.

Meiosis is the process by which a haploid cell is formed from a diploid cell. The difference between haploid cells and diploid cells is that haploid cells contain one complete set of chromosomes, whereas diploid cells contain two complete sets of chromosomes. Meiosis involves the division of a diploid (2n) parent cell. The chromosomes are duplicated, but carry out two consecutive divisions. The result is four haploid (n) cells, each with half the number of chromosomes as the parent cell due to the separation of homologous pairs in meiosis I.

In contrast, mitosis is the process by which a diploid parent cell produces two diploid daughter cells.

Meiosis involves two divisions and results in four unique daughter cells called gametes. Meiosis begins with one parent cell, after the first division there are two daughter cells, and then those each split, resulting in a total of four daughter cells.

In prophase I chromosomes become compact and homologous chromosomes pair up. Also during prophase I, the nuclear membrane begins to break down and the spindle apparatus begins to form.

In metaphase I, homologous chromosomes line up along the center of the cell in order to be pulled apart. Recall that during meiosis I, homologous chromosomes pair, cross over, and separate. Meiosis II is when the sister chromatids are separated.

Chromosome disjunction is the splitting up of paired of chromosomes. This occurs in anaphase I and anaphase II where the homologous chromosomes split and the sister chromatids split, respectively. Note that improper disjunction (nondisjunction) can be detrimental to the cell.

Chromatid disjunction occurs in anaphase II after the chromosomes line up along the equator during metaphase II. The chromosomes are then pulled apart, with one chromatid moving north, and one moving south. The next steps are telophase, and cytokinesis, which upon completion, will result in genetically distinct haploid gametes.

Telophase is the final stage of both meiosis I and meiosis II. So telophase II is the final step of the overall process of meiosis. In telophase II, the daughter cells begin to form, the DNA begins to decondense, the nuclear membrane reforms, and the spindle apparatus breaks down. Cytokinesis is the physical splitting of the cell that follows mitosis/meiosis.

Crossing over is the process in Meiosis I in which homologs line up on the metaphase plate and exchange genetic information, which changes the genetic make up of chromosomes. Independent assortment is a principle proposed by Gregory Mendel stating that genes assort independently during gamete formation, which creates genetic diversity.