what is the behaviour of chromosomes during meiosis?

Answer 1

The chromosomal behaviour in meiosis can be illustrated in each phase as follows:

Prophase I

The first stage of Prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads." During this stage, individual chromosomes begin to condense into long strands within the nucleus. However the two sister chromatids are still so tightly bound that they are indistinguishable from one another.

The zygotene stage, also known as zygonema, from Greek words meaning "paired threads," occurs as the chromosomes approximately line up with each other into homologous chromosomes. The combined homologous chromosomes are said to be bivalent. They may also be referred to as a tetrad, a reference to the four sister chromatids. The two chromatids become "zipped" together, forming the synaptonemal complex, in a process known as synapsis.

The pachytene stage, also known as pachynema, from Greek words meaning "thick threads," heralds crossing over. Nonsister chromatids of homologous chromosomes randomly exchange segments of genetic information over regions of homology. Exchange takes place at sites where recombination nodules have formed. The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.

During the diplotene stage, also known as diplonema, from Greek words meaning "two threads," the synaptonemal complex degrades and homologous chromosomes separate from one another a little. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing over occurred.

Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through." This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappears, the nuclear membrane disintegrates into vesicles, and the mitotic spindle begins to form.

Metaphase I

Homologous pairs move together along the phase plate: as kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate.

Anaphase I

Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome only has one kinetochore, whole chromosomes are pulled toward opposing poles, forming two diploid sets. Each chromosome still contains a pair of sister chromatids. Nonkinetochore microtubules lengthen, pushing the centrioles further apart. The cell elongates in preparation for division down the middle. In prophase 1 the DNA coils tightly and individual chromosomes become visible under the light microscope. Homologous chromosomes closely associated in synapsis and they exchange segments by crossing over.

Telophase I

The first meiotic division effectively ends when the centromeres arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. This effect produces a variety of responses from the neuro-synrchromatic enzyme, also known as NSE. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells.

Cells enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage. Note that many plants skip telophase I and interphase II, going immediately into prophase II.

Meiosis II

Prophase II takes an inversely proportional time compared to telophase I. In this prophase we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and are arranged by spindle fibres. The new equatorial plane is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plane.

In metaphase II, the centromeres contain three kinetochores, organizing fibers from the centrosomes on each side.

This is followed by anaphase II, where the centromeres are cleaved, allowing the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes, and they are pulled toward opposing poles.

The process ends with telophase II, which is similar to telophase I, marked by uncoiling, lengthening, and disappearance of the chromosomes occur as the disappearance of the microtubules. Nuclear envelopes reform; cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete.


For additional info and animations, click on to: http://www.biology.arizona.edu/CELL_BIO/tutorials/meiosis/main.html

Answer 2

During meiosis I the chromosomes synapse (homologous chromosomes pair up), cross over, and separate from their homologous partners. During meiosis II the individual chromosomes separate at their centromeres and sister chromatids are pulled to opposite poles of the spindle. The end result is that one diploid cell has become four haploid cells.

During mitosis the chromosomes line up on the equator of the spindle and split at the centromeres. Sister chromatids are pulled to the opposite poles of the spindle. The end result after cytokinesis is that one diploid cell has become two diploid cells.