Meiosis

When does Meiosis Occur?

  •  Meiosis occurs only in eukaryotes.
  •  Meiosis takes place at some point in the life cycle of the typical sexual organism because, by reducing the chromosome number by one half, it compensates for the doubling of the chromosome number caused by fertilization.
  •  In animals, meiosis occurs during the production of gametes (sperm and eggs).
  •  In plants, it takes place when spores are produced (plant gametes are produced by mitosis).
  •  Prokaryotes (i.e., archaeaand bacteria) reproduce via binary fission.

The names of the eight stages of meiosis are:

  1. 1.Prophase I,
  2. 2.Metaphase I,
  3. 3.Anaphase I,
  4. 4.Telophase I,
  5. 5.Prophase II,
  6. 6.Metaphase II,
  7. 7.Anaphase II, and
  8. 8.Telophase II.
  •  The first four steps (the ones ending in “I”) are the phases of the first meiotic division, meiosis I.
  •  The last four (those ending in “II”) are those of the second meiotic division, meiosis II.

Prophase I

  •  Prophase I is the first stage of meiosis. During this phase, the chromosomes (shown in red in the diagram, below right) become visible as they shorten, coil, and thicken.
  •  Also, the spindle (yellow strands in diagram) begins to extend outward from two centers of expansion.
  •  In animal cells a pair of centrioles can be seen in each of these centers (plants lack centrioles).
  •  The nuclear membrane (shown in white) breaks up and disappears.
  •  Each chromosome is composed of two sister chromatids containing identical genetic information.
  •  The information is the same because one sister chromatid is produced by copying the other.
  •  The sister chromatids are joined by a centromere (orange).
  •  The two members of each chromosome pair are called

    homologous chromosomes or simply homologs.

  •  During prophase I they join along their lengths (i.e., they synapse) to form a tetrad (or bivalent).
  •  Each of the two tetrads shown in the drawing, then, is represented as two xshaped chromosomes aligned along their lengths and connected with each other.
  •  This is a simplified diagram, the actual situation is more complicated: Prophase I is by far the most complicated phase of meiosis. It is much longer in meiosis than in mitosis.
  •  During this stage, homologous chromosomes join (synapse) along their lengths and exchange DNA.
  •  Prophase I is itself divided into the five substages which are explained and diagrammed below.

Leptotene

  •  The chromosomes have appeared within the nuclear membrane (shown in the diagram at right as a tan circle with a brown border), but are not yet fully condensed.
  •  In the diagram the two chromosomes of paternal origin are indicated in red, those of maternal origin, in blue.
  •  Each is a thin thread of DNA (lepto- is Greek for thin and -tene is Greek for ribbon or band) along which clearly defined beads of local coiling (chromomeres) can be seen.
  •  The chromosomes, while they have this threadlike form, are called chromatonemata (sing. chromonema; -nema is Greek for thread).
  •  The chromosomes appear single because the sister chromatids are still so tightly bound to each other that they cannot be separately seen.
  •  During this stage both ends (telomeres) of each chromosome are turned toward, and probably attached to, the same region of the nuclear membrane. Leptotene is also known as (1) leptonema; and as (2) the bouquet stage because all the telomeres tend to contact the nuclear membrane in one spot so that the looped chromosomes balloon out from that point like flower petals.

Zygotene (also known as zygonema)

  •  During this stage, homologous chromosomes (or homologs for short) begin to unite (synapse) by coming into approximate alignment (zygo- is Greek for union, fusing, or yoking).
  •  ,ynapsis, the process of fusion that occurs between homologous chromosomes, begins at various points along the chromosomes and extends outward, zipper-fashion, until complete.
  •  When synapsis is finished, the fused homologous chromosomes look like single chromosomes under the light microscope.
  •  These chromosomes that look single, but that are actually double, are called bivalents.
  •  The interface where two homologous chromosomes (“homologs” for short) unite is called a synaptonemal complex, which can be seen under an electron microcope In the diagram of early zygotene (above right), the regions where the paternal and maternal homologs have fused is shown in purple.
  •  In the next diagram, representing late zygotene, both homolog pairs have fused over their entire lengths (so they are shown entirely in purple).

  •  Once the the homolog pairs synapse they are called tetrads (each has four chromatids; tetra is Greek for four) or bivalents.
  •  Bivalent is the preferred term, but tetrad is, nonetheless, the word more commonly used in most introductory biology classes.
  •  Bivalent is the better choice because there are equivalent names for other situations.
  •  For example, an unfused homolog is called a univalent.
  •  Three fused homologs, a common situation in plants, is called a

    trivalent, etc.

Pachytene (also known as pachynema).

  •  During pachytene the two sister chromatids of each chromosome separate from each other.
  •  This makes the chromosomes look thicker (pachy- is Greek for thick). Homologs are still paired at this point.
  •  Non-sister chromatids remain in contact throughout pachytene and a kind of localized breakage of the DNA occurs, which is followed by exchanges of DNA between them.
  •  This process is called “crossing over.”
  •  Crossing over produces “cross-over chromatids” each composed of distinct blocks of DNA, some blocks derived from the mother, others from the father.

Diplotene (also known as diplonema)

  •  At the beginning of this stage each chromatid of each chromosome is still fused to a chromatid of that chromosome/s homolog (recall that sister chromatids are already separate at this point).
  •  As diplotene progresses, these initially fused non-sister chromatids begin to separate from each other.
  •  However, they cannot separate completely because they are still connected by two strands of DNA at each of the points where exchanges took place.
  •  At each cross-over site, the two strands form an x-shaped structure called a chiasma (pl. chiasmata).
  •  The chiasmata then begin moving toward the ends of the chromatids.
  •  This process of sliding toward the ends is known as

    terminalization.

  •  In oocytes, a special, extremely prolonged form of diplotene occurs called dictyotene.
  •  The primary oocyte undergoes the first three of the substages of prophase I (leptotene, zygotene, and pachytene) during late fetal life.
  •  The process is then suspended during diplotene until puberty or thereafter.
  •  Therefore, in oocytes dictyotene (and consequently prophase I) can last months or even years, depending on the type of organism in question.

Diakinesis

  •  During this, the last stage of Prophase I, the nucleolus disappears, terminalization reaches completion, and the chromosomes coil tightly, and so become shorter and thicker.
  •  The nuclear envelope begins to disappear.
  •  Note that the two homologs making up a chromosome pair are not expected to be genetically identical, as in the case of sister chromatids, because they are not direct copies of each other and they are inherited from different parents.
  •  When the chromosomes synapse during prophase, each gene in each chromosome is brought into contact with the same gene on that chromosome’s homolog.
  •  During this process of synapsis, the two homologs of each pair exchange segments of DNA in a process known as crossing over. As a result, the gene combinations on each chromosome can be changed.
  •  For example, suppose one homolog initially contained genes for brown eyes and brown hair.
  •  After crossing over, it could contain genes for blue eyes and brown hair, where the gene for blue eyes was taken from its homolog.
  •  While prophase I is proceeding, the spindle’s two centers of expansion move to the opposite ends of the cell (i.e., to the “poles”) and the spindle lengthens and extends toward the “metaphase plate,” an imaginary plane defining the middle plane of the cell, halfway between the centriole pairs.
  •  The tetrads also move toward the metaphase plate.

METAPHASE I

  •  In the second phase of the first meiotic division, metaphase I, the tetrads align on the “metaphase plate,” halfway between the poles of the cell.
  •  Next, the spindle fibers attach to the centromeres of each chromosome.
  •  Both spindle fiber attachment points (kinetochores) of each sister chromatid pair are turned toward the same pole.
  •  As a result, both kinetochores attach to spindle fibers from the same pole.
  •  This is a major difference between meiosis and mitosis.
  •  It causes the two members of each chromosome pair to be separated from each other during the next stage of meiosis, anaphase I (in mitotic metaphase, the two kinetochores of each sister chromatid pair attach to spindle fibers from opposite poles, so each chromatid separates from its sister during anaphase).

ANAPHASE I

  •  In the third stage, anaphase I, the cell lengthens as it begins the process of division.
  •   Homologs of each chromosome pair move toward opposite poles, drawn by the microtubles of the spindle apparatus (this contrasts with mitosis, where the sister chromatids from each homolog separate and move toward opposite poles).

TELOPHASE I

  •  In this, the fourth stage of meiosis, the chromosomes reach the poles.
  •  At each pole, now, there is a complete haploid set of chromosomes (but each chromosome still has two sister chromatids).
  •  During telophase I, a cleavage furrow appears.
  •  By the end of this stage the cell has divided in two along the plane defined by the furrow.
  •  This separation of the cytoplasm is called cytokinesis.
  •  In some organisms the nuclear membrane reappears briefly at this point (this intermediate stage is called interkinesis), but in others the daughter cells begin immediately to prepare for the second meiotic division.

CYTOKINESIS

  •  Cytokinesis is not one of the stages or phases of meiosis or mitosis.
  •  It is the process of division that the cytoplasm undergoes when it is distributed into daughter cells (as opposed to karyokinesis, which is division of the nucleus).
  •  The cytoplasm is all of the contents of the cell other than the nucleus.
  •  The cytoplasm lies outside the nucleus and is bounded by the plasma membrane.
  •  The liquid portion of cytoplasm is called cytosol.

Interkinesis

  •  In many organisms, after meiosis I, the daughter cells begin immediately to prepare for the second meiotic division.
  •  In others, however, the nuclear membrane reappears between telophase I and prophase II, and there is a period of rest.
  •  This period, during which the membrane is again visible, is called

    interkinesis.

  •  Each chromosome is still composed of two chromatids.

PROPHASE II

  •  Prophase II begins with the two daughter cells produced by the first meiotic division.
  •  As in Prophase I, the chromosomes are condensed and not yet attached to the spindle apparatus.
  •  If there was an interkinesis, then the nuclear membranes begin to break down again during this stage.
  •  The centrioles have replicated and are moving toward the poles.

METAPHASE II

  •  In metaphase II the chromosomes move to the equator (“metaphase plate”) of each of the two daughter cells produced by the first meiotic division.
  •  This time, unlike metaphase I, the two kinetochores of each centromere attach to spindle fibers from opposite poles.

ANAPHASE II

  •  During the seventh stage of meiosis, anaphase II (the third stage of the second division), the sister chromatids of each chromosome separate and move toward opposite poles.

TELOPHASE II

  •  During telophase II, the sister chromatids reach opposite poles, cytokinesis occurs, the two cells produced by meiosis I divide to form four haploid daughter cells, and the nuclear membranes (white in the diagram) reform.

Significance of Meiosis

    •  The long term survival of a species depends on its ability to adapt to a changing environment.
    •  To do this the offspring need to be different from their parents and each other.
    •  These are three ways in which variety occurs because of meiosis.
d.Production and fusion of haploid gametes
  •  The variety of offspring is increased by mixing the genotype of one parent with that of the other.
  •  It involves the production of special sex cells, called gametes, which fuse together to produce a new organism.
  •  Each gamete contains half the number of chromosomes of the adult.
  •  It is important that meiosis, which halves the number of chromosomes in daughter cells, happens at some stage in the life cycle of a sexually reproducing organism.
  •  Therefore Meiosis is important in order for variety in organisms, and allowing them to evolve.
a.The creation of genetic variety by the random distribution of chromosomes during metaphase 1
  •  When the pairs of homologous chromosomes arrange themselves on the equator of the spindle during metaphase 1 of meiosis, they do it randomly.
  •  Even though each one of the pair determines the same general

    features, they’re detail of the feature is different.

  •  The randomness of this distribution and independent assortment of these chromosomes produces new genetic combinations.
b.The creation of genetic variety by crossing over between homologous chromosomes
  •  During prophase 1 of meiosis, equal portions of homologous chromosomes may be swapped.
  •  In this way new genetic combinations are made and linked genes separated.
  •  The variety which meiosis brings vital for to the process of evolution.
  •  By providing a varied stock of individuals it allows the natural selection of those best suited to the existing conditions and makes sure that species constantly change and adapt when these conditions change.
  •  This is the main biological significance of meiosis.

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