MEIOSIS Flashcards
Formation of Gametes (Eggs & Sperm)
Replication
It is the process of chromosomal duplication that causes the preparation of the cell to divide and at the end of transcription the sister chromatids are formed linked together at the centromere.
prophase I
Leptotene – The chromosomes begin to condense and are attached to the nuclear membrane via their telomeres
Zygotene – Synapsis begins with a synaptonemal complex forming between homologous chromosomes
Pachytene – Crossing over of genetic material occurs between non-sister chromatids
Diplotene – Synapsis ends with disappearance of synaptonemal complex; homologous pairs remain attached at chiasmata
Diakinesis – Chromosomes become fully condensed and nuclear membrane disintegrates prior to metaphase I
Tetrads
The homologous chromosomes are linked at a point called Synapsis and called TETRAD
Crossing-Over or Chiasmata
Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes’ non-sister chromatids that results in recombinant chromosomes.
Metaphase I
After prophase I, crossing-over is complete. The tetrads move to a plane — called the “metaphase plate” — halfway between the two poles of the cell. Next, the spindle fibers attach to the centromeres of each chromosome. Both 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.
Anaphase I
During this stage of meiosis, the cell starts to lengthen. The two homologs of each chromosome pair separate and move toward opposite poles, drawn by the microtubules of the spindle apparatus. This contrasts with mitosis, where the sister chromatids of each homolog separate and move toward opposite poles. In meiosis at this stage, the chromatids remain together as one complete, replicated chromosome.
Telophase I
At each pole, during this stage, there is a complete haploid set of chromosomes (but each chromosome still has two sister chromatids). A cleavage furrow appears, and by the end of this stage the parent cell has divided into two daughter cells. This separation of the cytoplasm is called cytokinesis. In some organisms nuclear envelopes appear 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.
prophase II
Prophase II, the first step of meiosis II, begins with the two daughter cells produced by the first meiotic division (see figure right). As in prophase I, the chromosomes are condensed. During this stage of spermatogenesis, the cells are called secondary spermatocytes, or during oogenesis, secondary oocytes.
If there was an interkinesis, then the nuclear envelopes begin to break down again during this stage. The centrosomes have replicated and are moving toward the poles.
Again, the chromosomes are not yet attached to the spindle apparatus, which is growing outward from the centrosomes.
metaphase II
In metaphase II, the second stage of meiosis II, in each of the two daughter cells produced by the first meiotic division (which are known as secondary germ cells), the spindle again draws the chromosomes to the metaphase plate. This time, unlike metaphase I, the two kinetochores of each centromere bind to spindle fibers from opposite poles (as in mitotic metaphase). This results in separation of the sister chromatids of each chromosome during the next phase of meiosis, anaphase II.
anaphase II
During anaphase II, the third step of meiosis II, the sister chromatids of each chromosome separate and move toward opposite poles. Once they are no longer connected, the former chromatids are called unreplicated chromosomes. As the chromosomes are dragged along by the spindle apparatus, their arms can be seen dragging along behind so that the chromosomes form V-shapes. The poles themselves move further apart as cytokinesis begins and the cell lengthens (not shown in figure).
telophase II
During telophase II, the fourth step of meiosis II, the chromosomes reach opposite poles, cytokinesis occurs, the two cells produced by meiosis I divide to form four haploid daughter cells, and nuclear envelopes (white in the diagram at right) form. When telophase II is over, the two cells are entirely separated and their nuclear membranes are fully formed. Meiosis is then complete.
It occurs in the male testis and is called
Spermatogenesis
It occurs in the ovaries and is called
Oogenesis
