In eukaryotic cells, like those found in animals and plants, meiosis is a type of cellular division that is required for sexual reproduction. Meiosis shouldn’t be confused with mitosis as both are processes that divide the cell. In mitosis the cell duplicates itself to create a daughter cells and does not occur in sexual reproduction. In meiosis the cell actually divides into new cells called gametes (or spores when appropriate) and happens only during sexual reproduction. Meiosis occurs in two stages, Meiosis I and Meiosis II, both of which undergo a series of phases to complete the division. For an introduction to biology check out this course.
Before the cell undergoes Meiosis, it enters into the Interphase. In interphase cells grow and gain the nutrients they need for division. Throughout interphase, the cell goes through three sub-phases, one of which is the synthesis phase or S-phase. The S-phase is when the cell DNA is replicated. The genetic information, therefore, increases and identical sister chromatids are created adding to the existing amount of DNA in the cell. Sister chromatids are only sisters if they are joined at the center by a centromere. After interphase comes meiosis.
Meiosis is a process which produces sex cells that eventually fuse to create a zygote. A zygote is the precursor to an embryo, or the first stage in organism development. Each sex cell produced by Meiosis will have half the number of chromosomes as the parent cell. This would correlate to half of the fathers DNA and half of the mothers DNA. Once Meiosis is complete, there will be four haploid cells. Studying for the GCSE Biology exam? Take this GCSE Biology exam prep course to learn more about meiosis and mitosis. When the cell reaches Meiosis it is still considered a diploid cell because it still has the same amount of centromeres despite the amount of chromatids. Centromeres act as a connecting piece between two chromatids, or chromosome copies. When two chromatids are connected they are called sister chromatids.
Meiosis Prophase I
The first stage of meoisis I is prophase I. Prophase I is subdivided into five phases, leptotene, zygotene, pachytene, diplotene and diakinesis. We won’t go into these here but if you need to know, check out this GCSE Biology exam prep course for a full review of cellular development and division. During prophase I, the chromatids begin to twist to form microscopically visible chromosome structures. These structures, or pairs of sister chromatids begin to seek out their “match”. Once they find their homologous chromosome and bond the cell becomes a tetrad cell. The sister chromatids create a connecting point called the chiasma. Eventually, this will be the location where genetic information is shared or crossed over. During prophase I there can be some crossing over.
During metaphase I, the centromeres of each chromosome attach themselves to spindle fibers extending from the nuclear envelope. The spindle fibers are created by a structure called a centrosome. The centrosome replicates and moves to opposite ends of the cell to what are called poles. The spindle fibers line up the chromosomes at the metaphase plate. This plate is basically a line drawn down the middle of the tetrad with spindle fibers running parallel to the plate on the nuclear envelope.
In anaphase I, the spindle fibers pull apart the tetrad. The chromosomes that are pulled to the different poles is completely random. This event is called crossover at it occurs at the chiasma.
In telophase I, nuclear envelopes begin to develop around the separate chromosomes to create two separate cells. Sometimes during telophase I, cytokinesis occurs. Cytokinesis is when the cytoplasm of a cell divides to form two daughter cells. Read more about the function of cytoplasm in this article.
During anaphase I the tetrad was torn apart. The result are dyads (or a half of tetrad). Dyads are sister chromatids connected in the middle by a centromere just like they were in the early phases of meiosis. Again, the centrosomes (the ones that create the spindle fibers) begin to move to the poles of the cells.
Metaphase II looks almost exactly like metaphase I. The dyads line up on the metaphase plate and spindle fibers begin to reach out from the poles located on the nuclear envelopes.
Anaphase II is also similar to anaphase I except for this time, the spindle fibers pull the dyads apart creating four individual chromatids. Each one of these sister chromatids ends up in a corner of the cell to prepare for further separation.
The result of telophase II is four haploid cells. The nuclear envelopes begin to surround the four individual sister chromatids. These four separate cells are produced through meiosis of one diploid cell. Each one of these new haploid cells has one-half the genetic information of the original diploid cell. Meiosis may seem super confusing and complicated but it’s really rather simple. If you need to write a biology paper on meiosis or other biology research topics, check out this online course for A+ Research papers. To simplify meiosis without highlighting the phases the process looks like this: Each cell replicates it’s chromosomes to form two sister chromatids connected by a centromere. The centromeres of these two entities meet up to form a tetrad and exchange information at random. Fibers reach in to the cell from the nuclear envelope and pull the tetrad apart back into sister chromatids. The nuclear envelope realizes this and begins to separate the two sister chromatids into different cells. The spindle fibers engage the sister chromatids in each respective cell from each pole and pull them into individual chromatids. The nuclear envelop further differentiates and splits the individual chromosomes into their individual cells – now called haploid sex cells. Haploid cells eventually becomes gametes which become zygotes and then embryos. And that’s all there is to it. Plants and animals do this naturally. It’s amazing to think that microscopic structures are programmed to go through meiosis which in the end creates life. Learn more about cellular development and growth in the course Biotech Basics.