Describe the behavior of chromosomes throughout meiosisDescribe cellular events during meiosisExplain the differences between meiosis and mitosisExplain the mechanisms in ~ meiosis the generate genetic variation among the commodities of meiosis

Sexual reproduction needs fertilization, a union of 2 cells from 2 individual organisms. If those 2 cells each contain one collection of chromosomes, then the result cell contains two set of chromosomes. The variety of sets the chromosomes in a cell is referred to as its ploidy level. Haploid cells contain one set of chromosomes. Cell containing two sets of chromosomes are called diploid. If the reproductive cycle is come continue, the diploid cell should somehow alleviate its variety of chromosome sets before fertilization can take place again, or there will certainly be a constant doubling in the variety of chromosome to adjust in every generation. So, in enhancement to fertilization, sexual reproduction consists of a atom division, known as meiosis, the reduces the number of chromosome sets.

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Most animals and plants are diploid, containing 2 sets of chromosomes; in every somatic cell (the nonreproductive cell of a multicell organism), the nucleus consists of two duplicates of each chromosome that are referred to as homologous chromosomes. Somatic cells are periodically referred to together “body” cells. Homologous chromosomes are matched bag containing genes for the same traits in identical locations along their length. Diploid biology inherit one copy of each homologous chromosome from each parent; all together, lock are considered a full collection of chromosomes. In animals, haploid cell containing a solitary copy of each homologous chromosome are uncovered only in ~ gametes. Gametes fuse with another haploid gamete to create a diploid cell.

The nuclear division that forms haploid cells, i beg your pardon is referred to as meiosis, is regarded mitosis. As you have actually learned, mitosis is part of a cabinet reproduction cycle that results in the same daughter nuclei that are additionally genetically the same to the original parent nucleus. In mitosis, both the parent and also the daughter nuclei save on computer the same variety of chromosome sets—diploid for most plants and also animals. Meiosis employs many of the exact same mechanisms as mitosis. However, the starting nucleus is always diploid and also the nuclei that an outcome at the end of a meiotic cell department are haploid. To attain the palliation in chromosome number, meiosis consists of one ring of chromosome duplication and also two rounds of atom division. Because the events that occur during each the the department stages room analogous to the events of mitosis, the exact same stage names space assigned. However, since there are two rounds of division, the stages room designated through a “I” or “II.” Thus, meiosis ns is the first round the meiotic department and is composed of prophase I, prometaphase I, and also so on. Meiosis i reduces the number of chromosome sets from 2 to one. The hereditary information is likewise mixed during this department to create unique recombinant chromosomes. Meiosis II, in i beg your pardon the 2nd round the meiotic department takes ar in a way that is comparable to mitosis, has prophase II, prometaphase II, and so on.


Meiosis is came before by an interphase consisting of the G1, S, and G2 phases, i beg your pardon are virtually identical to the phases coming before mitosis. The G1 phase is the first phase of interphase and also is concentrated on cabinet growth. In the S phase, the DNA of the chromosomes is replicated. Finally, in the G2 phase, the cell undergoes the last preparations for meiosis.

During DNA duplication that the S phase, each chromosome becomes composed of two identical copies (called sister chromatids) that are held together at the centromere until they are pulled apart throughout meiosis II. In an pet cell, the centrosomes that organize the microtubules of the meiotic spindle also replicate. This prepares the cell for the first meiotic phase.

Meiosis I

Early in prophase I, the chromosomes have the right to be seen plainly microscopically. Together the atom envelope begins to break down, the proteins linked with homologous chromosomes bring the pair close to every other. The chop pairing the the homologous chromosomes is referred to as synapsis. In synapsis, the gene on the chromatids the the homologous chromosomes are precisely aligned through each other. An exchange of chromosome segments between non-sister homologous chromatids occurs and is called crossing over. This process is revealed visually after the exchange as chiasmata (singular = chiasma) (Figure 7.3).

As prophase ns progresses, the close association between homologous chromosomes begins to rest down, and the chromosomes proceed to condense, although the homologous chromosomes continue to be attached to each other at chiasmata. The variety of chiasmata varies with the varieties and the size of the chromosome. At the finish of prophase I, the pairs are organized together just at chiasmata (Figure 7.3) and are called tetrads due to the fact that the four sister chromatids of every pair of homologous chromosomes are now visible.

The crossover events are the first source of hereditary variation created by meiosis. A solitary crossover event in between homologous non-sister chromatids leader to a reciprocal exchange of tantamount DNA between a maternal chromosome and a paternal chromosome. Now, when that sisters chromatid is moved right into a gamete, the will bring some DNA indigenous one parental of the individual and also some DNA indigenous the other parent. The recombinant sister chromatid has actually a mix of maternal and paternal genes that did not exist prior to the crossover.

Figure 7.3 In this illustration of the impacts of crossing over, the blue chromosome came from the individual’s father and also the red chromosome came from the individual’s mother. Crossover occurs between non-sister chromatids of homologous chromosomes. The an outcome is one exchange of hereditary material in between homologous chromosomes. The chromosomes that have actually a mixture that maternal and paternal succession are dubbed recombinant and also the chromosomes that are totally paternal or maternal are dubbed non-recombinant.

 The an essential event in prometaphase ns is the attachments of the spindle fiber microtubules come the kinetochore proteins at the centromeres. The microtubules assembled native centrosomes at opposite poles the the cell grow toward the center of the cell. In ~ the finish of prometaphase I, every tetrad is attached come microtubules indigenous both poles, through one homologous chromosome attached in ~ one pole and also the other homologous chromosome attached to the various other pole. The homologous chromosomes space still organized together at chiasmata. In addition, the nuclear membrane has broken down entirely.

During metaphase I, the homologous chromosomes are arranged in the facility of the cell v the kinetochores facing opposite poles. The orientation of each pair of homologous chromosomes in ~ the facility of the cell is random.

This randomness, called independent assortment, is the physical basis for the generation the the second kind of genetic variation in offspring. Think about that the homologous chromosomes the a sexually reproducing organism are originally inherited together two different sets, one from each parent. Using humans as an example, one set of 23 chromosomes is current in the egg donated by the mother. The father gives the other collection of 23 chromosomes in the sperm the fertilizes the egg. In metaphase I, this pairs heat up at the midway suggest between the 2 poles the the cell. Due to the fact that there is one equal opportunity that a microtubule fiber will certainly encounter a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase bowl is random. Any type of maternally inherited chromosome may confront either pole. Any paternally inherited chromosome may likewise face one of two people pole. The orientation of every tetrad is live independence of the orientation that the various other 22 tetrads.

In each cell that undergoes meiosis, the setup of the tetrads is different. The number of variations depends on the variety of chromosomes consisting of a set. There space two possibilities because that orientation (for each tetrad); thus, the possible variety of alignments equates to 2n where n is the variety of chromosomes per set. Humans have 23 chromosome pairs, which results in end eight million (223) possibilities. This number go not include the variability previously developed in the sister chromatids through crossover. Offered these 2 mechanisms, it is very unlikely that any two haploid cells resulting native meiosis will have actually the same hereditary composition (Figure 7.4).

To summarize the genetic consequences of meiosis I: the maternal and paternal genes room recombined by crossover events occurring on each homologous pair throughout prophase I; in addition, the arbitrarily assortment of tetrads in ~ metaphase produce a unique combination of maternal and also paternal chromosomes that will certainly make their way into the gametes.

Figure 7.4 To demonstrate random, live independence assortment at metaphase I, think about a cell with n = 2. In this case, there space two possible arrangements at the equatorial airplane in metaphase I, as shown in the top cell of every panel. These two possible orientations lead to the production of genetically different gametes. With much more chromosomes, the variety of possible arrangements rises dramatically.

 In anaphase I, the spindle fibers traction the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere. That is the chiasma connections that are damaged in anaphase I as the yarn attached to the fused kinetochores traction the homologous chromosomes apart.

In telophase I, the separated chromosomes arrive at opposite poles. The remainder that the usual telophase events may or may not occur relying on the species. In some organisms, the chromosomes decondense and also nuclear envelopes form around the chromatids in telophase I.

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Cytokinesis, the physical separation the the cytoplasmic contents into two daughter cells, wake up without improvement of the nuclei in various other organisms. In virtually all species, cytokinesis separates the cell materials by either a cleavage furrow (in animals and also some fungi), or a cabinet plate the will at some point lead to formation of cell walls that different the 2 daughter cell (in plants). At each pole, over there is simply one member of each pair the the homologous chromosomes, so only one full set of the chromosomes is present. This is why the cell are thought about haploid—there is just one chromosome set, also though there are duplicate copies of the set because each homolog still is composed of 2 sister chromatids that space still attached to every other. However, return the sisters chromatids were as soon as duplicates the the exact same chromosome, they room no longer similar at this stage since of crossovers.