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Prophase I

During prophase I, chromosomes condense and become visible inside the nucleus. Together the nuclear envelope begins to break down, homologous chromosomes relocate closer together. The synaptonemal complex, a lattice that proteins between the homologous chromosomes, creates at details locations, dispersing to covering the entire length that the chromosomes. The chop pairing of the homologous chromosomes is called synapsis. In synapsis, the gene on the chromatids the the homologous chromosomes are aligned through each other. The synaptonemal complicated also supports the exchange the chromosomal segments between non-sister homologous chromatids in a process called cross over. The crossover events are the very first source of genetic variation developed by meiosis. A solitary crossover event between homologous non-sister chromatids leader to one exchange that DNA in between chromosomes. Complying with crossover, the synaptonemal facility breaks down and the cohesin connection in between homologous bag is likewise removed. In ~ the finish of prophase I, the pairs are hosted together only at the chiasmata; they are dubbed tetrads since the four sister chromatids of each pair that homologous chromosomes are currently visible.

You are watching: Homologous chromosomes are aligned at the equator of the spindle.

Figure \(\PageIndex1\): Crossover between homologous chromosomes: Crossover occurs between non-sister chromatids the homologous chromosomes. The result is one exchange of genetic material between homologous chromosomes.Figure \(\PageIndex1\): Synapsis holds pairs of homologous chromosomes together: beforehand in prophase I, homologous chromosomes come with each other to kind a synapse. The chromosomes space bound strict together and in perfect alignment by a protein lattice called a synaptonemal complex and through cohesin protein at the centromere.


Metaphase I

During metaphase I, the tetrads relocate to the metaphase plate with kinetochores dealing with opposite poles. The homologous pairs orient us randomly at the equator. This occasion is the 2nd mechanism the introduces variation right into the gametes or spores. In each cell the undergoes meiosis, the arrangement of the tetrads is different. The variety of variations is dependency on the number of chromosomes comprising a set. There room two possibilities for orientation in ~ the metaphase plate. The possible variety of alignments, therefore, equals 2n, wherein n is the number of chromosomes per set. Offered these two mechanisms, the is very unlikely that any kind of two haploid cells resulting from meiosis will have the same genetic composition.

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Figure \(\PageIndex1\): Meiosis i ensures unique gametes: Random, elevation assortment during metaphase I can be demonstrated by considering a cell v a collection of two chromosomes (n = 2). In this case, there room two feasible arrangements in ~ the equatorial plane in metaphase I. The full possible number of different gametes is 2n, where n amounts to the number of chromosomes in a set. In this example, there are four feasible genetic combinations for the gametes. With n = 23 in human being cells, there are over 8 million feasible combinations of paternal and also maternal chromosomes.