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Prophase I

During prophase I, chromosomes conthick and come to be visible inside the nucleus. As the nuclear envelope starts to break down, homologous chromosomes move closer together. The synaptonemal complex, a lattice of proteins between the homologous chromosomes, develops at certain areas, spanalysis to cover the whole length of the chromosomes. The tight pairing of the homologous chromosomes is referred to as synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned with each various other. The synaptonemal complex likewise supports the exchange of chromosomal segments between non-sister homologous chromatids in a procedure referred to as crossing over. The crossover occasions are the initially source of hereditary variation developed by meiosis. A single crossover occasion in between homologous non-sister chromatids leads to an exchange of DNA between chromosomes. Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is additionally rerelocated. At the finish of prophase I, the pairs are organized together just at the chiasmata; they are dubbed tetrads bereason the four sister chromatids of each pair of homologous chromosomes are now visible.

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

Figure (PageIndex1): Crossover in between homologous chromosomes: Crossover occurs in between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes.Figure (PageIndex1): Synapsis holds pairs of homologous chromosomes together: Early in prophase I, homologous chromosomes come together to create a synapse. The chromosomes are bound tightly together and also in perfect alignment by a protein lattice dubbed a synaptonemal complicated and by cohesin proteins at the centromere.

Metaphase I

Throughout metaphase I, the tetrads relocate to the metaphase plate with kinetochores dealing with opposite poles. The homologous pairs orient themselves randomly at the equator. This occasion is the second mechanism that introduces variation right into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The variety of variations is dependent on the variety of chromosomes comprising a collection. Tright here are 2 possibilities for orientation at the metaphase plate. The possible number of alignments, therefore, equals 2n, where n is the variety of chromosomes per set. Given these 2 mechanisms, it is very unlikely that any kind of two haploid cells resulting from meiosis will have actually the very same genetic composition.

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Figure (PageIndex1): Meiosis I ensures distinctive gametes: Random, independent assortment in the time of metaphase I have the right to be demonstrated by considering a cell via a set of 2 chromosomes (n = 2). In this instance, tbelow are 2 possible arrangements at the equatorial plane in metaphase I. The full feasible variety of various gametes is 2n, where n equates to the variety of chromosomes in a set. In this instance, there are four possible genetic combinations for the gametes. With n = 23 in humale cells, tright here are over 8 million possible combicountries of paternal and maternal chromosomes.