This complex extends the length of the chromosome pair and is attached to the nuclear envelope. There appears to be a thin space between the two chromosomes which contains a multiply-threaded structure called a synaptonemal complex. It is difficult to see what the paired chromosomes are doing if a light microscope is the only instrument used, but if these complexes are viewed using an electron microscope, the close association between them can be seen. Such chromosome pairs are said to be homologous and the pair is said to be a pair of homologous chromosomes.Īs this process of pairing continues (also called synapsis), the homologous chromosomes come into a tighter and tighter arrangement. During the second stage, these chromosomes start to pair up with their complementary partner (the other chromosome that is carrying the same set of genes). Each half was once called a chromatid, and is now known to be one partner of the doubled DNA molecules of each chromosome made back in S-phase.Īt some point in the thickening process it is possible to make out that each thread represents a double-DNA chromosome. These threads often have "bead-like" swellings along their length, but their significance is unknown.Ĭlose examination of these threads, which continue to shorten and thicken, shows that they are already doubled. (In red, below).Ĭondensation: threads of DNA wrapped in nuclear proteins and histones gradually become visible. These names will also be used here for clarity. However, we now have a much better understanding of the function that is taking place at all stages of Prophase I, and newer names have been devised which better described the mechanisms that are taking place. These names have lasted a long time in the history of science, and are given here for comparison.
(These are the names given in brown, below). Since no one at that time understood exactly what was going on, the names are more descriptive of what was seen rather than what was happening. Once upon a time the only tool for scientists studying meiosis was the light microscope, so the various stages of Prophase I were given names based only on what could be seen this way. This is the most complex of the various stages of meiosis. Although there is only one chromosome set, each homolog still consists of two sister chromatids.All cells undergoing a meiotic division pass through two distinct periods called First division, in which the homologous chromosomes are separated, and the Second division, in which the duplicated DNA molecules are separated.
Therefore, only one full set of the chromosomes is present. The cells are haploid because at each pole there is just one of each pair of the homologous chromosomes. Two haploid cells are the end result of the first meiotic division. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells. In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow (constriction of the actin ring that leads to cytoplasmic division). Then cytokinesis, the physical separation of the cytoplasmic components into two daughter cells, occurs without reformation of the nuclei. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. In telophase I, the separated chromosomes arrive at opposite poles. With n = 23 in human cells, there are over 8 million possible combinations of paternal and maternal chromosomes. In this example, there are four possible genetic combinations for the gametes. The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set. In this case, there are two possible arrangements at the equatorial plane in metaphase I. \( \newcommand\): Meiosis I ensures unique gametes: Random, independent assortment during metaphase I can be demonstrated by considering a cell with a set of two chromosomes (n = 2).