Gametes with a missing chromosome usually die quite fast however gametes with an extra chromosome can survive. This leads the production of gametes that either have a chromosome too many or too few. This is when the chromosomes do not separate properly during meiosis, either in meiosis I (in anaphase I) or meiosis II (in anaphase II). Cytokinesis then follows to divide the cytoplasm of the two cells and so the result is four daughter cells each with a haploid set of chromosomes.Ĥ.2.4 Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).Ī number of problems can arise during meiosis. Here the nuclear membrane reforms around the four sets of daughter chromosomes. Each sister chromatid then becomes a chromosome. Here the centromere devised as a result of the spindle microtubules pulling each sister chromatid to opposite poles in both cells. Here the chromosomes in each cell line up at the equator and the spindle microtubules attach to the centromere of each chromosome. The spindle microtubules stretch out from each pole again and the nuclear membrane breaks down as in prophase I. Here the cell has divided into two daughter haploid cells however the process does not end here as these two cells immediately start to divide again. The first stage of meiosis II is prophase II. There is a brief interphase stage before the start of meiosis II. The membrane then divides through citokinesis. Here each chromosome from the homologous pair are found at opposite poles and the nuclear membrane reforms around each daughter nucleus. The cell membrane starts to prepare for its separation at the equator to form two cells. This halves the chromosome number however each chromosome is still composed of two sister chromatids. This is the stage where the homologous chromosomes are separated and pulled to opposite poles. Here the paired up homologous chromosomes line up at the equator and the spindle fibbers attach to the chromosomes in a way that ensures that for each homologous pair, one chromosome moved to one pole and the other moves to the opposite pole. The nuclear membrane also starts to break down and the spindle microtubules stretch out from each pole to the equator. Crossing over is the exchange of genetic material between non-sister chromatids. Once the homologous chromosomes are paired up, crossing over occurs. In prophase I the chromosomes pair up so that the chromosomes in each pair are homologous. The first stage of meiosis I is prophase I. (See notes on mitosis) After meiosis I there is another brief interphase stage which is followed by meiosis II. Meiotic cells have an interphase stage before the start of meiosis I which is similar to mitosis. 4.2.3 Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells. Homologous chromosomes: chromosomes with the same genes as each other, in the same sequence but do not necessarily have the same allele of those genes. Meiosis is a reduction division of a diploid nucleus to form haploid nuclei. Meiosis 4.2.1 State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei. Chromosomes, genes, alleles & mutations.
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