Meiotic Errors & Chromosomal Mutations

Meiotic errors lead to abnormal chromosome structures and/or numbers Meiosis is a complex process, and things occasionally go wrong. For example, chromosomes may break, homologs may fail to separate at anaphase I, or chromatids may fail to separate at anaphase II.

How is this different from errors that take may take place in mitosis? The gametes formed from meiotic errors carry abnormal chromosomes, and when abnormal chromosomes take part in fertilization, the consequences for offspring can be significant.

Nondisjunction Failure of a homologous chromosome pair to separate during anaphase I, or a pair of chromatids fail to separate during anaphase II If this happens during anaphase I, two of the four daughter nuclei will each end up with both members of that homologous pair, and the other two will have neither member of the pair.

Nondisjunction continued… If nondisjunction occurs at anaphase II, only two of the four daughter nuclei will be affected; one will have an extra chromosome and the other will have one less than the full complement of chromosomes.

Nondisjunction cont. Nondisjunction results in aneuploidy: a condition resulting in an abnormal number of chromosomes. Having one extra chromosome is called trisomy, while missing one chromosome is called monosomy. Aneuploidy is a common and harmful condition (about 20% of all conceptions show aneuploidy, but most of the embryos that develop from such zygotes do not survive to birth, and those that do often die before the age of 1 year).

Even more nondisjunction The most common form of aneuploidy in humans is trisomy 16, but almost none of these embryos survive to birth. Among the few aneuploidies that allow survival is Down syndrome - trisomy 21; these individuals, despite intellectual disabilities, can lead long and productive lives.

Polyploidy Polyploidy is when cells have more than two paired (homologous) sets of chromosomes. Most eukaryotic species are diploid, so they have two sets of chromosomes. Under some circumstances, triploid (3n), or tetraploid (4n), or higher- order polyploid nuclei may form.

Polyploidy Polyploidy Polyploidy Polyploidy (get it? Haha) Polyploidy can occur in a number of ways. There could be an extra round of DNA replication preceding meiosis There could be no spindle formed in meiosis II.

Translocation During crossing over in meiosis I, chromatids from homologous chromosome pairs break and rejoin. Occasionally this can happen between non-homologous chromosomes. The result is a translocation, and these are quite common (even in mitotic cells). Since the location of genes relative to other DNA sequences is important, translocations can have profound effects on gene expression.

More Translocation An example of a translocation known to occur in humans is a swap of material between chromosomes 9 and 22. In this case part of the BCR gene sequence on chromosome 22 comes to lie adjacent to part of the ABL gene sequence, which was translocated from chromosome 9. If the translocation occurs in a mitotic cell forming white blood cells, the result of this combination is a form of leukemia, a cancer of white blood cells.

Chromosomal Mutations Mutations are stable and inherited changes in the genetic material. A mutation may result in a new allele of a gene, and different alleles may produce different phenotypes Causes: Spontaneous: Mistakes in base pairing during DNA replication Environmental factors: gamma radiation, X rays, ultraviolet light, nuclear radiation, asbestos, cyanide and other mutagens Mutagen: any agent that can cause a change in DNA

There are four kinds of Chromosomal Mutations a. deletion – portion of chromosome is missing b. duplication – portion of chromosome is on the chromosome twice. c. inversion – when fragment reattaches to original chromosome but in reverse order d. translocation – broken fragment attaches to a nonhomologous chromosome

Figure 15.13 Alterations of chromosome structure