Mutations continued

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During meiosis, the homologous maternal and paternal domains pair. Normally, these domains and homologous chromosomes are identical. This is different here. Due to the regular, balanced translocations, the chromosomes of Oenothera and a few other species form a ring when pairing: domain 2 of the maternal chromosome 1.2 pairs with domain 2 of the paternal chromosome 2.3. This again pairs with chromosome 3.4 and so on. The ring is closed by the pairing of chromosome 14.1 with chromosome 1.2. During anaphase I, the centromeres are distributed onto the daughter cells in a strictly alternating way. The result is that the chromosomes of the original maternal set stay together in one daughter cell and those of the paternal in the other. This explains the occurrence of just one coupling group.

The system works usually very well, though it is more prone to disruptions than normal meiosis is, leading to an increased mutation rate in Oenothera. This may be the reason, why H. de VRIES detected the phenomenon mutation in just this genus. Similar conditions were found in //Rhoeo discolor//, Paeonia californica and in species of the genus Datura.

The DNA sequence of a gene can be altered in a number of ways. Gene mutations have varying effects on health, depending on where they occur and whether they alter the function of essential proteins. The types of mutations include:

Missense mutation (illustration)
This type of mutation is a change in one DNA base pair that results in the substitution of one amino acid for another in the protein made by a gene.

Nonsense mutation (illustration)
A nonsense mutation is also a change in one DNA base pair. Instead of substituting one amino acid for another, however, the altered DNA sequence prematurely signals the cell to stop building a protein. This type of mutation results in a shortened protein that may function improperly or not at all.

Insertion (illustration)
An insertion changes the number of DNA bases in a gene by adding a piece of DNA. As a result, the protein made by the gene may not function properly.

Deletion (illustration)
A deletion changes the number of DNA bases by removing a piece of DNA. Small deletions may remove one or a few base pairs within a gene, while larger deletions can remove an entire gene or several neighboring genes. The deleted DNA may alter the function of the resulting protein(s).

Duplication (illustration)
A duplication consists of a piece of DNA that is abnormally copied one or more times. This type of mutation may alter the function of the resulting protein.

Frameshift mutation (illustration)
This type of mutation occurs when the addition or loss of DNA bases changes a gene’s reading frame. A reading frame consists of groups of 3 bases that each code for one amino acid. A frameshift mutation shifts the grouping of these bases and changes the code for amino acids. The resulting protein is usually nonfunctional. Insertions, deletions, and duplications can all be frameshift mutations.
Repeat expansion (illustration)
Nucleotide repeats are short DNA sequences that are repeated a number of times in a row. For example, a trinucleotide repeat is made up of 3-base-pair sequences, and a tetranucleotide repeat is made up of 4-base-pair sequences. A repeat expansion is a mutation that increases the number of times that the short DNA sequence is repeated. This type of mutation can cause the resulting protein to function improperly.


Sources:
-Biology 8th edition textbook, Solomon Berg Martin
-www.course-notes.org
-Mt. Sinai library database

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