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Lecture 19, part 4 of 4
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5. Ring chromosomes

The cellular mechanisms for repariing double-stranded breaks will sometimes cause chromosmes with broken ends to fuse, forming ring chromosomes. Ring chromosomes have been found in several plant species: maize, tobacco, Antirrhinum, Petunia, barley, as well as Drosophila and Homo sapiens. McClintock in papers from 1931 - 1941 studied the behaviour of ring chromosomes in mitosis and meiosis. Ring chromosomes are not stable during cell division and are often eliminated.

McClintock developed the breakage-fusion-bridge cycle hypothesis to explain the changes in chromosome size which occurred as a result of the ring chromosomes. If the ring reproduces itself in interphase with no sister strand cross-over in prophase, the ring chromatids can separate in anaphase without difficulty resulting in two equal sized ring-chromosomes, the same size as the original. If sister chromatid exchange occurs, a ring of twice the size is produced with two centromeres

Normally, ring chromosomes can segregate evenly during anaphase.  

However, if a crossover event occurs, the two daughter chromosomes effectively act as a single, circular, dicentric chromosome.  The two centromeres move to opposite poles in anaphase and form an anaphase double bridge. As the centromeres get pulled to opposite poles, the chromosomes break at two places. During telophase, chromosome ends tend to recircularize, in the absence of telomeric sequences.  If the break occus as shown below, balanced chromosomes will result, with neither deletions nor insertions.

Breaks are possible at different points along the ring chromosome. If breaks occur asymmetrically, deletions and duplications can result.

D. Transposition

1. Structure

Transposition is the movement of a region of chromosomal segment from one location to another. DNA is excised from one location by an enzyme called a transposase, and inserted elsewhere, either on the same chromosome or a different chromosome. The DNA that is  transposed is referred to as a transposon.  Transposons are typically very small pieces of DNA, ranging from less than 1 kb to several kb.

Transposases can cleave and rejoin double-stranded DNA at specific recognition sequences. When these sequences occur in an inverted repeat orientation, a transposase can remove the target sequence from its original location, and insert it elsewhere. The insertion is essentially a reversal of the excision.

The Ac element of maize is illustrated at right. It contains a transposase gene flanked by  11bp Terminal Inverted Repeats that are recognized by the transposase. Thus, the Ac element can cause its own transposition.

 5' end (C/TAGGGATGAAA)                  3' end (TTTCATCCCTA)

Other transposeable elements, such as Ds in maize, are flanked by the required 11bp repeats, but do not encode a transposase. Therefore, Ac must be present at another locus to produce the transposase. In maize lines with Ds but no Ac, Ds elements do not transpose. When an Ac-bearing line is crossed with a Ds line, the Ds elements can transpose. This is referred to as "mobilization of Ds elements".

Du C (2011) The complete Ac/Ds transposon family of maize. BMC Genomics 12:588. DOI:   [10.1186/1471-2164-12-588]

2. Discovery of transposition

Ds was first discovered by Barbara McClintock because one of the Ds elements on chromosome 9 contained a sequence that led to frequent chromosome breaks. These breaks always occurred at the same place on chromosome 9, leading to a breakage-fusion-bridge cycle as described above. Surprisingly, when McClintock crossed a Ds line with other maize lines, Ds was mobilized to other loci, causing breaks at those loci.

ACTIVATOR, or Ac , locus was named by Barbara McClintock for its ability to activate chromosome breakage at another locus; Dissociation, or Ds (a). The two loci are shown here on the same chromosome, but they can be on different chromosomes. Ac is able to promote its own transposition (b), or that of Ds (c)  to another site either on the same chromosome or on a different one. Ds cannot move unless Ac is present in the same cell. Ac is an autonomous transposable element and Ds is a nonautonomous element of the same family.

McClintock's Activator-Dissociator system in maize as a possible origin of chromosome structural changes. This is a two gene system: the Ac locus and the Ds locus. When both loci are present, chromosome breakage increases leading to an increase in chromosome structural changes: deficiencies, duplications, inversions, translocations and ring chromosomes. Ac and Ds are visualised as blocks of heterochromatin that move by transposition between different sites in the chromosomes. Ac does not have a mutating effect alone but promotes the movement of Ds (Ac= activator).

3. Transposon insertions can cause reversible mutations.

Example: The C locus produces an anthocyanin pigment, resulting in colored aleurone ie. wild-type.   Insertion of a transposon into C blocks pigment production, resulting in the colorless (c) phenotype.  During seed development, excision of the Ds element can result in patches of cells  (each progeny of a single cell) expressing the colored phenotype. This is referred to as variegation. If the Ac locus moves to a position adjacent to Ds, it promotes its movement away from the C locus during the development of the kernel and the locus reverts to normal expression. The larger the number of Ac factors present, the greater expression of variegation in the tissue. Larger coloured patches are formed when Ds is transposed early in development; smaller patches when Ds is transposed late.

MUTATION OF C LOCUS,a gene required for synthesis of a purple pigment in the aleurone (a), takes place when Ds moves into the locus (b). The mutation disables the gene, the pigment is not made and the aleurone is colorless. If Ac is present in the genome, however, it promotes the transposition of Ds away from the locus in some cells during kernel development (c). The mutation reverts when the element leaves, giving rise to cells in which the C locus is functional. Each such cell gives rise in turn to a pigmented sector in the aleurone.

In 1983, Barbara McClintock won the Nobel Prize in Physiology and Medicine for her discovery of mobile genetic elements (ie. transposons).

For an insightful  look into not only what Dr. McClintock did, but also, for how she thought, see:


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Lecture 19, part 4 of 4
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