Exercise2

..Group 4

Name of group: Hello Vera Group secretary plus email: Ayesha Begum bt09180@qmul.ac.uk Group members plus email: Shilpi Sheth bt09376@qmul.ac.uk Aditi Tanna bt09017@qmul.ac.uk Karmdeep Shoker bt09119@qmul.ac.uk Maureen Islam bt09462@qmul.ac.uk Ado Gubaljevic a.gubaljevic@stu10@qmul.ac.uk George Ebunam ef08028@qmul.ac.uk

An idiogram of Aloe vera. is shown below:
 * 1. Aloe karyotype.



The picture is a root-tip metaphase from an individual plant that is both polyploid and aneuploidy. a) Arrange the chromosomes into a karyotype as best you can. (i.e. print out the picture, cut it up and arrange it, then scan in the resulting picture and upload it to this page) b) Under the karyotype explain what you think has happened to generate this unusual karyotype.

__Karyotype__
 * [[image:triploid_aloelow.jpg]]



.. ....NO this is not correct, there are 7 chromosomes in the karyotype, not 9 as above. It is in fact an aneuploid at the triploid level, i.e. there should be three of most chromosome types, one of the large chromosomes is missing the third copy

__b)__

Aneuploidy is when a cell arises with an unusual number of chromosomes e.g. trisomy is an example of aneuploidy because the cell has a three copies of a particular chromosome. Polyploidy is when the cell has two sets of the same chromosome. Aloe barbadensis ....italics , known by it’s common name Aloe vera ....no this is another species, and should be in italics , has a chromosome number of 2n=14 shown in the ideogram above. However, in root tip metaphase, this individual plant gave rise to the chromosome number 2n=10 as shown in figure 1.

There are two sets of chromosomes in the karyotype, which have paired together, and this pairing of chromosomes results in polyploidy ....unclear . The most plausible reason for this is due to the addition of a chromosome set which is identical to its normal haploid compliment of the same species, resulting in autoploidy. However, polyploidy can arise by a combination of chromosome sets from a different species as a result of inter-specific mating but this is not the case as Aloe vera generally reproduces asexually.

Chromosome 5, 7 and 9 are examples of aneuploidy in the Aloe vera plant cell. There are many possible reasons as to why aneuploidy may have arisen. These include, lagging chromosomes, bridges with/without fragments, and meiotic abnormalities as in asynapsis or maybe even errors in disjunction of some bivalents.

Chromosome lagging occurs when the chromosomes do not segregate properly, in that one chromosome is not found in the nucleus of one of the daughter cells, leading to a triploid and a monoploid in the progeny. However, we know this is not the case here, as we are told specifically that this karyotype is taken from a root-tip metaphase, whereas chromosome lagging occurs during anaphase. ....yes but it could hav eoccured in a preceding anaphase

Aneuploidy can sometimes arise when two non-sister homologous chromatids become the subject of a chromosome bridge with fragments. Chromosome bridges occur when the cell cannot detect the telomeres signifying the end of chromosomes so the cell using its DNA repair mechanism attaches two chromosomes together resulting in a chromosome bridge. This is not the case in this example because it is not observable in the karyotype.

Asynapsis is the failure of homologous chromosomes to pair during meiosis. Chromosome 5 could be the result of asynapsis because it is a lone chromosome. It could also be caused by non-disjunction, which is when homologous chromosomes fail to separate during meiosis. However, the evidence shows there is no triploid so it cannot be caused by non-disjunction.

Picture 7 and 9 portray trisomy, which is caused by non-disjunction. When a cell has two of the chromosomes fuses with a normal gamete, the resulting cell has three copies of that chromosome, which results in trisomy as shown for chromosomes 7 and 9.

__References __

Taji, A., Kumar, P.P., Lakshmanan, P. (2001). In Vitro Plant Breeding. New York: The Haworth Press

Vig, B.K. (1968) Spontaneous chromosome abnormalities in roots and pollen mother cells in Aloe vera. Bulletin of the Torrey Botanical Club, 95(3), 254-261 || Discussion on Exercise 2

Posted Oct 17, 2007 3:16 am