MadSci Network: Biochemistry |
Dear Jauwad,
An interesting, and very detailed, question!
The first thing to say is that the substance you extracted is amost certainly NOT DNA at all, but pectin! The techniques used in schools/colleges to extract 'DNA', by using a combination of detergents and salts, followed by precipitating the 'DNA' with ethanol are simply extracting the wrong substance! This can easily be proved - add pectinase to your extract and watch the jelly-like substance you extracted disappear before your very eyes......!
The fact is that this 'DNA' is no such thing and the procedure keeps getting copied from one book to another, without anyone bothering to check what the extracted substance actually IS! Lets hear it for 'peer review'! :)
The second point is that DNA in eukaryotes is found in a complex with proteins (histones and chromatin) to form chromosomes. The DNA itself only makes up a small percentage of the complete molecule and separating the (acid) DNA from the (basic) protein is problematical, since 'pure' DNA is unstable!
Finally, the quantity of DNA in a cell is miniscule, so 14g of banana (90% water!) would yield an infinitessimally small yield of DNA (see below!).
Now, bananas are members of the family Musaceae (genus Musa) and this includes a number of tropical plants - all related to grasses, for they are Monocotes. Edible bananas, like yours, have no seeds and are infertile, since they are TRIPLOID (i.e. have 3 copies of the chromosomes and so cannot produce gametes by meiosis). Typically, normal, diploid, Musa plants have 22 chromones (2N), so edible bananas have 33 chromosomes (3N).
[It is common for different cultivars of plants to have different numbers of copies of the chromosomes, with wheat having 6 copies. In contrast, virtually ALL animals have 2 sets, the exceptions being colonial animals like bees, wasps and termites, which have haploid individuals as well as diploid (all male bees are haploid, for instance, whilst teh workers and queens are diploid)]
For this reason, the DNA content is measured for each full set of chromosomes, and for Musa spp. it is around 0.6 picograms (pg) and each cell has 3 copies, giving a value of about 1.8 pg (or 0.0000000000018 g) /cell! Not very much!
A better way of thinking about the value is to use the number of base- pairs that would be present, and that is about 550-600 million base pairs for each of the 3 sets of (identical) chromosomes in each cell.
There are several databases on the internet giving the size of the DNA in animal and plant species: www.genomesize.com does animals, and www.rbgkew.org.uk does plants.
Both sites give links to other databases - though domestic/crop species have been studied more commonly, for obvious reasons.
Using 14g of banana and making some wild guesstimates for the size and number of cells in a banana, it would appear that the total mass of DNA in your sample would be in the range of 0.001g - 0.00001g, (0.1 - 0.001mg) but they are probably the MAXIMUM possible values, since the yield you obtauin will be well below 100%.
Hope that is not too disappointing for you!
ian
Try the links in the MadSci Library for more information on Biochemistry.