Friday, November 04, 2005

DNA methylation, parthenogenesis and hermaphrodism

Have you ever thought why there are no true mammal hermaphrodites or why mammals or vertebrates for that matter cannot reproduce parthenogenetically?

Today, reviewing gene regulation in eukaryotes, I’ve come to understand few basic principles which govern these events.

I’ve always thought that the only thing fertilization accomplishes is returning the number of chromosomes to the diploid number (2n). therefore, I also thought that if these was the case then, a zygote can be formed from a container and two egg cell nuclei or two sperm nuclei because it satisfies the requirement for the diploid number.

Apparently, it is not possible(as of the moment)because of an occurrence called DNA methylation. Cytosine residues in vitro are occasionally methylated and this determines whether a certain gene will be actively transcribed, repressed or do other functions. Now, the methylation pattern is usually preserved in the chromosomes you inherit from your mother and father. Remember that you have two sets of chromosomes, one from your mother and one from your father. Some genes are active only on one of the chromosomes… how is it determined which chromosome does the job? Is it the paternal of maternal chromosome? Some processes have an inherent order, for example, the gene for the enzyme ILGF-II (insulin like growth factor-II) is paternally unmethylated and maternally methylated. And in this case methylation serves as a negative regulator. Therefore, all your ILGF-II principally comes from your father.

The understanding of the principles which govern the pattern and inheritance of the methylation pattern is not yet fully understood. But this leads us to the point of my exhausting story, two egg nuclei thefore cannot be used to produce a zygote because the chromosomes have the same methylation pattern and in turn many of the genes will not function normally.

God is saying… "don’t mess with me, I’ve planned ahead" ^_^

Terms for the layman :
Parthenogenesis – zygotes develop without fertilization
Diploid – bearing two copies of chromosomes, humans have a diploid number(46 chromosomes in all), plants can be polyploid meaning they can have more than two sets, fungi and lower animals are usually monoploid/haploid
Cytosine – one of the four nucleotides which make up the genetic code
Methylated – a methyl group (-CH3) is attached to the molecule as a side-chain

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