A Real Ice Monster

July 4, 2014

By Medical Discovery News

From the ice, scientists hauled a monster of unimaginable size. It was larger than any of its kind, and it was alive. Luckily, it wasn’t the Yeti, but Pithovirus sibericum, an abominable snow virus of sorts.

P. sibericum is the largest virus ever discovered. It’s about 1.5 micrometers, larger than some bacterium (a single-celled organism). All things considered though, it’s still microscopic – 1,333 copies of P. sibericum would fit on top of a pin. Luckily, this gigantic virus only infects amoebas, single-celled protozoans that live in bodies of water including lakes, ponds, streams, rivers, and even puddles. Some amoebas are associated with diseases such as dysentery.

This newly discovered virus was named P. sibericumbecause it was found in a sample of permafrost from Siberia, hence the word sibericum. The scientists who discovered it were French, and they were inspired by its shape to call it a Pithovirus from the ancient Greek word pithos, which were large containers used to store wine. They estimate the virus had been in the deep freeze for at least 30,000 years before they resurrected it this year. In 2012, the French scientists also resurrected an ancient plant from fruits buried in the same Siberian permafrost, which led them to search for the virus.

P. sibericum is unique in many ways beyond its record-breaking size. It is oval-shaped with a thick wall and a hole in one end. It has a distinctive honeycomb structure that caps the opening. Most viruses tightly pack their genetic information inside, but P. sibericum has a surprisingly small genome for a virus that big. Viruses one-third its size store two to three times more gene bases. Only about one-third of its proteins have any similarity to those of other viruses.

It is not, however, the first giant virus. Mimivirus was the first large virus ever found, reported in 2003. Previously, the record for largest virus went to Megavirus chilensis, which was found in water samples from Chile.

Just like Mimiviruses and Megaviruses, Pithoviruses are taken up by their amoebic hosts and once inside, they release their proteins and their genetic information. They then commandeer the host cell to produce hundreds of new viral particles, which are released when the host cell ruptures.

Interestingly, another giant virus called Marseillevirus also infects amoebas. Its genome contains a collection of genes found in similar viruses, bacteriophage viruses that infect bacteria, amoebas, and cells from the animal, plant, and fungus kingdoms. This suggests that amoebas may be acting as vessels for mixing genetic information from multiple forms of life. An amoeba could simultaneously be infected with Marseillevirus and bacteria, making it possible to produce complex genomes such as those of the giant viruses.

The resurrection of P. sibericum, a DNA virus long frozen in the now-thawing permafrost, has scientists wondering about undiscovered viruses that might be future threats to human or animal health.

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The Little Denisovan Girl and Us

Jan. 4, 2013

Medical Discovery News

We are now able to analyze the genomic sequences of prehistoric humaniods to learn how they differed from modern humans

The words screamed off the page:  “Genomic Sequencing Brings Ancient Girl to Life!” While such headlines bring to mind reanimated mummies, they actually referred to the successful extraction and complete genomic sequencing of a young girl who lived 80,000 years ago. This is the first time the quality of the genomic sequence of any ancient species has rivaled that of the living people today.

This study pioneered a new approach in DNA sequencing. DNA, while known to be hardy, cannot usually be extracted in the double stranded form from fossils like it can be from living organisms today. Researchers used a small piece of the knuckle of her pinky finger to extract the girl’s ancient DNA.

Researchers developed an effective strategy utilizing the single-stranded DNA fragments they extracted. This new approach generated up to 20 times more readable DNA sequences than previous methods could have. In the end, researchers put together a high-resolution genomic sequence.

The quality of the sequence amazed scientists because it matches the resolution of genome sequencing from living organisms today. For example, they determined that the girl had brown skin, hair, and eyes. 

The girl’s sequence holds important hints of human evolution. She belonged to a lesser-known sister species to Neanderthals called Denisovans, which have only been found in Southern Siberia. It was obvious that she had 23 pairs of chromosomes, the same as modern humans. Chimpanzees have 24 chromosome pairs, so Denisovans were definitely more human than ape-like.

By aligning differences in the DNA sequences, scientists estimated that the Denisovans split from humans between 170,000 and 700,000 years ago. When compared to chimpanzee sequences, there were fewer differences than those between modern humans and chimpanzees.  Due to the new sequencing technique, scientists can more accurately date when a fossilized person or animal lived and died. Such advances may alter the time frame of human and animal evolution.

Other recent studies indicate 1 to 4 percent of European and Asian DNA came from Neanderthals. Since Africans have no Neanderthal DNA, interbreeding between Neanderthals and humans occurred after humans left Africa and migrated to Eurasia. Only humans from Papua, New Guinea, shared DNA with Denisovans, but a trace amount of similarity was seen in the genomes of the Han and Dai peoples in mainland China. Scientists don’t yet know the connection between these groups.

Overall, researchers documented more than 100,000 DNA sequence changes between Denisovans and modern humans. Of high interest were eight changes in genes that play a role in the “wiring” of the human nervous system, including one linked to autism and one associated with speech defects, raising questions about the speech and mental capabilities of these early ancestors. Scientists will continue to learn more about early humans and their predecessors using this new approach to DNA sequencing as a “molecular time machine.” 

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