There is a new record for the largest genome to be sequenced, set at a massive 90 billion letters of DNA. It belongs to the South American lungfish.

“It was a technical challenge, of course, to do this,” says at the University of Konstanz in Germany. “It is the largest of all animal genomes.”

Each cell in the South American lungfish (Lepidosiren paradoxa) has two copies of the genome, comprising 180 gigabases (Gb) of DNA in total that would stretch for 55 metres if arranged in a line. That is 30 times more than the 6 Gb of DNA in each human cell.

The South American lungfish has 19 chromosomes, 18 of which are each larger than a single copy of the human genome, says Meyer.

His team also sequenced the 40 Gb of a single copy of the genome of the African lungfish (Protopterus annectens), meaning the researchers have now sequenced all six lungfish species found globally, which all have abnormally large genomes.

“It’s a real enigma how these fish are able to tolerate such a large genome,” says Meyer. The nucleus inside each cell has to be very large to fit in so much DNA, he says, which means each cell is larger than normal. Replicating so much DNA also takes a lot of energy each time cells divide.

There is no evidence that all this extra DNA does anything useful. Rather, it appears to be a result of “genetic parasites” making endless copies of themselves. It is probably mostly junk, says Meyer.

His team found that mechanisms that other organisms use to curtail the spread of genetic parasites seem to be damaged or missing in all the lungfish species. As a result, the genome of the South American lungfish has been growing by 3.7 Gb – more than a single copy of the human genome – every 10 million years.

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The reason for sequencing all the lungfish species is to get a better idea of what their shared ancestor was like. That ancestor was a close relative of the lungfish that evolved into the first four-legged land animal.

“They are our closest relative among the fishes,” says Meyer. As their name implies, lungfish breathe air and drown without it.

They can also live for more than 100 years and regenerate their fins and tails, he says. Meyer’s team hopes to find out how they achieve this.

Some plant genomes are even larger than that of the South American lungfish. A small fern found on a few Pacific islands is thought to have 321 Gb of DNA in each cell, but there are no plans to try to sequence it.

A database of proteins, dubbed the “unknome”, that ranks proteins according to how much we have learned about them has revealed that we still know next to nothing about thousands of human proteins. The team behind the database has also shown that at least some of these proteins are essential for survival.

To create the unknome, at the MRC Laboratory of Molecular Biology in Cambridge, UK, and his colleagues started with the 20,000 or so genes for proteins that have been identified in humans. They grouped together closely related human genes or proteins on the basis that they probably have similar functions, resulting in around 7500 protein clusters.

Next, they added closely related proteins found in commonly studied animals, such as mice or fruit flies, to these clusters, as these probably also have the same function. They then gave each protein cluster a score based on how many entries there were about its members in the main repository of information on the functions of genes, known as the Gene Ontology Resource.

A human protein that hasn’t been directly studied still scores highly if an equivalent protein has been well studied in another animal. Proteins also get higher scores for entries that are regarded as more reliable, such as having been published in a journal. The scoring is slightly arbitrary, says Munro, but this is inevitable when trying to work out what we don’t know.

The best-studied proteins have scores of well over 100. For instance, a protein called sonic hedgehog, which is involved in embryonic development, scores 168, while p53, which helps stop cells turning cancerous, scores 126. However, more than 2200 proteins have scores below 2, 1100 score below 1 and more than 800 score 0.

In theory, these low-scoring proteins might not have been studied because they don’t do anything important. To get an idea of whether the proteins matter, the team used a technique called RNA interference (RNAi) to reduce the levels of 260 proteins with scores below 1 in fruit flies. In 60 cases, the flies died, showing that these particular proteins have an essential function.

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That was a big surprise to the members of the team who study fruit flies, says Munro. “They just assume that every possible important gene has been found, which turns out, of course, not to be true.”

The number of unknown proteins is slowly going down, he says, but he hopes the findings will accelerate the pace of discovery. The problem at the moment is that both funding bodies and individual researchers are reluctant to risk studying unknown proteins in case they turn out not to do anything important.

“There may even be biological processes that we don’t know about,” says Munro. “No one is looking for the proteins involved in them because no one knows about them.” That may sound surprising, he says, but the gene-editing technique known as CRISPR is based on bacterial proteins whose function was uncovered only in 2012.