On the handedness of DNA

Double-stranded DNA forms a helical structure where the two strands are twisted into a helical shape. If you think of the base pairs forming a ladder then imagine that the entire ladder could be distorted by rotating the ends relative to each other. The result would be the helical shape of DNA. The twisting results mainly from the attraction between the planar base pairs (rungs of the ladder.) They are "happier" when they are stacked close together right on top of each other. (The "force" is called "stacking interactions.")

This is not how DNA is actually built since there's never a time inside the cell when the DNA forms a ladder-like structure that's not helical, but you get the picture. [The Three-Dimensional Structure of DNA]

The final form of double-stranded DNA on the right is a cartoon used to illustrate certain features. I've deliberately drawn it with about 10-11 base pairs per turn so you can see the shape of the helix.

A physicist tries to understand junk DNA: Part II

Yesterday I posted some comments on a blog post by physicist Rob Sheldon [A physicist tries to understand junk DNA ]. My comments were based on what I had seen on Uncommon Descent but it turns out that was only a summary of a longer post that appeared on Evolution News & Views (sic): More on Junk DNA and the "Onion Test".

The longer post doesn't add very much to the argument but it does have something interesting at the bottom. Here's what Rob Sheldon says about the Onion Test and junk DNA.

A physicist tries to understand junk DNA

Rob Sheldon has a PhD. in physics and a M.A. in religion.¹ With two strikes against him already, he attempts to understand biology by discussing evolution, junk DNA, and the Onion Test [Physicist suggests: “Onion test” for junk DNA is challenge to Darwinism, not ID]. As you might imagine, posting on Uncommon Descent in support of Intelligent Design Creationism leads directly to strike three.

The Onion Test was created by Ryan Gregory in 2007 [The Onion Test] and published in the scientific literature by Palazzo and Gregory in 2014. It goes like this. Take your favorite hypothesis suggesting that most of the DNA in the human genome is functional and use it to explain why the onion, Allium cepa, needs a genome that is five times larger than the human genome. Then explain why closely related species of onion need twenty times more DNA than humans.

Learn to think like a scientist

There's a course at MIT (Boston, USA) called "7.00x Introduction to Biology - The Secret of Life." It's a very popular MOOC (online course). Here's the trailer for the course. In it, Eric Lander tells you that if you take his introductory biology course you will learn to think like a scientist and you will be able to understand the latest breakthroughs.


Here are the week one lectures that focus on biochemistry. I don't have time to go through it all but check out the description of ATP beginning at 2:13. This is not how good teachers explain the importance of ATP in the 21st century but it is how it was taught 40 years ago.

Don't misuse the word "homology"

Here's the latest science news from The Allium": Evolutionist Loses It As Colleague Conflates Homology and Similarity Yet Again.

Evolutionary biologist Dr. Constance Noring shot and killed her microbiology colleague and formerly good friend, Dr. Dan Deline when, for the umpteenth time he used the word homology when he really should have said similarity.

Read the rest. I sympathize with Professor Noring. This could have been me if Canadians were allowed to buy handguns.

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Genomes and junk DNA

Here's your chance to hear about genomes and junk DNA from one of the world's leading experts. The seminar is at the University of Toronto (Toronto, Canada) in the Medical Sciences Building. It's on Wednesday, March 4th—only two days form today! The seminar room (Rm 2172) is right around the corner from Tim Hortons. Ryan is from the University of Guelph. How Canadian can you get, eh?

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Nessa Carey and New Scientist don't understand the junk DNA debate

There's a new book on junk DNA due to be published at the end of March. It's called Junk DNA: A Journey through the Dark Matter of the Genome. The author is someone named Nessa Carey. Here's her bio ....

Nessa Carey has a virology PhD from the University of Edinburgh and is a former Senior Lecturer in Molecular Biology at Imperial College, London. She worked in the biotech and pharmaceutical industry for thirteen years and is now International Director for the UK's leading organisation for technology transfer professionals. She lives in Norfolk and is a Visiting Professor at Imperial College.

Pretty impressive.

Here's how she describes her view of the human genome.

Francis Collins rejects junk DNA

Francis Collins is the Director of the National Institutes of Health (NIH) in the USA. He spoke recently at the 33rd Annual J.P. Morgan Healthcare Conference in San Francisco (Jan. 12-15, 2015). His talk was late in the afternoon on Tuesday, January 13, 2015. You can listen to the podcast on the conference website [J.P. Morgan Healthcare Conference].

The important bit is at the 30 minute mark where he comments on a question about junk DNA. This is what Francis Collins said last week ...

I would say, in terms of junk DNA, we don't use that term any more 'cause I think it was pretty much a case of hubris to imagine that we could dispense with any part of the genome as if we knew enough to say it wasn't functional. There will be parts of the genome that are just, you know, random collections of repeats, like Alu's, but most of the genome that we used to think was there for spacer turns out to be doing stuff and most of that stuff is about regulation and that's where the epigenome gets involved, and is teaching us a lot.

What seems like "hubris" to Francis Collins looks a lot like scientific evidence to me. We know enough to say, with a high degree of confidence, that most (~90%) of our genome is junk. And we know a great deal about the data that Collins is probably referring to (ENCODE)—enough to conclude that it is NOT saying what he thinks it says.

It would be bad enough if this were just another confused scientist who doesn't understand the data [see Five Things You Should Know if You Want to Participate in the Junk DNA Debate] but he's not just any scientist. He's a powerful man who talks to politicians all the time and deals with the leaders of large corporations (e.g. the J.P. Morgan Conference). If Francis Collins doesn't understand the fundamentals of genome science then he could mislead a lot of people.

Collins has many colleagues surrounding him at NIH and other agencies in Washington. These scientists also make important decisions about American science. I'm assuming that he reflects their opinion as well. If not, then why aren't they educating Francis Collins?

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Hat Tip: Ryan Gregory

Functional RNAs?

One of the most important problems in biochemistry & molecular biology is the role (if any) of pervasive transcription. We've known for decades that most of the genome is transcribed at some time or other. In the case of organisms with large genomes, this means that tens of thousand of RNA molecules are produced from regions of the genome that are not (yet?) recognized as functional genes.

Do these RNAs have a function?

Most knowledgeable biochemists are aware of the fact that transcription factors and RNA polymerase can bind at many sites in the genome that have nothing to do with transcription of a normal gene. This simply has to be the case based on our knowledge of DNA binding proteins [see The "duon" delusion and why transcription factors MUST bind non-functionally to exon sequences and How RNA Polymerase Binds to DNA].

If you have a genome containing large amounts of junk DNA then it follows, as night follows day, that there will be a great deal of spurious transcription. The RNAs produced by these accidental events will not have a biological function.

Evolutionary biochemistry and the importance of random genetic drift

I urge you to read an important paper that has just been published in PNAS.

Lynch, M., Field, M.C., Goodson, H.V., Malik, H.S., Pereira-Leal, J.B., Roos, D.S., Turkewitz, A.P., and Sazer, S. (2014) Evolutionary cell biology: Two origins, one objective. Proc. Natl. Acad. Sci. (USA) 111:16990–16994. [doi: 10.1073/pnas.1415861111]

Here's the bit on random genetic drift. It will be of interest to readers who have been discussing the importance of drift and natural selection in a previous thread [How to think about evolution].

Do you think Lynch et al. are correct? I do. I think it's important to emphasize the role of random genetic drift and I think it's true that most biochemists and cell biologists are stuck in an adaptationist mode of thinking.

A commonly held but incorrect stance is that essentially all of evolution is a simple consequence of natural selection. Leaving no room for doubt on the process, this narrow view leaves the impression that the only unknowns in evolutionary biology are the identities of the selective agents operating on specific traits. However, population-genetic models make clear that the power of natural selection to promote beneficial mutations and to remove deleterious mutations is strongly influenced by other factors. Most notable among these factors is random genetic drift, which imposes noise in the evolutionary process owing to the finite numbers of individuals and chromosome architecture. Such stochasticity leads to the drift-barrier hypothesis for the evolvable limits to molecular refinement (28, 29), which postulates that the degree to which natural selection can refine any adaptation is defined by the genetic effective population size. One of the most dramatic examples of this principle is the inverse relationship between levels of replication fidelity and the effective population sizes of species across the Tree of Life (30). Reduced effective population sizes also lead to the establishment of weakly harmful embellishments such as introns and mobile element insertions (7). Thus, rather than genome complexity being driven by natural selection, many aspects of the former actually arise as a consequence of inefficient selection.

Indeed, many pathways to greater complexity do not confer a selective fitness advantage at all. For example, due to pervasive duplication of entire genes (7) and their regulatory regions (31) and the promiscuity of many proteins (32), genes commonly acquire multiple modular functions. Subsequent duplication of such genes can then lead to a situation in which each copy loses a complementary subfunction, channeling both down independent evolutionary paths (33). Such dynamics may be responsible for the numerous cases of rewiring of regulatory and metabolic networks noted in the previous section (34, 35). In addition, the effectively neutral acquisition of a protein–protein-binding interaction can facilitate the subsequent accumulation of mutational alterations of interface residues that would be harmful if exposed, thereby rendering what was previously a monomeric structure permanently and irreversibly heteromeric (8, 36–39)¹. Finally, although it has long been assumed that selection virtually always accepts only mutations with immediate positive effects on fitness, it is now known that, in sufficiently large populations, trait modifications involving mutations with individually deleterious effects can become established in large populations when the small subset of maladapted individuals maintained by recurrent mutation acquire complementary secondary mutations that restore or even enhance fitness (40, 41).

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1. Note the brief description of how irreversibly complex structures can evolve. This refutes Michael Behe's main point, which is that irreversibly complex structures can't have arisen by natural processes and must have been designed. We've known this even before Darwin's Black Box was published.

The textbooks are wrong about protein synthesis according to a press release from the University of Utah

A recent paper in Science provides evidence that when protein synthesis is stalled a protein called Rqc2 ("conserved from yeast to man") catalyzes the addition of random amounts of alanine and threonine the the C-terminus of the proteins that's about to be destroyed (Shen et al., 2015).

Here's the editorial summary of the work ...

During the translation of a messenger RNA (mRNA) into protein, ribosomes can sometimes stall. Truncated proteins thus formed can be toxic to the cell and must be destroyed. Shen et al. show that the proteins Ltn1p and Rqc2p, subunits of the ribosome quality control complex, bind to the stalled and partially disassembled ribosome. Ltn1p, a ubiquitin ligase, binds near the nascent polypeptide exit tunnel on the ribosome, well placed to tag the truncated protein for destruction. The Rqc2p protein interacts with the transfer RNA binding sites on the partial ribosome and recruits alanine- and threonine-bearing tRNAs. Rqc2p then catalyzes the addition of these amino acids onto the unfinished protein, in the absence of both the fully assembled ribosome and mRNA. These so-called CAT tails may promote the heat shock response, which helps buffer against malformed proteins

This is mildly interesting. We've known about ubiquitin ligase for decades but this is a different way of tagging proteins for destruction.

We'll have to see if this work stands up to verification but even if it does, it's not going to make it into the textbooks.

Let's see what the University of Utah Press Office has to say ...

Defying Textbook Science, Study Finds New Role for Proteins

Open any introductory biology textbook and one of the first things you’ll learn is that our DNA spells out the instructions for making proteins, tiny machines that do much of the work in our body’s cells. Results from a study published on Jan. 2 in Science defy textbook science, showing for the first time that the building blocks of a protein, called amino acids, can be assembled without blueprints – DNA and an intermediate template called messenger RNA (mRNA). A team of researchers has observed a case in which another protein specifies which amino acids are added.

"This surprising discovery reflects how incomplete our understanding of biology is,” says first author Peter Shen, Ph.D., a postdoctoral fellow in biochemistry at the University of Utah. “Nature is capable of more than we realize." ...

Mathew Cobb, writing on Jerry Coynes blog, explains why this isn't really a big deal [CAT tails weaken the central dogma – why it matters and why it doesn’t]. Let me just add that the synthesis of peptides with defined sequences in the absence of mRNA and ribosomes has been described in most textbooks since the 1980s. The best examples are the peptides involved in pepditogylcan synthesis (cell walls) and peptide antibiotics.

Here's a figure from my book.

What this means is that the statement, "... showing for the first time that the building blocks of a protein, called amino acids, can be assembled without blueprints – DNA and an intermediate template called messenger RNA (mRNA)" is simply not true.

We really, really, need to do something about university press releases.

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Shen, P.S., Park, J., Qin, Y., Li, X., Parsawar, K., Larson, M.H., Cox, J., Cheng, Y., Lambowitz, A.M., Weissman, J.S., Brandman, O., and Frost, A. Rqc2p and 60S ribosomal subunits mediate mRNA-independent elongation of nascent chains. Science 347:75-78. [doi: 10.1126/science.1259724 ]

Thinking critically about the Central Dogma of Molecular Biology

Our department is preparing to review our undergraduate courses and programs. Part of the review will be to examine our fundamental goals and objectives and determine if we are meeting them. In preparation for this exercise, I've been going over some papers that have been sitting around my office.

One of them concerns teaching the Central Dogma of Molecular Biology (Wright et al., 2014). It was just published last year. The authors have discovered that students have a "weak conceptual understanding" of information flow. Here's how they describe it in the abstract.

The central dogma of molecular biology, a model that has remained intact for decades, describes the transfer of genetic information from DNA to protein though an RNA intermediate. While recent work has illustrated many exceptions to the central dogma, it is still a common model used to describe and study the relationship between genes and protein products. We investigated understanding of central dogma concepts and found that students are not primed to think about information when presented with the canonical figure of the central dogma. We also uncovered conceptual errors in student interpretation of the meaning of the transcription arrow in the central dogma representation; 36% of students (n = 128; all undergraduate levels) described transcription as a chemical conversion of DNA into RNA or suggested that RNA existed before the process of transcription began. Interviews confirm that students with weak conceptual understanding of information flow find inappropriate meaning in the canonical representation of central dogma. Therefore, we suggest that use of this representation during instruction can be counterproductive unless educators are explicit about the underlying meaning.

Simulated meteorite impact produces RNA bases. So what?

A group of Czech scientists have fired a big laser at a solution of formamide and found traces of adenine, guanine, cytosine, and uracil (Ferus et al., 2014). The result is reported in an article in Science [From hell on Earth, life's building blocks]. The image is from the Science article.

Here's the abstract of the PNAS article ...

The coincidence of the Late Heavy Bombardment (LHB) period and the emergence of terrestrial life about 4 billion years ago suggest that extraterrestrial impacts could contribute to the synthesis of the building blocks of the first life-giving molecules. We simulated the high-energy synthesis of nucleobases from formamide during the impact of an extraterrestrial body. A high-power laser has been used to induce the dielectric breakdown of the plasma produced by the impact. The results demonstrate that the initial dissociation of the formamide molecule could produce a large amount of highly reactive CN and NH radicals, which could further react with formamide to produce adenine, guanine, cytosine, and uracil. Based on GC-MS, high-resolution FTIR spectroscopic results, as well as theoretical calculations, we present a comprehensive mechanistic model, which accounts for all steps taking place in the studied impact chemistry. Our findings thus demonstrate that extraterrestrial impacts, which were one order of magnitude more abundant during the LHB period than before and after, could not only destroy the existing ancient life forms, but could also contribute to the creation of biogenic molecules.

In case you don't appreciate the significance of this research, PNAS provides you with a brief summary ...

This paper addresses one of the central problems of the origin of life research, i.e., the scenario suggesting extraterrestrial impact as the source of biogenic molecules. Likewise, the results might be relevant in the search of biogenic molecules in the universe. The work is therefore highly actual and interdisciplinary. It could be interesting for a very broad readership, from physical and organic chemists to synthetic biologists and specialists in astrobiology.

The problem with all these studies is that they don't answer the most important question; what happens next?

Let's assume that the four bases were created in the atmosphere as meteorites crashed into Earth four billion years ago. Let's assume there was water in the form of early oceans or big lakes. Then what happens? Do these researchers imagine that the concentrations of these bases built up gradually over thousands of years until there were spontaneous reactions with five-carbon sugars and phosphate to form nucleotides? Then did these nucleotides assemble into short RNA molecules?

It's a very large step from demonstrating that RNA bases can be made from formamide under extreme conditions to showing that their concentrations could have been high enough to make RNA spontaneously.

We need to demand more of these researchers. If they are going to postulate that life arose in a primordial soup then it's no longer sufficient to publish one more paper on how you can make organic molecules from inorganic precursors. Enough already. That's the easy part of the hypothesis. Let's see some evidence for the hard part.

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Ferus, M., Nesvorný, D., Šponer, J., Kubelík, P., Michalčíková, R., Shestivská, V., Šponer, J.E., and Civiš, S. (2014) "High-energy chemistry of formamide: A unified mechanism of nucleobase formation." Proceedings of the National Academy of Sciences Published online before print December 8, 2014. [doi: 10.1073/pnas.1412072111]

How do we teach our students that basic research is important?

There's a fabulous editorial in the Toronto Star today. It's critical of the Prime Minister and the Conservative Party of Canada for the damage they are doing to science in Canada [Canada needs a brighter federal science policy: Editorial].

Here's some excerpts ...

Finding a fan of Canada’s current science policy among those who care about such things would be a discovery worthy of Banting and Best. Few if any would contend that Ottawa’s approach is sound; rather, the debate in 2014 has been over what in the world would possess a government to pursue such a catastrophic course.

According to one school of thought, the answer is simple: the Conservatives are cavemen set on dragging Canada into a dark age in which ideology reigns unencumbered by evidence. Let’s call this the Caveman Theory.

The other, more moderate view holds that Prime Minister Stephen Harper et al are not anti-science – that they at least understand the importance of research and development to their "jobs and growth" agenda – but are instead merely confused about how the enterprise works and about the role government must play to help it flourish. Let’s call this the Incompetence Theory.

The rest of the editorial describes how Stephen Harper and his Conservative buddies have directed funding agencies to concentrate on research that will be of direct benefit to Canadian for-profit companies.

It concludes with ...

Whatever the government’s motives, whatever it understands or does not about how science works, it has over the last eight years devastated Canadian research in a way that will be hard to reverse. Private sector R&D continues to lag, but in our efforts to solve that problem we have seriously reduced our capacity for primary research, squandering a long-held Canadian advantage. Meanwhile, we have earned an international reputation for muzzling scientists, for defunding research that is politically inconvenient and for perversely conflating scientific goals with business ones, thus dooming both. Our current funding system is less well placed than it was in 2006 to promote innovation and our science culture has been so eroded that we are unlikely to attract the top talent we need to compete in the knowledge economy.

Whether it was anti-intellectualism, incompetence or both that led us to this dark place, let this coming election year bring the beginning of a climb back into the light.

How can the government of Canada be so ignorant? It's because they have a huge amount of support from the general public who see all research as technology. They are only willing to support research that helps the economy.

Most people are not interested in research that simply advances our knowledge of the natural world.

What are we doing as educators to reverse this trend? Not very much, as it turns out. Many of our courses in biochemistry focus on how biochemistry can benefit medicine as though this was the only reason for learning about biochemistry.¹ Our department is discussing whether we should have undergraduate courses on drug discovery and how drugs are brought to market. We are considering a co-op program where students will spend some time working in the private sector. We are toying with the idea of creating an entirely new program that will train students to work in the pharmaceutical industry.

It's no wonder that the general public thinks of science as the servant of industry. We are not doing a very good job of teaching undergraduates about the importance of knowledge and the value of scientific thinking. In fact, we are doing the opposite. We are supporting the Stephen Harper agenda.

Don't be surprised if it comes back to bite you in the future.

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1. We teach medical case studies in our introductory biochemistry course for science undergraduates!

Questions about alternative splicing

Alternative splicing is a mechanism where am intron-containing gene is transcribed and the primary transcript is spliced in two or more different ways to produce different functional RNAs. If it's a protein-coding gene then the idea is that different forms of the protein are produced in this way and each of them is functional.

It's important to emphasize that the products of alternative splicing must be functional because we know that splicing is error-prone and that mispliced, nonfunctional, RNAs will be quite common. Every gene will produce a bunch of these aberrantly spliced variants but that doesn't mean that every primary transcript is alternatively spliced.

It's important to distinguish between real functional alternative splicing and junk RNAs that arise from splicing errors. One of the ways to do this is to report on the concentrations of the various transcripts but that's rarely done in papers that promote alternative splicing [see: The most important rule for publishing a paper on alternative splicing].

The importance of alternative splicing is related to the debate over the importance of pervasive transcription and junk DNA since advocates of alternative splicing are often the same people who object to junk DNA [see: Vertebrate Complexity Is Explained by the Evolution of Long-Range Interactions that Regulate Transcription?]. I call this The Deflated Ego Problem because these scientists are usually looking for way to "explain" the complexity of humans in light of the fact that we seem to have the same number of genes as many other species.

If it's true that most human genes are alternatively spliced then let's see the evidence. That means actually demonstrating that different proteins with different functions are produced from the same gene. We've known for 35 years that this is possible but that's not the point. The point is whether all, or most, human RNAs are alternatively spliced. I've issued a simple challenge to those who use the alternative splice databases [A Challenge to Fans of Alternative Splicing]. So far, nobody has stepped up to the plate.

Some of the examples that are promoted in those databases make no sense whatsoever [Two Examples of "Alternative Splicing"] [The Frequency of Alternative Splicing ].

Someone raised this issue in the comments to another post and send me a link to a paper published in 2010. Here's the paper and part of the introduction.

Keren, H., Lev-Maor, G. and Ast, G. (2010) Alternative splicing and evolution: diversification, exon definition and function. Nature Reviews Genetics 11:345-355 [doi: 10.1038/nrg2776].

Splicing of precursor mRNA (pre-mRNA) is a crucial regulatory stage in the pathway of gene expression: introns are removed and exons are ligated to form mRNA. The inclusion of different exons in mRNA — alternative splicing (AS) — results in the generation of different isoforms from a single gene and is the basis for the discrepancy between the estimated 24,000 protein-coding genes in the human genome and the 100,000 different proteins that are postulated to be synthesized.

... Comparing species to see what has changed and what is conserved is proving valuable in addressing these issues and has recently yielded substantial progress. For example, new high-throughput sequencing technology has revealed that >90% of human genes undergo AS — a much higher percentage than anticipated. Such technological progress is providing more comprehensive studies of splicing and genomic architecture in an increasing number of species, and these studies have extended our evolutionary understanding.

I'd like you to answer two questions.

1. Do you believe that there are about four (4) different, functional, proteins produced on average from every human protein-encoding gene?
2. Do you believe that more than 90% of human genes produce a transcript that can be alternatively spliced, where alternative splicing is restricted to producing different functional RNAs and not just noise?

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Ann Gauger moves the goalposts

We've been discussing Ann Gauger's claim that evolution is impossible because she was unable to transform a modern enzyme into another related one by changing a small number of amino acids.

I pointed out that this is not how evolution works. In some cases, you can easily show that two enzymes with different specificities can evolve from a common ancestor that could carry out both reactions. Such enzymes are said to be "promiscuous."

Here's Ann Gauger's latest post: In Explaining Proteins (and Life), Here's What Matters Most. She says ...

So now, let's address enzyme evolution and the divergence of enzymes to produce related families and superfamilies. Larry Moran says that modern enzymes evolved by specializing from a promiscuous ancestor. As evidence, he says modern enzymes can sometimes catalyze reactions with several substrates (the chemicals they bind to and change), and that it is possible to shift these enzymes to favor one substrate over another. He gives several examples or provides links to them.

Here's another place where he and I agree. Promiscuous enzymes can be shifted with just a few mutations to a new reaction specificity, provided the capacity for the reaction already exists in the starting enzyme, and each step is small and selectable. They can evolve easily, because they can already carry out the reaction in question. Larry Moran's description of the process is actually quite good, despite the digs he takes at us.

It strikes me that Larry Moran would know we agree with him on these points if he had read our papers.

So, what's the problem?

Turns out that changing one related enzyme into another with a different specificity wasn't the goal of her experiment. Here's what she was really trying to do ...

The Big Problem

Here's the big problem -- the arrival of novelty.

Novelty or innovation means the appearance of something not already present. It's the opposite of promiscuity. So a way to create novelty is absolutely essential to explain modern cells, as I will demonstrate.

...

Here's the heart of the matter. Promiscuity cannot solve the problem of novelty. Mutation, natural selection, and drift cannot drive the creation of novelty of all those new protein folds. That's what Doug Axe and I have been testing all along, from Doug Axe's 2004 paper to this most recent one. Based on our experiments, the problem of how innovation originates remains unsolved.

Now I get it (not). What she and Doug Axe were really trying to do was to intelligently design an entirely new enzyme.

They failed.

Therefore Intelligent Design Creationism is falsified.

That seems logical to me.

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On the meaning of pH optima for enzyme activity

The students in my lab course measured the activity of trypsin at different pH's. They discovered that the enzyme was most active at a pH of about 8.0-8.5 and that activity fell off rapidly at pH values above and below this optimum. This is consistent with results in the published literature (see figure from Sipsos and Merkel, 1970). Here's the exam question ...

What was the pH optimum of trypsin activity? Can you explain this in terms of the normal biological function of the enzyme and the physiological conditions under which it is active? Do you expect there to be a strong correlation between the optimal pH of an enzyme’s activity and the pH of the cell/environment where it is active?

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Sipos, T., and Merkel, J. R. (1970) An effect of calcium ions on the activity, heat stability, and structure of trypsin. Biochemistry, 9:2766-2775 [doi: 10.1021/bi00816a003]

On the specificity of enzymes

Most biochemistry students are taught that enzymes are highly specific. It's certainly true that the stereospecificity of some enzymes is extraordinary but is it true in general? Here's one of the exam questions that the students in my course had to answer ....

All three of the enzymes (trypsin, alcohol oxidase, β-galactosidase) that you assayed in the past three months are active with several different substrates substrates. Is this behaviour typical or are most enzymes highly specific? Aminoacyl tRNA synthetases are the classic examples of enzymes that are highly specific. Why? Do aminoacyl-tRNA synthetases ever make mistakes?

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Using mass spec to find out how many protein-encoding genes we have

One of the other exam questions is based on an experiment students did with an enzyme they purified. They digested the enzyme with trypsin and then analyzed the peptides by mass spectrometry. They were able to match the peptides to the sequence databases to identify the protein and the species. The exam question is ...

Nobody knows for sure how many functional protein-encoding genes there are in the human genome. About 20,000 potential protein-encoding genes have been identified based on open reading frames and sequence conservation but it is not known if all of them are actually expressed. How can you use Mass Spec to find out how many functional protein encoding genes we have? [see the cover of Nature from May 29, 2014: click on about the cover]

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King Dick and PCR

The students in my lab course are writing their final exam. Prior to the exam they were given 22 questions and they knew that five of them would be on the exam. I thought that Sandwalk readers might enjoy coming up with answers to some of the questions.

The possible remains of King Richard III of England have recently been discovered. His identity has been confirmed by DNA PCR analysis. Descendants of his mother in the female line have the same mitochondrial DNA as King Richard. However, the results with the Y chromosome were surprising. None of the descendants in the all-male lineage had the same Y chromosome markers as King Richard. This is almost certainly due to something called a "false-paternity" event. (There are other ways of describing this event.) Given what you know about PCR, what are some possible sources of error in this analysis? Would you be prepared to go back in time and accuse one of the Kings of England of being a bastard? [Identification of the remains of King Richard III]

(The lab experiment was to analyze various foods to see if they were made from genetically modified plants.)

Note: It's extremely unlikely that the "false-paternity" event occurred in the lineage leading directly to any of the Kings and Queens of England.

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