October Update

Bought a standard poodle puppy.  Bringing him home October 5, so October will be full of housebreaking, and FUN.



Entries in DNA (7)


How Can Actual Animals Be So Tiny?

          Bryozoans are animals so tiny you almost can’t see them at all.  They were first discovered In the 1500’s, when microscopes became a popular novelty for people who could afford them. 

         Those early microscope hobbyists gathered samples of fluids and tissues and examined them out of pure curiosity.  In drops of sea water, or on stones or seaweed gathered from sea water, they found infinitesimal, delicate, plant-like organisms.  The early microscopists thought these were plants.  But as the centuries passed, and microscopes and microscope techniques improved, investigators found that these organisms were animals, with nervous systems, muscles, and digestive systems.

         The Bryozoans are similar in size and habitat to corals, but they are quite different.  Corals are more primitive: they have nerve nets instead of nervous systems, and contain only a simple digestive cavity with a single opening to take in food and give off waste.

         Now researcher Judith Fuchs, at the University of Gothenburg, has been able to study a large group of Bryozoans using DNA data.  She has discovered that these animals come from a common ancestor, even though some species live in salt water and some in fresh water.

         To me, the Bryozoans are just mind-boggling: Imagine packing all those animal systems into a creature too small to really see with the naked eye.  Imagine their tremendous variety of shapes, all of which evolved to enhance feeding, reproduction, survival.  Imagine the discovery and clarification of this whole animal group paralleling the development of biological technology, from early through advanced microscopy, from early to advanced genetics.

         And there’s more.  Stay tuned…


Photograph 51: Rosalind Franklin and the DNA Double Helix, Part 3

In my last two posts I introduced the history of Rosalind Franklin’s revelation of the molecular structure of DNA.

         Once Franklin saw that her colleague and nemesis Maurice Wilkins had two different forms of DNA mixed together in his sample, she figured out a way to produce pure forms of both DNA crystal types, the A form (dehydrated) and the B form (hydrated).  Then she set about getting X-ray photos of each crystal type.

         After less than two years, by the beginning of 1953, Franklin not only had produced superb X-ray photos of both forms, she had figured out roughly the helical structure of DNA, and had become fairly certain the molecule was not single or triple, but double. 

         She knew that the phosphate-sugar backbones of the two DNA strands were on the outside of the molecule, and that the 4 bases, adenine (A), thymine (T), guanine (G), and cytosine (C), were tucked inside.  She knew the dimensions of both forms.  And she was just beginning to suspect how the molecule might work to allow inheritance of genes.

         It was at this point that Wilkins handed over a copy of Franklin’s Photograph 51, a superb X-ray photo of the B form, to James Watson and Francis Crick.  The latter two also got their hands on a report Franklin had written about the crystal form and dimensions. 

         Combining the photo and the report, Crick and Watson built a model of the DNA double helix, with single chains running in opposite directions.  But the model didn’t fit together well. 

         Jerry Donohue, another fellow scientist, pointed out to Watson that he was modeling with the wrong forms of the bases.  Once Watson corrected that problem, he found that his DNA model fit together perfectly to match Rosalind Franklin’s photograph.

         Although the women’s movement later deplored Franklin’s not being credited at the time, Franklin seems not to have cared.  She went on to do excellent and important work with tobacco mosaic and polio viruses at Birkbeck College, London.

         The Crick/Watson model was not altogether correct and was not accepted immediately.  Franklin, alas, died prematurely at 37 of ovarian cancer, so the Nobel Prize she should have shared was awarded some time after her death.

         But what a wonderful scientist and person she was.  I highly recommend the Brenda Maddox book pictured above.


Photograph 51: Rosalind Franklin and the DNA Double Helix, Part 2

My last post introduces Rosalind Franklin.  She’s the unsung heroine whose name should go right along with “Watson & Crick.”  We should be saying “Franklin, Watson, and Crick.”  But alas, Franklin’s contribution to the discovery of DNA”s structure came at a time when women weren’t even considered in any professional work, even if their accomplishments were outstanding. 

         Also, Franklin died young, and Nobel Prizes are never awarded posthumously, so Crick and Watson and, of all people, Franklin’s nemesis Maurice Wilkins, received the Nobel for DNA.  (Barbara McClintock suffered from this same lack of considerations, but fortunately, she outlived it, and received the Nobel Prize at the age of 81.)

         Franklin earned her doctorate in chemistry from Cambridge University during WWII, went on to make important discoveries about carbon, then learned X-ray crystallography in Paris.

         X-ray crystallography is a technique for figuring out the structure of a molecule.  The substance in question must first be crystallized.  Then a miniscule sample of the crystallized material is mounted on a pin, and an X-ray beam is aimed at the crystal; behind the crystal is a piece of photographic film.  Most of the X-rays pass right through the empty space within the crystal and expose the film.  But when an X-ray hits the nucleus of one of the atoms in the crystal, the ray gets deflected.  The pattern of deflections shows dark on the photographic film after it is developed.  This pattern gives strong clues about the three-dimensional positions of the atoms in the molecule.

         After her work in Paris, Franklin went to King’s College in London to figure out the structure of the DNA molecule.  At King’s, Wilkins had been unable to get a clear X-ray photograph of DNA. 

         The first thing Franklin did at King’s was to figure out that Wilkins’ DNA crystals weren’t pure.  There were actually two crystalline forms of DNA, and Wilkins had been trying to X-ray a mixture of these, so of course he had been unsuccessful at finding the DNA structure.

         The plot thickens.  More about this in my next post; stay tuned.



Photograph 51: Rosalind Franklin and the DNA Double Helix, Part 1

Rosalind Franklin was the superb X-ray crystallographer who figured out the molecular structure of the DNA double helix.  Her accomplishment was almost completely hidden in 1953 when Francis Crick and James Watson announced that they had solved the structure of DNA.

         Rosalind Franklin died young, of ovarian cancer, possibly caused by years of exposure to the X-rays she used in her work.  But since her death, the women’s movement, and the increasing number of women in science, have brought her story to public attention.  Books, such as Rosalind Franklin: The Dark Lady of DNA, by Brenda Maddox, and articles about Franklin tell a far different story from the one in James Watson’s The Double Helix.

         Now Anna Ziegler has written a play about Rosalind Franklin, called Photograph 51 after Franklin’s famous X-ray diffraction photo that led directly to the discovery of the double helix.  I’ll have more to say about Rosalind Franklin in my next post.  Meanwhile, here is some film about the new play.


RNA Miracles: First Genetic Molecule! Who Knew?  

Every new biological discovery opens the door to a world of undreamed of miracles.

         Back when Rosalind Franklin elucidated the structure of DNA and Francis Crick and James Watson figured out how DNA genes might work, no one really knew much about RNA.  Gradually researchers discovered that a molecule christened “messenger RNA” (mRNA) transcribed genes from DNA.  Two other types of RNA then translated the mRNA into proteins. 

         And what was the purpose of these proteins?  In plants, animals, fungi, and various single-celled organisms, some proteins form rigid or moving structures, some provide storage or transport, some fight off infection, and some proteins catalyze the myriad chemical reactions that keep cells alive and doing their jobs.

         So genes turned out to build organisms and run them by this method: DNA makes RNA, and RNA makes protein.

         But this was just the tip of the iceberg.

         For instance, when DNA makes RNA, and RNA makes protein, each step of this process is engineered by proteins (enzymes).  Yet each protein was translated from genes (DNA).  And the genes got transcribed and translated by RNA.  So biologists began thinking that these molecules and this process couldn’t have sprouted from nothing.  Something had to come first.  Was it the DNA?  The RNA?  The protein? 

         Whatever came first had to have two functions.  It had to be able to store the plans for each living organism—genes do this.  And it had to be able to catalyze biochemical reactions in cells—enzymes do this.

         At last, Nobel Prize winners Thomas R Cech and Sidney Altman came upon RNA enzymes that could do both the job of a gene and the job of an enzyme.  Such a molecule was dubbed a “ribozyme.”  Ribozymes may well be the ancestors of both DNA and enzymes made of protein.  After the first ribozymes made it possible for successful cells, or perhaps mere successful molecules, to be duplicated, how might DNA and proteins have come about?

         Even today, there are RNA viruses whose RNA gets “back-copied” into DNA.  Perhaps the first DNA came about in such a way.  And even today many ribozymes join with proteins to modify their catalytic activity.  Perhaps the first protein enzymes came about in such a way.  It’s exciting to imagine such molecular evolutions happening billions of years ago.

         But RNA turns out to have many talents: More on this in my next post.