On Geometry And Genomes

Biology concepts – linear chromosomes, circular chromosomes, taxonomy, replication, telomere

Everyone (teenagers excepted) knows that getting organized helps you to learn and work. When you group tasks, items, or facts, it helps in remembering or working with them. In biology, grouping organisms has a history as old as language.

In the older grouping systems, the name of an organism was a phrase that described some characteristic of the organism. When a new relative was identified, the name phrase had to be lengthened to separate this new organism from those similar to it. As you can imagine, the names got very long very fast.

In the 1750’s, Carolus Linnaeus developed a much easier system of naming. In his “trivial system,” each organism had two descriptors in its name; a binary naming system. Linnaeus’ system (and others) of taxonomy (taxis is Greek for “arrangement”) is based on shared characteristics.

At first, it was the characteristics people could see that were used to group organisms. Then it was the characteristics on the macroscopic and the microscopic levels. Now it is based on molecular characteristics, forming both a taxonomic classification and an evolutionary tree; this is now called the science of phylogenetics.

Molecular characteristics usually mean DNA. Differences in DNA sequence and in the number of mutations that have occurred provide a relationship between organisms. Using these factors, a time line for their divergence can be estimated. We changed the ways we determine similarity, and that changed the rules. With new rules come new exceptions.

Many of the DNA rules start with chromosomes (chromo = color and soma = body, this comes from the dark and light banding pattern of stained DNA). Cellular DNA is very long and very thin, perhaps only 12-22 nanometers wide (about 1/5000 the width of a human hair). In this form, it can only be seen with an electron microscope.

In eukaryotes, this DNA becomes complexed with many proteins during cell division so that all the DNA can be packed up and moved more easily to the daughter cells.  Called chromosomal packaging, the DNA is wound around proteins called histones, then folded many times over, so that the finished chromosome is packed 10,000 times more compact than the original DNA helix. This is the packed DNA that we see as dark and light bands and gives it its name.

By definition, a chromosome is a piece of DNA that contains genes that are essential for the survival and function of the organism. This implies that there may be other pieces of DNA that contain genes that are not necessary for survival.

The molecular rules of biology state that prokaryotes have one chromosome, a single piece of double stranded DNA that contains all the genes that the prokaryote (archaea or bacteria) needs. This is efficient for the organism; it is one stop shopping for replication of all its instructions and only two chromosomes (after replication) need to be segregated to the two daughter cells that are being made.

And here begins our exceptions. There are several prokaryotic organisms that have more than one chromosome. That is to say, their essential genes are located on more than one piece of DNA.

The first identified example of multiple chromosomes in a prokaryote was Rhodobacter sphaeroides, a photosynthetic species of true bacteria that can also break down carbohydrates it takes up. This bacterium was found to have two chromosomes, although one was more than three times the size of the other.

Genes encoding essential products for making proteins and carrying out day-to-day functions are located on each of the two R. sphaeroides circular chromosomes. There are other genes that exist on both of the chromosomes, but appear to be turned on and off via different signals. This implies that the same gene may serve its function at different times in the organism's life, or under different environmental conditions.

R. sphaeroides is by no means the only prokaryote that possesses multiple chromosomes. More than a dozen different groups of bacteria have at least some members with more than one chromosome. This includes Vibrio cholerae, the causative organism of the disease cholera. V. cholerae is responsible for a diarrheal infection that affects more than 3-5 million people per year and causes 130,000 deaths each year.

In addition to these organisms there is Agrobacterium tumefaciens, whose name was recently changed to Rhizobium radiobacter. This is a very interesting two chromosome bacterium. It usually is a pathogen of plants, forming galls (tumors) on several cash crops, such as nut trees and grape vines. This is an important tool in the molecular biologist’s toolbox, since it has been found that R. radiobacter easily transfers DNA between itself and the plants it infects, via lateral gene transfer (a subject we have discussed in depth, When Amazing Isn’t Enough and Evolution of Cooperation). But R. radiobacter goes further, it can also cause disease in humans who have poorly functioning immune systems. For folks battling cancers, HIV, or other diseases that wreak havoc with their ability to fight off infections, R. radiobacter can cause bacteremia (bacteria colonizing the blood) or endopthalmitis (infection of the two hollow cavities of the eye).

The second molecular rule of biology is that prokaryotic chromosomes take the shape of a circle; the DNA forms a single loop. This shape is helpful in terms of replicating the prokaryotic chromosome prior to cell division. Start anywhere, and you can keep going to replicate the entire thing.  In point of fact, they don’t start just anywhere, but one start point (called an origin of replication) leads to complete replication.

There are advantages to having a circular chromosome. Prokaryotic chromosomes do not complex with proteins to become more densely packed, so it remains as a thin, long molecule. This means that fewer proteins are needed to maintain a circular, prokaryotic chromosome. In addition, since replication requires the doubling of just one piece of DNA from one origin of replication, this takes less time and fewer proteins to accomplish. Together, these features of a circular chromosome result in a more efficient and simpler process, with fewer chances for mistakes to be made.

However, there are exceptions in which prokaryotes have linear chromosomes. The Borrelia burgdorferi bacterium has a single chromosome, but it has the geometry of eukaryotic chromosomes, a line segment with two ends. This was the first prokaryote found to have a linear genome, way back in 1989. This lyme disease pathogen has one major linear chromosome and other pieces of smaller DNA that are circular or linear (which we will discuss in the next post); you just can’t trust a pathogen to follow the rules. Other prokaryotes that have linear chromosomes include our friend R. radiobacter. Even more interesting, while this pathogen has two chromosomes; one is circular and one is linear. How does that happen?

The previous discussions do not mean that all prokaryotes with multiple chromosomes or linear chromosomes are disease-causing agents, just the interesting ones. Since they cause pathology in animals or crops, they hit us in the wallet. It makes sense that we have studied them in more detail and have discovered their hidden exceptions. There are probably thousands of innocuous prokaryotes that have more than one chromosome or have linear chromosomes, we just don’t have a reason to look at them in that much detail.

There may be more than one way that prokaryotes end up with linear chromosomes. In some cases, the linear chromosomes still have bacterial origins of replication, indicating that they may have evolved from circular chromosomes. There is also evidence that some linear chromosomes might have developed from other linear DNAs in the cell, something we will talk about next time.

The rules of defining prokaryotes and eukaryotes also state that eukaryotes have linear chromosomes. The essential genes are stored on more than one piece of DNA, and these pieces have two ends apiece, like a line segment in geometry.

Linear chromosomes are a disadvantage because it is hard to replicate the ends. Because of the way that DNA replicates, the ends of the chromosomes, called telomeres, end up being shortened every time the DNA is replicated. Over time, this leads to shorter chromosomes that might lose DNA sequences that the cell needs in order to function.

Some lines of evidence suggest that telomere shortening is a direct cause of ageing. The loss of important sequences at the ends of chromosomes cases cells to perform at less than optimal levels, and mistakes and toxic products then build up and lead to larger dysfunctions of cells, organs, and systems, ie. getting old.

On the other hand, linear chromosomes may promote genetic diversity. In eukaryotes, the division of the cell requires each chromosome to be replicated, then the matching chromosomes of a pair (one from mom and one from dad) line up together. This is a prime opportunity for the chromosome to exchange some sequences in a process called homologous recombination; a mixing of genes beyond just getting one from each parent.

However, a study published in 2010 indicates that the geometry of the chromosome doesn’t matter when it comes to recombination rates. Scientists took a circular chromosome organism and linearized its genome (they cut it so it had ends). They also did the reverse experiment, taking a linear chromosome organism and circularizing its DNA.

In both cases, there was no change in the rate that its DNA recombined and produced slightly different offspring (the two circular chromosomes after replication can swap some pieces). So geometry does not appear to affect genetic diversity – so why did each type evolve? Good question – that can be your Nobel Prize project.

Next week we will continue the discussion of exceptions in DNA structures, including DNA that isn’t part of a chromosome, and mitochondrial and chloroplast genomes that don’t look like they should.