Sunday, March 30, 2008

DNA genetic material

We All Share the Same Building Blocks

DNA is a winning formula for packaging genetic material. Therefore almost all organisms – bacteria, plants, yeast and animals – carry genetic information encapsulated as DNA. One exception is some viruses that use RNA instead.

Different species need different amounts of DNA. Therefore the copying of the DNA that precedes cell division differs between organisms. For example, the DNA in E. coli bacteria is made up of 4 million base pairs and the whole genome is thus one millimeter long. The single-cell bacterium can copy its genome and divide into two cells once every 20 minutes.

The DNA of humans, on the other hand, is composed of approximately 3 billion base pairs, making up a total of almost a meter-long stretch of DNA in every cell in our bodies.

In order to fit, the DNA must be packaged in a very compact form. In E. coli the single circular DNA molecule is curled up in a condensed fashion, whereas the human DNA is packaged in 23 distinct chromosome pairs. Here the genetic material is tightly rolled up on structures called histones.

A New Biological Era

This knowledge of how genetic material is stored and copied has given rise to a new way of looking at and manipulating biological processes, called molecular biology. With the help of so-called restriction enzymes, molecules that cut the DNA at particular stretches, pieces of DNA can be cut out or inserted at different places.

In basic science, where you want to understand the role of all the different genes in humans and animals, new techniques have been developed. For one thing, it is now possible to make mice that are genetically modified and lack particular genes. By studying these animals scientists try to figure out what that gene may be used for in normal mice. This is called the knockout technique, since stretches of DNA have been taken away, or knocked out.

Scientists have also been able to insert new bits of DNA into cells that lack particular pieces of genes or whole genes. With this new DNA, the cell becomes capable of producing gene products it could not make before. The hope is that, in the future, diseases that arise due to the lack of a particular protein could be treated by this kind of gene therapy.

Was Franklin Nominated?

Rosalind Franklin.
Photo: Cold Spring Harbor Laboratory Archives

Many voices have argued that the Nobel Prize should also have been awarded to Rosalind Franklin, since her experimental data provided a very important piece of evidence leading to the solving of the DNA structure. In a recent interview in the magazine Scientific American, Watson himself suggested that it might have been a good idea to give Wilkins and Franklin the Nobel Prize in Chemistry, and him and Crick the Nobel Prize in Physiology or Medicine – in that way all four would have been honored.

Rosalind Franklin died in 1958. As a rule only living persons can be nominated for the the Nobel Prize, so the 1962 Prize was out of the question. But she may have been a nominee while she was still alive. The Nobel archives, that among other things contain the nominations connected to the prizes, are held closed. But 50 years after a particular prize had been awarded, the archives concerning the nominees are released. Therefore, in 2008 it will be possible to see whether Rosalind Franklin was ever a nominee for the Nobel Prize concerning the DNA helix.

The DNA-Helix

The sugar-phosphate backbone is on the outside and the four different bases are on the inside of the DNA molecule.

The two strands of the double helix are anti-parallel, which means that they run in opposite directions.

The sugar-phosphate backbone is on the outside of the helix, and the bases are on the inside. The backbone can be thought of as the sides of a ladder, whereas the bases in the middle form the rungs of the ladder.

Each rung is composed of two base pairs. Either an adenine-thymine pair that form a two-hydrogen bond together, or a cytosine-guanine pair that form a three-hydrogen bond. The base pairing is thus restricted.

This restriction is essential when the DNA is being copied: the DNA-helix is first "unzipped" in two long stretches of sugar-phosphate backbone with a line of free bases sticking up from it, like the teeth of a comb. Each half will then be the template for a new, complementary strand. Biological machines inside the cell put the corresponding free bases onto the split molecule and also "proof-read" the result to find and correct any mistakes. After the doubling, this gives rise to two exact copies of the original DNA molecule.

The coding regions in the DNA strand, the genes, make up only a fraction of the total amount of DNA. The stretches that flank the coding regions are called introns, and consist of non-coding DNA. Introns were looked upon as junk in the early days. Today, biologists and geneticists believe that this non-coding DNA may be essential in order to expose the coding regions and to regulate how the genes are expressed.

No comments: