Friday, January 13, 2012

Structure and Function of the DNA Molecule

fragment of DNA double helix spinning, backbone of sugar and phosphate in green and red, nitrogenated bases in blue
Figure 1. DNA double helix.
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You probably know that the DNA is the physical support of our genetic information and that it is a double helix molecule. Lets see its structure and function with a little bit more of detail.
In figure 1 you can see a fragment of DNA. The backbone is in the outside in green (deoxyribose sugar) and red (phosphate) whereas the nitrogenated bases are in the inside in blue. There are four different nitrogenated basis and they always pair up the same way due to their chemical structure: adenine with thymine (figure 2) and guanine with cytosine (figure 3). If we cut a section of the DNA, one nitrogenated base plus one deoxyribose plus one phosphate is called a nucleotide. And a nucleotide and the complementary one in the other strand is called a base pair.
adenine and thymine pairing by hydrogen bonds
Figure 2. Adenine - Thymine.

The pairing of the nitrogenated basis is what holds together the two strands of the DNA double helix. And it is very important for the replication of the DNA: since the pairing of nitrogenated basis is always A-T and C-G, you can separate the two strands of DNA and build a complementary strand for each as there is only one complementary nitrogenated base for each position.
guanine and cytosine pairing by hydrogen bonds
Figure 3. Guanine - Cytosine.

The DNA is a very long molecule and it is highly packed in the nucleus of the cells. In figure 4 we can see how the double helix coils around the histones (the gray balls in the diagram) and then this filament coils and forms the chromatin, that can coil again and again like a rope until it takes the form of a chromosome. However the degree of packing of the DNA varies as the regions that are actively being read or replicated need to be uncoiled.

Functions of the DNA: replication and expression of the genetic information.

  • Replication.
When a cell divides during the growth of the organism, it must pass the same genetic information on to the two daughter cells. As we mentioned above, thanks to the double helix structure of the DNA and the fact that the two strands are complementary, this can be easily achieved. The two strands separate and the enzyme DNA polymerase starts building a complementary strand for each of the parent strands. This way, using each strand as a template to build its complementary the DNA is replicated with a very low error rate (the errors in the replication that are not corrected will cause mutations), and each daughter cell will receive a copy of the genome.

  • Expression.
Thanks to replication and cell division each new cell has a complete set of genetic information, now the important matter is what it does with this information. To grow, develop and carry on with its functions the cell has to read the genes in the DNA and produce proteins and enzymes that are important for the cell's structure and metabolism. This is done in two steps. First the information of the DNA is copied to a fragment of  RNA (a molecule very similar to DNA although it is usually single stranded) by the RNA polimerase, this is called transcription. Then the RNA goes fron the nucleus to the citoplasm and there occurs the second step, the translation of the genetic information by the ribosomes to a sequence of aminoacids that forms a protein. Each gene encodes for a different protein and they are expresed at a different rates and at different times during the life of the cell according to what it is needeed at every moment. In a next post we will explain the expresion and its regulation with more detail.

diagram of DNA packing: double helix, histones, chromatin, chromosome
Figute 4. Packing of the DNA into a chromosome.


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  2. I appreciate all of the information that you have shared. Thank you for the hard work!
    Klenow (3′→ 5′ exo-) is a mesophilic dna polymerase deficient in both proofreading (3′→ 5′) and nick-translation (5′→ 3′) nuclease activities, and that displays a moderate strand displacement activity during DNA synthesis.