Eukaryotic Chromosomes

• Eukaryotic organisms are those whose cells possess sub-cellular organelles such as the nucleus, mitochondria and the like. These include all organisms except bacteria. Their genetic material (DNA) is organized in structures known as chromosomes that, in eukaryotes, are linear. This means that, as opposed to the circular chromosomes found in prokaryotes, they have "ends." As we will see this poses problems when duplicating the DNA.
  
  

DNA Structure

• In the majority of living organisms DNA is composed of two long chains of nucleotides, each one complementary to the other, known as a double helix because the two strands twist around a central axis. This complementary structure provides a template for duplication, because each chain contains information about the other one and can serve as a template to build it from loose nucleotides. The portion of the nucleotide that must match the complementary one is called a base, and the fact that the bases of the two strands are complementary is known as base pairing.
  
  

DNA Replication

• In order to divide, all cells must first duplicate their DNA so that the resulting two daughter cells will have a complete set of chromosomes. The majority of this task is performed by an enzyme called DNA polymerase. This enzyme works by matching each nucleotide to its complementary one. However, DNA polymerase cannot duplicate the DNA close to the ends of linear chromosomes. Because of this, each replication of DNA would produce a shorter chromosome with the ends missing, and the corresponding loss of genetic information.
  
  

Telomeres and Telomerase

• Telomeres are short nucleotide sequences repeated in tandem on the extremes of eukaryotic chromosomes like bookends on a shelf. The number of repeats of the sequence is not important, but the sequence itself is conserved within a species. Because they are at the ends of the chromosomes they cannot be fully replicated, but since they do not carry genetic information this is not important. Telomerase is the enzyme responsible for generating the telomeres, and can do so because it carries a complementary template that allows it to place the required nucleotides in the correct sequence.
  
  

Telomeres and Disease

• One of the characteristics of cancer cells is that they divide uncontrollably, and this led to the idea that their telomerase should be active to avoid losing genetic information. It has since been found that this is indeed true of many, but not all, cancer cells. On the other hand, telomeres that are too short are an indication of cell senescence.