Why are genomes unstable? What factors in the environment can cause a DNA strand to be replicated? Well the answer comes from the fact that an enzyme called DNA polymerase epsilon plays a significant role in replicating DNA in higher organisms such as yeast and perhaps even humans.
An enzyme is a protein that acts as a catalyst for chemical reactions. A protein is a molecule made up of a sequence of amino acids. They are the unit molecular building blocks of proteins. They occur in a certain sequence. And there are 20 main amino acids in the proteins of living things, and the properties of a protein are determined by its particular amino acid sequence.
In 1953 Watson and Crick first described the structure of DNA, they also pointed out that the two DNA strands, referred to as leading and lagging, pair with each other to form the now familiar double helix.
Researchers at the Umeå University in Sweden found that in baker's yeast, the primary role in replicating the leading strand of DNA was the enzyme called DNA polymerase epsilon. This enzyme was found to be a key determinant providing genome stability and it also is responsible for cellular responses to DNA damage resulting from exposures to environmental stress.
In the mid 50's researchers discovered the first enzymes capable of replicating DNA. This is an important process required to make new genomes for cell division. So the enzymes, called DNA polymerases, were shown to copy the two DNA strands in only one of two possible directions. One strand of the double helix must be replicated first by a dedicated leading strand polymerase, then it was followed by replication from the lagging strand by a different polymerase.
In lower organisms like the E. coli bacteria one DNA polymerase can accomplish both tasks. But with humans and related higher organisms, such as baker's yeast, the DNA polymerase function is more complicated. Some discoveries, which emerged from the human genome project, indicate that the human genome encodes at least 15 DNA polymerases that can copy DNA. Their tasks appear to be different. Some are thought to perform genomic replication, but others operate under special circumstances, such as to repair DNA damage resulting from environmental exposures.