Question

Discuss the repetitive elements of genome structure placing particular emphasis on mobile element and how such elements can influence evolution

biochem

Answer 1

The repetitive sequences unlike the non-repetitive sequences have more than one copy in a haploid genome. They are completely absent in prokaryotes and are found in eukaryotes. The amount of repetitive DNA increases with increase in genome size and contributes to its complexity. As a result, as we move forward from lower eukaryotes to more complex organisms (plants and animals), repetitive DNA sequences contribute to about 80% of the genome. There are 2 types of repetitive sequences based on their renaturation kinetics:

1. Highly Repetitive sequences : The DNA sequences which undergo renaturation most rapidly fall into this category. These sequences are very short (less than 100bp)
2. Moderately Repetitive sequences : These sequences are repeated 10 to 1000 times and are dispersed throughout the genome.

A major component of these sequences includes the mobile transposable elements. Transposable elements are short DNA sequences which have the ability to move between different locations in the genome and make multiple copies of themselves during this process (called transposition). This duplication event often results in the generation of new genes which is one of the driving forces of evolution.

Mobile elements are of two types : Transposons which uses a DNA intermediate in a direct cut and paste mechanism and Retrotranposons which involves transcription into an RNA intermediate and then reverse transcription of this RNA intermediate into DNA (copy and paste). In both the cases, the resulting DNA intermediate in integrated into the target site of another location.

Transposition is responsible for mutations which are considered to be a major factor for genome diversity and evolution.