What are some evidences that support the “intron early” and “Intron late” models

Question

What are some evidences that support the “intron early” and “Intron late” models of development?

Assignment Chapter 4 I have set up a Discussion area on Blackboard that willlet you post your ideas on this assignment with your classmates. You will be responsible for formulating and posting your own thoughts on this question by next Monday Introns are found in eukaryotes and some archaea but not generally in bacteria. There are two thoughts as to the source of introns. The "intron carly' model says that introns have always been an integral part of genes and those organisms without The "intron late" model supposes that the ancestral protein-coding units consisted of uninterrupted sequences and introns have lost them in the course of evolution. that introns were subsequently inserted into them. Discuss the evidences that support the "intron early" and "intron late" models of intron development. Include thoughts on the evolutionary forces that may be involved to eliminate introns in bacteria (intron early) or insert introns in non-bacterial species (intron late). You may want to look at the mechanisms and difficulty of precisely removing an intron vs inserting an intron. The evolutionary tree below represents the current thought on the relative distance and grouping of various EUKARYA Stremenopiles Alveolates Plentee Red algae Slime molds Animalia Entamoebae Heterolobosea BACTERIA Fungi Myroplasima Physarum Plant Chlorophsts Cyanobacteria Kinetoplastids Eugl Microsporidians Trichomonads lgrobacteriun Diplomonads Pla nt Mitochondria Enterobacte ria Themnop lasma Methanobacteria ARCHAEA

Explanation / Answer

Introns or intervening sequences were discovered in 1977 by walter Gilbert. The word intron is derived from the term 'intragenic' which means 'region inside the genes' and the genes containing them are called 'split genes'

Introns have been only found in eukaryotes and they are absent in prokaryotes.

Introns are considered as the interruptions in the eukaryotic genes because they are also replicated every time with the genome and unless they are not removed a functional protein is not formed.

because Intron is a non-coding part and exon is a coding part.Therfore all these introns have to be removed so that all the exons can join back again to form a functional protein.

Generation of introns -

Introns are believed to arise by either of the two mechanisms : 'Intron-early' or 'Intron- late'

According to Intron-early hypothesis, genes were built of smaller pieces called minigenes. These minigenes were none other than exons.In precellular life, minigenes used to function the same way as gene would do today and then in later stages these minigenes would assemble to make a whole gene. Introns were the functionless part that would hold the exons together.

Bacteria have no introns, and single-celled eukaryotes have very few because they lost them in later evolutionary stages. This is the Intron-early hypothesis !

In contrary to this, 'Intron-late' hypothesis says that  the original common ancestor did not have introns and they only evolved in the eukaryote branch of the tree of life. This theory suggests that
introns are placed pretty much at random into genes, and they do not necessarily correspond to protein structural elements.

The possible evidence for Intron-early hypothesis -

Evidence to supports Intron-late hypothesis

It suggests that introns were acquired in eukaryotic organisms by evolution

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