Nucleic Acid Structure and General Features

ID #1052

In the video we watched in discussion, it says that only .5% of the DNA that we have codes for Genes. Other 99.5% DNA is in a sense a junk DNA. And it's also true that we have just twice as many genes as the fruitflies. so, does this mean that fruitflies have all the DNA but it isn't coding for the genes that humans have? And is this concept of Junk DNA universally true in every living thing? And it's also hard for me to grab the concept that the most of our DNA is kind of junk. So, is it really true that it's junk or whatever it's function is, is so small or negligible that we just ignore it and call it a junk. If so, what is that function?

We'll touch on some of these subjects later in the semester, but these are good questions. The fact is, over the last billion years of eukaryotic evolution, genomes have aquired repeated sequences, duplications of genes, DNA inserted from other sources and "trapped," and so forth. There is simply far more DNA than is needed to code for the proteins that an organism can make. Since it is largely assumed that the genome is the information storage for the proteome, the non-coding DNA has traditionally been called "junk." But this term can be misleading. Some of these non-coding regions have important control or regulatory roles that influence the production of proteins from the coding regions, and recently we've become aware of just how important some of that "junk" DNA can be. But it will probably hold on to the name "junk DNA" because it does not technically code for proteins. And as we'll see in a few lectures, some of that non-coding DNA (introns) can even be found within a coding region (a gene). I'll explain the relevance of that when we visit that material in class.

Regarding whether all living organims have junk DNA, there isn't an organism with 100% of its bases that are parts of actual genes, but the prokaryotes seem to have by far the least amount of junk DNA. This is by necessity -- a relatively simple, single-celled organism has to be more efficient and streamlined than its multicellular counterpart. There is very little junk DNA in an E. coli cell, while as you've seen, a very small percentage of a human cell's DNA is coding regions. But consider the raw numbers in addition to considering the percentages. A human cell (other than a sperm or egg) contains 6 billion base pairs of DNA. 0.5% of that is 30 million base pairs of coding DNA, still a vastly higher amount of coding DNA than a typical bacterial species like E. coli which, while streamlined, still only has 4.5 million base pairs. That extra coding capacity is what makes us more complex organisms than bacteria, even though 99.5% of our genomes doesn't have information in it for making proteins.

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