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May 31, 2010 00:02DNA and non-coding partsI know that over 50% of our DNA does not contain any genes, so it is not transcribed. Do these non-coding parts have a role and, if yes what is that role? Moreover, eukaryotic genes may include streches of DNA called introns which are not translated into protein and they are sliced out before translation. Why do they exist? Could our DNA contain "useless parts"??? |
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“Only” 20.000 in human, encode all the proteins in an organism. These genes are about 1–2% of the human genome.
The regions that have no genes that create proteins may be:
1. Promoters. Promoter is recognized by the transcription machinery when a gene is about to be transcribed and expressed.
2. Codons, (except for these which help to create a specific amino acid) that create a signal that alerts the translation machinery that the end of the gene has been reached. You could say that it is the opposite of promoters.
3. Introns which are transcribed but never translated into protein as they are spliced out before translation, as you said Angeliki. Splicing can also occur in PROKARYOTIC genes too, but is less common than in eukaryotes. One role of introns is the transcription of the introns to small RNA, such as miRNA (microRNA). These "small" RNAs regulate the expression of genes.
You asked why they are sliced. After intron splicing, mRNA is created which consists of exon sequences, which are translated into a protein.
4. Retrotransposons. These are genetic elements that can amplify themselves. They increase the copy numbers of elements rapidly and the genome size.
5. A non-coding RNA (ncRNA) which is not translated into a protein too. Non-coding RNA genes include tRNA) and ribosomal rRNA and microRNAs, piRNAs. Since most of the newly identified ncRNAs have not been validated for their function, it is possible that many are non-functional.
What means that they are not functional? That they MIGHT be useless parts. But this is not strange. Imagine the first DNA molecule that has ever been created. Its elements would came next to each other randomly. The greatest part of it would be useless. That means that because of luck some genes were created that are functional. Life is an accident... :D
I don't know any other categories of non-functional (regarding the creation of proteins by them).
Orfea, thanks for your huuuuuge answer and for the time you obviously spent to write it :P!!! I have one question though: we know that introns are removed from pre-mRNA with the help of small nuclear RNA (snRNA) which is found within the nucleus of eykaryotic cells. Prokaryotic cells do not have a nucleus so they do not have snRNA. How does splicing take place in that case?
The truth is that I had never thought of live as an accident but you are right :D (although I think it is more probable that an RNA and not DNA molecule was created first :P). However wouldn't most of these non-functional sequences disappear through evolution IF they are useless? Or it would be impossible to "get rid" of ALL of them?
I cannot answer your first question... I don't know, it is too specialized for me... :D
Now, about your second question. I have to explain the mechanism of evolution. In the past, there was a theory about evolution that seems right, but it isn't. This theory was set by Lamark. I will give you an example which will make clear what it states. Suppose that in the past girrafes were short. If the trees of the savana start becoming taller, then this species will choose to change. As the result the descendance will become taller and taller IN ORDER TO SURVIVE.
Then Darwin came and said that this theory is wrong and set a new theory. I will give you again an example. In the beginning, only short girrafes exist. The food at the low levels of the trees starts disappearing for some reason. BY ACCIDENT, a descendance has WRONG DNA, which gives it a new characteristic: being tall. All short girrafes will die because of luck of food and only the WRONG girrafe will survive. If short girrafes die, there will be no competitors for the wrong giraffe, it will survive and have descendants. A new species will be created.
Evolution is a result of "wrong" genes, when something goes wrong at the reproduction of a species. Life is he same. DNA or RNA was made by accident (we don't know which was first created).
By the time useless characteristics don't affect so much the chances of the species to survive, these characteristics will not be lost. For example, suppose that a man with three hands is born. This causes him problems and he has no chance to compete normal people. A new species will not be created, because it will not survive for a long. Now, suppose because of evolution, only 2% of DNA is usefull. The fact that we have 98% useless DNA doesn't affect us, it causes no problem. If a new species has more advantages because of having 50% usefull DNA, then this species will survive and the species with more useless DNA will be lost. In this way we get rid of it ( not exactly we, because we will not survive and a new species will appear).
This doesn't mean that every "old" species disappear when a new appears. But, many times this is the case.
Ok, I think I got it!!!!!! Thanks!!!!!! :D
(I can see that you learn from your mistakes Orfea...!!!)
Are you refering to the split of the text? :D
Yeap :D!!! It was a wise choice!!!!!!!!!!!!!