Naked Science Forum
Life Sciences => Plant Sciences, Zoology & Evolution => Topic started by: bizerl on 19/07/2012 01:07:18
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I'm just wondering how DNA started. As far as I know, it is an enormous molecule that hides in our cells and tells them what to do. And there are genes and chromosomes (I don't pretend to know how it all goes together!).
My question is: Is DNA the same length for all living things? Is variety dictated only by the GATC code that makes up the molecule, or is it also dictated by how long it is?
Stemming from that point, how did it start? It seems improbable that the first life just happened by amino acids combining into the complex DNA molecules we have today (again, I welcome corrections), so is there any evidence for an evolution of DNA and does it stand to reason that DNA itself has evolved out of life?
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My question is: Is DNA the same length for all living things?
No (http://www.genomesize.com/statistics.php), some of the the older species have larger genomes and more chromosomes (https://en.wikipedia.org/wiki/List_of_organisms_by_chromosome_count) : they have had more time to accumulate junk DNA (https://en.wikipedia.org/wiki/Junk_DNA) by the random processes involved in evolution.
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DNA length varies enormously between different organisms. This is usually referred to in terms of base pairs (bp) or Kilo-base pairs (Kbp). According to Mr Wikipedia, the (haploid) human genome is believed to be about 3.2billion base pairs long, so in a standard cell, excepting sperm or eggs, you will have 6.4billion base pairs of DNA as one set of DNA is inherited from your mother and the other from your father. This supposedly contains somewhere around 20,000 distinct genes, but this DNA will also contain large amounts of 'junk' (that may yet be found to play some role) and repeats and all sorts. To contrast, E.coli has about 4.6million base pairs. Having said all that though, the amount of DNA doesn't necessarily reflect the complexity, or lack thereof, of the organism.
DNA contains 4 different bases, GATC, and every 3 base pairs is referred to as a codon. Each codon codes for an amino acid (some amino acids are coded for by more than one codon). As you have 4 different bases, and there are 3 pairs per codon, you have 4*4*4 = 64 possible codons which between them code for the 20 different naturally-occurring amino acids. The codons come together as genes to produce amino acids in various sequences, such that particular proteins are produced. There's no real limit to the size of a protein, so in theory the length of DNA can affect variety in that it enables more codons and hence more genes to 'fit on'. But, in practise this is not the case because DNA can contain huge amounts of junk.
The origins of life are completely unknown, with some interesting hypotheses. I think the big steps towards life are the emergence of a system of replicating nucleic acids (be it RNA, DNA or some other NA), and the emergence of some kind of membrane to protect it all and enable it to happen. In all life-forms we know about, nucleic acids are fundamental as they enable the transfer of genetic information from one generation to the next. Without this, the whole basis of life doesn't exist. For that reason, I think it's extremely unlikely that nucleic acids have emerged after life. Amino acids I suspect would have been a later addition. Nucleic acids aren't actually composed of them. DNA is made up of a deoxyribose sugar, some phosphate groups, and then either the G,A,T or C base. The DNA is a way of encoding the sequence information for all the necessary proteins in an organism in a readable and manageable form.
Apologies for the length of the post, but I hope it helps.
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schneebfloob: Some amino acids have been found in carbonaceous chondrite meteorites. Interestingly, these appear to be chiral - something mainly seen in living organic chemicals. http://en.wikipedia.org/wiki/Murchison_meteorite#Organic_matter
Amino acids were also made in the famous Urey & Miller experiment. http://en.wikipedia.org/wiki/Miller-Urey_experiment
We can expect amino acids to have been present in the initial ingredients of the earth.
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I realise that they're perfectly capable of existing before life appeared, but I suspect they would have been a later addition to life (or what 'life' then was). They're not the imperative components to get life going, that's what I meant by them being a later addition.
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(in addition to what have been said by schneebfloob)
There is a good webpage on this topic: http://exploringorigins.org (http://exploringorigins.org).
It is quite sure that DNA did not come first (nobody is actually claiming this).
But most of the scientists think RNA came first. (but there are other theories)
DNA is a kind of storage medium and does not form complex enough three-dimensional structures which are needed to be active (= catalysing chemical reactions). RNA can do both: act as a template to be copied and catalyse reactions (for example the protein producing catalytical step in the ribosom is done by RNA).