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Chromosome number differs across species.

Is the amount of DNA comparable between organisms, just being split into smaller chunks in those species with more chromosomes, or do species have different genome sizes? If so, does the genome size correlate roughly with the complexity of the species?

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    $\begingroup$ Please clarify your question. As it is written now, you ask multiple, very broad questions. $\endgroup$ – Luigi Jun 21 '15 at 20:41
  • $\begingroup$ To briefly answer all four of your questions: no, no, yes, and kind of but not really. If you narrow it down a little more there are great answers to this question. $\endgroup$ – Resonating Jun 21 '15 at 23:03
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    $\begingroup$ @Resonating - now, see why comments are bad as answers? I think I can make a good case for "yes, yes, yes, and not really". $\endgroup$ – anongoodnurse Jun 21 '15 at 23:41
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    $\begingroup$ There are exactly two questions: do different species have the same genome size and, if they don't, does size relate to complexity. This is pretty straightforward and can be well answered in a paragraph, which is hardly broad. $\endgroup$ – canadianer Jun 22 '15 at 2:07
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    $\begingroup$ I guess you're right. I retracted my close vote and removed my comment. $\endgroup$ – Remi.b Jun 22 '15 at 2:49
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Chart of C-values (the mass of DNA in a single haploid cell); there is no logical order to the groups:

enter image description here

[source]

Base pairs in haploid genome (some examples):

  • Escherichia coli (bacterium): ~4.5 million
  • Caenorhabditis elegans (nematode worm): ~100 million
  • Homo sapiens (we all know what these are): ~3 billion
  • Pinus taeda (coniferous tree): ~22 billion
  • Prorocentrum micans (single-celled algae): ~245 billion

From these data we can conclude:

  • Different species do not have the same genome size.
  • Genome size is not correlated with complexity. Organismal complexity can be hard to define but, qualitatively, I think we can all agree that a human is more complex than a single-celled alga. And yet, humans have a genome that is 80 times smaller. This is known as the C-value paradox. Note, however, that this paradox has been resolved after it was found that the genomes of most eukaryotes contain a large proportion of non-coding and repetitive DNA.

Further reading:

The C-value paradox, junk DNA and ENCODE by Eddy SR

Eukaryotic Genome Complexity by Pray L

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It is already mentioned by canadianer that genome size differs between organisms. But what about complexity?

First we should define what complexity is: complexity can be defined as number of different cell types that a multicellular organism can produce, with the same genome. Yes complexity does not correlate with genome size. However, it seems to correlate with number of genes. According to Kauffman, the number of cell types i.e. complexity is linearly correlated (direct correlation) with the square root of number of genes.

enter image description here

Though Kauffman says that complexity increases with DNA content, which is not actually true, it is certainly possible that that complexity is correlated with number of genes. The book is a little old and certainly molecular and cell biology has seen a lot of progress in this time. However, theoretically higher number of distinct genes should produce more complex phenotypes which Kauffman justifies using his NK-model. This assumption has again been disproved. The catch would be that exact duplicates/polyploid genes should not be counted as different genes. Moreover metabolic genes should also not be counted (plants/bacteria have a higher number of functional metabolic pathways). Organismal complexity arises because of the complexity of gene regulatory network which in-turn is dependent on the number of regulatory genes. NK model also in a way assumes regulatory genes (i.e. genes that can interact with each other). Surely complexity cannot arise out of nothing. I can't find the data for this at the moment but the theory is quite strongly plausible.

Another point I would like to add is that complexity need not only mean spatial complexity. Complexity can also be temporal.

Reference:
    Stuart A Kauffman (1993) The Origins of Order, Chapter 12

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    $\begingroup$ That graph seems quite outdated. Man, for instance, doesn't have ~600,000 genes (or am I reading it wrong, I can't imagine an estimate that high in the 90s). Also, the selection of organisms isn't exactly inclusive. Plants are conspicuously absent. It would be interesting to see the comparison done with contemporary estimates and a broader range of species. From what I've read, complexity and number of genes don't correlate. $\endgroup$ – canadianer Jun 22 '15 at 6:48
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    $\begingroup$ @canadianer I agree and I said that this is outdated. However, number of unique genes can theoretically (perhaps also in real) give rise to complexity. It is also intuitive that with increased functionality, you need a bigger repertoire of genes. The complexity of gene network also matters, though. I must also indicate that metabolic genes would not play a role and it is the regulatory genes that would matter. Complexity cannot just arise from nothing (if it has no connection with genome whatsoever). I'll add this to the answer. $\endgroup$ – WYSIWYG Jun 22 '15 at 7:59
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I can show the facts here.

Human:

total length: about 3,000,000,000

coding genes: about 50,000 (included predicted ones)

3,000,000,000/50,000=60,000

Chromosome number: 23

Zebrafish:

total length: about 1,400,000,000

coding genes: about 36,000 (included predicted ones)

1,400,000,000/36,000=38,889

Chromosome number: 25

Fruitfly:

total length: about 1,400,000,000

coding genes: about 19,000 (included predicted ones)

1,400,000,000/19,000=73,684

Chromosome number: 4

Saccharomyces cerevisiae:

total length: about 12,000,000

coding genes: about 7,000 (included predicted ones)

12,000,000/7,000=1,714

Chromosome number: 16

http://useast.ensembl.org/Homo_sapiens/Info/Annotation http://useast.ensembl.org/Danio_rerio/Info/Annotation http://useast.ensembl.org/Drosophila_melanogaster/Info/Annotation http://useast.ensembl.org/Saccharomyces_cerevisiae/Info/Annotation

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    $\begingroup$ and how do these numbers answer the question? The answer is in there, but it would be good to end with a concluding few sentences addressing the question. $\endgroup$ – AliceD♦ Jun 22 '15 at 0:30

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