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Author Topic: What is the relationship between number of chromosomes and organism complexity?  (Read 29810 times)

Offline Super Hans

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Looking down list of chromosome numbers (Link below) it follows the general pattern that more complex organisms have more chromosomes, and less complicated organisms have fewer chromosomes.

There are several bucks to this trend though, the potato for example having 48 in comparison to our 46.

How is it that complex creatures can have the same number or less chromosomes then a very simple organism?

Just find it odd that cotton can have many times more chromosomes than a kangaroo, I don't understand this correlation.

Help me understand this in Laymans terms if possible.

Thanks.

http://en.wikipedia.org/wiki/List_of_organisms_by_chromosome_count [nofollow]
« Last Edit: 10/06/2013 01:33:45 by Super Hans »


 

Offline RD

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Q. "How is it that complex creatures can have the same number or less chromosomes then a very simple organism?"

i.e. the "C value enigma" ... http://en.wikipedia.org/wiki/C-value_enigma

Possibly species which are older, or which have a short life-cycle, have accumulated more junk DNA ... http://en.wikipedia.org/wiki/Junk_DNA
« Last Edit: 10/06/2013 08:55:33 by RD »
 

Offline CliffordK

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Chromosomes can split, or fuse, leading to larger or smaller chromosomes.  Chimps & Apes have 48 chromosomes, while humans only have 46 due to a 2a/2b fusion.  However, we may have a larger brain than a chimp, but otherwise it would be hard to say whether we are more or less evolved than them.

Anyway, you could look at:
  • Chromosome Number
  • Number of Genes
  • Number of Base Pairs or Nucleotides
  • Number of Base Pairs per gene

To a large extent, the number of genes and base pairs does increase with the more complex organisms. 

However, as RD noted, the "Junk DNA" can also be quite variable, or in some cases, genes might be repeated.

Here is an interesting Wikipedia chart on Genome Size.  To some extent, the complexity of the organism follows the genome size, but for example Flowering Plants covers a very wide range of genome sizes.

In the middle of this page, there is a nice chart on the number of genes and base pairs for a few select organisms.  It is interesting that the gene density for Mammals is relatively low.  Not necessarily more complex genes, but rather more non-coding DNA.
 

Offline alancalverd

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Can someone please define complexity, and give a numerical parameter for it, before attempting to answer the question? The word is often a creationist trap.

Some wheat strains run to 90 chromosomes or more. Drosophila, if memory serves, have about four. Like us, drosophila can see, walk, and eat, but unlike us they can also fly. So in terms of functional diversity they are way ahead of most mammals.   
 

Offline Lmnre

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Here is an interesting Wikipedia chart on Genome Size.  To some extent, the complexity of the organism follows the genome size, but for example Flowering Plants covers a very wide range of genome sizes.

This Wikipedia chart got me to thinking that, maybe for organisms (such as humans) who can adapt to variations/changes in their environment using non-genetic means (that is, intelligence/sapience, although there may be other means), their genome size can atrophy (in the extreme to only the bare essential material) and they will easily survive. It would be the low-intelligence/sapience lifeforms that require a large genome size, many offspring and plenty of mutations in order to survive the perils of environmental variations/changes.
 

Offline cheryl j

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I wonder if it genome size is always related to complexity. If an organism didn't have a mechanism for substituting new genes for old ones, or getting rid of inactive ones, it might explain things like polyploidy in plants. Or perhaps it allows organism to be adaptable to abrupt changes, and old genetic material is kept just in case the environment changes back to previous conditions? Would it speed up natural selection in those cicumstances if these old inactive genes were still hanging around?
 

Offline evan_au

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As Cheryl mentions, polyploidy is relatively common in plants. This is where a viable plant includes all chromosomes that existed in both the "parent" plants.
This doubles the number of chromosomes & quantity of DNA, without an increase in the "complexity" of the plant (however you define this).

Sexually reproducing mammals are generally diploids (2 sets of chromosomes), and cases where this is violated (eg 2 sperm simultaneously fertilising a single egg) normally results in a miscarriage.

As with everything in biology, there are exceptions to every rule...
So a comparison of chromosome numbers should allow for polyploidy (where it can be identified).
 

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