2 - Nuclear Genome Size Flashcards
What is the C-value? What is the C-value paradox?
The amount of DNA in the genome
It varies enormously across organisms, even within taxa.
In eukaryotes there is little correlation between the size of the genome and the complexity of the organism/cell.
Before knowing the content of the genome, the C-value enigma or C-value paradox was the complex puzzle surrounding the extensive variation in nuclear genome size among eukaryotic species.
HOWEVER: the c-value does correlated (roughly) organismal complexity in bacteria!
What are the two best correlates with genome size?
- Cell volume
- Nuclear volume
Suggests that DNA has skeletal roles in the nucleus
Bulk DNA exerts a strong causative influence on cell size and division rate. As a result, genome size may be linked to one or more features at the organismal level, depending on the biology of the group in question. These, in turn, can have important ecological impacts.
How is protein coding gene % and transposable element % (of genome) correlated with genome size?
Protein coding gene %: negatively
Transposable elements %: positively
Is intronic DNA amount correlated with organismal complexity?
A little bit.
Is coding DNA amount correlated with organismal complexity?
Only for Animal nuclear genomes
Is Intergenic DNA amount correlated with organismal complexity?
Yes, though perhaps indirectly.
More complex organisms (eg. humans) have LOADS of non-coding DNA, and prokaryotes have very little.
How does genome size change over time?
Very rapidly sometimes.
This is why a closely related group such as dinoflagellates, can have genomes ranging from 2-200 gbp!
What is the possible function of junk DNA? (recall Doolittle’s comments)
To survive within the genome (selfish gene).
When a given DNA or class of DNAs of unproven phenotypic function can be shown to have evolved a strategy (such as transposition), which ensures its genomic survival, then no other explanation for its existence is necessary.
Give four mechanisms which act to increase genome size
- Polyploidy (aka. whole genome duplication)
- Short range duplications
- Spread of transposable elements
- Insertional biases
These work in tandem with mutation pressure, which may drive selection pressure (if there are physiological effects or constraints) or random genetic drift.
Give two mechanisms which act to decrease genome size
- Deletional biases
- Long range deletions (eg. via recombination)
These work in tandem with mutation pressure, which may drive selection pressure (if there are physiological effects or constraints) or random genetic drift.
Is there a bias towards genome increase or reduction?
Give an exceptional case to this
There is an intrinsic bias for small deletions. IN the absence of strong forces favouring DNA increase, the default mode would be slow reduction in genome size.
One exception is salamanders, which have increasing genomes because of the expansion of long terminal repeat (LTR) retrotransposons.
