My Opinion:
I Love the new logo, and have to agree with it, as the DNA is one thing that can be represented in the restrictions by the SE design philosophy very easily.
Not a big fan of the coloured background though.I like my text on a white parchment background with a coloured header for the page, but then again, its just me.
Biology:
We all must realise that OP has a very daunting task ahead of himself. There are multiple scales wherein biologists operate at (ignoring the whole zoology/botany) routine (and yes, that exists too). In short
The first is the molecular level, where we deal in DNA, RNA, proteins, Cellular pathways
The second major level is the tissue and organ level where we deal with well... tissues and organs (muscles, xylem, nervous etc).
After looking at each individual, biologists also look at interactions between individuals in a population, species and environment, including the ecosystem and biospheres as a whole.
This picture represents what I am talking about: http://www.chesterfield.k12.sc.us/cheraw%20intermediate/DaveEvans/BiologyICP/Notes--LevelsOfOrganization.jpg
(for some reason, I am not able to upload it).
Images in the banner:
Keeping in mind with OP's original design concept, of pencil sketches as background and the numerous ideas from the community, I went hunting in google images and found a few sketches, perhaps more in line with Biology ?.
I am not a graphics designer, so I don't know if these are useful, but figured I'll post them anyway.
We can perhaps remove the chemistry elements from here and instead add elements by @rg255, @terdon and @fileunderwater ? I'll keep updating as I find more.
If anyone wants to know what each of the images are, let me know and I'll edit the post an include the names of the organisms/sketches.