Wednesday, September 1, 2010

Fireflies and DNA Transcription

Two new figures for chapter 2: a schematic of how DNA keeps cranking out near-perfect cells, moment by moment, and the complexities of mate-recognition in the world of a female firefly. Captions below. These images are inverted because I prefer them that way; in the book, the DNA image will be black-on-white. About 29 days to go, now.

CAPTION for Figure 2-1: Multicellular life forms (metazoa), such as a human (A) are composed of a wide variety of cells (B). Each cell contains a nucleus (dark spots in B) which contains chromosomes (C), the ‘nuclear threads’ observed under the microscope even in the 1800’s. Each chromosome (D) is actually a long ‘thread’ of the DNA molecule, tightly wound like a telephone cord (E). The twisting-ladder-shape of the DNA is composed of the phosphate rails (white and black ‘strips’ in E) that connect the rungs (F) of adenine, cytosine, thymine and guanine (A,T,G and C, becoming visible just to the left of label (G)). Although there isn’t room to show a whole gene in this figure, animal genes are normally around 1,200 ‘base-pairs’ long, schematically shown as a set of base pairs from (F1) to (F2). Base pairs are arranged in triplets (codons) such as (G), the codon TAC, which specifies the production of Tyrosine. When DNA is used to make a protein (e.g. to build cells to replace dead cells) an enzyme called helicase (the long gray oval, (H)) runs down the length of the DNA (moving to left in this image), ‘unzipping’ the DNA double helix, separating the base pairs. The helicase copies the base pair codons of one side of the DNA ladder (lower and middle rows) and streams them off in a tail-like chain. That chain (I) eventually moves out of the cell nucleus, merging with a ribosome (J) which attaches free-floating chemicals (A,T,G and C) that will only bind to the appropriate codons, together in the appropriate sequence (K). The resulting amino acids (in this case, lysine, aspargine and glutamine, assembled in (L)) eventually assemble in the production of proteins, assembled as body cells (M). This figure is schematic, and a universe of detail lies beyond every single thing I’ve shown. Having said that, what I depict here is enough to understand the system.

CAPTION for Figure 2-3: The night-light world of the firefly. To find a potential mate, a female has to sort through the light signals of (in this case) nine other species. Typical tracks of those other species are shown; some fly higher, some low to the ground. Some flash as the ascend (top line), some flash in repeated sequences (the eight 'blips' repeated occasionally on the fourth line down, on the left). A slight variation in the female's ability to perceive these signals, and sort through them, could prevent her from mating successfully.

(c) 2010 Cameron M. Smith

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