2006-10-17 00:45:48 · 3 answers · asked by haru1290 2 in Science & Mathematics ➔ Biology
DNA exists in either the A form, the B form or the Z form of the helix.
2006-10-17 01:06:46 · answer #1 · answered by Gene Guy 5 · 1⤊ 0⤋
Highly dense secondary structure: eukaryotic chromosomal DNA of length 1-20cm is packed into chromosome of length 1 to 10um.
DNA is packed into a dense and regular struture called chromatin.
There are three levels of packaging: nucleosomes, 30nm fibers, and loops of fibers.
Highly compact structure of packaged DNA inhibits transcription, DNA must be unpacked in order to be expressed.
DNA is a polymer of nucleotide bases, i.e. a chain with a backbone of sugar-phosphate with bases attached via the #1 carbon of the deoxy-sugar moiety. Two such chains are combined by base pairing with Adenine-Thymine paired by two hydrogen bonds and Cytosine-Guanine paired by three bonds. The two chains usually form a right handed helix (B-helix) and run in opposite directions (antiparallel).
The sodium salt of duplex DNA at about 90% hydration is 20A wide with one turn of the helix every 34 A and containing 10 base pairs. This, of course, indicates a spacing between base pairs of about 3.4 A. This configuration is referred to as the B configuration or structure of DNA. In physiological solution (complete hydration) the B-structure changes such that there are 10.4 base pairs per turn of the helix.
2006-10-17 12:40:27 · answer #2 · answered by veerabhadrasarma m 7 · 0⤊ 0⤋
nucleosomes, 30nm fibers, and loops of fibers
2006-10-17 07:50:48 · answer #3 · answered by mysterious_gal1984 3 · 0⤊ 1⤋