DNA twisting

A constant number of chromosomes is present in each cell. The somatic cells (i.e. not sperm or egg) are described as diploid because they contain two complete sets of chromosomes. There are 23 pairs of chromosomes in each cell, 22 pairs of somatic chromosomes (one of each pair derived from each parent) and one pair of sex chromosomes, either two Xs in the female or an X and Y in the male. Together, the 23 chromosomes contain about two metres of linear DNA or about three billion pairs of nucleotides. The linear structure of bases in DNA strands is called the primary structure of the chromosome. The secondary structure is the double heUx, in which the two complementary strands of DNA twist about each other. One turn of the helix is called a pitch and consists of ten nucleotides. 

Morse, R.H. and Cantor, C.R. (1986) Effect of trypsinization and histone H5 addition on DNA twist and topology in reconstituted minichromosomes. Nucl. Acids Res. 14, 3293-3310. 

The major alternative mechanism for nucleosome redistribution to the bulge propagation mechanism is twist diffusion. Supporting this, it is clear that the nucleosome core particle is capable of accommodating alterations to DNA twist of 1-2 bp per double helical turn without significant changes to the... 

See also DNA, Nucleic Acid Structures, Structure of B-DNA, Twists, Linking Number, Supercoiling, Writhing, Topoisomerases... 

Many of these proteins (e.g., the TATA-box binding protein, catabolite activator protein, Cro, Arc repressor) bind to DNA with some sequence specificity and are involved in transcription initiation or repression. Others, involved in packaging (the histone core, Sac7d), are relatively sequence neutral in binding. Pro-tein-DNA interactions important in disease are also represented human immunodeficiency virus (HIV) type 1 reverse transcriptase bends DNA considerably upon binding [131], as does the tumor suppressor protein p53, which also increases DNA twisting [132]. 

Wang JY, Syvanen M. DNA twist as a transcriptional sensor for environmental changes. Molec. Microbiol. 1992 6 1861-1866. 

Genes are constructed from sets of deoxyribonucleic acids (DNA), which in turn consist of chains of nucleotides. These chains occur in matched pairs, twisted around each other (a double helix). 

The linking difference can be considered in terms of changes in the twist, Tiv, and writh, Wr, of the molecule. Twist describes how the DNA strands are coHed around each other writh describes the coiling of the hehcal axis. The change in linking number is related to changes in twist and writh ... 

In complexes with Cro, the overall bend and twist of the DNA are similar to those in the repressor complexes, but there is a significant difference in the local structure of two of the nucleotides in each half-site. Binding of 434 Cro or repressor fragment thus imposes a distinct local structure (Figure 8.13), as a result of differences in both the identity and conformations of various amino acid residues that interact with the DNA. The DNA conformational details are significant for the relative affinities of Cro and repressor for various sites, as we describe in a later section. 

FIGURE 12.9 (a) Double-stranded DNA as an imaginary ladderlike structure, (b) A simple right-handed twist converts the ladder to a helix. 

DNA duplex of 400 bp, L is 40 (assuming 10 bp per turn in B-DNA). The linking number for relaxed DNA is usually taken as the reference parameter and is written as Lq. L can be equated to the twist (T) and writhe (W) of the duplex, where twist is the number of helical turns and writhe is the number of supercoils ... 

Bacterial as well as eukaryotic chromosomes contain too much DNA to fit easily into a cell. Therefore, the DNA must be condensed (compacted) to fit into the cell or nucleus. This is accomplished by supercoiling the DNA into a highly condensed form. When relaxed circular DNA is twisted in the direction that the helix turns, the DNA becomes positively supercoiled, if it is twisted in the opposite direction, it is called negatively supercoiled. Bacterial DNA is normally found in a negatively supercoiled state. Supercoiling reactions are catalyzed by topoisomerases. 

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