Light strand

The Meselson-Stahl experiment resulted in a single band of DNA on CsCl gradient centrifugation after one round of cell division. Its density was intermediate between heavy and light DNA. This could only occur if the newly synthesized dsDNA contained an old light strand and a new heavy strand. 

FIGURE 10.3 The experimental evidence for semiconservative replication. Heavy DNA labeled with forms a band at the bottom of the tube, and light DNA with N forms a band at the top. DNA that forms a band at an intermediate position has one heavy strand and one light strand. 

The semiconservative replication of DNA means that a newly formed DNA molecule has one new strand and one strand from the original DNA. The experimental evidence for semiconservative replication comes from density-gradient centrifugation (Figure 10.3). If replication were a conservative process, the original DNA would have two heavy strands and all newly formed DNA would have light strands. 

Species Tissue Reference" Circular duplex (gm/ml) Buoyant densities Light strand Heavy strand H-L (alkaline) A (mg/ml)... 

The small size of the mt DNA, the possibility of isolating the different forms of this DNA species in a highly purified state, and the ability to separate the heavy and the light strand of this DNA makes this polynucleotide a convenient model for the study of fundamental questions in molecular biology. In contrast to the nuclear material, mt DNA... 

As stated previously, the complementary strands of AAV DNA have different thymidine contents (Rose and Koczot, 1971). Therefore, when BUdR is substituted for thymidine the complementary strands have different boyant densities in neutral CsCl density gradients and may be separated by physical methods (Berns and Rose, 1970). AAV-specific RNA isolated from infected cells was able to hybridize only to the heavy BUdR-substituted DNA strand and had an average base composition which was identical to that of the light strand (low thymidine-content) (Rose and Koczot, 1971). Thus, the authors concluded, that, within the limits of detectability, AAV RNA was transcribed... 

Yeast mitochondrial DNA occurs as double-stranded 26-/im closed circles, a molecular size corresponding to about 50 x 10 daltons. The number of circles per mitochondrion may range from zero to about five the total amount of cellular mtDNA per wild-type cell varies with the strain and accounts for 10-25% of the total cellular DNA. RNA-DNA hybridization studies indicate that yeast mtDNA contains one cistron of each of the 15 S and 21 S RNA species and probably 20 tRNA cis-trons. It has been reported that mitochondria from HeLa cells contain only 12 tRNA cistrons, 9 on the heavy DNA strand and 3 on the light strand. These authors suggested that since the proteins formed by mitochondrial ribosomes are enriched in hydrophobic amino acids, an array of 12 tRNAs may be sufficient for the complete synthesis of the inner-membrane proteins by mitochondria. Alternatively, some nuclear coded tRNAs may be available to the mitochondrial protein-synthesizing system. 

Alternatively, a fiber optic bundle can be used in place of the pipe. In a fiber optic bundle, a matrix of smaU (50—100 -lm) fiber optic strands are arranged such that the ordering of the strands at one end is equivalent to that on the other end. Therefore an image focused on one end with lenses is transmitted to the other end. Light is typicaUy sent down some of the fibers not used for image transmission to provide illumination. 

Modified oligonucleotides can be used to cross-link DNA sequences via a reactive group tethered to an oligonucleotide. When irradiated with uv light, psoralens (31) reacts with thymine bases, and the reaction yields a cross-link if the thymine residues are adjacent to each other on opposite strands. Psoralen linked to oligonucleotides have been shown to induce site-specific cross-links in vitro (51). 

Figure 4.11 Schematic diagram of the structure of the ribonuclease inhibitor. The molecule, which is built up by repetitive P-loop-a motifs, resembles a horseshoe with a 17-stranded parallel p sheet on the inside and 16 a helices on the outside. The P sheet is light red, a helices are blue, and loops that are part of the p-loop-(x motifs are orange. (Adapted from B. Kobe et al.. Nature 366 7S1-756,...

Figure 13.15 Schematic diagram of the heterotrimeric Gap complex based on the crystal structure of the transducin molecule. The a suhunit is hlue with some of the a helices and (5 strands outlined. The switch regions of the catalytic domain of Gq are violet. The (5 suhunit is light red and the seven WD repeats are represented as seven orange propeller blades. The 7 subunit is yellow. The switch regions of Gq interact with the p subunit, thereby locking them into an inactive conformation that binds GDP but not GTP.

Figure 15.12 Schematic diagram of the barrel arrangement of four p strands from each of the variable domains in Fab. The six hypervariable regions, CDR1-CDR3 from the light chain (L1-L3) and from the heavy chain (H1-H3), are at one end of this barrel. (From J. Novotny et al., /. Biol. Chem. 2S8 14433-14437, 1983.)...

The constant domain has a stable framework structure composed of two antiparallel sheets comprising seven p strands, four in one sheet and three in the other. The variable domains have a similar framework structure but comprising nine p strands, five in one sheet and four in the other. The three hypervariable regions are in loops at one end of the variable domain. The variable domains from the heavy and light chains associate through their five-stranded p sheets to form a barrel with the hypervariable loop regions from both domains close together at the top of the barrel. 

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