1. Unfractionated RNA from reticulocyte ribosomes was hydrolysed with pancreatic ribonuclease at 25 degrees . The molecular weight decreased rapidly to about 3s when about 6% of the residues were soluble in 0.5n-perchloric acid. In the early stages 60-80% of the hydrolysed linkages were ;hidden'. The denaturation spectrum was affected. Continued hydrolysis led to slow changes in S value, in the electrophoresis pattern in polyacrylamide gels and in the denaturation spectrum. 2. Hydrolysis of RNA with alkali to fragments of between 2.8s and 5.9s led to changes in the denaturation spectrum similar to those observed in the early stages of enzymic hydrolysis. 3. A theory was developed to relate changes in secondary structure with main-chain scission. 4. The results agree with the ;hairpin-loop' model for RNA. The denaturation studies are consistent with the presence of more than one species of hairpin loop that differ in their denaturation spectra. The average length of the hairpin loop was estimated to be 10-20 residues and an upper limit of 35 residues was established. 5. It is inferred, on the basis of studies with model compounds, that the stability of single-stranded stacked structures is hardly dependent on salt concentration. 6. The denaturation spectrum of the fragments obtained on hydrolysis became less dependent on ionic strength, suggesting that double-helical structures revert to a single-stranded stacked form on denaturation.

1. RNA was isolated from virus-like particles found in Penicillium chrysogenum and resolved into two fractions by gel filtration through agarose columns. 2. Fraction 1 was excluded and had the following properties: 50.9% G+C [AMP 0.246, UMP 0.246, CMP 0.252, GMP 0.255 (mole fraction)]; mol.wt. about 1.2x10(6) daltons; s(20,w) 12.3S and ;melting' temperature about 100 degrees C (solvent 0.15m-sodium chloride-0.015m-sodium citrate pH7.2); optical rotation [alpha](max.) 6000 degrees at 278nm; circular dichroism (epsilon(L)-epsilon(R))(max.)=8.181mol(-1) cm(-1) at 260nm. 3. Properties of fraction 2 include 37.8% G+C [AMP 0.313, UMP 0.312, CMP 0.186, GMP 0.189 (mole fraction)]; mol.wt. about 140000 daltons; s(20,w) 7.3S, T(m) about 85 degrees C (solvent 0.15m-sodium chloride-0.015m-sodium citrate, pH7.2); optical rotation [alpha](max.) 6000 degrees at 278nm; circular dichroism (epsilon(L)-epsilon(R))(max.)=8.241mol(-1) cm(-1) at 260nm. 4. The properties of both fractions were consistent with a double-helical conformation.

1. On the basis of studies with model compounds it was concluded that in 8m-urea-m-potassium chloride (or 4m-guanidinium chloride) in 0.01m-potassium phosphate buffer, pH7.0, multi-helical structures have about the same stability as in 0.1m-potassium phosphate buffer, pH7.0, whereas the tendency of base residues to ;stack' along a single polynucleotide chain is much decreased. 2. Base-pairing was eliminated whereas base-;stacking' persisted after RNA in 1% formaldehyde-0.1m-potassium phosphate buffer, pH7.0, was heated to 95 degrees . 3. From a study of the thermal denaturation of unfractionated transfer RNA from Escherichia coli and of RNA from the fractionated sub-units of rabbit reticulocyte ribosomes in 8m-urea-m-potassium chloride (or 4m-guanidinium chloride) in 0.01m-potassium phosphate buffer, pH7.0, it was inferred that ;stacked' residues may account for up to 25% of the increase in E(260) found on heating RNA in solvents such as 0.1m-potassium phosphate buffer, pH7.0. 4. Changes in the spectrum with temperature were analysed on the basis of the assumptions that (a) the polynucleotide chain is amorphous on denaturation (which is probable in 8m-urea-m-potassium chloride-0.01m-potassium phosphate buffer, pH7.0) and that (b) the polynucleotide chain adopts a single-stranded ;stacked' conformation on denaturation (which is probable when ordinary solvents such as 0.1m-potassium phosphate buffer, pH7.0, are used).

1. RNA isolated from the sub-units of rabbit reticulocyte ribosomes was hydrolysed by 0.4n-potassium hydroxide at 20 degrees . The probability of main-chain scission was calculated from the number-average chain length, which was obtained from S(25,w) in 0.01m-phosphate buffer. 2. The fraction, f, of the original secondary structure that the fragments re-formed at neutral pH in 4m-guanidinium chloride, as well as in 0.01m- and 0.1m-phosphate buffer, was derived from changes in extinction over the range 220-310mmu on thermal denaturation. 3. The secondary structure of RNA is regarded as an assembly of hairpin loops each of 2N+b residues on average, where N is the number of base-paired residues and b is the number of unpaired residues. 4. If chain scission takes place at random then 2N+b=logf/log(1-p). 5. For RNA from the smaller sub-unit 2N+b was estimated as 25+/-5 residues, compared with 30+/-5 residues for the less stable species and 35+/-5 residues for the more stable species of hairpin loop of RNA from the larger sub-unit.

1. The thermal denaturation of DNA from rat liver was studied spectrophotometrically. In sodium phosphate buffers denaturation led to a single-stranded form having, at 25 degrees , about 25% of the hypochromism of the intact double helix. 2. The hypochromism of the denatured form was the same in 1mm- as in 10mm-sodium phosphate buffer and was scarcely affected by reaction with formaldehyde. The hypochromism was decreased by about 40% in the presence of 8m-urea. 3. The hypochromism of denatured DNA at low ionic strengths was about the same as that of fragments of reticulocyte ribosomal RNA that were too short to form double-helical secondary structure and about the same as that of RNA after reaction with formaldehyde. 4. The spectrum of DNA was slightly affected by the presence of 8m-urea or 4m-guanidinium chloride. The differences in the spectrum of the native and denatured forms of DNA in 0.1m-sodium phosphate buffer, in 8m-urea-10mm-sodium phosphate buffer and in 4m-guanidinium chloride-10mm-sodium phosphate buffer, pH7.6, were similar but not identical. 5. Denatured rat liver DNA appears to have no double-helical character at 25 degrees in 10mm-sodium phosphate buffer, pH7.6; increasing the buffer concentration to 0.1m leads to a more compact form in which about 40% of the residues form base pairs.