Search PubMed⌕ Search

Biomedical subjects

C T Zhang

Publications and source records attributed to C T Zhang.

63 records · Page 4Linked to original sources
PubMed1991

The study of stacking energy for natural DNA sequences.

The stacking energies between bases of DNA for both A and B conformation have been calculated by Aida & Nagata (1986, Int. J. Quantum Chem. 29, 1253-1261). For naturally occurring DNA sequences, by assuming that the average stacking energy of A conformation is completely identical with that of B, the lowest average stacking energy for double strand DNA has been calculated and found to be equal to -7.38 kcal M-1. This conclusion has been confirmed by the data of stacking energy of 112 promoter sequences of Escherichia coli and some other natural DNA sequences. Our result shows that the natural DNA sequences are bi-stable. One stable-state is the A conformation, the other is B. It is shown that the result is only applicable to the transcription processes that regulate the gene expression. Through the interaction with RNA polymerase in the processes of transcription, the conformation of DNA might undergo a transition of B----A----B.

Animals↗
PubMed1991

Diagrammatic representation of the distribution of DNA bases and its applications.

The frequencies of occurrence of the bases A, C, G and T (or U) in a DNA or mRNA sequence are denoted by a, c, g, t (or u), respectively. Since a + c + g + t = 1, the four real numbers are mapped into a point within a regular tetrahedron. The mapping points are then projected to some planes for further study. We have given several examples of application of this technique. As a by-product of an application we have found an empirical formula on the limit distribution of DNA bases for different kind of organisms, i.e. 1/4 less than or equal to a2 + c2 + g2 + t2 less than 1/3.

Adenine↗
PubMed1990

Analysis of patterns of twist angles in DNA double helix.

We have analysed theoretically the patterns of twist angles of B-DNA by the Tung-Harvey model mainly. It is shown that for a sequence of twist angles a smaller twist angle tends to follow a larger one and vice versa. Therefore the sequence of twist angles always takes a gentle zig-zag form. For simplicity we convert the sequence of twist angles to a symbolic sequence consisting of L and S, where L or S represents a large or a small angle, respectively. The -10 and -35 regions of 112 well-defined promoters for E. coli RNA polymerase, which were compiled by Hawley and McClure, have been analysed in terms of LS sequences in detail. The results shows that the number of LS sequences for promoters is considerably limited and the promoter mutations do not change the patterns of LS sequences in most cases. Several new ideas, which are believed to be useful in the further study, have been presented.

Base Sequence↗
PubMed1989

The study of DNA sequences by their sequences of twist angles.

To study the properties of DNA sequences we have transformed the sequences of bases into the sequences of twist angles along the chain of DNA double helix by using the Dickerson sum function. The Fourier transform and the auto-correlation function of the twist angles sequences have been used to study the periodicity and randomness of the original DNA sequences. Basing on the correlation coefficient, a "distance" between two DNA fragments has been defined and used to compare some realistic DNA sequences. It is hoped that the techniques developed here could be used to analyze more realistic DNA sequences.

Base Sequence↗
PubMed1989

Upper limit for the variances of some helical parameters in DNA double helix.

Assuming that the realistic DNA chains are random sequences of purines and pyrimidines and, by using the Dickerson sum functions, it is shown that there exists an upper limit for the variance of helical twist angles, the base-plane roll angles and the other helical parameters respectively in the DNA sequences. The estimates of the variances of all the above helical parameters for the DNA sequences in the Los Alamos data base have been performed and found to be in good agreement with the theoretical results obtained in this paper.

Animals↗
PubMed1989

Analysis of sequences of twist angles in DNA double helix.

By assuming that the realistic DNA chains are random sequence of bases and using the Tung-Harvey formula for the prediction of twist angles, it is shown that the mean value of the sequence of twist angles is almost sequence-independent. In general the variance for the A, T-rich sequence is larger than that of G, C-rich sequence. There exists an upper bound for the variance of all possible sequences, i.e., the variance is not greater than 27 deg2. It is pointed out that the large conformational deviation from ideal DNA is an important factor for the recognition of DNA with protein/enzyme.

Base Sequence↗
PubMed1988

Thalassemia in the Silk Road region of China.

This paper summarizes data obtained during a screening program involving 11,563 persons from the Silk Road region in China. The mean incidence of thalassemia is 1.62% with an increase from east to west. The incidence in the Hui population (3.01%) is higher than in Kazaks (2.92%), and in Uygur (2.22%). The Han population also has a higher incidence (0.98%) than seen for other regions in Northern China. The thalassemias observed are classified into seven groups; beta-thalassemia accounts for 89.48% of the total, and alpha-thalassemia for 10.52%.

China↗