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PubMed1992

DNA looping.

DNA-looping mechanisms are part of networks that regulate all aspects of DNA metabolism, including transcription, replication, and recombination. DNA looping is involved in regulation of transcriptional initiation in prokaryotic operons, including ara, gal, lac, and deo, and in phage systems. Similarly, in eukaryotic organisms, the effects of enhancers appear to be mediated at least in part by loop formation, and examples of DNA looping by hormone receptor proteins and developmental regulatory proteins have been found. In addition, instances of looped structures have been found in replication and in recombination in both prokaryotes and eukaryotes. DNA loop formation may have different functions in different cellular contexts; in some cases, the loop itself is requisite for regulation, while in others the increase in the effective local concentration of protein may account for the effects observed. The ability of DNA to form loops is affected by the distance between binding sites; by the DNA sequence, which determines deformability and bendability; and by the presence of other proteins that exert an influence on the conformation of a particular sequence. Alteration of the stability of DNA loops and/or protein-DNA binding by extra- or intracellular signals provides responsivity to changing metabolic or environmental conditions. The fundamental property of site-specific protein binding to DNA can be combined with protein-protein and protein-ligand interaction to generate a broad range of physiological states.

DNA↗
PubMed1996

Loop gain of reflexes controlling human standing measured with the use of postural and vestibular disturbances.

1. In this study we measured the loop gain of postural reflexes in standing human subjects. Reflex activity is conventionally described in terms of the muscle activation arising from a perturbation, but in this study the ability of the evoked muscle activity to correct the perturbation was also measured, and the behavior of the entire feedback loop is described. 2. A weak continuous random perturbation was applied at waist level to standing subjects. The effects of the perturbation on body sway and soleus electromyogram (EMG) were identified by cross-correlation, and spectral analysis was used to estimate the open-loop reflex transmission characteristics (i.e., sway to EMG). Under the same conditions, activity in the leg muscles was evoked by galvanic vestibular stimulation with the use of a continuous randomly varying current. The effects on soleus EMG and the subsequent body sway were identified by cross-correlation. This allowed calculation of the open-loop muscle and load behavior (i.e., EMG to sway). From these open-loop reflex and muscle and load transfer functions, the loop gain and phase were calculated. 3. In addition to the gain of the feedback loop, the study describes the transmission characteristics of reflex responses in the leg muscles associated with body sway and the effects of excluding visual and proprioceptive contributions to the response; the transfer function of human soleus with a stimulus that preserves the normal recruitment of motoneurons, including the effects of different load conditions on the muscle; and the transmission characteristics of vestibular pathways that evoke responses in the leg muscles during standing in situations that might modify the reflexes. 4. When standing, the loop gain of reflex feedback is approximately unity and is unchanged by eye closure and stability of support. Reflex transmission introduced a marked phase advance, and this served to offset most of the phase lag introduced by muscle and load. The residual phase lag could explain the frequency of tremor observed during standing (6-8 Hz). 5. The gain of the feedback loop (approximately 1) is higher than suggested by both previous estimates and theoretical considerations, but is still insufficient to explain the stability of normal human standing. This implies that, although sensory information is used to control posture, it does not do so exclusively through a negative feedback control process. The experimental findings are consistent with a reflex response based on a feed-forward process, and this would result in prediction of the response necessary to counteract a postural disturbance.

Adult↗
PubMedDate not supplied

Myoelectric activity of an ileal isoperistaltic or antiperistaltic loop interposed between colon and rectum: an experimental study in pigs.

The myoelectric activity of 25-30 cm ileal loops interposed isoperistaltically or antiperistaltically between colon and rectum was investigated 1-3 and 20-22 weeks after surgery, in growing pigs chronically fitted with intraparietal electrodes on the loop and the ileum. The fundamental motor profile of the small intestine consisting of migrating myoelectric complexes (MMC) was present on both loops; the MMCs were propagated on the antiperistaltic loop in the original anatomical direction. There was no coordination between the occurrence of phase 3 on the loop and the ileum. Some minor differences concerning the duration, the frequency and the velocity of propagation of phase 3 were observed between the loops and the ileum. Moreover propagated clusters of spike bursts and propagated long duration spike bursts appeared on the isoperistaltic and antiperistaltic loop, respectively, but not on the ileum. No prominent differences in the motor profiles of the loops have been observed between 1-3 and 20-22 weeks after surgery. It is concluded that despite the presence of semisolid contents with an abundant microflora the fundamental intestinal motor profile persists in ileal loops interposed between colon and rectum.

Animals↗
PubMed2000

Dual-loop intra-atrial reentry in humans.

BACKGROUND: Dual-loop atrial reentrant tachycardias have not been clinically described. METHODS AND RESULTS: Five patients (3 men, 2 women; mean age, 48+/-16 years) were studied 24+/-15 years after surgical closure of an ostium secundum atrial septal defect for drug-resistant atrial tachycardia. Complete tachycardia mapping was performed in the right atrium with multipolar catheters and a 3-dimensional electroanatomic mapping system (Biosense), followed by linear radiofrequency ablation of the narrowest part of each complete loop. Six tachycardias with a typical flutter morphology, a cycle length of 262+/-40 ms, and a superior f-wave axis (-77+/-11 degrees ) were mapped, 4 with a Biosense map including 106+/-32 points. Five figure-8 tachycardias had a counterclockwise loop around the tricuspid valve sharing a common anterior channel with a clockwise loop around the lateral atriotomy scar. One tachycardia was thought to have 2 counterclockwise loops around the same obstacles. Radiofrequency delivery in the cavotricuspid isthmus in each case transformed the tachycardia without any pause in a different morphology tachycardia with an inferior P-wave axis (50+/-42 degrees ) and nearly the same cycle length (272+/-39 ms) but with the periatriotomy loop alone. This arrhythmia required ablation of a second isthmus: between the lower end of the atriotomy and the inferior vena cava in 4 and the superior tricuspid annulus in 1. After a follow-up of 19+/-6 months, there were no recurrences. CONCLUSIONS: Figure-8 double-loop tachycardias mimicking the ECG pattern of a common atrial flutter occur in some patients after a surgical atriotomy. Ablation of 1 loop produces a sudden transformation to a new reentrant tachycardia formed of the remaining loop that requires ablation at a second isthmus.

Adult↗
PubMed2006

The mid-thigh loop arteriovenous graft: patient selection, technique, and results.

As patient longevity on hemodialysis has increased, surgeons are increasingly challenged to provide vascular access to patients who have exhausted options for access in the upper extremity. A common operation performed on these patients has been the loop thigh arteriovenous (AV) graft based off the common femoral vessels. However, there are several disadvantages of placing prosthetic grafts in proximity to the groin. Our group has modified the thigh loop AV graft procedure by moving the anastomoses to the mid-superficial femoral artery and vein. The advantage of this location is that it preserves the proximal femoral vessels for graft revision and avoids the node-bearing tissue and overhanging panniculus of the groin. The purpose of this study was to review our technique, patient selection, and experience with the mid-thigh loop AV graft procedure. Between 2001 and 2003, 46 mid-thigh loop AV grafts were placed in 38 patients. Patient hospital, office, and dialysis clinic records were reviewed. The primary and secondary patency for AV grafts in this study by life-table was 40 per cent and 68 per cent at 1 year and 18 per cent and 43 per cent at 2 years. There were 10 infections (21%) requiring graft removal. Four patients underwent subsequent placement of a proximal loop thigh AV graft after mid-thigh graft failure. Patient survival was 86 per cent at 1 year and 82 per cent at 2 years. There were no patient deaths related to thigh graft placement. Our results with the mid-thigh loop AV graft compare favorably with published results for thigh loop AV grafts. The procedure preserves the proximal vasculature, permitting graft revision or subsequent proximal graft placement, and may be associated with fewer infectious complications. The mid-thigh loop AV graft procedure should be considered before placement of a thigh loop AV graft based off the common femoral artery and vein.

Adult↗
PubMedDate not supplied

Auditory and vestibular system findings in patients with vascular loops in the internal auditory canal.

Many anatomic studies have shown that a loop of the anterior inferior cerebellar artery is frequently found in the cerebellopontine angle and internal auditory canal. The concept of vascular cross-compression has been extended to the eighth cranial nerve, and patients with symptoms of hearing loss, tinnitus, and vertigo have been treated surgically by separating the vascular loop from the nerve. Previous reports have emphasized vascular anatomy, surgical approaches, and treatment results. Our study provides details of audiometric and vestibular system test results in a series of ten patients with prominent vascular loops in the internal auditory canal diagnosed by computerized tomography after subarachnoid space air injection (pneumo-CT). All patients had a unilateral (or asymmetric) hearing loss on the side of the lesion, and no vascular loops were detected on the contralateral sides. Hearing losses ranged from mild to profound. Audiometric findings were generally of a cochlear type, and most patients had excellent speech discrimination. Spontaneous nystagmus was detected in all patients during neurotologic testing, and half of the patients had normal caloric test results. The variability of audiometric and vestibular system test results is probably a reflection of anatomic variations of the vascular loop and its branches. Auditory and vestibular symptoms may be due to a complex interaction between the eighth cranial nerve and the vascular loop, in which the loop compresses the nerve and the nerve compromises circulation to the inner ear. Although symptoms from vascular loops and eighth nerve tumors are similar, the findings of a cochlear type of hearing loss, excellent speech discrimination, and normal caloric test results should raise the suspicion of a vascular loop.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMedDate not supplied

Cold knife conization and loop excision for cervical intraepithelial neoplasia.

AIMS AND BACKGROUND: Our aim was to investigate whether loop excision is an acceptable alternative to traditional cold knife conization of the cervix. PATIENTS AND METHODS: 240 with cervical intraepithelial neoplasia (CIN) were randomly assigned to loop excision (n = 120) or cold knife conization (n = 120). Success and complication rates of both methods were analysed. RESULTS: 100% of cold knife conization and 98% of loop excision surgical specimens were positive for dysplasia. The rate of complete resection was 91% in the cold knife and 82% in the loop excision group, but histologic confirmation of residual CIN was obtained in only 2 (1.7%) women after cold knife conization and in 5 (4.2%) after loop excision. Loop excision cones were significantly shallower than those obtained by a cold knife. Secondary surgical procedures due to early hemorrhage were performed in 9 (7.5%) patients treated with cold knife conization and in 8 (6.7%) treated with loop excision. Elevated temperature postoperatively was observed in 16.4% of patients after cold knife conization and in 13.9% after loop excision. There were no other postoperative complications. CONCLUSIONS: The results suggest that cold knife conization and loop excision are comparable and equally effective diagnostic and therapeutic procedures for CIN.

Adult↗
PubMed1994

The middle portion in the second cytoplasmic loop of the thyrotropin receptor plays a crucial role in adenylate cyclase activation.

We have examined the role of the 2nd cytoplasmic loop of the TSH receptor (TSHR) in TSH- and TSHR autoantibody-stimulated cAMP and inositol phosphate formation using mutants created by substituting sequences from the alpha 1- or beta 2-adrenergic receptors (AR). Unlike similar substitution mutants involving the 3rd cytoplasmic loop that lose agonist-induced inositol phosphate but not cAMP increase after transfection into Cos-7 cells, mutants involving the 2nd loop showed significant change in generating both signals. Mutant B525, which substitutes residues 525-527 with a comparable beta 2-AR sequence, exhibited a complete loss in TSH- or Graves' immunoglobulin G-increased cAMP signaling and a lesser loss in phosphoinositide signaling. This is a unique mutant in which cAMP response was completely lost in all those involving the 2nd or 3rd cytoplasmic loop. On the other hand, mutant B528, in which residues 528-532 are substituted with a comparable beta 2-AR sequence, exhibited the most profound loss in phosphoinositide signaling. Mutants involving portions surrounding residues 528-532 in the 2nd cytoplasmic loop had milder losses in agonist-increased phosphoinositide signaling and much lesser losses in agonist-increased cAMP generation. The transfection efficiency of all transfectants was the same. All transfectants with mutant or wild type TSHR had a similar amount and identical profile of TSHR mRNA in Northern blots and TSHR forms on Western blots. Thus, the 2nd cytoplasmic loop is important for agonist-induced cAMP as well as for phosphoinositide signal generation, whereas the 3rd loop appears to be important only for the latter. The most important determinant for agonist-increased cAMP signal generation is in the middle of the 2nd loop, around residues 525-527. In contrast, the determinants most critical for agonist-induced phosphoinositide signaling are also located in the middle of the 2nd loop, around residues 528-532, and those with less importance are broadly distributed.

Adenylyl Cyclases↗
PubMed2005

NMR structure of the apoB mRNA stem-loop and its interaction with the C to U editing APOBEC1 complementary factor.

We have solved the NMR structure of the 31-nucleotide (nt) apoB mRNA stem-loop, a substrate of the cytidine deaminase APOBEC1. We found that the edited base located at the 5' end of the octa-loop is stacked between two adenosines in both the unedited (cytidine 6666) and the edited (uridine 6666) forms and that the rest of the loop is unstructured. The 11-nt "mooring" sequence essential for editing is partially flexible although it is mostly in the stem of the RNA. The octa-loop and the internal loop in the middle of the stem confer this flexibility. These findings shed light on why APOBEC1 alone cannot edit efficiently the cytidine 6666 under physiological conditions, the editing base being buried in the loop and not directly accessible. We also show that APOBEC1 does not specifically bind apoB mRNA and requires the auxiliary factor, APOBEC1 complementary factor (ACF), to edit specifically cytidine 6666. The binding of ACF to both the mooring sequence and APOBEC1 explains the specificity of the reaction. Our NMR study lead us to propose a mechanism in which ACF recognizes first the flexible nucleotides of the mooring sequence (the internal loop and the 3' end octa-loop) and subsequently melts the stem-loop, exposing the amino group of the cytidine 6666 to APOBEC1. Thus, the flexibility of the mooring sequence plays a central role in the RNA recognition by ACF.

APOBEC-1 Deaminase↗
PubMed1993

Cardiac arrhythmias during performance of the flow-volume loop.

The purpose of our study was to determine the frequency of cardiac ectopy during the performance of flow-volume loops. The performance of an adequate flow-volume loop entails significant shifts in intrathoracic pressure and oxyhemoglobin saturation that could heighten myocardial arrhythmogenicity and lead to increased cardiac ectopy. Forty-two male patients referred for performance of flow-volume loops were studied with an ambulatory cardiac monitor and an arterial oxyhemoglobin saturation monitor during the performance of three flow-volume loops before and after administration of an aerosolized, selective beta 2-agonist. For comparison, the 60 min prior to and after completion of the loop testing was monitored with the continuous cardiac monitor. The frequency of cardiac ectopy was determined in four patient populations delineated by the presence or absence of cardiac, pulmonary, or cardiopulmonary disease. Each group was further divided based on the occurrence of arterial oxyhemoglobin desaturation during the performance of the flow-volume loop. We found that, despite the presence or absence of underlying cardiopulmonary disease or the occurrence of oxyhemoglobin desaturation during flow-volume loop performance, the number of cardiac ectopies diminished significantly during loop performance when compared with the 60-min study period prior to testing. This is in contrast to what had been reported previously. We conclude that the flow-volume loop is safe to perform in the general population, including those individuals with an increased risk of arrhythmias such as those with cardiac or pulmonary disease.

Aged↗
PubMed2005

Disruption of protein-mediated DNA looping by tension in the substrate DNA.

Protein-mediated DNA looping is important in a variety of biological processes, including gene regulation and genetic transformation. Although the biochemistry of loop formation is well established, the mechanics of loop closure in a constrained cellular environment has received less attention. Recent single molecule measurements show that mechanical constraints have a significant impact on DNA looping and motivate the need for a more comprehensive characterization of the effects of tension. By modeling DNA as a wormlike chain, we calculate how continuous stretching of the substrate DNA affects the loop formation probability. We find that when the loop size is >100 bp, a tension of 500 fN can increase the time required for loop closure by two orders of magnitude. This force is small compared to the piconewton forces that are associated with RNA polymerases and other molecular motors, indicating that intracellular mechanical forces might affect transcriptional regulation. In contrast to existing theory, we find that for loops <200 bp, the effect of tension is partly dependent on the relative orientation of the DNA-binding domains in the linker protein. Our results provide perspective on recent DNA looping experiments and suggestions for future micromechanical studies.

Computer Simulation↗
PubMed1979

Computer analysis of vectorcardiograms in myocardial infarction with special reference to polar vector and planarity of the QRS and T loops.

Spatial characteristics of the QRS and T loops in 110 patients with old myocardial infarction were analysed in comparison with 221 normal subjects. Measured were (1) QRS and T polar vectors, (2) initial 20- and 30-msec segmental QRS polar vectors, and (3) length, width, thickness, and ratios of width/length and thickness/length of the QRS and T loops in edgewise and broadside projections. Broadside and edgewise projections were obtained by transformation of the reference frame of the Frank lead system to a patient's own frame based on the polar vector. The recognition rates of abnormality in the QRS and T polar vectors were 66% and 60% of a total of 110 patients with myocardial infarction. The initial 30 msec segmental QRS polar vector showed the highest recognition rate of abnormality in myocardial infarction, i.e., 87% in anterior myocardial infarction, 100% in extensive anterior myocardial infarction and 78% in inferior myocardial infarction. The initial segmental QRS polar vector was abnormally deviated posteriorly and superiorly in inferior myocardial infarction. In anterior myocardial infarction, the initial segmental polar vector was directed inferiorly in more than 50% of the cases, while the vector in normal subjects was located superiorly and to the left. The QRS loop of anterior myocardial infarction was significantly smaller in the width and width/length ratio and significantly larger in the thickness and thickness/length ratio than those of the normal. Poor planarity of the QRS loop was one of the characteristics of myocardial infarction, especially of extensive anterior myocardial infarction. The T loop of myocardial infarction was significantly larger in the width/length ratio than that of the normal. More than 50% of the cases with anterior myocardial infarction showed abnormally wide T loops. The polar vector was a useful index to characterize the spatial orientation and sense of rotation of the spatial loop. In addition, the initial segmental QRS polar vector represented the mild localized abnormalities of the spatial loop. The loop configuration in space was characterized in edgewise and broadside projections.

Acute Disease↗
PubMed2000

Closed-loop identification of carotid sinus baroreflex transfer characteristics using electrical stimulation.

Although random aortic pressure (AOP) perturbation according to a binary white noise sequence enables us to estimate open-loop dynamic characteristics of the carotid sinus baroreflex under closed-loop conditions, the necessity of arterial catheter implantation limits the applicability of this method in freely moving animal experiments. Thus, we explored a closed-loop system identification method using electrical stimulation. In 6 anesthetized and vagotomized rabbits, we stimulated the aortic depressor nerve with a binary white noise sequence (0-10 Hz) under baroreflex closed-loop conditions while measuring cardiac sympathetic nerve activity (SNA) and AOP. We used a closed-loop identification method to estimate the peripheral arc transfer function from SNA to AOP. The peripheral arc transfer function approximated a second-order low-pass filter and its fitted parameters did not differ from those obtained by an open-loop identification method (dynamic gain: 1.16+/-0.32 vs. 1.02+/-0.11; natural frequency: 0.08+/-0.03 vs. 0.09+/-0.03 Hz; damping ratio: 1.53+/-0.15 vs. 1.57+/-0.21). In 6 different rabbits, we applied intermittent rapid pacing (396 beats/min) under baroreflex closed-loop conditions to estimate the neural arc transfer function from AOP to SNA. The neural arc transfer function approximated a first-order high-pass filter and its fitted parameters did not differ from those obtained by an open-loop identification method (dynamic gain: -1.15+/-0.45 vs. -1.06+/-0.05; corner frequency: 0.12+/-0.05 vs. 0.13+/-0.03 Hz). In conclusion, the closed-loop identification method using electrical stimulation is effective to estimate the neural and peripheral arc transfer functions.

Animals↗
PubMed1990

Gut formation after transection of the midgut loop in the chick embryo.

The primitive gut in vertebrates can be divided into the foregut, midgut, and hindgut. The midgut forms the midgut loop or the intestinal loop, which rotates as it develops the small and large intestines. We examined the effects of transection of the midgut loop on the subsequent development of the intestine in chick embryos by their out-of-the-shell incubation. The development of the embryo and the intestines out of the shell was nearly identical to that usual in-the-shell incubation. The rotation of the midgut loop began in stage 27 (5 days of incubation) and was completed in stage 36 (10 days of incubation) after counterclockwise rotation through 180 degrees. In experimental groups, the midgut loop was transected immediately before or in the early stages of rotation (stages 27-33) proximally or distally to the apex of the loop. Transecting of the midgut loop caused little effects on its subsequent rotation regardless of the time or the site of transection although the secondary loop formation of intestine was poor in some cases. The stumps of the transected intestine were repaired and closed. After closure of the stumps, the proximal segment of the intestine was dilated in some cases. The secondary loop formation, or convolution of the intestine didn't occur in some dilated cases.

Animals↗
PubMed1995

Involvement of loop 2 of platelet-derived growth factor-AA and -BB in receptor binding.

Platelet-derived growth factor (PDGF) is a disulfide-bonded antiparallel dimer of A- and B-polypeptide chains. Each subunit contains two loops (loops 1 and 3) which point in the same direction, and which are located close to a region (loop 2) from the other subunit of the dimer. Previous studies have shown that epitopes in loops 1 and 3 are important for binding to PDGF alpha- and beta-receptors. The aim of the present investigation was to determine the importance of loop 2 for receptor interactions. PDGF A- and B-chain cDNA:s were mutated in the loop 2 regions and transfected into COS cells. Analyses of conditioned media of such cell cultures revealed that PDGF B-chain mutated in the loop 2 region lost its ability to compete with 125I-PDGF for binding to PDGF beta-receptors, but retained 2-5% of its binding of alpha-receptors. The A-chain binds only to alpha-receptors; 2-5% of this binding was also retained after mutation of the loop 2 region. In conclusion, the loop 2 region of PDGF is important for receptor binding, but appears to be more important for binding to the PDGF beta-receptors than to the alpha-receptors.

Amino Acid Sequence↗
PubMed1999

Loop diathermy or laser excisional conization for cervical intraepithelial neoplasia.

BACKGROUND: Cervical intraepithelial neoplasia (CIN) can be managed by ablative or excisional procedures. We have compared the excision time, effectiveness, and safety of loop diathermy (loop) against laser conization. METHODS: In a prospective study in two hospital departments 222 women were randomized to loop or laser conization. Data were collected by questionnaires after operation and at two follow-up examinations. RESULTS: At department A (122 women), two physicians performed 27% of the loop and 35% of the laser excisions; at department B (100 women), the corresponding figures were 69% and 59%. Loop was quicker than laser conization in both departments (median 3-4 min versus 10-20 min), while laser conization was more time consuming in department A (median A/B = 20/10 min). Peroperative bleeding dominated during the laser procedure in both departments and complicated the loop procedure more frequently in department A. Postoperative bleeding occurred with equal frequency in the four groups (41.8%, 52.7%, 59.2%, 64.7%). At both departments, bleeding for more than two weeks was reported twice as often after laser conization (A:13.8%, B:24.2%), when compared to loop excision (A:7.1%, B:13.7%). Residual CIN was found in all of three re-conizations and in one of eight hysterectomy specimens. CONCLUSIONS: Loop was quicker than laser excision, per- and postoperative bleeding diminished, and the success rates were comparable. Physicians mastered Loop excision after a few attempts. However, the results improved, when performed by a restricted number of physicians. Histological incomplete excision indicates close colposcopic and cytologic follow-up to identify residual CIN.

Adult↗
PubMed2004

[Influence of loops on the torsion stiffness of rectangular wire].

OBJECTIVE: To study the influence of loops on the torsion stiffness of rectangular wire. METHODS: The torsion moments and angles of 0.018 inch x 0.025 inch rectangular orthodontic arch wires in single 0.022 inch bracket of the first maxillary incisor were measured with a torsion measurement apparatus, and the measured values were analyzed by two-way ANOVA. The wires included straight stainless steel rectangular wires, and 3.5 mm, 7 mm, 10.5 mm, 14 mm high vertical loop wires, and 3 mm, 6 mm, 9 mm, 12 mm long horizontal loop wires. RESULTS: 1. The torsion stiffnesses of straight wire and vertical and horizontal loop wires were 4.909, 4.544, 4.137, 3.851, 3.454, 3.448, 3.024, 2.686, 2.378 N.mm/degree respectively. 2. The arch wires will be twisted 19.143 degrees, 19.377 degrees, 19.851 degrees, 20.949 degrees, 21.632 degrees, 21.812 degrees, 22.418 degrees, 23.254 degrees, 24.316 degrees respectively to obtain 20 N.mm force. CONCLUSION: The design of loop can reduce the torsion stiffness of rectangular wire. Compared with the length of loop, the shape of loop has more important effect on the reduction of torsion stiffness. The horizontal loop design as a factor for the control of the tongue is more effective than the vertical loop design.

Analysis of Variance↗
PubMedDate not supplied

DNA loop organization and DNA fragmentation during radiation-induced apoptosis in human lymphocytes.

Apoptotic DNA fragmentation induced by gamma-rays has been compared with the DNA loop sizes in G0-human lymphocytes using pulsed field gel electrophoresis (PFGE). Genomic DNA was cleaved into the DNA loops at the topoisomerase II mediated attachment points using short treatment of cells with etoposide. The apoptotic fragmentation, with a distinct cut-off around 50 kb for a maximum length of fragments, appeared 5 h after irradiation when the most part of radiation-induced DNA double strand breaks (DSBs) have been repaired. The data indicate that apoptotic fragmentation of DNA in the G0-human lymphocytes begins when repair of radiation-induced DSBs has been completed. Similar apoptotic DNA fragmentation was also observed following the treatment of cells with etoposide. All genomic DNA was fragmented into 50-kb fragments during the final stages of apoptosis. Most of the DNA in resting lymphocytes is organized into Mb-size loops but loops of sizes down to 50 kb were also observed. A sharp border between the size distributions of DNA loops and apoptotic fragments was found. The data suggest that 50 kb apoptotic fragmentation is not based on excision of the DNA loops. No apoptotic fragments with the sizes more than 5.7 Mb were seen during the whole course of apoptosis. This observation indicates that despite intensive apoptotic fragmentation into the 50-kb fragments the chromosomes maintain integrity during radiation-induced apoptosis in human lymphocytes. We propose a model for radiation-induced apoptotic fragmentation in human lymphocytes that involves four stages: induction of DNA breaks and relaxation of DNA loops; DNA repair followed by reorganization of the DNA loops into the 50-kb units of condensed chromatin; co-operative fragmentation of the reorganized DNA loops into the distinct 50-kb fragments and resealing of the chromosome ends at the sites of this fragmentation; cleavage of the 50-kb fragments at the internucleosomal spacers.

Apoptosis↗