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Wilmington, North Carolina once had a thriving Black middle class and an elected interracial government. In 1898 white supremacists used violence to overthrow that government, kill Black residents and drive many others from the city.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
D-loop replication is a proposed process by which circular DNA molecules like those found in many (though not all) chloroplasts and mitochondria replicate their genetic material. These circular chromosomes often contain D-loops, short regions of triple-stranded DNA where the double-stranded duplex molecule is opened and one of the strands is displaced by a third, independent strand of variable length. Not all circular genomes use D-loop replication, however, nor do all chloroplasts and mitochondria use circular chromosomes; in those with linear chromosomes, D-loop replication does not occur.[1]
In most double-stranded DNA molecules, one of the two strands tends to be composed of heavier nucleotides with higher molecular weights (i.e. relatively more purines: adenine and guanine). This strand is called the H (heavy) strand. The complementary L (light) strand comprises lighter nucleotides (i.e. the pyrimidines: thymine and cytosine). Replication begins with replication of the heavy strand starting at the D-loop (also known as the control region). This structure consists of an intervening third strand which is complementary to the light strand and displaces the heavy strand to form a displacement loop (D-loop).[2] Circular DNA is stable with this small D-loop and can remain in this formation more or less indefinitely, but the middle strand, or the displacing strand, must be replaced frequently due to its short half-life, which is very energetically expensive to the cell.[3][4] When diagrammed, the resulting structure looks like the letter D. The D-loop was first discovered in 1971 when researchers noticed that many DNA in the mitochondria they were examining under the microscope contained a short segment that was triple-stranded.[2]
Each D-loop contains an origin of replication for the heavy strand. Full circular DNA replication is initiated at that origin and replicates in only one direction. The middle strand in the D-loop can be removed and a new one will be synthesized that is not terminated until the heavy strand is fully replicated, or until the middle strand can serve as a primer for the heavy strand's replication. As the heavy strand's replication reaches the origin of replication for the light strand, a new light strand will be synthesized in the direction opposite to that of the heavy strand.[3][5][6]
There is more than one proposed process through which D-loop replication occurs, but in all of the models, these steps are agreed upon. The portions not agreed upon regard the importance of maintaining a D-loop when replication is not in progress, because it is energetically expensive to the cell, and what mechanisms, during replication, preserve the detached strand of DNA that is waiting to be replicated.[7][8][9]
The D-loop region is important for phylogeographic studies. Because the region does not code for any genes, it is not imperative for this region to remain conserved over time; therefore, it is free to mutate with only a few selective limitations on size and heavy/light strand factors. The mutation rate for the D-loop region is among the fastest in either the nuclear or mitochondrial genomes in animals. Using these mutations in the D-loop, recent and rapid evolutionary changes can effectively be tracked such as within species and among very closely related species. Due to the high mutation rate, it is not effective in tracking evolutionary changes that are not recent. This is a very common use of the D-loop in genomics.[10]
One example of the use of D-loop mutations in phylogeographic studies was the phylogeny assembled using the largely unstudied Iberian red deer. Scientists tracked D-loop polymorphisms within these red deer and determined the genetic relationship that these deer had among each other. They were also able to determine the relationships, based on D-loop similarities and differences, between these red deer and other species of deer throughout Europe.[11] In another example, scientists used variations in the D-loop, along with microsatellite markers, to study and map genetic diversity among goats in Sri Lanka.[12]
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In molecular biology, a displacement loop or D-loop is a DNA structure where the two strands of a double-stranded DNA molecule are separated for a stretch and held apart by a third strand of DNA. An R-loop is similar to a D-loop, but in that case the third strand is RNA rather than DNA. The third strand has a base sequence which is complementary to one of the main strands and pairs with it, thus displacing the other complementary main strand in the region. Within that region the structure is thus a form of triple-stranded DNA. A diagram in the paper introducing the term illustrated the D-loop with a shape resembling a capital "D", where the displaced strand formed the loop of the "D". D-loops occur in a number of particular situations, including in DNA repair, in telomeres, and as a semi-stable structure in mitochondrial circular DNA molecules.
D-loop replication is a proposed process by which circular DNA molecules like those found in many (though not all) chloroplasts and mitochondria replicate their genetic material. These circular chromosomes often contain D-loops, short regions of triple-stranded DNA where the double-stranded duplex molecule is opened and one of the strands is displaced by a third, independent strand of variable length. Not all circular genomes use D-loop replication, however, nor do all chloroplasts and mitochondria use circular chromosomes; in those with linear chromosomes, D-loop replication does not occur.
A closed-loop controller or feedback controller is a control loop which incorporates feedback, in contrast to an open-loop controller or non-feedback controller. A closed-loop controller uses feedback to control states or outputs of a dynamical system. Its name comes from the information path in the system: process inputs (e.g., voltage applied to an electric motor) have an effect on the process outputs (e.g., speed or torque of the motor), which is measured with sensors and processed by the controller; the result (the control signal) is "fed back" as input to the process, closing the loop. In the case of linear feedback systems, a control loop including sensors, control algorithms, and actuators is arranged in an attempt to regulate a variable at a setpoint (SP). An everyday example is the cruise control on a road vehicle; where external influences such as hills would cause speed changes, and the driver has the ability to alter the desired set speed. The PID algorithm in the controller restores the actual speed to the desired speed in an optimum way, with minimal delay or overshoot, by controlling the power output of the vehicle's engine. Control systems that include some sensing of the results they are trying to achieve are making use of feedback and can adapt to varying circumstances to some extent. Open-loop control systems do not make use of feedback, and run only in pre-arranged ways. Closed-loop controllers have the following advantages over open-loop controllers: disturbance rejection (such as hills in the cruise control example above) guaranteed performance even with model uncertainties, when the model structure does not match perfectly the real process and the model parameters are not exact unstable processes can be stabilized reduced sensitivity to parameter variations improved reference tracking performance improved rectification of random fluctuations In some systems, closed-loop and open-loop control are used simultaneously. In such systems, the open-loop control is termed feedforward and serves to further improve reference tracking performance. A common closed-loop controller architecture is the PID controller.
The D arm is a conserved structural element of transfer RNA (tRNA) composed of a short D stem and a D-loop that characteristically contains the modified nucleotide dihydrouridine. It plays important roles in stabilizing tRNA tertiary structure and in molecular recognition, particularly by aminoacyl-tRNA synthetases during protein synthesis. Although broadly conserved across Bacteria, Archaea, and Eukaryota, the D arm exhibits structural variation among different tRNA types and organisms, and is absent from certain mitochondrial tRNAs.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →The Greenwood District.