Discontinuous leading-strand synthesis: a stop–start …
This type of replication is termed semiconservative replication because each newly formed molecule of DNA has one strand conserved from the parent molecule and one newly synthesized strand (depicted left).
Simultaneous continuous and discontinuous synthesis …
For a given piece of DNA, replication begins at numerous origins of replication. Each origin of replication is composed of a team of enzyme proteins that are involved in DNA replication. Helicases unwind the DNA helix, single-strand binding proteins keep the strands separate while primases initiate replication, and DNA polymerase adds nucleotides to the unwound parent molecule. While the fundamentals of replication are simple, there is a feature of DNA structure that makes things a bit more complicated; the strands have opposite chemical polarities. This can be hard to comprehend because the strands seem identical. However, a close inspection reveals that the H-bonding that occurs between bases is only achieved if the strands have opposite polarities. This arrangement of strands is antiparallel, with one strand designated the 3'-to-5' strand and the other the 5'-to-3' strand.
DNA polymerase can only add nucleotides to the 3' end of a parent strand. Therefore, nucleotide addition is a smooth, continuous process along one of the strands (the leading strand) of DNA. The other strand (the lagging strand) has a discontinuous mode of replication because DNA polymerase can only work by starting from the replication fork(where DNA is unwinding) and progressing outward (until it runs into a previously synthesized fragment). An added wrinkle to the process on the lagging strand is created by the lack of a continuous new strand; DNA polymerase can only add nucleotides to an existing 3' nucleotide. How is this lagging strand started? Primase has the ability to synthesize a short primer made of a few nucleotides of RNA. DNA polymerase can then add DNA nucleotides to the end of this primer sequence and synthesize relatively short stretches of DNA known as Okazaki fragments. An additional enzyme (ligase) seals the fragments into a continuous strand of DNA. The figure above provides an overview of the enzymes involved, antiparallelism, and the overall direction of DNA replication. All of this takes place in the nucleus of eukaryotic cells (cells that have membrane-bound nuclei and organelles). Be sure that you study this figure carefully. You could see this figure unlabeled and be asked to name the enzymes and their functions on an exam.
SparkNotes: DNA Replication and Repair: DNA Replication
As mentioned, the process of transcription occurs in the nucleus of eukaryotic cells and requires that the two strands of DNA separate, or open up sufficiently enough so that complementary RNA nucleotides can be added to one side of the DNA molecule. The strand that is copied is the template strand. The enzyme RNA polymeraseseparates the DNA strands and joins the RNA nucleotides along the exposed DNA template. This process is initiated when certain proteins, transcription factors, bind to a specific starting point, the promoter. The promoter is actually a sequence of DNA bases that signals the beginning of RNA synthesis. RNA polymerase then adds nucleotides to the 3' end of the elongating RNA molecule. The enzyme then moves down the DNA strand, unwinding as it goes and allowing the DNA helix to reform after a sequence has been transcribed. This continues until a specific RNA sequence is transcribed. This sequence, the terminator sequence, signals the end of RNA synthesis. Transcription is broken down into three stages: initiation, elongation, and termination.
A summary of DNA Replication in 's DNA Replication and Repair
Recall that we discussed promoter regions of DNA and how these promoters bind specific proteins to initiate transcription, thereby setting the stage for protein synthesis. Different genes may have different promoters that respond to different transcription factor proteins. Gene regulation describes how genes can be "turned on" to synthesize a needed protein, or "turned off" to stop synthesis of a protein that is no longer needed.
Molecular Biology | Biochemistry for Medics – Lecture …
This tutorial examined DNA replication, RNA transcription, and protein translation. These processes are pivotal to life, so it will be important that you have a firm grasp on the basic aspects of these processes (i.e., to the level presented in this tutorial).
Replication Fork | Science Primer
All cells that divide need to replicate their DNA so that each daughter cell contains a full complement of all the parent's genetic information. In the process of replication, the two strands are replicated with remarkable fidelity. To appreciate this process, keep a couple of things in mind. First, DNA is comprised of two antiparallel strands. The polarity of each strand is due to the manner in which the nucleotides are linked together. Second, DNA is synthesized in only one direction; 5' to 3'. Be sure that you understand that one strand is synthesized continuously, and that the other is synthesized discontinuously. Be sure that you understand why this is so, and be sure that you are familiar with the basic steps involved in this process.