This is accomplished by having multiple replication sites on each strand. The rate at which DNA strands are replicated at a rate of up to 4000 nucleotides per second. When the Okazaki fragments are assembled, an enzyme called DNA ligase attaches the sugar phosphate backbone of the lagging strand fragments to form a single DNA strand. They are only needed once for the leading strand.
Primer, primase, and DNA polymerase III are required to begin the formation of each Okazaki fragment of the lagging strand, and to begin the formation of the continuously produced leading strand. The fragments of the lagging strand are called Okazaki fragments. The lagging strand lags because it's being assembled in reverse and it takes a little more time.įormation of the lagging strand is assembled by fragments being produce moving away from the replication fork in the 5' to 3' direction. In the above image, the leading daughter strand (top) is formed from left to right and the lagging strand is formed from right to left. DNA can only be formed in the 5' to 3' direction because of polymerase III.
#DNA SUGAR PHOSPHATE BACKBONE COVALENT BOND FREE#
Then DNA polymerase III allows the addition of free nucleotides in a 5' to 3' direction to produce the growing strand.
It is the small portion in red in the above image. In order for this process to work normally, another enzyme called RNA primase (which is a short strand of RNA), precedes DNA polymerase by preparing the molecule in order for it to receive the nucleotides from DNA polymerase. Two phosphate molecules are lost to provide energy for the chemical bonding of the molecules. This molecule contains deoxyribose, a nitrogen base, and three phosphate groups. DNA polymerase reconstructs two daughter strands.Įach nucleotide that is added to the DNA chain is actually a deoxynucleoside triphosphate dNTP molecule. Remember: Helicase splits the parent strand. DNA polymerase is an enzyme that helps put together new DNA strands from the original strand, nucleotide by nucleotide. This occurs at specific regions, called replication origins, that create a replication fork of two polynucleotide strands in antiparallel directions (the middle portion of the diagram). Helicase breaks the hydrogen bonds that link the nitrogen bases and allows the molecule to split. The original strand, shown on the right side of the image, is unzipped or unwound by the enzyme helicase. As a result, this dual-stranded molecule forms a double helix when twisted. Single-rings always pair with double-rings which form complementary base pairs based on the distance between the sugar-phosphate backbones. Adenine and guanine are known as purines (double ring structures) and guanine and thymine are known as pyrimidines (single ring structures). These hydrogen bonds are very weak, which allows for the molecule to be split in replication.
Adenine forms a double hydrogen bond with thymine and guanine forms a triple hydrogen bond with cytosine. There are four nitrogen bases in DNA (adenine, thymine, guanine and cytosine). So a strand runs 5' - 3' and the opposing strand runs 3' - 5'. The sugar-phosphate backbones are held together by the nitrogen bases and the backbones run in opposite directions (or are antiparallel). This is the sugar-phosphate backbone of the molecule.Īs the nucleotides link, a sequence of nitrogen bases becomes clear. No matter how many nucleotides are added, there will always be a free 5' carbon with a phosphate group and a 3' carbon with a hydroxyl group. Each nucleotide attaches to another by this method. A covalent bond called a phosphodiester bond links the C5 phosphate group to the C3 hydroxyl group. The structure of DNA includes an alternating sequence of phosphates and deoxyribose sugars. These orientations must be anti-parallel for DNA's double helix structure to work. The nitrogen base is attached to C1 and the phosphate is attached to C5.Īlso recall that the orientation of the sugars change: the 5' carbon has the oxygen at the "top" and the 3' carbon has the oxygen at the bottom. The carbon immediately to the right of the oxygen atom is #1, and goes around the pentagon structure as shown to the right. It is a 5-carbon sugar with a phosphate and nitrogen base attached.
Before we go any further, it is important that you recall the structure of the nucleotide that makes up DNA.
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