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| DNA replication |
DNA, deoxyribonucleic acid, is the hereditary material of most living creatures. It carries genetic information that determines all types of plant lives. DNA replication is a process by which a single DNA molecule is copied, resulting in two identical molecules prior to the cell division. The accuracy and precision in DNA replication has ensured the continuity of life from generation to generation.
Following James Watson and Francis Crick’s landmark proposal for the structure of the deoxyribonucleic acid (DNA) molecule in 1953, many scientists turned their attention to how this molecule is replicated.
The process of replication is the key to continuity of plant as well as other forms of life. The DNA molecule consists of two polymer chains (strands) forming a double helix.
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Each strand is made up of a five-carbon sugar (2′-deoxyribose), phosphoric acid, and four nitrogen-containing bases. Two bases are purines, which have a double ring structure; the other two are pyrimidines, which contain a single ring. The purine bases are adenine (A) and guanine (G), while the pyrimidine bases are thymine (T) and cytosine (C).
The two strands of a DNA molecule running in opposite directions are held together by hydrogen bonding between the A-T and G-C base pairs, which form the “rungs” of the ladder that makes the double helix. Such complementary base pairing is the foundation for the DNA double helix as well as its replication.
Semiconservative Replication
The replication mechanism first proposed by Watson and Crick was that the strands of the original (parental) duplex separate, and each individual strand serves as a pattern or template for the synthesis of a new strand (daughter).
The daughter strands are synthesized by the addition of successive nucleotides in such a way that each base in the daughter strand is complementary to the base across the way in the template strand. This is called semiconservative replication.
Although the mechanism is simple in principle, replication is a complex process, with geometric problems requiring energy, a variety of enzymes, and other proteins. The end result, nevertheless, is that a single double-stranded DNA is replicated into two copies having identical sequences.
Each of the two daughter double-stranded DNA copies is made up of one parental strand and one newly synthesized strand, hence the name semi-conservative (literally, “half conserving”) replication.
DNA Synthesis
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| DNA Synthesis |
Then each parental strand is used as a template for the formation of a new daughter strand. Finally, one parental strand and its newly synthesized daughter strand wind together into one double helix, while the other parental strand and its daughter strand wind together into a second double helix.
The process begins with the separation and unwinding of segments of the parental double helix. To accomplish this, an enzyme named DNA helicase, powered by adenosine triphosphate (ATP), works its way between the two strands. As this enzyme “plows” its way through the double helix, it breaks the hydrogen bonds that hold together the “rungs” of the ladder, formed by the base pairs.
DNA helicase then “walks” along one strand, nudging the other strand out of its way as it goes. The result is that the two DNA strands separate, thus exposing their bases, as though the ladder had been split vetically, down through the rungs.
The second step of DNA synthesis requires the enzyme DNA polymerase, which performs a dual function during the replication. First, it recognizes bases exposed in a parental strand and matches them up with free nucleotides that have complementary bases.
Second, DNA polymerase bonds together the sugars and phosphates of the complementary nucleotides to form the backbone of the daughter strand. Because the DNA polymerase can travel in only one direction on a DNA strand, from the 3′ end to 5′ end, the two DNA polymerase molecules, one on each parental strand, move in opposite directions.
Only one daughter strand is synthesized continuously, while the duplication of another is done piece by piece. As the DNA helicase molecule continues to separate the parental strands, one polymerase simply follows behind it, synthesizing a long, continuous complementary daughter strand as it goes.
The polymerase on the second parental strand travels away from the DNA helicase. As the helicase continues to separate the parental strands, this polymerase cannot reach the newly separated segment of the second strand.
Hence a new DNA polymerase attaches to the second strand close behind the helicase to synthesize another small piece of DNA. These pieces are “sewn” together by another enzyme called DNA ligase. This process is repeated many times until the copying of a second parental strand is completed.
Proofreading and End-Sealing
Hydrogen bonding between complementary base pairs makes DNA replication highly accurate. However, the process is not perfect. This is a result partly of the fast pace (fifty to five hundred nucleotides added per second) and partly to the chemical flip-flops in the bases occurring spontaneously.
DNA polymerase occasionally matches bases incorrectly, making on average one mistake in every ten thousand base pairs. Even this low rate of errors, if left uncorrected, would be devastating to the continuity of life.
In reality, replicated DNA strands contain only one mistake in every billion base pairs. This incredible accuracy is achieved by several DNA repair enzymes, including DNA polymerase. Mismatches and errors are corrected through “proofreading” by DNA polymerase or other repair enzymes.
A separate problem in DNA replication lies at the end of a linear DNA molecule,which is not suitable for replication by polymerase.
Yet another enzyme, DNA telomerase, is involved in solving this problem. It attaches a long stretch of repeating sequence of nucleotide 5′-TTGGGG-3′ to the strand already synthesized by polymerase. This ending sequence (called the telomere) offers protection and provides stability for plant DNA molecules.
Plant Propagation
Overall, the faithful replication and the amazing stability of each DNA molecule ensure the continuity and survival of species from generation to generation. DNA replication precedes every cycle of mitosis, so that two daughter cells derived from cell division can inherit a full complement of genetic material.
The replication is also an essential prerequisite for plant sexual reproduction, a process by which the two sexual cells (pollen and egg) produced via meiosis are united to start a new generation.
The faithful DNA replication and subsequent cell divisions (mitosis) form the basis for plant growth as well as for vegetative propagation in many plant species. Growth in size and volume of the plant results from a combination of mitosis, cell enlargement, and cell differentiation. Mitosis is also essential in wound healing through the production of a mass of cells called callus.
Vegetative propagation, an asexual process through mitosis, plays an important role in agriculture. Through vegetative propagation, individual plants of the progeny population are genetic copies both of the original source plant and of one another.
Such plants are known as clones, and the process is called cloning. Examples of cloning include grafting hardwood cuttings of grapevines and apple trees and rapid propagation of liriope by crown division.
The best-known example of vegetative propagation is probably the production of Macintosh apples via grafting. More recently, micro propagation via direct cell cultures and related biotechnology has played a critical role in agriculture.
