Mechanism of DNA Replication

Mechanism of DNA Replication

DNA replication involves four important stages. These are

1. Initiation of DNA replication
2. Unwinding of DNA helix
3. Formation of Primer strand
4. Elongation of new strand

1.Initiation of DNA replication:-

Replication of DNA always begins at a definite site called replication origin. In virus and bacteria there is only one such replication origin present. e.g E-coli.But in eukaryotes there are several thousands of replication origins are present. These linearly arranged units are known as replicons.e.g-

                               Organisms                                      No. of replicons
                                Yeast                                                    500
                                 Drosophila                                          3500
                                 Toad                                                    1500
                                 Mouse                                                 25000
                                 Bean                                                    35000

In Escherichia coli replication is due to multienzyme complex known as replication apparatus or repliosome.

2.Unwinding of DNA helix:-

Now unwinding occurs in the  parental DNA molecule in such a manner that its internal bases are exposed to the replication enzymes.Unwinding  occurs due to a specific enzyme called Helicase or rep protein. This leads to formation of super coils which are then removed by the action of another enzyme called DNA gyrase or Topoisomerase.

Unwinding of DNA molecule separates the two strands to form a Y-shaped structure called replication fork.Then the exposed single strands ae stabilised by a protein called single stranded binding protein (SSB) or the helix destabilizing protein. The SSB proteins holds open the two separated strands to form replication fork.

3.Formation of Primer strand:-

After that opposite to each of the parental strand a new strand is synthesized. The parental strand acts as a template strand. The enzyme which is meant for replication is DNA Polymerase. In prokaryotes it is of three types i.e DP-I ,DP-II  and DP-III. Out of which DP-I and DP-II are concerned with DNA repair where DP-III takes part in actual DNA replication .But in eukaryotes  DNA polymerase is of five types i.e- DP-𝞪, DP-𝜷  ,DP-𝛄, DP-𝞭 and DP-Ɛ. Out of which  DP-Ɛ  takes part in DNA replication.However in both the cases DNA polymerase is incapable of initiating DNA synthesis. It is unable to deposit the first nucleotide in a daughter strand. For that reason another enzyme primase is initiating the synthesis by producing a short primer strand of RNA . Some other enzymes are also involved in this process and are collectively known as primasomes.

This primer strand then elongates and removed enzymatically once the initiation of DNA synthesis is completed. In E.coli DP-I helps in removal of RNA primer. In eukaryotes DP-𝛂 synthesizes primer strand.

4.Elongation of new strand:-

DNA replication takes place in 5'-3' direction. It occurs after the formation of primer strand. At this time deoxyribonucleotides are  added only to the 3'OH end of the new strand. These are ATP,GTP,CTP and TTP. In each of the addition a molecule of pyrophosphate is simultaneously released. Thus the nucleotide at 3' carbon of sugar is always the most recently added nucleotide to the chain.

As proceeds of the two parental strands synthesis of daughter or new strand occurs continuously along the upper strand in 5'-3'OH direction initially  known as leading strand. Similarly synthesis of another daughter strand along the lower parental strand in 3'OH-5'P direction occurs in the form of short pieces known as lagging strand. These pieces are called as okasaki fragments. Those are 1000 to 2000 in number (nucleotide pieces). Later on they joined together by the enzyme DNA ligase to form a continuous daughter strand.

So at the end two DNA molecules are formed from one molecule where each of the daughter DNA is made of one parental or old strand and other new or complementary strand.

Replication of DNA

Replication of DNA

DNA replication means synthesis of DNA.It occurs during the S-phase (synthesis phase) of cell cycle. DNA molecule performs 2 major functions i.e   Autocatalytic  function and Heterocatalytic function.

Whenever a DNA molecule synthesizes another DNA molecule similar to  it at that time it is called autocatalytic function. It is the replication of DNA.But whenever the DNA molecule directs the synthesis of protein, dissimilar in structure compared  to its own structure involving different types of RNAs it is called heterocatalytic function.

There are three possible ways of DNA replication i.e Dispersive,Conservative and Semiconservative.In the dispersive and conservative methods of DNA replication there is no experinmental proof available. In dispersive method of replication the two strands of mother DNA break at several points resulting in several pieces of  DNA.Each piece replicates and pieces are reunited randomly ,so the 2 copies of DNA  molecules are formed from single copy The new DNA molecules are hybrid which have  old and new DNA in patches .In conservative method of replication two DNA molecules are formed. One molecule has both parental strands and the other contains both newly synthesized strands.

Semiconservative method of DNA replication was proposed by Watson and Crick.It  states at the time of DNA replication the two parental strands are seperated.

Then opposite to each of the parental strand a new strand a new strand is synthesized.At this time the separated parental strand serves as a template or guide or model for the formation of new but  complementary strand.So at the end of the DNA replication  from a parent DNA molecule. Two daughter  DNAs  are synthesized where each one contains one parental strand or old strand and other newly formed complementary strand.

This semiconservative mode of DNA replication was experimentally proved by Meselson  and stahl in 1958, from E.coli.

Telome Theory

Telome Theory

It is now widely accepted that the  present day plants with green leaves and branches have evolved from the axial sporophyte of the earliest and most primitive land plants. Devinian Psilophytales which are considered to be the earliest vascular land plants, had a simple dichotomously branched axis with leaves and roots. Some of their terminal branches bore sporangia while other were sterile.The Telome Theory was 1st proposed by the German palaeobotanist Walter Zimmermann in his1930 book entitled "Die Phylogenic de  Pflanzen.

What is Telome?

The name telome has been given to the simple ultimate terminal portions of a dichotomously branched axis. These axes are undifferentiated and single nerved.Two telomes of a dichotomizing axis are united below the point of dichotomy to form a fused structure, called mesome. There are two types of telomes on the basis of their function.

1. Vegetative or sterile telomes-These telomes are without sporangia and they are also called phylloids.

2. Fertile telomes- Those telomes which bore terminal sporangia are called fertile telomes.

During the time of evolutionary development, telomes become grouped together and form a complex structure,is called syntelome or telome truss. When a syntelome consisted of only sterile telomes is called phylloid truss and a fertile truss if it consisted of only fertile telomes and a mixed telome truss or mixed syntelome when it consisted of both sterile and fertile telomes.

Process of  Telome Theory-

Zimmermann suggested that the following five elementary process were responsible for the development of higher vascular plants from the early vascular criptogams.

1. Overtopping
2. Planation
3. Syngenesis
4. Reduction
5. Curvation

1.Overtopping:

In this process one of the two dichotomizing brnches of an axis become laarger, stronger and grew vertically upward as the main axis. The shorter dichotomy was displaced laterally and it served as precursor of megaphylls.

Thus in this process the weaker branch was overtopped by the stronger branch. It resulted in the development of monopodial branches from equal dichotomies. Evolutionary studies have also shown that dichotomous branching is frequent in primitive pteridophytes and monopodial in advanced forms.

2.Planation-:

The equal dichotomies of a system,(cruciate dichotomy) which were in more than one plane come to lie in a single plane (fan-shaped dichotomy).This process, known as planation, helps in the interpretation of the development of organs of bilateral symmetry from those of radial symmetry. Thus, planation must have led to the evolution of leaf.

3.Syngenesis:

In this process, telomes and mesomes came to lie within a common parenchymatous tissue. This process is also known as fusion or webbing. Syngenesis also involves the fusion of vascular strands of telomes. It is an important process which explains the formation of (i)leaves with open dichotomous,pinnatified and reticulate venation and(ii)polystelic condition found in Selaginella and some fossil members of the Devonian and Lower Carboniferous period.

4.Reduction:

It involved transformation of a syntelome into a single needle like leaf. This process thus accounts for the evolution of simple microphyllous leaves of the lycopods.

5.Curvation:

It is brought about by unequal growth of tissues on two opposite flanks of the telome. There are two types of curvation process have been recognized.

• Recurvation:

When telomes bent downwards, it is called recurvation. It is believed that recurved position of sporangia in the Sphenopsida is the result of this process.

• Incurvation:

This process accounts for the shifting of sporangia to the ventral surface of the leaf in ferns.

Merits of Telome Theory:

Telome theory provids an excellent interpretation of the origin and evolution of sporophyte of land plants. It is based on phyletic relationship between the various groups of plants,both living and fossils. The Elementary processes proposed by Zimmermann provide a basis  of interpretation which remove outstanding morphological difficulties in the lower vascular plants.According to Eames (1936),though the theory is built upon structure in the lowest known vascular plants, higher plants can also be safely interpreted in this way. Bierhorst (1971) is of the view that the theory is too simple and too easily applicable but unfortunately its excessive use has greatly diminished its value.

Demerits of Telome Theory:

1. An important drawback of the telome concept is that Zimmermann has taken telome as a ready-made structural unit, without explaining as to how such a structure really came into existence.Although this problem was later realized by Zimmermann himself and he put forward several other elementary processes, but they do not satisfy plant morphologists.
2. Many fossil plants of much greater compllexity than Rhynia have been discovered in the beds of the same age. They bore whorled or lateral sporangia instead of terminal sporangia. The telome theory does not explain such arrangements of sporangia.
3. According to the telome theory ,all leaves in plants are telomic in nature. But Bower(1935) condidered that microphyllous leaves are simply outgrowths of the stem, i.e, they are not the end product of reduction.
4. According to the telome theory,the polystelic condition , such as found in the stem of Selaginella, has developed due to syngenesis and siphonostelic and actinostelic conditions are supposed to be the products of tangential or redial fusion of vascular systems of polystelic axis. Such an explanation is opposed to the widely accepted concept of stelar theory.