This 3D animation shows you how DNA is copied in a cell. It shows how both strands of the DNA helix are unzipped and copied to produce two identical DNA molecules. Show Transcript DNA is a molecule made up of two strands twisted around each other in a double helix shape. Each strand is made up a sequence of four chemical bases represented by the letters A, C, G and T. The two strands are complementary. This means that wherever there's a T in one strand there will be in an A in the opposite strand, and wherever there's a C there will be a G in the other strand. Each strand has a 5' end and a 3' end. The two strands run in opposite directions. This determines how each strand of DNA is replicated. The first step in DNA replication is to separate the two strands. This unzipping is done by an enzyme called helicase and results in the formation of a replication fork. The separated strands each provide a template for creating a new strand of DNA. An enzyme called primase starts the process. This enzyme makes a small piece of RNA called a primer. This marks the starting point for the construction of the new strand of DNA. An enzyme called DNA polymerase binds to the primer and will make the new strand of DNA. DNA polymerase can only add DNA bases in one direction, from the 5' end to the 3' end. One of the new strands of DNA, the leading strand, is made continuously, the DNA polymerase adding bases one by one in the 5' to 3' direction. The other strand, the lagging strand, cannot be made in this continuous way because it runs in the opposite direction the DNA polymerase can therefore only make this strand in a series of small chunks called Okazaki fragments. Each fragment is started with an RNA primer. DNA polymerase then adds a short row of DNA bases in the 5' to 3' direction. The next primer is then added further down the lagging strand. Another Okazaki fragment is then made and the process is repeated again. Once the new DNA has been made the enzyme exonuclease removes all the RNA primers from both strands of DNA. Another DNA polymerase enzyme then fills in the gaps that are left behind with DNA. Finally the enzyme DNA ligase seals up the fragments of DNA in both strands to form a continuous double strand. DNA replication is described as semi- conservative because each DNA molecule is made up of one old, conserved strand of DNA and one new one. DNA replication is the first step of the central dogma where the DNA strands are replicated to make copies. During the process of replication the double stranded DNA is separated from each other by the help of enzymes like topoisomerases and helicases. The separated DNA strands form a replication fork, where both the DNA strands
get replicated forming a lagging and leading strand. The major difference between a lagging and leading strand is that the lagging strand replicates discontinuously forming short fragments, whereas the leading strand replicates continuously. Find out more such differences between a lagging and leading strand, in the table below.
Lagging Strand Leading Strand Description The strand that opens in the 3’ to 5’ direction towards the replication fork is referred to as the lagging strand. The strand that runs in the 5’ to 3’ direction in the replication fork is referred to as the leading strand. Replication The strand is replicated discontinuously. The strand is replicated continuously. Fragments Short stretches called okazaki fragments are formed during replication. No short fragments are formed. Primer Requirements Each fragment requires its own set of primers. It requires only one primer. Ligase Requirement It requires DNA ligase enzymes for the joining of short okazaki fragments. It does not require DNA ligase. Direction of Growth It grows away from the replication fork. It grows in the direction of the replication fork. Speed The synthesis of new strands is slow. The synthesis of new strands is fast. Explore BYJU’S Biology to learn more. Also Read:
Frequently Asked QuestionsReplication is continuous in a leading strand because the template strand runs in the 3’ to 5’ direction, therefore it is replicated continuously as DNA polymerase catalyses polymerisation in only one direction, that is 5’ to 3’. The lagging strand has the DNA polymerase running away from the fork, so it has to come off and reattach every time to the newly exposed strand. Yes, the lagging strand needs a new primer everytime a new fragment is synthesised. What is the lagging strand in DNA replication?The lagging strand is a single DNA strand that, during DNA replication, is replicated in the 5′ – 3′ direction (opposite direction to the replication fork). DNA is added to the lagging strand in discontinuous chunks called 'okazaki fragments'.
Is the leading strand from 5 to 3?Leading and lagging strands
One new strand, the leading strand, runs 5' to 3' towards the fork and is made continuously. The other, the lagging strand, runs 5' to 3' away from the fork and is made in small pieces called Okazaki fragments.
Is the leading strand 5?The direction of Strand Growth
Leading strand grows in the 5' to 3' direction while the lagging strand grows in the 3' to 5' direction.
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