Why are RNA primers necessary during the DNA replication of a lagging strand?

Answer 1

Because the synthesis of DNA only occurs in one direction, different processes must occur on the two strands. These two strands are termed the leading and lagging strands. The leading strand is synthesised continuously 5′→3′. However, the other, 'lagging' strand is still synthesised 5′→3′ but in discrete chunks called Okazaki fragments, from the replication fork back towards the origin.

DNA polymerase will not synthesise dsDNA from ssDNA: it needs a short double stranded section. RNA primers are therefore added by DNA primase. The primase plus the helicase are often called the lagging strand primosome. In eukaryotes, DNA polymerase α has a primase subunit, rather than a separate enzyme.

Since priming is the most likely point for mismatch to occur because the short helix is unstable, DNA polymerase has been evolutionarily tuned to refuse to bind to ssDNA, and will only bind to dsDNA or a DNA/RNA hybrid. RNA is distinguishable from DNA and can be removed later, whereas a DNA primer would not allow this erasibility. Hence RNA is used.

DNA primases catalyse the synthesis of the short RNA primers on single stranded (ss) DNA templates that are used by DNA polymerase to initiate the synthesis of Okazaki fragments on the lagging strand.
Primase is an essential enzyme in all well characterised systems of DNA replication.
DNA polymerase produces Okazaki fragments on the lagging strand:

* DNA primase adds 10 bp RNA primer to DNA.
* DNA polymerase extends the primer to 200 bp (eukaryotes - nucleosome size?) or 1000 bp (prokaryotes) with DNA.

Answer 2

The lagging strand does not have free 3' ends of the DNA for 5' to 3' replication to proceed. The DNA polymerase requires a free 3' end for DNA replication. RNA polymerase does not. Thus, RNA polymerase creates RNA primers complimentary to the sequence in the lagging strand so that the DNA polymerase can have its free 3' end and DNA replication of the lagging strand can proceed.

Answer 3

the leading strand is encoded straight through with no breaks 3' to 5', but the lagging strand (5' to 3') is encoded backwards. It has to be encoded in sections (DNA polymerase must always go 3' to 5'). You can look up Okazaki fragments for more on this.

Thing is, DNA polymerase can't start encoding by itself. It needs an RNA primer, so each fragment or backwards encoded section needs an RNA primer to start that sections DNA polymerase encoding.

Answer 4

RNA molecule, known as a primer, complementary to one strand of the open DNA, is synthesized by a DNA-dependent, RNA polymerase - this enzyme is sometimes known as primase.

Answer 5

RNA polymerase is the enzyme that is needed to "unzip" the DNA helix into 2 separate strands. Once there are 2 strands, the DNA replicates through transcription. Then it moves on to translation where the proteins are made.