Chapter 11, Problem 1RE

RECALL What is the difference in the requirement for a primer in RNA transcription compared to DNA replication?

Interpretation Introduction

Interpretation:

The difference between the requirement of primer in RNA transcription and DNA replication is to be explained.

Concept introduction:

Primer is a short stretch of oligonucleotide needed for the initiation of DNA replication.

Primer is synthesized by the enzyme primase at the beginning of DNA replication.

Primase copies a small stretch of template DNA strand to produce an RNA primer.

The process of producing two identical copies of DNA molecule from original DNA molecule is called DNA replication.

The process of copying of DNA into a new molecule of mRNA is called transcription.

Answer to Problem 1RE

Solution: Primer is not required for the activity of RNA polymerase, the enzyme that carries out RNA transcription. However, DNA polymerases need an RNA primer during initiation step of DNA replication to be able to start the polymerization reaction.

Explanation of Solution

Given information: Primer, RNA transcription, and DNA replication

RNA transcription is carried out by an enzyme called RNA polymerase, which is a DNA-dependent polymerase that needs DNA as a template in order to synthesize RNA. Only one strand of DNA is used for transcription. The strand of DNA, which is read in the 3’ to 5’ direction by the RNA polymerase, is known as the template strand or antisense strand. The template DNA sequence, which signals the start of RNA transcription, is known as a promoter. RNA polymerase binds to the promoter sequence and synthesizes RNA using all four ribonucleoside triphosphates (ATP, GTP, UTP, and CTP). In the process of RNA synthesis, RNA polymerase does not require primer due to its ability to de novo synthesize RNA by positioning a ribonucleoside triphosphates (usually a purine) complementary to the template strand.

During replication, both the strands of DNA are required as template by DNA polymerase to synthesize the leading and lagging strand. DNA polymerase needs a free 3’- hydroxyl group already in place in order to synthesize the DNA chain. The RNA primer synthesized by the enzyme primase at the beginning of replication helps provide a free 3’- hydroxyl group for the initiation of DNA replication. In the absence of a primer, DNA polymerase cannot catalyze the DNA polymerization reaction. Once the DNA strands have been completely synthesized, primer is removed and replaced with deoxynucleotides by DNA polymerase I using the exonuclease and polymerase activity, respectively. DNA polymerases possess proof reading activity to check for any errors in the newly synthesized DNA strands. The presence of proofreading activity in DNA polymerase is compensated by the absence of primer synthesis ability, thereby making sure that DNA replication is an error proof process.

Conclusion

RNA polymerase can initiate a primer free transcription, whereas DNA polymerase needs primer to initiate the replication of DNA.