Chapter 12, Problem 15STP

Summary Introduction

To explain:

The difference in number of bases in a strand of mRNA and DNA from which it was synthesized.

Introduction:

Transcription results in the synthesis of mRNA molecule from DNA molecule. The RNA produced undergoes processing before it is ready for translation. In eukaryotes, the original mRNA produced is called pre- mRNA. It contains all of the DNA code.

Answer to Problem 15STP

There is a difference in number of bases in a strand of mRNA and DNA from which it was synthesized because pre- mRNA has some intervening sequence of nucleotides called introns which are excised before they are ready to move out of nucleus. So the mature mRNA has less number of bases than the original DNA.

Explanation of Solution

The mRNA produced in the nucleus from DNA is called pre- mRNA. It contains all of the DNA code. The pre- mRNA undergoes processing before it is ready to attach to ribosomes for translation. After processing the mRNA formed is shorter than the DNA from which it was made. The code in DNA was found to be interrupted by sequences that were missing in the final RNA. These intervening sequences were called introns and the final sequences that remained in the mRNA were called exons. Before the pre- mRNA leaves the nucleus, introns are removed from it. Further, processing adds a protective cap on the 5’ end and a pony A tail to the 3’ end of the mRNA. The cap helps in recognizing the ribosome and the importance of pony A tail is unknown. Now the mRNA is ready to reach the ribosome. Hence, there is lesser number of bases in a strand of mRNA than in the DNA from which it was synthesized.