Chapter 12, Problem 28A

Summary Introduction

To compare and contrast:

Transcription and translation and indicate where they occur in prokaryotic and eukaryotic cells

Introduction:

The central dogma of biology is that the DNA codes for RNA which guides the synthesis of proteins. The first step involves the synthesis of mRNA from DNA in a process called transcription. The next step is translation of codes in mRNA to protein.

Answer to Problem 28A

The following table compares transcription and translation.

Transcription	Translation
It involves synthesis of RNA from DNA.	It involves synthesis of proteins from mRNA.
Code in DNA is converted to complementary code in mRNA.	Code in mRNA is converted to amino acid sequence in protein.
It occurs in nucleus of a eukaryotic cell.	It occurs in cytoplasm of a eukaryotic cell.
It needs the enzyme RNA polymerase.	It uses various reagents to make a polypeptide chain

Both the processes need a template to function and both have the same nucleotides; adenine, uracil, cytosine and guanine.

Location of transcription in prokaryotic cells- cytoplasm of the cell.

Location of translation in prokaryotic cells- cytoplasm of the cell.

Location of transcription in eukaryotic cells- inside nucleus of the cell.

Location of translation in eukaryotic cells- on ribosomes in cytoplasm of the cell.

Explanation of Solution

Transcription:

Each DNA molecule consists of two strands. Both the strands have complementary base pairs. During transcription, the two strands of DNA unwind. One strand serves as a template to synthesize new mRNA molecule. Through transcription, the DNA code is transferred to mRNA. The mRNA transcript is manufactured in a 5’ to 3’ direction adding each new nucleotide to the 3’ end. Free RNA nucleotides base pairs with their complementary nucleotides on the DNA strand. Uracil is incorporated instead of thymine as the mRNA molecule is made.

RNA polymerase is the enzyme that initiates transcription. It binds to promoter sites in DNA, then separates the strands of DNA and helps in synthesizing mRNA using template strand of DNA. It then releases mRNA at stop signal.

In prokaryotes transcription occurs in the cytoplasm of the cell.

In eukaryotes it occurs in the nucleus of the cell.

Translation:

Once the mRNA is synthesized it moves out of the nucleus and enters the cytoplasm where it connects to the ribosome. tRNA molecules act as interpreters of the the mRNA codon sequence. An enzyme activates tRNA by attaching a specific amino acid to each tRNA. tRNAs add their amino acids to the polypeptide chain as the mRNA moves through the ribosome one codon at a time. When a stop codon is reached, translation terminates and the polypeptide chain is released.

In prokaryotes translation occurs in the cytoplasm of the cell.

In eukaryotes it occurs on the ribosomes in the cytoplasm of the cell.