Question

Why is the strand of DNA with 3’ → 5’ polarity transcribed and not the other strand of 5’ → 3’ polarity?

Answer 1

Step 1: Direction of RNA Synthesis.
During transcription, RNA polymerase synthesizes RNA in the 5’ → 3’ direction. This means that it reads the DNA template strand in the 3’ → 5’ direction to ensure that the RNA is complementary to the DNA template.
Step 2: Template and Coding Strands.
The strand of DNA that runs in the 3’ → 5’ direction is called the template strand, which serves as the template for RNA synthesis. The other strand, running in the 5’ → 3’ direction, is called the coding strand, which is not used for transcription.
Step 3: Why Only One Strand is Used.
Only one strand is transcribed because it is the only strand that contains the necessary genetic information in the correct orientation for RNA synthesis. Transcribing both strands would lead to confusion and incorrect RNA sequences.

Answer 2

Step 1: Introns and Exons.
Heterogeneous nuclear RNA (hnRNA) contains both introns (non-coding regions) and exons (coding regions). The introns must be removed through splicing to produce a functional mRNA that can be translated into a protein.
Step 2: Splicing Process.
Splicing is the process by which the introns are excised, and the exons are joined together. This results in a mature mRNA molecule that contains only the coding regions, which can then be translated into a protein.

Answer 3

Step 1: Capping.
After splicing, hnRNA undergoes a process called capping. A 5' cap is added to the mRNA molecule, which helps in the stability, protection, and recognition of the mRNA during translation.
Step 2: Polyadenylation.
The second process is polyadenylation, where a poly-A tail is added to the 3' end of the mRNA. This tail protects the mRNA from degradation and helps in the transport of the mRNA from the nucleus to the cytoplasm.

Answer 4

Step 1: Direction of RNA Synthesis.
Only one strand of DNA is transcribed because RNA polymerase synthesizes RNA in the 5’ → 3’ direction. The DNA template strand runs in the 3’ → 5’ direction, allowing RNA polymerase to read it correctly and synthesize a complementary RNA strand.
Step 2: Gene Expression.
The two DNA strands have different functions. One serves as the template for transcription (the template strand), while the other strand (the coding strand) has the same sequence as the RNA and is not involved in transcription.