Transcription

Transcription in Prokaryotes: - Initiation: RNA polymerase binds to the promoter region of DNA, initiating the process of transcription. - Elongation: RNA polymerase synthesizes RNA in the 5’ to 3’ direction, creating an RNA sequence complementary to the DNA template strand. - Termination: Transcription stops when RNA polymerase reaches the terminator sequence. The newly formed RNA transcript is released.
Differences between Prokaryotic and Eukaryotic Transcription:
- RNA Polymerase: - Prokaryotes use a single RNA polymerase for transcription. - Eukaryotes have three distinct RNA polymerases (RNA Polymerase I, II, and III), each responsible for transcribing specific types of RNA.
- Location: - In prokaryotes, transcription occurs in the cytoplasm, and translation begins immediately after transcription. - In eukaryotes, transcription occurs in the nucleus, and the RNA transcript must undergo processing before it is transported to the cytoplasm for translation.
- RNA Processing: - Prokaryotic RNA does not require processing and is directly functional as mRNA. - Eukaryotic RNA undergoes capping (addition of a methylated guanosine cap at the 5’ end), tailing (addition of a poly-A tail at the 3’ end), and splicing (removal of non-coding introns and joining of coding exons) to become a mature mRNA.

Step 1: Relationship between the coding strand and RNA transcript.
- The RNA transcript is complementary to the template strand and identical to the coding strand, except uracil (U) replaces thymine (T).
Step 2: Base sequence of the transcript. - Coding strand: 5’ – ATGAATG – 3’ - mRNA transcript: 5’ – AUGAAUG – 3’

- In prokaryotes, transcription occurs in the cytoplasm.
- In eukaryotes, transcription occurs in the nucleus.
- Prokaryotes lack a defined nucleus, so transcription occurs directly in the cytoplasm where translation can occur simultaneously.
- Eukaryotes have a defined nucleus where transcription occurs, separating it from translation which happens in the cytoplasm.

Location of (C) and (D) in a Transcription Unit

(C) = Promoter

(D) = Terminator

1. Location of (C) - Promoter:

• The promoter is located at the 5′ end of the coding strand of DNA.
• It lies upstream of the transcription start site.
• It is the region where RNA polymerase binds to initiate transcription.

2. Location of (D) - Terminator:

• The terminator is located at the 3′ end of the coding strand.
• It lies downstream of the coding region.
• This is the site where transcription ends, and the RNA polymerase detaches.

Summary:

In a transcription unit:

• Promoter (C) marks the beginning of transcription.
• Terminator (D) marks the end of transcription.

These regulatory sequences ensure that the correct gene segment is transcribed into RNA.

Identification of (C) and (D) in the Transcription Unit

(C) → Promoter

(D) → Terminator

Significance in Transcription:

1. Promoter (C):

• A specific DNA sequence where RNA polymerase binds to initiate transcription.
• Located upstream (toward the 5' end of the coding strand).
• Helps in determining the starting point and direction of transcription.

2. Terminator (D):

• A sequence that signals the end of transcription.
• RNA polymerase detaches from the DNA and releases the newly formed RNA molecule.
• Ensures that only the desired segment is transcribed.

Conclusion:

Both the promoter and terminator regions are essential for correct initiation and termination of gene transcription.

Identification of Coding and Template Strands in Transcription

Answer: In a transcription unit:

• The template strand (also called antisense strand) is the DNA strand that is used by RNA polymerase to synthesize RNA. It is complementary to the RNA sequence.
• The coding strand (also called sense strand) is the DNA strand that has the same sequence as the RNA transcript (except it has thymine (T) instead of uracil (U)).

How to Identify from the Diagram:

• The strand with the same direction and sequence as the mRNA (except T in place of U) is the coding strand.
• The strand that is complementary to the mRNA is the template strand.

Note:

Typically, in diagrams, the template strand is labeled 3' to 5' (because RNA polymerase reads in this direction), and the coding strand is 5' to 3'.

Enzyme Involved in Transcription

Answer: The main enzyme involved in the process of transcription is:

RNA Polymerase

Function of RNA Polymerase:

• Binds to the promoter region on the DNA.
• Unwinds the DNA helix to expose the template strand.
• Reads the DNA template and synthesizes a complementary RNA strand.
• Replaces thymine (T) with uracil (U) in RNA.

Additional Notes:

In prokaryotes: A single RNA polymerase performs all types of transcription.
In eukaryotes:

• RNA Polymerase I – synthesizes rRNA
• RNA Polymerase II – synthesizes mRNA
• RNA Polymerase III – synthesizes tRNA and 5S rRNA