Transcription and RNA Processing

Overview of Transcription

Transcription is the process by which genetic information from DNA is copied into messenger RNA (mRNA) for protein synthesis. This process is fundamental to gene expression in both prokaryotic and eukaryotic cells, but there are important differences between the two.

• Transcription: The synthesis of RNA from a DNA template.

• mRNA (messenger RNA): The RNA molecule that carries genetic information from DNA to the ribosome for protein synthesis.

• RNA polymerase: The enzyme responsible for synthesizing RNA from the DNA template.

Transcription in Prokaryotes (Bacteria)

Steps of Transcription

Transcription in bacteria involves a series of steps that result in the production of mRNA. The process is relatively simple compared to eukaryotes.

1. Initiation: RNA polymerase binds to the promoter region of DNA, aided by sigma factors.

2. Strand Separation: RNA polymerase separates the DNA strands at the promoter.

3. Elongation: RNA polymerase synthesizes a complementary RNA strand in the 5' to 3' direction.

4. Termination: Transcription ends when a termination signal is reached. In bacteria, this often involves the formation of a hairpin structure (a region of RNA that folds back on itself due to complementary base pairing), which causes the RNA polymerase to detach from the DNA.

• Hairpin Structure: A secondary structure formed in the RNA transcript that signals the end of transcription in many bacterial genes.

• After termination, the DNA strands re-anneal and RNA polymerase can initiate transcription elsewhere.

Coupling of Transcription and Translation

In bacteria, transcription and translation are coupled, meaning translation of mRNA can begin before transcription is complete. This is possible because bacteria lack a nuclear membrane.

• Multiple ribosomes can translate a single mRNA simultaneously, allowing rapid protein production.

• This coupling enables bacteria to respond quickly to environmental changes.

Transcription in Eukaryotes

Key Differences from Prokaryotes

Eukaryotic transcription is more complex due to the presence of a nucleus and additional regulatory elements.

• There are three main RNA polymerases (I, II, III), each transcribing different types of genes.

• Promoters are more diverse and include elements like the TATA box (a common promoter sequence).

• General transcription factors are required for RNA polymerase binding (no sigma factors as in bacteria).

• Transcription and translation are separated by the nuclear membrane; transcription occurs in the nucleus, translation in the cytoplasm.

Steps of Eukaryotic Transcription

1. Initiation: General transcription factors and RNA polymerase bind to the promoter (e.g., TATA box).

2. Strand Separation: RNA polymerase separates the DNA strands at the promoter.

3. Elongation: RNA polymerase synthesizes the pre-mRNA (primary transcript) in the 5' to 3' direction.

4. Termination: Transcription ends at a polyadenylation signal (a sequence of many adenines, called the poly-A signal).

5. RNA Processing: The pre-mRNA undergoes several modifications before becoming mature mRNA.

RNA Processing in Eukaryotes

RNA processing is essential for producing a mature mRNA that can be translated. It involves several steps:

• 5' Capping: Addition of a modified guanine nucleotide to the 5' end of the mRNA. This cap protects the mRNA from degradation and assists in ribosome binding during translation.

• Polyadenylation: Addition of a poly-A tail (100-250 adenine nucleotides) to the 3' end. This tail also protects the mRNA and aids in export from the nucleus.

• Splicing: Removal of non-coding sequences called introns from the pre-mRNA, and joining of coding sequences called exons. The spliceosome (a complex molecular machine) carries out this process.

After processing, the mature mRNA is exported from the nucleus to the cytoplasm for translation.

Table: Comparison of Transcription in Prokaryotes and Eukaryotes

Introns and Exons

In eukaryotic genes, exons are coding regions that remain in the mature mRNA, while introns are non-coding regions that are removed during splicing.

• Exons are expressed and code for proteins.

• Introns are intervening sequences that are spliced out.

• The function of introns is not fully understood; some may have regulatory roles or be remnants of evolutionary history (sometimes called "junk DNA").

• Some introns may have originated from ancient viral infections.

Summary of Steps in Eukaryotic Transcription and RNA Processing

1. Binding of general transcription factors and RNA polymerase to the promoter (e.g., TATA box).

2. Separation of DNA strands by RNA polymerase.

3. Synthesis of complementary pre-mRNA in the 5' to 3' direction.

4. Termination at the poly-A signal.

5. RNA processing: splicing out introns, joining exons, adding 5' cap and poly-A tail.

6. Export of mature mRNA from the nucleus to the cytoplasm.

Example: mRNA Processing

• After transcription, a eukaryotic pre-mRNA contains both introns and exons.

• The spliceosome removes introns and joins exons together.

• A 5' cap and a poly-A tail are added to protect the mRNA and facilitate translation.

• The mature mRNA is then exported to the cytoplasm for translation by ribosomes.

Additional info: The notes reference future chapters for more detail on the regulation and evolutionary significance of introns and RNA processing. The concept of "junk DNA" is discussed, with the suggestion that some non-coding regions may have originated from ancient viruses.