The Process and Mechanics of Transcription and RNA Processing

The Purpose and Function of Transcription

• Definition and Purpose: Transcription is the process by which a specific section of DNA, referred to as a gene, is copied or transcribed into a messenger RNA ($mRNA$) strand. This allows the genetic information to leave the nucleus and enter the ribosome, where it can be synthesized into a polypeptide protein.

• The mRNA Transcript: Transcription results in a single-stranded transcript of the gene composed of messenger RNA ($mRNA$).

• Reasons DNA Remains in the Nucleus: DNA molecules cannot leave the nucleus for two primary reasons:

  • The DNA must be protected from potential damage and degradation.

  • DNA is a very large, double-stranded molecule that is physically too big to fit through the nuclear pores.

Structural Regions of a Gene

• The Promoter: This region acts as a switch to turn a gene on and off and serves as the signal to initiate the start of transcription. RNA polymerase ‘knows’ to bind and begin the process at this specific nucleotide sequence.

• The Coding Region: This section contains the specific base sequence that codes for the protein.

• The Terminator: This region signals the end of the genetic sequence and indicates where transcription should stop.

The Detailed Process of Transcription

• Initiation:

  • The enzyme RNA polymerase binds to the promoter region of a gene.

  • The DNA double-helix unzips and separates as the enzyme breaks the bonds, exposing the internal bases/nucleotides and forming a "transcription bubble."

• Elongation and Synthesis:

  • The RNA polymerase moves along the template strand of the DNA.

  • The two strands of DNA are antiparallel; consequently, the RNA polymerase moves along the template strand in the $3'$ to $5'$ direction.

  • The enzyme links ribonucleotides together to form the RNA strand. Ribonucleotides are added to the $3'$ end of the growing RNA molecule, meaning the $mRNA$ chain grows in the $5'$ to $3'$ direction.

  • The resulting $mRNA$ strand is a copy of the non-template strand (also known as the coding strand or sense strand), with the exception that Uracil ($U$) replaces Thymine ($T$).

  • As RNA polymerase moves along the gene, it unwinds the DNA ahead of its path and rewinds the DNA behind it.

• Complementary Base Pairing:

  • Free RNA nucleotides are matched to the exposed bases on the template strand.

  • The rules for complementary base pairing are: Adenine ($A$) pairs with Uracil ($U$), and Guanine ($G$) pairs with Cytosine ($C$).

• Termination:

  • Transcription is complete when the RNA polymerase reaches the terminator sequence.

  • The RNA polymerase detaches from the template strand.

  • The two DNA strands anneal back together to reform the double helix structure.

  • The $mRNA$ strand detaches from the template.

RNA Processing: Pre-mRNA to Mature mRNA

• Pre-mRNA Production: During the initial transcription, the entire gene is copied into a form called pre-mRNA. This molecule contains both exons and introns.

• Exons: These are the coding regions; sections that specify the actual amino acid sequence of the protein.

• Introns: These are non-coding regions that have no obvious function in the final protein synthesis.

• RNA Splicing: This is the process by which introns are removed from the pre-mRNA and exons are joined together to create a contiguous coding sequence.

• Chemical Modifications:

  • 5' Cap: A guanine nucleotide is added to the $5'$ end of the pre-mRNA strand.

  • 3' Poly(A) Tail: Multiple adenosine nucleotides are added to the $3'$ end, forming a polyadenylation tail.

  • Purpose of Modifications: These additions serve to slow down the degradation of the $mRNA$ by enzymes during its transport from the nucleus to the cytoplasm.

• Mature mRNA: Once splicing and modifications are complete, the "mature" $mRNA$ is ready to leave the nucleus for translation.

Cellular Localization and Movement

• Compartmentalization in Eukaryotes: In eukaryotic cells, the two primary steps of protein synthesis occur in separate areas: transcription takes place in the nucleus, while translation occurs in the cytoplasm.

• Nuclear Export: The processed $mRNA$ moves out of the nucleus and into the cytoplasm by passing through nuclear pores in the nuclear membrane.

• Ribosome Attachment: Upon entering the cytoplasm, the $mRNA$ attaches to a ribosome in preparation for the translation phase of protein synthesis.

Questions & Discussion

• Question: What is the purpose of transcription?

• Response: The purpose is to copy a section of DNA/gene into an $mRNA$ strand so it can leave the nucleus and enter the ribosome to be made into a polypeptide protein.

• Question: What is the process of transcription? Write the steps of transcription.

• Response:

  1. An enzyme (RNA polymerase) unzips and separates the DNA strand, exposing the gene/bases.

  2. Free RNA nucleotides match to the exposed bases on the template strand using complementary base pairing ($A-U$ and $G-C$).

  3. A single $mRNA$ strand forms and detaches when the RNA polymerase reaches the terminator. The mRNA then moves to the cytoplasm to attach to a ribosome.