6.3 Transcription and RNA Processing

Transcription and RNA Processing: The Journey from DNA to Protein

Meet the RNA

In the world of genetics, structure and function are closely related. The sequence of RNA bases, along with the structure of the RNA molecule, plays a critical role in its function. Let’s break down the key players involved:

mRNA: Messenger RNA

mRNA, or messenger RNA, is transcribed from DNA and carries genetic information to the ribosome, where it is translated into a specific protein sequence. Think of mRNA as the message that carries the instructions for making a protein. Its base sequence determines which amino acids are used to construct the protein.

tRNA: Transfer RNA

tRNA, or transfer RNA, is responsible for bringing the correct amino acids to the ribosome during protein synthesis. Each tRNA molecule has a specific binding site for an amino acid and an anticodon sequence that pairs with the mRNA codon. This ensures that the right amino acid is added to the growing peptide chain.

rRNA: Ribosomal RNA

rRNA, or ribosomal RNA, forms the structural backbone of ribosomes, the cellular structures responsible for synthesizing proteins. rRNA helps position tRNA molecules and catalyze the formation of peptide bonds during translation.

The Central Dogma: DNA to RNA to Protein

The central dogma of molecular biology describes how genetic information flows from DNA to protein. It begins with transcription, where DNA is copied into mRNA, which then serves as a template for translation, resulting in protein formation.

During transcription, RNA polymerase reads the DNA sequence and synthesizes a complementary mRNA strand. This mRNA carries the genetic code that specifies which amino acids will be assembled to create a protein. Translation occurs on the ribosome, converting the mRNA sequence into a functional protein that can perform a wide range of cellular tasks.

RNA Polymerase: The Ace of Transcription

Transcription is initiated by RNA polymerase, which binds to a specific region of the DNA known as the promoter. RNA polymerase then reads the template strand of DNA and adds complementary nucleotides to build the mRNA molecule. The resulting primary RNA transcript is complementary to the template strand and nearly identical to the coding strand (except for uracil replacing thymine).

Template Strand

The DNA strand that serves as the template during transcription is called the noncoding strand, antisense strand, or minus strand. The coding strand, on the other hand, has the same sequence as the mRNA, with thymine (T) replaced by uracil (U).

Processing the mRNA Transcript

In eukaryotic cells, the primary RNA transcript undergoes several modifications to become a mature mRNA molecule that is ready for translation:

1. Addition of a Poly-A Tail: A string of adenine nucleotides (poly-A tail) is added to the 3′ end of the transcript. This tail helps protect the mRNA from degradation and aids in its transport out of the nucleus.

2. Addition of a GTP Cap: A modified nucleotide, known as a GTP cap, is added to the 5′ end of the transcript. This cap protects the mRNA from degradation and facilitates ribosome binding during translation.

3. Excision of Introns and Splicing of Exons: The primary RNA transcript contains both coding regions (exons) and non-coding regions (introns). The introns are removed through RNA splicing, and the exons are joined together to form a mature mRNA molecule that contains only the coding sequences.

4. Alternative Splicing: In some cases, exons can be spliced together in different combinations, resulting in different versions of the mRNA. This process, known as alternative splicing, allows a single gene to produce multiple proteins, increasing protein diversity and gene regulation complexity.