mRNA Structure and Production

Structure of mRNA

• A 5' untranslated region (leader sequence) is present at the 5' end of the mRNA.
• The protein-coding sequence specifies the amino acid sequence of a protein during translation.
• A 3' untranslated region (trailer sequence) is present at the 3' end of the mRNA.

mRNA Production in Eukaryotes

• The RNA transcript (pre-mRNA) is modified in the nucleus by RNA processing to produce the mature mRNA.
• The mRNA must migrate from the nucleus to the cytoplasm.
• mRNA is always transcribed completely and then processed before it is translated.
• Eukaryotic mRNAs are typically monocistronic, meaning they contain the amino acid-coding information from just one gene.

Modification of mRNA in Eukaryotes

5' Capping

• A capping enzyme adds a guanine nucleotide (most commonly, 7-methyl guanosine (m7G)) to the 5' end of the pre-mRNA.
• The 5' cap remains throughout the lifetime of the mRNA and plays a crucial role in protecting the mRNA against degradation by exonucleases.
• The 5' cap is also important for the binding of the ribosome as an initial step of translation.

3' Modification

• A poly(A) tail is added to the 3' end of the pre-mRNA.
• The poly(A) tail is required for efficient export of the mRNA from the nucleus to the cytoplasm.

Transcription and Translation

• Transcription is the synthesis of a single-stranded RNA copy of a segment of DNA.
• Translation is the conversion of the mRNA base-sequence information into the amino acid sequence of a polypeptide.

Gene Expression - The Central Dogma

• The central dogma is the two-step process of DNA → RNA → protein (transcription followed by translation).
• In transcription, only one of the two DNA strands is transcribed into RNA.

Types of RNA Molecules

• There are four main types of RNA molecules: mRNA, rRNA, tRNA, and snRNA.
• Each type of RNA is encoded by its own type of gene, but only mRNA is translated.

The Transcription Process

• Gene regulatory elements are sequence elements associated with each gene that are involved in regulating transcription.
• The enzyme RNA polymerase (RNA Pol) catalyzes the process of transcription.
• The DNA double helix unwinds for a short region next to the gene before transcription begins.
• RNA is synthesized in the 5'-to-3' direction.
• The 3'-to-5' DNA strand that is read to make the RNA strand is called the template strand.
• The 5'-to-3' DNA strand complementary to the template strand is called the non-template strand.

RNA Polymerase

• RNA polymerase is responsible for the synthesis of RNA from DNA.
• RNA polymerase catalyzes the linkage of ribonucleoside triphosphates (ATP, GTP, CTP, and UTP) to form a complementary RNA strand.
• RNA polymerase can start an RNA chain without a primer.
• RNA polymerase makes about one mistake for every 10^4 nucleotides copied into RNA.

Transcription in Eukaryotes

• Three different RNA polymerases (RNA Pol I, RNA Pol II, and RNA Pol III) transcribe the genes for four main types of RNAs.
• RNA Pol I is located in the nucleolus and synthesizes the RNAs found in ribosomes.
• RNA Pol II is located in the nucleoplasm and synthesizes mRNAs and some snRNAs.
• RNA Pol III is located in the nucleoplasm and synthesizes tRNAs, 5S rRNA, and the snRNAs not made by RNA Pol II.

Promoters and Enhancers

• Promoters of protein-coding genes encompass about 200 base pairs upstream of the transcription initiation site and contain various sequence elements.
• The core promoter is the set of cis-acting sequence elements needed for the transcription machinery to start RNA synthesis at the correct site.
• The promoter-proximal elements (PPEs) are upstream from the TATA box and are involved in the regulation of transcription.

Steps in Transcription

• Promoter recognition
• Preinitiation complex formation
• Initiation
• Elongation
• Termination