The process of mRNA synthesis involves two key steps: transcription and translation - How It Works?

May 22, 2024

The process of mRNA synthesis involves two key steps: transcription and translation. Here's a detailed explanation of each process:

Transcription

Transcription is the process by which the genetic information encoded in DNA is copied into messenger RNA (mRNA). This occurs in the nucleus of eukaryotic cells. The steps involved are as follows:

Initiation:

The process begins when RNA polymerase, an enzyme responsible for synthesizing RNA, binds to a specific region of the DNA called the promoter.
Transcription factors help RNA polymerase recognize the promoter and initiate transcription.
The DNA double helix unwinds to expose the template strand.

Elongation:

RNA polymerase moves along the template strand of the DNA, reading the nucleotide sequence and synthesizing a complementary mRNA strand.
RNA polymerase adds ribonucleotides (A, U, C, G) in a sequence complementary to the DNA template strand (with uracil (U) replacing thymine (T)).
As RNA polymerase progresses, the newly synthesized mRNA strand detaches from the DNA template, and the DNA helix re-forms.

Termination:

Transcription continues until RNA polymerase reaches a termination sequence on the DNA, signaling the end of the gene.
The RNA polymerase enzyme releases the newly synthesized mRNA strand and detaches from the DNA.

Processing (in eukaryotes):

The primary mRNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA.
Capping: A 5' cap (modified guanine nucleotide) is added to the beginning of the mRNA for protection and ribosome recognition.
Polyadenylation: A poly-A tail (a string of adenine nucleotides) is added to the 3' end of the mRNA for stability and export from the nucleus.
Splicing: Introns (non-coding regions) are removed, and exons (coding regions) are joined together by the spliceosome complex.

Translation

Translation is the process by which the mRNA sequence is used to synthesize a corresponding protein. This occurs in the cytoplasm on the ribosomes. The steps involved are as follows:

Initiation:

The small ribosomal subunit binds to the mRNA at the 5' end and scans for the start codon (AUG).
The start codon signals the beginning of the protein-coding sequence.
A transfer RNA (tRNA) with the complementary anticodon (UAC) and carrying the amino acid methionine binds to the start codon.
The large ribosomal subunit joins to form the complete ribosome, with the initiator tRNA positioned in the P site of the ribosome.

Elongation:

The ribosome moves along the mRNA, reading the codons (three-nucleotide sequences) and matching them with the appropriate tRNAs carrying amino acids.
Each tRNA's anticodon pairs with its corresponding mRNA codon at the A site of the ribosome.
Peptide bonds form between the amino acids, linking them together into a growing polypeptide chain.
The ribosome shifts, moving the tRNA in the P site to the E site (exit site) and the tRNA in the A site to the P site, making room for the next tRNA.

Termination:

Translation continues until the ribosome encounters a stop codon (UAA, UAG, or UGA) on the mRNA.
Stop codons do not code for any amino acids but signal the release of the newly synthesized polypeptide chain.
Release factors bind to the stop codon, prompting the ribosome to release the completed polypeptide and dissociate from the mRNA.

Post-Translational Modifications:

The newly synthesized polypeptide often undergoes further modifications, such as folding, cleavage, and addition of functional groups, to become a functional protein.

In summary, mRNA is synthesized from DNA during transcription, processed to become mature mRNA, and then translated into a protein by the ribosome in the cytoplasm. These processes ensure that genetic information is accurately transferred from the DNA to functional proteins, enabling the cell to perform various vital functions.

Expand all make it comprehensive and long detailed scientific technical

ChatGPT

Certainly! Let's delve into the detailed scientific processes of mRNA synthesis, including both transcription and translation, with a comprehensive exploration of the mechanisms involved.

Transcription: From DNA to mRNA

Transcription is the process by which genetic information from DNA is transcribed into messenger RNA (mRNA), which can then be translated into a protein. This process occurs in the nucleus of eukaryotic cells and involves several steps:

1. Initiation

Promoter Recognition: The transcription process begins at a specific sequence on the DNA known as the promoter. The promoter contains specific DNA sequences, including the TATA box (in many eukaryotes), that signal the start site for transcription.
Binding of RNA Polymerase: RNA polymerase, the enzyme responsible for RNA synthesis, binds to the promoter region with the help of transcription factors. In eukaryotes, RNA polymerase II is responsible for mRNA synthesis.
Transcription Factors: These proteins help in the assembly of the transcription initiation complex. General transcription factors (e.g., TFIIA, TFIIB, TFIID) facilitate the binding of RNA polymerase II to the promoter. TFIID, which contains the TATA-binding protein (TBP), recognizes and binds to the TATA box, causing a local unwinding of the DNA.
Formation of the Transcription Initiation Complex: The assembly of RNA polymerase II and transcription factors forms the transcription initiation complex, which positions the enzyme at the correct start site.

2. Elongation

Unwinding of DNA: As RNA polymerase moves along the DNA template strand, it unwinds the DNA helix ahead of it, creating a transcription bubble.
Synthesis of mRNA: RNA polymerase synthesizes the mRNA strand by adding ribonucleotides (A, U, C, G) complementary to the DNA template strand. The addition of ribonucleotides occurs in the 5' to 3' direction.
RNA Polymerase Dynamics: The enzyme catalyzes the formation of phosphodiester bonds between ribonucleotides, releasing pyrophosphate with each addition. The energy from this reaction drives the polymerization process.
RNA-DNA Hybrid: A short region of the newly synthesized RNA remains base-paired with the DNA template, forming an RNA-DNA hybrid until the RNA strand is displaced and the DNA re-anneals.

3. Termination

Termination Sequences: Transcription continues until RNA polymerase encounters a termination sequence in the DNA, which signals the end of transcription. In eukaryotes, this often involves the polyadenylation signal sequence (AAUAAA).
Cleavage and Release: The mRNA transcript is cleaved from the RNA polymerase complex, releasing the pre-mRNA (precursor mRNA).

4. mRNA Processing

In eukaryotes, the primary mRNA transcript (pre-mRNA) undergoes several modifications before it becomes mature mRNA:

5' Capping: A 7-methylguanosine cap is added to the 5' end of the pre-mRNA shortly after transcription begins. This cap protects the mRNA from degradation and assists in ribosome binding during translation.
Polyadenylation: A poly-A tail, consisting of 50-250 adenine nucleotides, is added to the 3' end of the pre-mRNA. This tail stabilizes the mRNA and facilitates its export from the nucleus.
Splicing: The pre-mRNA contains non-coding regions called introns and coding regions called exons. The spliceosome, a complex of small nuclear RNAs (snRNAs) and proteins, removes the introns and joins the exons together. Alternative splicing can occur, allowing a single gene to encode multiple proteins.
Editing: In some cases, the mRNA undergoes RNA editing, where specific nucleotides are modified or inserted to alter the mRNA sequence and the resulting protein.

Translation: From mRNA to Protein

Translation is the process by which the sequence of nucleotides in mRNA is decoded to produce a specific polypeptide or protein. This occurs in the cytoplasm on ribosomes, which are composed of ribosomal RNA (rRNA) and proteins. The translation process involves the following steps:

1. Initiation

Ribosome Assembly: The small ribosomal subunit binds to the mRNA near the 5' cap and scans along the mRNA until it finds the start codon (AUG), which codes for methionine.
Initiator tRNA: A transfer RNA (tRNA) molecule with an anticodon (UAC) complementary to the start codon binds to the ribosome's P site, carrying methionine.
Large Ribosomal Subunit: The large ribosomal subunit joins the complex, forming the complete ribosome with the initiator tRNA positioned in the P site. This assembly requires initiation factors and GTP hydrolysis for energy.

2. Elongation

Codon Recognition: The ribosome reads the next codon on the mRNA, and a corresponding tRNA with a complementary anticodon binds to the A site, bringing the appropriate amino acid.
Peptide Bond Formation: Peptidyl transferase, an enzymatic activity of the ribosome's large subunit, catalyzes the formation of a peptide bond between the amino acid in the P site and the amino acid in the A site. The growing polypeptide chain is transferred to the tRNA in the A site.
Translocation: The ribosome moves one codon down the mRNA (toward the 3' end), shifting the tRNA in the A site to the P site and the tRNA in the P site to the E site (exit site). The tRNA in the E site is released.
Cycle Repetition: This process repeats for each codon on the mRNA, elongating the polypeptide chain until a stop codon is encountered.

3. Termination

Stop Codon Recognition: Translation continues until the ribosome encounters a stop codon (UAA, UAG, or UGA). Stop codons do not code for any amino acids and do not have corresponding tRNAs.
Release Factors: Release factors (proteins) bind to the ribosome when a stop codon is present in the A site. These factors trigger the hydrolysis of the bond between the polypeptide chain and the tRNA in the P site, releasing the newly synthesized protein.
Ribosome Disassembly: The ribosomal subunits dissociate from the mRNA and are available for another round of translation.

4. Post-Translational Modifications

After translation, the newly synthesized polypeptide often undergoes further modifications to become a functional protein:

Folding: The polypeptide folds into its specific three-dimensional structure, a process assisted by molecular chaperones.
Cleavage: Some proteins are synthesized as inactive precursors and require cleavage of specific segments to become active.
Chemical Modifications: Various chemical groups (e.g., phosphate, methyl, acetyl) may be added to specific amino acids to regulate the protein's function, stability, and localization.
Formation of Disulfide Bonds: In proteins that are secreted or located in the extracellular space, disulfide bonds form between cysteine residues, stabilizing the protein structure.