Lecture #9 | Transcription & Translation

Nucleic Acids

Large biomolecules important for information storage and exchange, including DNA and RNA.

Central Dogma

The flow of genetic information from DNA replication to transcription and translation.

Transcription

The process of reading a DNA template to produce an RNA copy.

Core RNA Polymerase

Contains αI, αII, β, β′ subunits required for elongating an RNA chain.

Sigma Factor

A factor needed only for the initiation of RNA synthesis.

Promoter

A specific DNA sequence that directs the RNA polymerase to begin transcription.

Holoenzyme

The combination of the core enzyme and the sigma factor.

𝜎70 sigma factor

The housekeeping sigma factor that recognizes most promoters.

𝜎32 sigma factor

Recognizes promoters for heat-shocked-induced genes.

𝜎28 sigma factor

Recognizes promoters for genes involved in motility and chemotaxis.

𝜎38 sigma factor

Recognizes promoters for stationary phase and stress response genes.

𝜎54 sigma factor

Recognizes promoters for nitrogen metabolism genes.

Transcription Initiation

Transcription initiation involves the binding of the sigma factor to a specific DNA sequence called the promoter. This interaction facilitates the recruitment of the core RNA polymerase enzyme to the promoter region, forming the RNA polymerase holoenzyme complex. The binding of sigma factor ensures that the RNA polymerase is oriented properly to begin transcription, which is essential for accurate synthesis of

Transcription Elongation

Core RNA polymerase synthesizes RNA from the DNA template strand by adding complementary RNA nucleotides in a 5' to 3' direction. During this phase, the RNA polymerase unwinds the DNA helix, exposing the DNA template strand for the synthesis of the RNA transcript. As the RNA polymerase moves along the DNA, it catalyzes the formation of phosphodiester bonds between the RNA nucleotides, progressing the elongation of the RNA chain.

Rho-dependent Termination

Termination mechanism involving rho protein binding to mRNA to halt transcription.

Rho-independent Termination

Termination method that requires a GC-rich region to form a stem loop, halting transcription.

mRNA

Messenger RNA, a type of RNA that conveys genetic information.

rRNA

Ribosomal RNA, a component of ribosomes.

tRNA

Transfer RNA that carries information from DNA to protein.

sRNA

Small RNA that regulates mRNA stability or translation.

Codons

Triplets of nucleotides coding for amino acids; 61 codons exist for 20 amino acids.

Stop Codons

Codons that signal termination: UAA, UAG, UGA.

tRNA Structure

Cloverleaf structure that binds to amino acids and has an anticodon complementary to mRNA codons.

Translation

The process of decoding RNA into a protein.

Ribosome structure

Large rRNA-protein complexes with 30s and 50s subunits.

30s subunit

Contains 16s rRNA and binds to mRNA.

50s subunit

Contains 5s and 23s rRNA.

Translation Initiation

Translation initiation involves the binding of the ribosome to the mRNA strand at the start codon (AUG). This process requires several key components including the ribosomal subunits, messenger RNA (mRNA), and the initiator transfer RNA (tRNA) that carries the first amino acid, methionine. The small ribosomal subunit (30s) recognizes the Shine-Dalgarno sequence in prokaryotic mRNA, which aligns the mRNA with the ribosome. Once the start codon is positioned in the ribosome’s P site, the large ribosomal subunit (50s) then binds, completing the initiation complex and preparing for

Translation Elongation

The translation elongation phase is characterized by the binding of aminoacyl-tRNA to the ribosome at the A (aminoacyl) site, where each tRNA carries a specific amino acid corresponding to the mRNA codon. The ribosome facilitates the formation of peptide bonds between the amino acids, linking them together into a growing polypeptide chain. This process involves the movement of the ribosome along the mRNA strand, shifting the tRNA from the A site to the P (peptidyl) site, while the tRNA that was in the P site is moved to the E (exit) site and released. The elongation continues until a stop codon is reached, with the rate and accuracy being essential for proper protein synthesis.

Translation Termination

Translation termination is the final step in protein synthesis that occurs when the ribosome encounters a stop codon (UAA, UAG, or UGA) on the mRNA strand. This event triggers the binding of release factors that promote the disassembly of the ribosomal complex, leading to the release of the newly synthesized polypeptide chain (the complete protein). This process is crucial as it ensures that translation does not continue past the intended protein length, thus maintaining the fidelity of protein synthesis.

Coupled Process in Prokaryotes

Transcription and translation occur simultaneously, allowing rapid protein synthesis.

Open Reading Frame

The protein-coding region in mRNA from the start codon (AUG) to a stop codon.