Cell Biology | Translation: Protein Synthesis 🧬

The video introduces the topic of translation or protein synthesis, requesting viewers to support the channel through subscription and donations. It starts by defining translation - converting mRNA, a result of transcription from DNA, into proteins. The key RNAs in this process are mRNA, tRNA, and rRNA. The genetic code, composed of nucleotide triplets known as codons, is also introduced. RNA nucleotides differ from DNA in having uracil instead of thymine.

It explains the concept of codons in mRNA, detailing that there are 64 different codons. Out of these, 61 codons code for amino acids while 3 are stop codons indicating the termination of protein synthesis. The codon AUG is noteworthy for encoding methionine, marking it as a start codon. The role of tRNA, which carries specific amino acids matching mRNA codons, is emphasized along with the introduction of terms like anticodons and their complementary nature.

The section discusses complementary relationships between codons and anticodons in mRNA and tRNA, exemplifying with the AUG codon and UAC anticodon. The importance of enzymes in reading anticodons and attaching corresponding amino acids is highlighted. It explains the charging of tRNA with the specific amino acid methionine at the 3' CCA domain for translation initiation.

The video details the characteristics of the genetic code, describing it as continuous (comless) and non-overlapping, except in viruses. The nature of redundancy and degeneracy in the genetic code is explained, where different codons can code for the same amino acid. The wobble effect allows tRNA flexibility in binding, reducing mutation risks by enabling one tRNA to recognize multiple codons that code for the same amino acid.

The process by which tRNA gets charged with amino acids is explained. This involves ATP binding to amino acids, leading to the formation of aminoacyl-AMP, which tRNA synthetase enzymes attach to tRNA. This results in a charged tRNA, ready to participate in translation. The structure of tRNA is reviewed, detailing its anticodon and amino acid binding sites, and its interaction with ribosomes facilitated by the D-arm and T-arm regions.

Further insights into ribosomes prepare for understanding their role in translation. The differences between eukaryotic and prokaryotic ribosomes, which affect protein synthesis inhibition in bacteria, are detailed. Eukaryotic ribosomes consist of a 60S large subunit and 40S small subunit, functioning as an 80S ribosome. Prokaryotic ribosomes comprise a 50S large subunit and 30S small one, forming a 70S ribosome. Their roles in translation highlight clinical relevance, like antibiotic targeting.

The initiation phase of translation involves binding of the small ribosomal subunit and initiation factors to the mRNA's Shine-Dalgarno sequence in prokaryotes, followed by tRNA and fMet (or Met in eukaryotes) binding to the start codon (AUG). GTP hydrolysis propels large ribosomal subunit binding, forming the complete initiation complex. The segment outlines differences in initiation processes between eukaryotes and prokaryotes.

In the elongation phase, the video explains the sequential addition of amino acids to the growing polypeptide chain. The process involves tRNAs bringing amino acids to the A site of the ribosome, the formation of peptide bonds facilitated by peptidyl transferase, and translocation moving the ribosome along mRNA. Energy from GTP is crucial, and this step is cyclic, adding one amino acid at a time to grow the polypeptide.

Elongation continues with the process of translocation where the ribosome moves the mRNA forward one codon length. This involves transitioning the peptidyl-tRNA from the A site to the P site, the deacylated tRNA to the E site, and its eventual exit. This cycle repeats with extended polypeptides moving into the P site, and another charged tRNA entering the A site. The continual cycle builds the growing polypeptide chain.

Termination of translation occurs once a stop codon enters the A site. No tRNA corresponds to stop codons, so release factors bind, initiating the disassembly of the translation complex. The completed polypeptide is released from the ribosome, which then dissociates from the mRNA to be reused or degraded. Release factors facilitate this process and ensure accurate halting of protein synthesis upon encountering stop codons.

The video transitions into discussing translation on free ribosomes and membrane-bound ribosomes on the rough ER. The primary determinant for rough ER-based synthesis is if the protein is to be secreted, integrated into a membrane, or sent to lysosomes. The signal sequence on peptides dictates binding to the rough ER where further synthesis and insertion occur. This process is critical for protein targeting to specific cellular compartments.

The ribosomal translocation to the rough ER is guided by signal sequences on nascent peptides, recognized by Signal Recognition Particles (SRP), which direct the ribosome to SRP receptors on the ER membrane. After attachment, the translocon opens, allowing peptide entry into the ER lumen. GTP hydrolysis plays a key role in translocon gating, and the signal peptide is cleaved by signal peptidase, finalizing incorporation into the ER lumen.

The video elaborates on protein targeting within the cell, clarifying that certain proteins synthesized on the rough ER are intended for secretion, membrane integration, or lysosomal pathways. In contrast, proteins produced by free ribosomes are typically retained for cytosolic functions, nuclear roles, or mitochondrial enzyme components. These distinctions underlie fundamental pathways of cellular protein distribution and functional specialization.

Protein post-translational modifications ensure functional maturity and proper cellular location. Examples include glycosylation for cellular recognition, lipidation for membrane anchoring, and phosphorylation for regulatory functions. Hydroxylation aids in collagen stability, essential for structural components of tissues. Trimming activates zymogens - precursor enzymes like trypsinogen - through precise proteolytic cleavage to yield active forms.