TRANSLATION IN EUKARYOTES

Protein Synthesis in Eukaryotes

Protein synthesis is a fundamental biological process through which cells generate proteins, essential for various cellular functions. In eukaryotic cells, this process occurs in two main stages: transcription and translation. Each stage involves specific key proteins and enzymes that facilitate the accurate synthesis of proteins based on genetic information encoded in DNA.

1. Transcription

Transcription is the first step of protein synthesis, where the DNA sequence of a gene is transcribed into messenger RNA (mRNA). This process takes place in the nucleus of eukaryotic cells and involves several key components:

• RNA Polymerase II: This enzyme is crucial for synthesizing mRNA from the DNA template. It binds to the promoter region of a gene, unwinds the DNA strands, and catalyzes the addition of ribonucleotides complementary to the DNA template strand.
• Transcription Factors: These are proteins that assist RNA polymerase in binding to the promoter region. They help regulate gene expression by either promoting or inhibiting transcription.
• Enhancers and Silencers: These regulatory sequences can be located far from the promoter and interact with transcription factors to enhance or silence gene expression.

The process begins when RNA polymerase II binds to the promoter region with the help of transcription factors. Once bound, it unwinds the DNA helix and synthesizes a single strand of mRNA by adding ribonucleotides complementary to the DNA template strand (A pairs with U, C pairs with G). After elongation, RNA polymerase reaches a termination signal, causing it to detach from the DNA and release the newly synthesized mRNA.

Following transcription, eukaryotic mRNA undergoes several modifications before it exits the nucleus:

• 5’ Capping: A modified guanine nucleotide is added to the 5’ end of mRNA, protecting it from degradation and assisting in ribosome binding during translation.
• Polyadenylation: A poly(A) tail consisting of adenine nucleotides is added to the 3’ end of mRNA, enhancing stability and facilitating export from the nucleus.
• Splicing: Introns (non-coding regions) are removed from pre-mRNA through splicing, while exons (coding regions) are joined together by spliceosomes composed of small nuclear RNAs (snRNAs) and proteins.

Once processed, mature mRNA exits through nuclear pores into the cytoplasm for translation.

2. Translation

Translation is the second stage of protein synthesis where ribosomes synthesize proteins based on mRNA sequences. This process occurs in three main phases: initiation, elongation, and termination.

• Ribosomes: Composed of ribosomal RNA (rRNA) and proteins, ribosomes serve as molecular machines that facilitate translation. They consist of two subunits (large and small), which come together during translation.
• Transfer RNA (tRNA): tRNAs are adapter molecules that transport amino acids to ribosomes during protein synthesis. Each tRNA has an anticodon that pairs with a corresponding codon on mRNA.
• Aminoacyl-tRNA Synthetases: These enzymes charge tRNAs with their respective amino acids by catalyzing their attachment. Each synthetase is specific for one amino acid and its corresponding tRNAs.

The translation process begins with initiation:

1. The small ribosomal subunit binds to the 5’ cap of mRNA.
2. The initiator tRNA carrying methionine recognizes the start codon (AUG) on mRNA.
3. The large ribosomal subunit then joins to form a complete ribosome complex.

During elongation:

1. The ribosome moves along mRNA; new tRNAs enter at the A site.
2. Peptide bonds form between adjacent amino acids facilitated by peptidyl transferase activity within rRNA.
3. The ribosome translocates along mRNA until it reaches a stop codon (UAA, UAG, or UGA).

Termination occurs when a stop codon enters the A site:

1. Release factors bind to stop codons instead of tRNAs.
2. The completed polypeptide chain is released from tRNA at the P site.
3. The ribosomal subunits disassemble, freeing up mRNA for potential re-use or degradation.

After translation, newly synthesized polypeptides may undergo post-translational modifications such as phosphorylation or glycosylation before folding into functional proteins or being transported to their final destinations within or outside of cells.

In summary, protein synthesis in eukaryotes involves intricate processes governed by various enzymes and proteins that ensure accurate transcription and translation based on genetic information stored in DNA.

Comparison of Protein Synthesis in Prokaryotes vs. Eukaryotes

Protein synthesis is a fundamental biological process that translates genetic information into functional proteins. While both prokaryotes and eukaryotes perform this essential function, there are significant differences in the mechanisms and processes involved.

1. Location of Protein Synthesis

In prokaryotes, protein synthesis occurs in the cytoplasm since they lack membrane-bound organelles. The ribosomes, which are the sites of protein synthesis, float freely within the cytoplasmic space. In contrast, eukaryotic cells compartmentalize their cellular functions; thus, protein synthesis occurs primarily in the cytoplasm but is closely associated with the endoplasmic reticulum (ER). The rough ER has ribosomes attached to its surface, facilitating the translation of proteins destined for secretion or for use within membranes.

2. Ribosome Structure and Size

Prokaryotic ribosomes are smaller than those found in eukaryotes. Prokaryotic ribosomes are 70S (composed of 50S and 30S subunits), while eukaryotic ribosomes are larger at 80S (composed of 60S and 40S subunits). This difference in size affects the binding of antibiotics that target bacterial ribosomes without affecting eukaryotic ribosomes.

3. mRNA Processing

In prokaryotes, mRNA is synthesized directly from DNA and is immediately available for translation as it does not undergo extensive processing. There is no intron removal or capping; instead, transcription and translation can occur simultaneously due to the absence of a nuclear membrane.

Conversely, eukaryotic mRNA undergoes several processing steps before it can be translated. These include:

• Capping: A methylated guanine cap is added to the 5’ end.
• Polyadenylation: A poly-A tail is added to the 3’ end.
• Splicing: Introns (non-coding regions) are removed, and exons (coding regions) are joined together.

These modifications help stabilize mRNA and facilitate its export from the nucleus to the cytoplasm for translation.

4. Initiation of Translation

The initiation phase also differs between prokaryotes and eukaryotes:

• In prokaryotes, initiation begins when the small ribosomal subunit binds to a specific sequence on the mRNA called the Shine-Dalgarno sequence located upstream of the start codon (AUG). The initiator tRNA carrying formylmethionine (fMet) then binds to this start codon.
• In eukaryotes, initiation involves a more complex process where the small ribosomal subunit recognizes a modified cap structure at the 5’ end of mRNA. The initiator tRNA carries methionine rather than fMet and scans along the mRNA until it finds an AUG start codon.

5. Elongation and Termination

The elongation process is similar in both groups but varies slightly in terms of factors involved:

• In both prokaryotes and eukaryotes, amino acids are added one by one to form a polypeptide chain as dictated by codons on mRNA.

Termination occurs when a stop codon (UAA, UAG, UGA) is reached:

• In prokaryotes, release factors recognize these stop codons directly.
• In eukaryotes, multiple release factors work together with additional proteins to facilitate termination.

6. Post-translational Modifications

Post-translational modifications also differ significantly:

• Prokaryotic proteins often undergo minimal modification after translation.

Eukaryotic proteins frequently undergo extensive post-translational modifications such as phosphorylation, glycosylation, ubiquitination, etc., which play critical roles in regulating protein activity and function.

Conclusion

In summary, while both prokaryotic and eukaryotic cells synthesize proteins based on genetic instructions encoded in DNA through transcription and translation processes, they differ significantly in terms of location within the cell, ribosome structure and size, mRNA processing requirements, initiation mechanisms for translation, elongation processes involving different factors, termination methods involving distinct release factors as well as post-translational modifications that occur predominantly in eukaryotic cells.