Protein Synthesis

PROTEIN SYNTHESIS NOTES

01 OVERVIEW

  • Video Preview: Discussion of the protein synthesis process to be explored in detail.

02 OVERVIEW: THE FLOW OF GENETIC INFORMATION

DNA Characteristics:

  • Sugar: Deoxyribose

  • Structure: Double helix

  • Location: Stays in the nucleus

  • Nucleotide Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)

RNA Characteristics:

  • Sugar: Ribose

  • Structure: Single stranded

  • Location: Can leave the nucleus

  • Nucleotide Bases: Adenine (A), Uracil (U), Cytosine (C), Guanine (G)

  • Types of RNA: Messenger RNA (mRNA), Transfer RNA (tRNA), Ribosomal RNA (rRNA)

Key Processes:

  1. Transcription:

    • Function: DNA → mRNA

    • Description: DNA provides a template for synthesis of a complementary RNA strand.

  2. Translation:

    • Function: mRNA → Protein

    • Description: Information in mRNA's nucleotide order is used to determine the amino acid sequence of a polypeptide. Occurs at ribosomes.

Prokaryotic vs Eukaryotic Cells:

  • Prokaryotes: Transcription and translation are coupled; ribosomes attach to the leading end of mRNA while transcription is ongoing.

  • Eukaryotes: Transcription occurs in the nucleus, translation at ribosomes, and RNA processing modifies the primary transcript before it exits the nucleus.

Central Dogma:

  • Definition: The molecular chain of command in a cell is summarized by the sequence: DNA → RNA → Protein.

03 TRIPLET CODE

Codon Chart:

  • DNA template strand example:

    • 5' ACCAAACCGAGT 3' (Template)

    • 5' TGGTTTGGCTCA 3' (Complementary)

Understanding Codons:

  • Definition: Codon is a triplet of nucleotides that corresponds to a specific amino acid.

  • Examples of Codons:

    • UUU → Phe

    • UAA, UAG → Stop

Genetic Code Characteristics:

  • Redundant: Multiple codons can encode the same amino acid.

  • Non-Ambiguous: Each codon corresponds to only one amino acid.

    • Example: GAA and GAG both specify glutamate.

  • Synonymous Codons: Often differ at the third base position.

Extraction of Genetic Message:

  • Reading Frame: Correct starting point is essential for determining the reading frame; subsequent codons are read as nonoverlapping triplets.

  • Encoding: Genetic information encodes as sequences of nonoverlapping triplets.

Evolution of the Genetic Code:

  • Universality: Similarities in the genetic code across various life forms; genes can be transcribed and translated across species.

  • Biotechnology Applications: Bacteria can be engineered to synthesize human proteins.

  • Exceptions: Certain single-celled eukaryotes exhibit minor codon variations.

  • Historical Remark: Suggests a common ancestor for all life due to universal genetic vocabulary.

04 TRANSCRIPTION

Transcription Process:

  • Definition: Messenger RNA is synthesized from the DNA template.

  • RNA Polymerase: Enzyme that starts transcription by separating DNA strands and bonding RNA nucleotides that complement the template.

  • Direction of Synthesis: New RNA synthesized in the 5' to 3' direction.

Stages of Transcription:

  1. Initiation:

    • RNA polymerase binds to the promoter region upstream of the transcription unit; the starting point for gene transcription.

    • Promoter Example: TATAAAA sequence in eukaryotes.

    • Transcription Factors: Recognize the promoter in eukaryotes, allowing RNA polymerase to form a transcription initiation complex.

  2. Elongation:

    • RNA polymerase unwinds the double helix and adds nucleotides to the 3' end of the growing RNA strand, re-forming the double helix behind the RNA synthesis.

    • Multiple RNA polymerases can transcribe a single gene simultaneously.

  3. Termination:

    • Prokaryotes: RNA polymerase stops transcription at the terminator sequence, releasing RNA and DNA.

    • Eukaryotes: RNA polymerase continues past the terminator sequence (e.g., AAUAAA) before cutting the pre-mRNA at 10-35 nucleotides beyond the terminator.

05 POST-TRANSCRIPTIONAL MODIFICATIONS

RNA Processing in Eukaryotic Cells:

  • 5' Cap: Modified guanine added to the 5' end of pre-mRNA for protection and ribosome attachment.

  • Poly(A) Tail: 50-250 adenine nucleotides added to the 3' end to inhibit hydrolysis and control mRNA export from the nucleus.

  • Leader and Trailer Segments: Nontranslated segments present in mRNA.

RNA Splicing:

  • Definition: Removal of introns (non-coding sequences) and joining of exons (coding sequences).

  • Spliceosome: Complex of proteins and small nuclear RNAs (snRNPs) that catalyzes splicing.

    • Steps in Splicing: 1. Formation of spliceosome; 2. Base-pairing of snRNA with intron ends; 3. Intron release and exons joining.

  • snRNA Role: Acts as a ribozyme during splicing.

  • Alternative RNA Splicing: The same gene encodes multiple polypeptides by varying which exons are included in the final mRNA.

Implications of RNA Splicing:

  • Introns can regulate gene activity.

  • May facilitate the evolution of new proteins through exon shuffling.

06 TRANSLATION

Overview of Translation:

  • Definition: The process where the cell interprets mRNA and synthesizes proteins.

  • tRNA Function: Transfers amino acids from the cytoplasm to the ribosomes for polypeptide chain assembly.

  • Ribosome Role: Joins amino acids carried by tRNA into the growing polypeptide chain.

tRNA Structure and Function:

  • Structure: tRNA is a folded strand with an anticodon region and an amino acid attachment site at the 3' end.

  • Wobble Hypothesis: Some tRNAs can recognize multiple codons due to relaxed base pairing rules at the third base position, aiding efficiency in translation.

Ribosome Structure:

  • Consists of two subunits made of proteins and rRNA.

  • Each ribosome contains three tRNA binding sites: A site (acceptor), P site (polypeptide), E site (exit).

Stages of Translation:

  1. Initiation:

    • The small ribosomal subunit binds to mRNA and an initiator tRNA with methionine.

    • The large subunit attaches, completing the initiation stage.

  2. Elongation:

    • Codon Recognition: Elongation factor facilitates correct tRNA recruitment based on the codon.

    • Peptide Bond Formation: Catalyzed by rRNA, joining the amino acid in the A site with the polypeptide in the P site.

    • Translocation: tRNA with polypeptide moves to P site, shifting mRNA for next codon.

  3. Termination:

    • Occurs when a stop codon reaches the A site, allowing a release factor to disassemble the complex and free the polypeptide.

Final Steps in Protein Folding and Modification:

  • Proteins fold into their functional shapes spontaneously, often with help from chaperone proteins.

  • Post-translational modifications may include the addition of functional groups or polypeptide cleaving.

Signal Peptides:

  • Distinction between free and bound ribosomes, determined by the presence of a signal peptide on polypeptides destined for endomembrane systems.

  • Signal Recognition Particle (SRP) binds to the signal peptide, directing ribosome to the ER for processing.

07 MUTATIONS

Definition:

  • Changes in the genetic material of a cell; can vary in scale (point mutations vs. large-scale mutations).

  • Point Mutations: Small-scale changes often leading to single amino acid alterations in proteins. Example: Sickle-cell disease caused by a single base pair mutation.

Types of Point Mutations:

  1. Silent Mutations: Do not affect protein function due to redundancy in the genetic code.

  2. Missense Mutations: Replace one amino acid with another, affecting the protein.

  3. Nonsense Mutations: Convert amino acid codons to stop codons, resulting in truncated and usually nonfunctional proteins.

Insertions and Deletions:

  • Effect on Genes: These mutations often lead to frameshift mutations unless in multiples of three, leading to improper codon grouping and significant functional changes in proteins.

RECAP: THE FLOW OF GENETIC INFORMATION

  • Flow: DNA → RNA → Polypeptide Chain.

  • Key Components: RNA polymerase, spliceosome, ribosomal subunits, tRNA, codons, and anticodons illustrate the detailed mechanisms of transcription, processing, and translation of genetic information into functional proteins.