Biology Chapter on Translation Processes

Lecture 10: How Cells Make Proteins

• Lecture 8 covered DNA as a store of biological information
• Lecture 9 covered RNA and how genes are expressed
• Lecture 10 details how cells make proteins

False Statement about DNA and mRNA

• DNA contains deoxyribose sugars, mRNA contains ribose sugars
• DNA uses thymine, mRNA uses uracil
• DNA has a double helix, mRNA is single-stranded
• DNA and mRNA do not ALWAYS reside in the nucleus of cells (false statement)

mRNA Processing

• RNA is modified before leaving the nucleus
• mRNA processing protects mRNA from degradation
• mRNA processing helps mRNA exit the nucleus
• mRNA processing does NOT help anchor mRNA to ribosomes
• mRNA processing does help the splicing process by attracting snRNPs (small nuclear ribonucleoproteins)

Ribosomes

• Ribosomes are the site of translation
• Ribosomes are not involved in transcription, RNA processing, or splicing

Learning Objectives - Lecture 14 (Protein)

• Cells make polypeptides of specific lengths and sequences.
• Accurate protein synthesis requires base-pairing
• Protein synthesis (translation) utilizes ribosomes, aminoacyl-tRNAs, and mRNA (instruction tape)
• The genetic code defines how codons associate with anticodons
• Proteins are processed and targeted to specific cellular locations for proper function

From DNA to Protein

• Transcription occurs in the nucleus (eukaryotes) or cytoplasm (prokaryotes)
• DNA strand (template) is transcribed into mRNA
• mRNA carries the genetic code to the ribosome
• Translation occurs at the ribosome in the cytoplasm
• mRNA codons (triplet code) are translated into amino acids
• Translation uses tRNA anticodons for recognition and amino acid delivery

Translation: The Basic Concept

• The cell reads and interprets a genetic message, building a polypeptide
• Messenger RNA (mRNA) contains codons, decoded by tRNA
• mRNA moves through a ribosome, translating codons into amino acids, one by one
• Transfer RNA (tRNA) carries amino acids to the ribosome, matching anticodons with codons
• tRNA delivers amino acids to the growing polypeptide chain, when the anticodon recognizes the codon

tRNA Structure

• tRNA is a key part of accurate information transfer, through base pairing using hydrogen bonds
• tRNA transfers amino acids to the ribosome, the site of protein synthesis
• Several tRNA types exist for each amino acid type
• The tRNA anticodon interacts with the mRNA codon to ensure correct amino acid addition

Aminoacyl-tRNA Synthetase

• Aminoacyl-tRNA synthetase is an enzyme that attaches the correct amino acid to a tRNA molecule
• 20 different synthetases exist, one for each amino acid
• This process requires energy (ATP) to activate the amino acid
• Accurate loading of tRNA ensures precise protein synthesis

The Ribosome (Structure and Function)

• Ribosomes are large complex structures made of rRNA and protein
• Ribosomes have three binding sites (A, P, and E sites) for tRNA
• Ribosomes catalyze peptide bond formation between amino acids, acting as ribozymes and central to the process
• Ribosomes facilitate translation of mRNA to polypeptide chains, from amino to carboxyl end

Initiation of Translation

• Translation begins when the small ribosomal subunit binds to mRNA at the start codon (AUG)
• An initiator tRNA carrying methionine (Met) binds to the start codon
• The large ribosomal subunit joins to complete the initiation complex - which is required for protein synthesis to start

Elongation During Translation

• Amino acids are added to the growing polypeptide chain, one at a time
• Codon recognition brings the correct tRNA into the A site
• Peptide bond formation links the new amino acid to the growing polypeptide chain, by rRNA (large subunit)
• Translocation moves the tRNA in the A site to the P site, and discharged tRNA to the E site to leave the ribosome

Termination of Translation

• Elongation continues until a stop codon is reached (UAA, UAG, or UGA)
• A release factor binds to the stop codon, releasing the polypeptide chain
• The ribosome and other components are then released

Microscopic Analysis of Transcription

• Ribosomes can work in clusters on an mRNA, synthesizing multiple polypeptides simultaneously (polyribosomes)
• A single ribosome makes an average sized polypeptide in about a minute

The Genetic Code

• The genetic code translates the nucleotide sequence of mRNA into the amino acid sequence of a polypeptide
• A triplet codon of mRNA translates to a single amino acid
• The genetic code is degenerate (multiple codons can code for the same amino acid), and not ambiguous
• Genetic code is universal (works in all organisms)

The Genetic Code (Detail)

• The genetic code is a triplet code, non-overlapping
• The reading frame must be correct for accurate translation
• The code is degenerate (redundant), multiple codons can specify the same amino acid
• There are start and stop codons (AUG for methionine, UGA, UAA, UAG)
• The code is nearly universal (with some minor exceptions in certain organelles)

Protein Post-Translational Modification

• Polypeptide chains fold into their 3D structures
• Additional components (sugars, lipids) might be added or some amino acids removed
• Some proteins (e.g. membrane proteins) are moved to their required location

Mutations

• Mutations in DNA sequence can lead to changes or no changes in proteins
• Mutations can be base-pair substitutions (no shift, missense, or nonsense mutations) or frameshift mutations (causing extensive missense)
• Mutations can be insertions or deletions

Protein Structure

• Proteins have specific 3D structures, essential for function
• The three-dimensional shape is dictated by the amino acid sequence, interacting by multiple bonds and forces.

Ribosomes (Composition Question)

• Ribosomes are made of ribosomal RNA (rRNA) and protein

The Start Codon in mRNA

• The start codon in mRNA is AUG

Which statement is false for tRNA?

• tRNA molecules are NOT held together by disulfide bridges

Which process in the elongation phase of transcription does not require energy?

• Peptide bond formation; between adjacent amino acids

Triplet Code Details

• The number of possible codons is 64 and not 9
• There are 20 amino acids, but 64 codons
• Different codons code for the same amino acid
• Some codons are start and stop signals

Lecture 10 Translation Summary

• Translation involves initiation, elongation, and termination phases
• Ribosomes are essential for translation and protein synthesis
• The genetic code is involved in accurate protein sequence determination
• Mutations can cause protein synthesis errors