Protein Synthesis ( summarized )
Protein Synthesis Notes (Comprehensive)
Proteins are the building blocks of all living materials
Examples include:
Muscles and muscle tissue
Hair and nails (alpha-keratin) and other keratin-based structures (feathers, wool, claws, scales, horns, hooves)
Hemoglobin in blood carries oxygen
Channel proteins regulate brain signaling by controlling molecule movement into/out of nerve cells
Receptor proteins on cell surfaces transmit signals inside
Antibodies defend against bacteria and viruses
Enzymes in saliva, stomach, and small intestine digest food
Large clusters of proteins form molecular machines that perform cellular work (e.g., copying genes during cell division and making new proteins)
Proteins are composed of amino acids
There are 20 different amino acids
Different proteins are formed by combining these 20 amino acids in various sequences and lengths
Protein construction requires ribosomes and RNA-based machinery
All biological proteins ultimately follow the same set of rules for making polypeptide chains from amino acids
The total number of amino acids in a protein varies by protein; the sequence determines structure and function
The process by which amino acids are linked is a dehydration synthesis forming peptide bonds
Protein synthesis machinery and organelles
Proteins are manufactured by ribosomes (made of ribosomal RNA, rRNA, and ribosomal proteins)
Ribosomes can be free in the cytosol or attached to the rough endoplasmic reticulum (RER)
The ribosome has two subunits: a large subunit and a small subunit
tRNA (transfer RNA) brings amino acids to the ribosome; each tRNA carries a specific amino acid
The ribosome has functional sites: A site (aminoacyl site), P site (peptidyl site), and E site (exit site)
The process occurs in two main stages: transcription (DNA -> RNA) and translation (RNA -> protein)
Newly formed proteins may be targeted to particular locations (e.g., secreted proteins, membranes) via the ER and Golgi apparatus
Function of proteins (summary by roles)
Fight disease (antibodies and immune-related proteins)
Build and repair body tissues
Enzymes catalyze digestion and other chemical reactions; they speed up reaction rates
Components of all cell membranes (proteins embedded in or associated with membranes)
Enzymes and structural proteins together maintain cell structure, signaling, and metabolism
Making proteins: Step 1 – Transcription
Transcription is the copying of genetic information from DNA to RNA
Why transcription happens:
DNA contains the genetic code for the needed protein
Ribosomes are in the cytoplasm (outside the nucleus) and synthesize proteins
DNA is too large and double-stranded to leave the nucleus; RNA is single-stranded and can leave
Part of DNA temporarily unzips to serve as a template for assembling complementary nucleotides into messenger RNA (mRNA)
The result is mature, export-ready mRNA that carries the genetic code to the cytoplasm
The DNA → mRNA pathway preserves the genetic code while allowing mRNA to travel to ribosomes for translation
Making proteins: Step 2 – Translation
Translation is the decoding of mRNA into a protein
Transfer RNA (tRNA) carries amino acids from the cytoplasm to the ribosome
The amino acids used come from the protein components of the diet; dietary proteins are broken down into amino acids and reused to build new proteins according to DNA instructions
A codon is a sequence of three adjacent bases in mRNA that codes for a specific amino acid
Each tRNA has an anticodon of three bases complementary to the codon and carries the corresponding amino acid
Each tRNA corresponds to a particular amino acid
Translation begins at the start codon: AUG
The start codon is recognized by its complementary tRNA anticodon, which carries methionine (Met in many organisms)
The ribosome binds the next codon and its anticodon, and the process continues along the mRNA
The ribosome orchestrates the joining of amino acids to form a growing polypeptide chain
The ribosome moves from codon to codon along the mRNA in the 5' to 3' direction
The elongation continues until a stop codon is encountered
A stop codon signals termination; no corresponding tRNA carries an amino acid for a stop codon
The resulting chain of amino acids folds into a functional protein (Polypeptide = Protein)
Key molecular players and their roles
DNA: holds genetic information in the nucleus
mRNA: carries the DNA code from nucleus to cytoplasm; serves as the template for protein synthesis
RNA polymerase (implied in transcription): enzyme that builds the mRNA strand from the DNA template
Ribosome: the molecular machine where translation occurs (large and small subunits; A, P, E sites)
tRNA: adapters that bring specific amino acids to the ribosome and match codons with anticodons
rRNA: structural and catalytic component of ribosomes (ribosomal RNA)
The central dogma and pathway flow
Flow of genetic information: DNA -> RNA -> Protein
Transcription occurs in the nucleus; translation occurs in the cytoplasm at ribosomes
The protein's primary sequence is determined by the mRNA codon sequence, which is derived from the DNA template
Codons, anticodons, and the genetic code (conceptual map)
A codon: three adjacent bases in mRNA that specify an amino acid or a stop signal
An anticodon: a three-nucleotide sequence on tRNA that is complementary to a codon on mRNA
The genetic code is read in a 5' to 3' direction on mRNA; tRNA anticodons pair with codons to deliver the correct amino acid
Start codon: AUG (codes for Methionine in initiation; often the first amino acid in new polypeptides)
Stop codons: UAA, UAG, UGA (do not code for amino acids; signal termination)
The same codon table is used to map codons to amino acids (examples provided below)
Examples from codon chart (demonstration of translation rules)
Example 1: CAC/CCA/UGG/UGA
CAC → Histidine
CCA → Proline
UGG → Tryptophan
UGA → Stop
Translation outcome: Histidine - Proline - Tryptophan (then termination at Stop)
Example 2: AUG/AAC/GAC/UAA
AUG → Methionine (start codon)
AAC → Asparagine
GAC → Aspartic acid
UAA → Stop
Translation outcome: Methionine - Asparagine - Aspartic acid (then termination at Stop)
Codon chart and reading frames (conceptual notes)
Reading frame matters: a shift changes the entire amino acid sequence
Codons are read in triplets from a defined starting point (the start codon, typically AUG)
There are 64 possible codons (4 bases ^ 3 positions) and 20 standard amino acids plus stop signals
Some codons encode the same amino acid (degeneracy of the code); multiple codons can specify the same amino acid
Stop codons do not encode amino acids and terminate translation
The chart typically shows which codons map to which amino acids and where Stop codons occur
The practical flow for protein synthesis (concise map)
DNA in nucleus contains gene sequence
Transcription creates mRNA in the nucleus (mRNA transcript exits the nucleus through nuclear pores)
Mature mRNA travels to the cytoplasm and binds to a ribosome
tRNA molecules bring specific amino acids to the ribosome
The ribosome matches codons on the mRNA with anticodons on tRNA and links amino acids together via peptide bonds
The polypeptide chain elongates until a stop codon is reached
The newly formed polypeptide folds into its functional three-dimensional structure and may be modified or targeted to its proper cellular location
Connections to broader biology and real-world relevance
Central dogma underpins molecular biology and genetics; mutations in DNA can alter mRNA codons and thus protein sequences, affecting function
Understanding transcription and translation is foundational for biotechnology, medicine, and forensic biology
Knowledge of protein synthesis informs drug design, gene therapy, and management of metabolic diseases
Numerical, symbolic, and LaTeX references (key facts)
Number of standard amino acids: 20
Codon length: 3 bases per codon
Start codon: AUG
Stop codons: UAA, UAG, UGA
Translation involves A, P, and E sites on the ribosome
The polypeptide produced is the protein
Quick practice questions (to test understanding)
Given the mRNA sequence AUG-GUU-CAA-UAA, identify the amino acid sequence before the stop
If a mutation changes UAA (Stop) to CAA (Glutamine), what potential effect could occur on the resulting protein?
Explain why mRNA leaves the nucleus while DNA does not
Practical implications and ethics (brief notes)
Advances in gene expression knowledge enable medical therapies (e.g., recombinant proteins, gene editing)
Ethical considerations include gene manipulation, animal models, and implications for cloning and personalized medicine
Summary
Protein synthesis is the coordinated process of transcription (DNA to mRNA) and translation (mRNA to polypeptide) that yields functional proteins
The ribosome, mRNA, and tRNA collaborate to convert the genetic code into a specific amino acid sequence
The genetic code is read in codons (three-base units), with start and stop signals guiding initiation and termination
The structure and function of proteins depend on the precise sequence and folding of amino acids, determined by the gene sequence
Proteins are fundamental building blocks of all living materials, including muscles, hair, hemoglobin, and enzymes. They are large clusters of proteins that form molecular structures that perform cellular work like copying genes or making new proteins.
Composed of 20 different amino acids, which dictate a protein's structure and function based on their sequence and length.
Amino acids are linked by dehydration synthesis, forming peptide bonds.
Protein Synthesis Machinery and Organelles
Proteins are manufactured by ribosomes (made of ribosomal RNA, rRNA, and ribosomal proteins).
Ribosomes consist of two subunits (large and small) and have A (aminoacyl), P (peptidyl), and E (exit) sites.
tRNA (transfer RNA) carries specific amino acids to the ribosome.
Newly formed proteins may be targeted to specific locations via the ER and Golgi apparatus.
Function of Proteins
Fight disease (antibodies)
Build and repair body tissues
Enzymes catalyze digestion and other chemical reactions
Components of all cell membranes
Making Proteins: Step 1 – Transcription (DNA -> RNA)
Occurs in the nucleus.
DNA (too large to leave the nucleus) temporarily unzips.
Genetic information is copied from DNA into a complementary, single-stranded mRNA molecule.
mRNA carries the genetic code out of the nucleus to the cytoplasm.
Making Proteins: Step 2 – Translation (RNA -> Protein)
Occurs in the cytoplasm at the ribosome.
mRNA sequence is decoded into a protein.
tRNA molecules carry amino acids to the ribosome.
A codon (three adjacent bases in mRNA) codes for a specific amino acid.
Each tRNA has an anticodon (three bases complementary to the codon) and carries the corresponding amino acid.
Translation begins at the start codon (AUG) with methionine.
Ribosome moves along mRNA, joining amino acids to form a polypeptide chain via peptide bonds.
Elongation continues until a stop codon (UAA, UAG, UGA) is reached, signaling termination.
The resulting amino acid chain folds into a functional protein.
Key Molecular Players and Their Roles
DNA: Holds genetic information.
mRNA: Carries DNA code from nucleus to ribosome.
Ribosome: Site of translation.
tRNA: Brings specific amino acids to the ribosome.
rRNA: Structural and catalytic component of ribosomes.
The Central Dogma and Pathway Flow
Genetic information flows: DNA -> RNA -> Protein.
Transcription in nucleus; translation in cytoplasm.
Codons, Anticodons, and the Genetic Code
Codon: Three adjacent bases in mRNA specifying an amino acid or stop signal.
Anticodon: Three-nucleotide sequence on tRNA complementary to a codon.
Genetic code is read in 5' to 3' direction on mRNA.
64 possible codons; 20 standard amino acids. Some amino acids have multiple codons (degeneracy).
Practical Flow for Protein Synthesis
DNA in nucleus contains gene sequence.
Transcription creates mRNA in the nucleus (mRNA exits).
mRNA travels to cytoplasm, binds to a ribosome.
tRNA brings specific amino acids to ribosome.
Ribosome matches mRNA codons with tRNA anticodons, linking amino acids.
Polypeptide chain elongates until a stop codon.
Polypeptide folds into functional protein.
Numerical, Symbolic, and LaTeX References
Number of standard amino acids: 20
Codon length: 3 bases
Start codon: AUG
Stop codons: UAA, UAG, UGA
Ribosome sites: A, P, E