Protein Synthesis Lesson: How Cells Build Proteins

Lesson Overview

• DNA to Protein: Core Process Overview
• Translation (RNA to Protein)
• The Genetic Code and Wobble
• Mutations and Their Effects
• Gene Regulation – Lac Operon Example

Cells rely on proteins to perform nearly every function, from building structures to carrying out chemical reactions. To make these proteins, cells follow a precise two-step process called protein synthesis. In this process, genetic information stored in DNA is first copied into RNA, and then translated into a chain of amino acids to form a protein. 

This lesson explains how transcription and translation work together to produce proteins, covering each step in detail and addressing key concepts needed for a strong understanding.

DNA to Protein: Core Process Overview

Protein synthesis occurs in two major steps: transcription and translation. Transcription converts DNA into messenger RNA (mRNA). Translation uses this mRNA to build a protein.

Transcription (DNA to RNA)

• RNA polymerase binds to the promoter region of DNA.
• The TATA box in the promoter helps position RNA polymerase.
• RNA is built by adding complementary RNA bases to the DNA template.
• Uracil (U) replaces thymine (T) in RNA.
• Transcription ends at a termination sequence.

mRNA Processing in Eukaryotes

• A 5' cap is added to the start of the mRNA.
• A poly-A tail is added to the end.
• Introns (noncoding sequences) are removed.
• Exons (coding sequences) are joined together.

This table outlines the key modifications that occur to eukaryotic mRNA before it exits the nucleus:

Key Table – mRNA Modifications

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Translation (RNA to Protein)

This step occurs in the cytoplasm where ribosomes use mRNA to assemble amino acids into proteins.

Initiation

• mRNA binds to the small ribosomal subunit.
• A start codon (AUG) signals where translation begins.

Elongation

• tRNA molecules carry amino acids to the ribosome.
• Each tRNA has an anticodon that matches an mRNA codon.
• Peptide bonds form between amino acids.
• Ribosome has A, P, E sites for tRNA binding:
  • A: Aminoacyl site (entry)
  • P: Peptidyl site (holds growing chain)
  • E: Exit site

Termination

• A stop codon (UAA, UAG, or UGA) enters the A site.
• The completed polypeptide chain is released.

The table below summarizes the roles of each ribosome site involved in translation:

Key Table – Ribosome tRNA Binding Sites

The Genetic Code and Wobble

Each codon (three mRNA bases) codes for one amino acid. Some amino acids are specified by more than one codon.

Wobble Base Pairing

• The third base of a codon may pair loosely with the tRNA anticodon.
• This allows fewer tRNAs to read multiple codons.

Codon Features

• Start codon: AUG (methionine)
• Stop codons: UAA, UAG, UGA

This table provides examples of different codons and the amino acids or signals they represent:

Codon Example Table

Mutations and Their Effects

Changes in the DNA sequence can affect the protein product.

Types of Mutations

• Substitution: One base is replaced.
• Insertion/Deletion: Bases added or removed.

Frameshift Mutation

• Insertion or deletion not in multiples of three shifts the reading frame.
• Alters all downstream amino acids.

This table compares the main mutation types and their typical impacts on protein structure:

Mutation Impact Table

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Gene Regulation – Lac Operon Example

Cells control when genes are expressed. The lac operon in bacteria is a model for gene regulation.

Key Features of Lac Operon

• Repressor protein blocks transcription in absence of lactose.
• Lactose presence removes repressor and allows gene expression.

This table outlines the major components of the lac operon and their roles in gene regulation:

Key Table – Lac Operon Components

Ribosomes, Plasmids, and Other Notes

• Ribosomes contain rRNA and proteins, not DNA.
• In bacteria, genes may also exist on plasmids (small circular DNA).

This table summarizes key differences between DNA and RNA:

Table – Nucleic Acids Comparison

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