Central Dogma Slides
Molecular Biology & Genetics
High School Biology Series
The Code of Life
The Central Dogma
How living cells read DNA instructions to manufacture functional proteins
1. REPLICATION DNA
Permanent master blueprint preserved in the nucleus
2. TRANSCRIPTION mRNA
Portable single-stranded copy sent out to ribosomes
3. TRANSLATION PROTEIN
Folded molecular nanomachines that carry out cellular work
Unit 6 • Gene Expression and Protein Synthesis Slide 1 of 10
The Grand Architecture
Information Flow in Living Cells
Francis Crick • 1958
THE ARCHIVE
DNA
Double-stranded master vault containing Deoxyribose sugar and Thymine (T).
Location: Nucleus Only
THE DISPATCH
mRNA
Single-stranded messenger containing Ribose sugar and Uracil (U) instead of Thymine.
Travels: Nucleus → Cytoplasm
THE WORKHORSE
Protein
Folded 3D chains of 20 amino acids doing cellular work (enzymes, antibodies, transport).
Active Everywhere in Cell
One-Way Traffic: Information moves nucleic acid → protein, NEVER backwards from protein → nucleic acid!
Phase 1 • Cellular Division Prep
DNA Replication: Copying the Code
S Phase • Nucleus
The Replication Fork Semi-Conservative
Helicase Pol Leading Strand (Continuous) Lagging (Okazaki fragments)
Meselson-Stahl Proof: Each daughter helix retains 1 parental strand (blue) + 1 newly synthesized strand (green).
1
DNA Helicase ("The Unzipper")
Breaks hydrogen bonds holding complementary base pairs together, opening the replication fork.
2
DNA Polymerase ("The Builder")
Adds matching free nucleotides in the 5' → 3' direction and proofreads for replication errors.
3
DNA Ligase ("The Gluer")
Seals gaps between Okazaki fragments on the lagging strand, completing the sugar-phosphate backbone.
Base Pairing Rules: Adenine (A) = Thymine (T) • Guanine (G) ≡ Cytosine (C) Slide 3 of 10
Phase 2 • The Message
Transcription: DNA → Messenger RNA
Inside the Nucleus
The Transcription Bubble in Action Enzyme: RNA Polymerase
5' Non-template DNA 3' Template DNA Strand RNA Polymerase 5' New mRNA A pairs with U • G pairs with C
1. INITIATION
RNA Polymerase binds gene promoter region (TATA Box).
2. ELONGATION
Enzyme builds mRNA matching template in 5' → 3' direction.
3. TERMINATION
Terminator sequence releases pre-mRNA transcript.
U Crucial Rule: RNA contains Uracil (U) instead of Thymine (T)!
DNA [A] → mRNA [U]
Eukaryotic Editing
RNA Splicing: Editing Pre-mRNA
Before Nuclear Export
Pre-mRNA Transcript (Uncut) Introns = Non-coding junk
5' Cap
Exon 1
Intron A
Exon 2
Intron B
Exon 3
Poly-A Tail 3'
Spliceosomes cut out Introns and paste Exons together!
Mature mRNA (Classroom-Ready Export) Exons = EXPRESSED genes
5' Cap
Exon 1
Exon 2
Exon 3
Poly-A Tail 3'
1. 5' Guanine Cap
Helps ribosome dock and protects from degradation.
2. RNA Splicing
Removes non-coding introns; joins coding exons.
3. Poly-A Tail
Adds 100-250 Adenines on 3' end to prolong mRNA lifespan.
Phase 3 • Protein Factory
Translation: RNA → Polypeptide
At the Ribosome
Ribosomal Assembly E • P • A Sites
E P A 5' mRNA Strand →
A site: Accepts incoming tRNA • P site: Holds growing peptide • E site: Empty tRNA exits.
Ribosome (rRNA + Proteins)
Clamps around mRNA like a workbench and catalytically forms covalent peptide bonds.
tRNA (Transfer RNA)
Bilingual molecular adapter: carries a specific amino acid top, complementary anticodon bottom.
Start & Stop Codons
AUG tells the ribosome where to start; UAA/UAG/UGA triggers peptide release.
Translation converts nucleic acid language (nucleotides) to protein language (amino acids) Slide 6 of 10
Universal Biological Cipher
Cracking the Genetic Code
64 Codons • 20 Amino Acids
How Ribosomes Read Codons Non-overlapping Triplets
START CODON
AUG
Met
Methionine
CODON 2
GCU
Ala
Alanine
CODON 3
AAU
Asn
Asparagine
STOP CODON
UGA
×
Release Factor
1. Nearly Universal
From bacteria to humans, identical codons code for identical amino acids.
2. Degenerate (Redundant)
Multiple codons code for the same amino acid, buffering against mutations.
3. Unambiguous
Each specific codon specifies one and only one amino acid.
Shape Equals Function
From Polypeptide Chain to 3D Machine
Hierarchical Folding
1° PRIMARY
Sequence
Linear amino acid chain bonded by covalent peptide bonds.
Covalent Bonds
2° SECONDARY
Local Coils
α-helices & β-sheets formed by backbone H-bonds.
Hydrogen Bonds
3° TERTIARY
3D Globule
Full 3D active shape stabilized by R-group interactions.
Active Enzyme!
4° QUATERNARY
Multi-Unit
Multiple folded chains assembling together (e.g. Hemoglobin).
Protein Complex
Why it matters: In Sickle Cell Anemia, 1 mistaken DNA nucleotide changes Glutamic Acid → Valine, causing hemoglobin to misfold into rigid sickled rods.
Classroom Guided Practice
Trace the Code: From DNA to Protein
Student Challenge
1. DNA Template Strand (3' → 5')
Unzipped and read by RNA Polymerase
3'- TAC CGA TTA GCT ACT -5'
Transcription (A pairs with U, T pairs with A, C pairs with G)
2. Transcribed mRNA (5' → 3')
mRNA codons transported to the ribosome
5'- AUG GCU AAU CGA UGA -3'
Translation by Ribosome & tRNA Anticodons
3. Polypeptide Chain
Amino acids connected by peptide bonds
Met — Ala — Asn — Arg — [STOP]
Check for Understanding: Why does this 5-codon sequence produce a chain of only 4 amino acids? (UGA is a STOP signal!) Slide 9 of 10
Unit Review & Synthesis
The Central Dogma Master Matrix
Summary Reference
Process
Starting Material
Finished Product
Primary Catalyst
Cell Location
Replication
Parental DNA
2 Identical DNA Duplexes
DNA Polymerase
Nucleus (S Phase)
Transcription
DNA Template Strand
Pre-mRNA Transcript
RNA Polymerase
Nucleus
RNA Splicing
Pre-mRNA with Introns
Mature mRNA + Caps
Spliceosome
Nucleus
Translation
Mature mRNA Codons
Folded Active Protein
Ribosome & tRNA
Cytoplasm / RER
The Takeaway: DNA provides the enduring blueprint, RNA delivers the operational transcript, and proteins execute every living function in the cell.
Classroom Presentation Ready • Biology Standards Aligned Slide 10 of 10
