IMAT Preparation: Biology Section
Lesson 12: The Absolute Complete Edition (DNA, Expression & Biotech)
Introduction: The Code of Life
Welcome to the definitive guide on Molecular Biology for IMAT, MCAT, and BMAT. This document covers the central dogma of biology with unprecedented depth, focusing on the mechanisms, the math, and the medicine. We move from the atomic structure of nucleotides to the cutting-edge of CRISPR-Cas9 editing.
Learning Objectives (IMAT Spec 2024/25)
- LO 12.0: Foundations: Nucleotide structure, Phosphodiester bonds, Chargaff's Rules, and Chromatin packing (Histones).
- LO 12.1: Replication: Semi-conservative model, Meselson-Stahl, Enzymes (Prok vs Euk), Telomeres, and Inhibitors.
- LO 12.2: Expression: Transcription (Promoters, TFs), RNA Processing (Capping, Tailing, Splicing), and the Genetic Code.
- LO 12.3: Translation: Ribosome structure, Charging, Initiation, Elongation, Termination, and Antibiotics.
- LO 12.4: Regulation: Operons (Lac, Trp), Epigenetics (Methylation, Acetylation), and Transcription Factors.
- LO 12.5: Mutations: Point mutations, Frameshifts, Mutagens, and Repair Mechanisms (NER, BER, MMR, NHEJ).
- LO 12.6: Biotechnology: PCR, Gel Electrophoresis, Sequencing (Sanger vs NGS), Cloning, CRISPR, and Blotting.
Part 0: The Molecular Foundations
DNA (Deoxyribonucleic Acid) is a biopolymer consisting of monomeric units called nucleotides. It acts as the repository of genetic information, stable enough to be passed down through generations yet flexible enough to allow for evolution.
0.1 The Nucleotide Structure
A single nucleotide consists of three functional groups, assembled via condensation reactions:
- Phosphate Group ($PO_4^{3-}$): Attached to the 5' carbon of the sugar. This confers a strong negative charge, making DNA acidic and allowing it to migrate towards the anode (+) in electrophoresis. It also links nucleotides together.
- Pentose Sugar: A 5-carbon sugar ring (furanose).
- DNA: Deoxyribose (Has an -H at the 2' Carbon). This lack of oxygen makes DNA more chemically stable and suitable for long-term storage.
- RNA: Ribose (Has an -OH at the 2' Carbon). This hydroxyl group makes RNA susceptible to alkaline hydrolysis, fitting its role as a transient messenger.
- Nitrogenous Base: Attached to the 1' carbon via an N-glycosidic bond.
Diagram: Atomic Structure of a Nucleotide
Purines vs. Pyrimidines
- Adenine (A)
- Guanine (G)
- Mnemonic: "Pure As Gold" (Purines = A, G)
- Cytosine (C)
- Thymine (T) - DNA only (has a Methyl group)
- Uracil (U) - RNA only (lacks Methyl group)
- Mnemonic: "CUT the Py"
Bonding Energetics
- Backbone: Phosphodiester bond (Covalent). Very strong. Forms the structural integrity. Occurs between 3' OH and 5' Phosphate.
- Rungs: Hydrogen bonds (Weak individually, strong collectively).
- A = T: 2 Hydrogen bonds. Easier to break (Lower melting point). Origin of Replication is usually A-T rich to facilitate opening.
- G $\equiv$ C: 3 Hydrogen bonds. Harder to break (Higher melting point).
Math Corner: Chargaff's Rules
For double-stranded DNA (dsDNA):
$$ \%A = \%T $$
$$ \%G = \%C $$
Therefore:
$$ \% \text{Purines} (A+G) = \% \text{Pyrimidines} (C+T) = 50\% $$
Example: If a DNA sample has 20% Adenine, what is the % of Cytosine?
Solution: If A=20%, then T=20%. A+T = 40%. Remaining = 60%. G+C = 60%, so C = 30%.
0.2 DNA Organization: The Packing Problem
Human DNA is ~2 meters long but fits in a 10 $\mu m$ nucleus. This requires extreme compaction through multiple levels of organization.
Diagram: Chromatin Structure
The Nucleosome Core
A nucleosome consists of 146 base pairs of DNA wrapped 1.65 times around a histone octamer.
- Core Histones: H2A, H2B, H3, H4 (Two of each = Octamer).
- Charge: Histones are rich in Lysine and Arginine (Positively charged basic amino acids) to bind tightly to the negatively charged Phosphate backbone of DNA.
- H1 Histone: The "Linker" histone. It sits outside the bead and pulls nucleosomes together to form the 30nm fiber (Solenoid).
- Euchromatin: Loose, Acetylated, Active ("Light").
- Heterochromatin: Tight, Methylated, Inactive ("Dark"). e.g., Barr Bodies (Inactive X chromosome).
Part 1: DNA Replication
Replication occurs during the S-Phase of the cell cycle. It is semiconservative, meaning each new daughter molecule contains one old parental strand and one newly synthesized strand.
- Grew bacteria in heavy Nitrogen ($^{15}N$). DNA was heavy.
- Switched to light Nitrogen ($^{14}N$).
- Generation 1: 100% Intermediate density. (Ruled out Conservative model).
- Generation 2: 50% Light, 50% Intermediate. (Ruled out Dispersive model).
1.1 The Replication Fork Machinery
Diagram: The Replication Fork
| Enzyme | Prokaryotic Name | Eukaryotic Name | Function |
|---|---|---|---|
| Helicase | DnaB Helicase | Mcm Complex | Unwinds DNA (Breaks H-bonds). Requires ATP. |
| SSB Proteins | SSB | RPA | Prevents re-annealing and protects ssDNA from nucleases. |
| Topoisomerase | DNA Gyrase (Topo II) | Topo I / Topo II | Relieves supercoiling tension ahead of the fork by cutting and resealing strands. |
| Primase | DnaG | Pol $\alpha$-Primase | Synthesizes RNA primers (provides the essential 3' OH group). |
| Polymerase (Main) | DNA Pol III | Pol $\delta$ (Lagging) / $\epsilon$ (Leading) | Synthesizes new DNA ($5' \to 3'$). Has $3' \to 5'$ exonuclease (proofreading) activity. |
| Polymerase (Removal) | DNA Pol I | RNase H / FEN1 | Removes RNA primers ($5' \to 3'$ exonuclease) and fills gaps. |
| Ligase | DNA Ligase | DNA Ligase | Seals nicks (creates phosphodiester bonds) between Okazaki fragments. |
- Fluoroquinolones (e.g., Ciprofloxacin): Inhibit Bacterial Topoisomerase II (Gyrase) and IV. Used for UTIs and respiratory infections.
- Etoposide / Doxorubicin: Inhibit Human Topoisomerase II. Used as chemotherapeutic agents (stop cancer cell replication).
- Zidovudine (AZT): A nucleoside analog (Thymidine analogue with an Azide $N_3$ group at the 3' position). It has no 3' OH, so when HIV Reverse Transcriptase adds it, the chain terminates.
1.2 The Telomere Problem (End Replication Problem)
Because DNA Pol requires a primer and synthesizes only $5' \to 3'$, the extreme 5' end of the lagging strand cannot be replicated when the primer is removed. DNA gets shorter with every division. This is the "Hayflick Limit".
Telomerase: An enzyme (Reverse Transcriptase, carrying its own RNA template) that extends telomeres (TTAGGG repeats in humans). It is active in:
- Germ Cells (Sperm/Egg)
- Stem Cells
- Cancer Cells (85-90% of cancers reactivate telomerase to achieve immortality).
Part 2: Transcription & RNA Processing
The synthesis of RNA from a DNA template. The DNA strand read is the Template (Antisense) strand. The other is the Coding (Sense) strand (identical to RNA, except T $\to$ U).
2.1 Initiation Complexes
Prokaryotes
- Promoter: -10 (Pribnow Box) and -35 sequences.
- Factor: Sigma Factor ($\sigma$) guides RNA Polymerase to the promoter.
- Holoenzyme: Core Enzyme + Sigma.
- Polycistronic: One mRNA can encode multiple proteins.
Eukaryotes
- Promoter: TATA Box (-25), CAAT Box (-75).
- Polymerases:
- Pol I: rRNA (except 5S)
- Pol II: mRNA, snRNA, miRNA
- Pol III: tRNA, 5S rRNA
- TFs: General Transcription Factors (TFIID binds TATA via TBP) recruit Pol II.
- Monocistronic: One mRNA = One Protein.
2.2 Post-Transcriptional Processing (Eukaryotes Only)
Pre-mRNA (hnRNA) must be processed before leaving the nucleus:
- 5' Capping: Addition of 7-methylguanosine via a $5'-5'$ triphosphate bridge. Protects from degradation and recruits the ribosome.
- 3' Poly-A Tail: Addition of ~200 Adenines. Prevents degradation.
- Splicing: Removal of Introns (non-coding) and joining of Exons (expressed).
Diagram: The Spliceosome Mechanism
Alternative Splicing
One gene does not equal one protein. By including or excluding certain exons, a single pre-mRNA can produce distinct protein isoforms.
Example: Tropomyosin gene in muscle vs. brain.
Clinical: 15% of genetic diseases are caused by splicing mutations (e.g., some forms of Beta-Thalassemia).
Part 3: Translation
The high-energy process of decoding mRNA into a polypeptide chain.
3.1 Energetics of Translation
Protein synthesis is expensive! For a protein of $n$ amino acids:
- Activation (Charging): 2 ATP equivalents per AA (ATP $\to$ AMP + PPi). Total: $2n$.
- Initiation: 1 GTP.
- Elongation: 2 GTP per step (1 for binding, 1 for translocation). Total: $2(n-1)$.
- Termination: 1 GTP.
Approximate Cost: $4n$ High Energy Bonds.
3.2 The Ribosome Mechanism
Sites: A (Aminoacyl), P (Peptidyl), E (Exit).
Diagram: Ribosomal Translocation
3.3 Post-Translational Modifications
Proteins are often not functional immediately after translation. They require modification:
- Phosphorylation: Adding phosphate to Ser/Thr/Tyr (Kinases). Activates/Deactivates enzymes.
- Glycosylation: Adding sugars in the ER/Golgi. Important for cell signaling and membrane proteins.
- Ubiquitination: Adding Ubiquitin tags to mark misfolded proteins for destruction in the Proteasome.
- Proteolysis: Cleaving inactive precursors (Zymogens) like Trypsinogen to Trypsin.
Bacterial ribosomes are 70S (30S + 50S), while human ribosomes are 80S (40S + 60S). This difference allows selective toxicity.
| Drug Class | Target | Mechanism |
|---|---|---|
| Aminoglycosides (Streptomycin) | 30S | Causes misreading of mRNA. |
| Tetracyclines | 30S | Blocks tRNA binding to A-site. |
| Chloramphenicol | 50S | Inhibits Peptidyl Transferase (bond formation). |
| Macrolides (Erythromycin) | 50S | Prevents Translocation. |
Part 4: Gene Regulation
4.1 The Lac Operon (Inducible System)
The Lac Operon has dual control: Negative (Repressor) and Positive (CAP-cAMP).
- Negative Control: LacI gene produces a Repressor that binds the Operator. Lactose (Inducer) removes it.
- Positive Control (Glucose Effect):
- Low Glucose $\to$ High cAMP.
- cAMP binds CAP (Catabolite Activator Protein).
- CAP-cAMP complex binds promoter $\to$ High Transcription.
- Logic: The cell prefers Glucose. It only ramps up Lac operon if Glucose is GONE and Lactose is PRESENT.
4.2 The Trp Operon (Repressible System)
The Trp operon controls the biosynthesis of Tryptophan. It is ON by default.
- High Tryptophan: Tryptophan acts as a Corepressor. It binds to the Trp Repressor, activating it. The Repressor binds the operator and stops transcription.
- Low Tryptophan: Repressor is inactive. Transcription proceeds.
- Attenuation: A second layer of control in prokaryotes.
- High Trp $\to$ Ribosome moves fast $\to$ Terminator loop forms (3-4) $\to$ Stop.
- Low Trp $\to$ Ribosome stalls $\to$ Anti-terminator loop forms (2-3) $\to$ Go.
4.3 Epigenetics
Heritable changes in gene expression that do not involve changes to the underlying DNA sequence.
Acetylation (On)
Histone Acetyltransferases (HATs) add acetyl groups to Lysine tails.
$\downarrow$ Positive charge.
$\downarrow$ Attraction to DNA.
Result: Euchromatin (Open). Transcription Possible.
Methylation (Off)
DNA Methyltransferases add methyl groups to Cytosine (CpG islands).
Recruits HDACs (Deacetylases).
Result: Heterochromatin (Closed). Gene Silencing.
Mnemonic: "Methylation Mutes"
Part 5: Mutations & Repair
5.1 Types of Mutations (Table)
| Type | Mechanism | Effect on Protein | Disease Example |
|---|---|---|---|
| Silent | Substition (Wobble position) | No change in Amino Acid (e.g., GGU $\to$ GGC are both Gly). | None usually. |
| Missense | Substitution | Amino acid is changed. Can be Conservative (similar properties) or Non-conservative. | Sickle Cell Anemia (Glu $\to$ Val). |
| Nonsense | Substitution to Stop Codon | Early termination. Truncated, usually non-functional protein. | Duchenne MD (Dystrophin gene). |
| Frameshift | Insertion or Deletion (not multiple of 3) | Reading frame shifts. Massive alteration of downstream amino acids. | Tay-Sachs (HexA gene). |
5.2 Mutagens (Table)
| Category | Mutagen | Mechanism |
|---|---|---|
| Physical | UV Light | Causes Pyrimadine Dimers (T-T bond on same strand), distorting the helix. |
| Physical | X-Rays / Gamma Rays | High energy causes Double Strand Breaks (DSBs). Very dangerous. |
| Chemical | Intercalating Agents (e.g., Ethidium Bromide) | Slip between bases, causing polymerase to stutter $\to$ Frameshifts. |
| Chemical | Base Analogs (e.g., 5-Bromouracil) | Mimic normal bases but pair incorrectly (Tautomeric shifts). |
| Biological | Viruses (HPV) | Viral proteins (E6, E7) inhibit tumor suppressors (p53, Rb). |
| Biological | Transposons | "Jumping genes" insert randomly, disrupting gene function. |
5.3 Repair Pathways (Table)
| Pathway | Repairs | Key Enzymes | Defect Associated Disease |
|---|---|---|---|
| Mismatch Repair (MMR) | Replication errors (G-T mismatch) | MutS, MutL | Lynch Syndrome (Colon Cancer) |
| Nucleotide Excision Repair (NER) | Bulky Lesions (UV Pyrimidine dimers) | Excision Endonuclease | Xeroderma Pigmentosum (Extreme UV sensitivity) |
| Base Excision Repair (BER) | Small damage (Deaminated bases like Uracil) | Glycosylase, AP Endonuclease | - |
| NHEJ / HR | Double Strand Breaks | Ku proteins / BRCA1, BRCA2 | Breast Cancer (BRCA mutations) |
Part 6: Advanced Biotechnology
6.1 PCR (Polymerase Chain Reaction)
A technique to exponentially amplify a specific DNA segment.
The 3 Steps of PCR
- Denaturation (95°C): Heat breaks Hydrogen bonds. Strands separate.
- Annealing (55°C): Temperature lowered. Specific DNA primers bind to the flanking regions of the target.
- Extension (72°C): Taq Polymerase (thermostable from Thermus aquaticus) synthesizes new DNA using dNTPs.
Yield: $2^n$ copies after $n$ cycles.
Diagram: PCR Cycle
6.2 Gel Electrophoresis
Separates DNA based on size.
- DNA is negatively charged (Phosphate). Moves toward Anode (+).
- Agarose gel acts as a molecular sieve. Small fragments move fast; large move slow.
- Visualization: Ethidium Bromide (intercalates and fluoresces under UV).
6.3 Recombinant DNA Technology (Cloning)
The process of inserting a gene of interest into a vector (plasmid) to be replicated by bacteria.
- Isolation: Isolate gene of interest (e.g., Insulin) and plasmid vector.
- Digestion: Cut both with the same Restriction Enzyme (e.g., EcoRI). This creates compatible "sticky ends".
- Ligation: Mix gene and plasmid. DNA Ligase seals the phosphodiester bonds. Result: Recombinant Plasmid.
- Transformation: Introduce plasmid into bacteria (e.g., E. coli) via Heat Shock or Electroporation.
- Selection (Blue/White Screening):
- Plate contains Ampicillin + X-gal.
- Ampicillin: Kills bacteria without plasmid (selection for plasmid).
- X-gal: If gene inserted correctly, LacZ is broken $\to$ No blue pigment $\to$ White Colony (Success).
- If gene NOT inserted, LacZ works $\to$ Blue Colony (Fail).
Diagram: Gene Cloning Workflow
6.4 Sanger Sequencing
Uses ddNTPs (dideoxynucleotides). They lack the 3' OH group, so when added, polymerization stops immediately ("Chain Termination").
Diagram: Sanger Sequencing Output
6.5 CRISPR-Cas9
A gene-editing tool derived from bacterial immunity.
- Cas9: Endonuclease (molecular scissors).
- gRNA (Guide RNA): Matches the specific target sequence.
- Mechanism: gRNA guides Cas9 to target $\to$ Double Strand Break $\to$ Repair (NHEJ causes deletion, or HDR allows insertion).
Diagram: CRISPR-Cas9 Mechanism
6.6 Immunological Analysis
| Method | Target | Key Components |
|---|---|---|
| Southern Blot | DNA sequences | Probes (Radiolabeled DNA) |
| Northern Blot | RNA sequences | Probes (Radiolabeled DNA/RNA) |
| Western Blot | Proteins | Antibodies + SDS-PAGE Gel |
| ELISA | Antigens or Antibodies | Enzyme-linked Antibodies + Color change substrate. (Quantitative). |
| Flow Cytometry | Cell surface markers | Fluorescent Antibodies + Laser (Single cell analysis). |
Biomolecules Visual Reference Gallery
A comprehensive visual compendium covering Carbohydrates, Proteins, Nucleic Acids, and Enzyme Kinetics to supplement cellular metabolism and molecular biology concepts.
Ultimate Review Quiz
This quiz tests the finest details of the lesson. Good luck!