Comprehensive Study Guide: DNA, Gene Expression, Cell Cycle, and Biotechnology

DNA as the Genetic Material

Historical Experiments Establishing DNA as Genetic Material

The identification of DNA as the genetic material was a pivotal moment in biology, established through a series of key experiments.

• Proteins vs. DNA: Early scientists believed proteins were the genetic material due to their complexity and diversity compared to DNA's simple structure. Chromosomes were known to contain DNA and protein, the 20 amino acids gave a lot more combinations than just four nucleotide bases.

• Griffith’s Transformation Experiment (1928): Demonstrated that a 'transforming principle' from dead virulent bacteria could make non-virulent bacteria pathogenic, suggesting a transferable genetic material.

• Avery, MacLeod, and McCarty (1944): Identified DNA as the 'transforming principle' by showing that only DNA, not proteins or RNA, could transform bacteria.

• Hershey and Chase (1952): Used radioactive labeling of DNA and protein in bacteriophages to show that DNA, not protein, enters bacterial cells and directs viral replication.

• Watson and Crick (1953): With data from Rosalind Franklin, proposed the double helix structure of DNA, explaining its capacity for replication and information storage.

Main Competing Models: Triple helix, parallel strands, and the now-accepted antiparallel double helix.

Key Characteristics of the Watson-Crick Model:

• Double helix with antiparallel strands

• Complementary base pairing (A-T, G-C)

• Hydrogen bonds between bases

• Consistent with Chargaff’s rules: amount$[A] = [T]$ amount and amount$[G] = [C]$amount

• Sugar Phosphate Backbone on the outside

DNA Replication

Models of Replication

• Three Proposed Models: Conservative, semi-conservative, and dispersive.

• Watson and Crick’s Suggestion: Semi-conservative model, where each new DNA molecule contains one old and one new strand.

• Meselson and Stahl Experiment (1958): Used isotopic labeling (14N as negative control and 15N as positive control) to confirm the semi-conservative model (and got dispersive too).

Chromosome Structure and Histones

• Main Protein Components: Histones (H1, H2A, H2B, H3, H4)

• Structural Similarities: All are basic, rich in lysine and arginine, and form octamers around which DNA wraps.

• Interaction with DNA: DNA wraps twice around histones to create a "bead". DNA between histones are called "Linker DNA".

• Post-Translational Modifications: Acetylation (makes neutral), methylation (makes more positive), phosphorylation (makes histone tails more negative)

• Effect: Modifications alter chromatin structure, influencing gene expression by making DNA more or less accessible.

• Chromosomes condensed or relaxed states: methylation or deacetylation makes chromosomes more condensed (less susceptible to transcription), acetylation makes chromosomes more relaxed (more susceptible to transciption).

DNA Replication Mechanism

• Prokaryotic vs. Eukaryotic Chromosomes: Prokaryotes have circular chromosomes with a single origin; eukaryotes have linear chromosomes with multiple origins.

• Replication Fork Proteins:

  • Topoisomerase: Relieves supercoiling, releases strain

  • Helicase: Unwinds DNA

  • Single-Stranded Binding proteins (SSBP’s): Stabilize single strands

  • Primase: Synthesizes RNA primers (on 3' end)

  • DNA Pol III: Main DNA synthesizer, Okazaki fragments

  • DNA Pol I: Replaces RNA primers with DNA

  • DNA Ligase: Seals nicks in DNA

• Leading vs. Lagging Strand: Leading strand synthesized continuously ( had DNA Pol III); lagging strand synthesized in Okazaki fragments, 5' -> 3' (has Primase, DNA Pol III, DNA Pol I, DNA Ligase).

• Energy Source: dNTP hydrolysis provides energy for DNA synthesis; helicase uses ATP; ligase uses ATP or NAD+; DNA POL III uses dNTP's with the cleavage of pyrophosphate.

• Trombone Model: Replication fork is a more complete complex, linking helicase to DNA POL III, allows for just one DNA POL III to synthesize all Okazaki fragments, functions like scaffolding proteins in signal transduction to increase efficiency.

Telomeres and Telomerase

• Problem: Linear chromosomes lose DNA (base pairs) at ends after each replication.

• Telomeres: Repetitive sequences of non-coding DNA at chromosome ends create a buffer (5' TTAGGG 3'); loss leads to cell aging, cell death (Apoptosis).

• Telomerase: Enzyme with RNA template extends telomeres; active in germ cells, stem cells, and many cancers.It can recognize and bind that Telomere sequence, TTAGGG, with its complementary RNA. That RNA provides a template for strand extension-on the template strand itself. Following extension, Primase and POL III can come back and finish the daughter strand. Errors in Telomerase regulation can leade to some cancers (its reactivation is what causes cancer in 90% of human cells).

DNA Proofreading and Repair

• DNA Pol III Proofreading: 3'→5' exonuclease activity reduces error rate. It can proofread itself, cut out mismatched bases, resynthesize a new base pair.

• Excision Repair: Enzymes remove and replace mismatched or damaged bases, DNA POL fills in gap, DNA Ligase seals the gap, further reducing errors.

The Central Dogma: Transcription and Translation

Central Dogma and Exceptions

• Central Dogma: DNA → RNA → Protein. Genetic info begets proteins in a two step process, Transcription and Translation.

• Exceptions: Reverse transcription (RNA → DNA), non-coding RNAs, and some viruses. Not all transcripts get translated, wrong way traffic: HIV, COVID, etc.

Transcription

• Definition: Synthesis of RNA from DNA template, to copy something down in the same language, a 1:1 transcription.

• Key Differences from DNA: RNA uses ribose (removes an oxygen from the 2nd Carbon), uracil replaces thymine.

• RNA Polymerase: Initiates at promoter, does not require primer, synthesizes 5'→3'.

• Termination: In eukaryotes, occurs after polyadenylation signal (AAUAAA).

mRNA Processing in Eukaryotes

• 5’ Cap and 3’ Poly-A Tail: Added for stability, export, and translation initiation.

• Splicing: Introns removed by spliceosomes (snRNPs and snRNAs); alternative splicing allows tissue-specific proteins.

• UTRs: 5’ and 3’ untranslated regions regulate translation and mRNA stability.

Translation

• Definition: to copy something in a different language

• tRNA Structure: Cloverleaf with anticodon and amino acid attachment site.

• Aminoacyl-tRNA Synthetases: Charge tRNAs using ATP; accuracy is critical for correct protein synthesis.

• Ribosome Structure: Eukaryotic ribosomes are larger and more complex than prokaryotic; mitochondrial ribosomes have unique origins for rRNA and proteins.

• Translation Phases: Initiation, elongation, termination; energy from GTP hydrolysis.

• Polyribosomes: Multiple ribosomes translate a single mRNA, amplifying protein production.

• Prokaryotes: Transcription and translation are coupled, allowing rapid response.

Protein Sorting and Signal Sequences

Protein Targeting and Transport

• Nuclear Pores: mRNA export signals; proteins use nuclear localization signals (NLS) and nuclear transport receptors.

• Pulse-Chase Experiments: Track protein synthesis and movement.

• Secretory Pathway: Signal sequences direct proteins to ER; ribosomes may become membrane-bound; signal sequence often cleaved.

• Transmembrane Proteins: Orientation determined by signal and stop-transfer sequences.

• Unfolded Protein Response (UPR): Cellular response to misfolded proteins in ER, affecting gene expression and signaling.

Golgi Protein Sorting

• Shipping Labels: Proteins are tagged (e.g., with sugars) for sorting.

• Vesicle Transport: Vesicles bud from Golgi and fuse with target membranes using specific recognition proteins.

Gene Regulation

Epigenetics and Chromatin Remodeling

• Epigenetics: Heritable changes in gene expression not involving DNA sequence changes.

• Genotype vs. Phenotype: Genotype is genetic makeup; phenotype is observable traits.

• Chromatin Remodeling: HATs (acetylate histones, activate genes), HDACs (deacetylate, repress genes), DNA methylation (often silences genes, can be reversible, influenced by environment).

Levels of Gene Regulation

1. Chromatin Remodeling

2. Transcriptional Regulation: General vs. specific transcription factors; 'combination lock' model; master TFs (e.g., MyoD).

3. RNA Processing: Alternative splicing creates protein diversity.

4. mRNA Stability: microRNAs (miRNAs) inhibit gene expression; prevalent regulatory mechanism.

5. Translational Regulation: mRNA secondary structures (e.g., ferritin, transferrin) respond to cellular conditions (e.g., iron levels).

6. Post-Translational Regulation: Protein degradation (e.g., ubiquitin-proteasome system); defects linked to diseases like cancer and neurodegeneration.

Cell Cycle and Regulation

Phases of the Cell Cycle

• Interphase: G1 (growth, 9 hours), S (DNA synthesis, 10 hours), G2 (preparation for mitosis, 4 hours)

• Mitosis: Prophase (chromosomes start to condense, nucleolus disappears, main site of transcription), Prometaphase (nuclear membrane starts to disintegrate, lamins get phosporylated, chromosomes condense further, centromere become visible and binds to microtubules at the kinetochore), Metaphase (chromosomes have fully condensed, line up perpendicular to the mitotic spindle of microtubles), Anaphase (the shortest of the phases and complicated, the cohesion complexes must be cleaved, proteases cut the cohesion proteins on all chromosomes at the same time, centromeres still connected), Telophase (repairing the damage, nuclear membrane reforms, the nucleolus reforms, chromosome relaxation), (<1 hour)

• Cytokinesis: Division of cytoplasm; cleavage furrow in animals (actin-myosin, muscle cells), cell plate in plants. The formation of a new cell membrane division between the two, new daughter cells.

Centrioles, Centrosomes, and Mitotic Spindle

• Centrosomes: Microtubule organizing centers; duplicated before mitosis.

• Spindle: Microtubules attach to chromosomes, pull them apart during anaphase.

• Differences: Plants lack centrioles; yeast have unique division mechanisms.

Cell Cycle Checkpoints and Regulation

• Checkpoints: G1/S, G2/M, M (spindle assembly)

• G0 Phase: Non-dividing state; cells can re-enter cycle with signals (e.g., PDGF).

• Cyclins and CdKs: Cyclins regulate CdKs, which drive cell cycle progression; cyclins degraded after use.

• Key Checkpoints:

  • G2/M: Ensures DNA replication is complete

  • M: Ensures chromosomes are properly attached to spindle

  • G1/S: Most critical; regulated by RB protein, CyclinE/CdK2, Cyclin D/CdK4/6

Tumor Suppressor Genes

• RB: Holds E2F protein, preventing S phase entry until cell is ready.

• P53: Short-lived, activates DNA repair or apoptosis if DNA is damaged; upregulates P21 to inhibit Cyclin/CdK complexes.

• Mutations: In tumor suppressor genes can lead to uncontrolled cell division (cancer).

Viruses, COVID-19, and DNA Technology

Coronavirus Biology

• Zoonotic Transfer: Transmission of viruses from animals to humans.

• COVID-19: SARS-CoV-2 is a positive-sense RNA virus with a large genome and spike protein for cell entry (via ACE2 receptor and a protease). Easiest and highest deaths: Covid-19, Hardest and lowest deaths: SARS, MERS

• SARS and MERS: infected individuals were only contagious after developing severe symptoms.

• Genome: Positive-sense RNA can be directly translated; differs from retroviruses (e.g., HIV) which require reverse transcription.

• Gene Regulation: Nested mRNAs produced by discontinuous transcription; allows complex regulation.

DNA Sequencing

• Dideoxy Nucleotides: Terminate DNA synthesis in Sanger sequencing.

• Gel Electrophoresis: Polyacrylamide gels used for high-resolution separation of DNA fragments.

CRISPR-Cas9 Technology

• CRISPR DNA: Bacterial immune system; stores viral DNA sequences.

• Cas9: Endonuclease guided by crRNA or gRNA to cut specific DNA sequences.

• Gene Editing: Used to correct genetic defects (e.g., cystic fibrosis) by introducing targeted cuts and providing repair templates.

Experiment	Main Finding	Year
Griffith	Transformation principle exists	1928
Avery, MacLeod, McCarty	DNA is the transforming principle	1944
Hershey & Chase	DNA, not protein, is genetic material	1952
Watson & Crick	Double helix structure of DNA	1953
Meselson & Stahl	Semi-conservative replication	1958

Additional info: This guide integrates foundational experiments, molecular mechanisms, and modern biotechnology, providing a comprehensive overview for exam preparation in introductory and intermediate college biology courses.