DNA Structure, Replication, RNA, and Molecular Methods: Cell Biology Study Guide

Lecture 11 – DNA Part 1 (Structure)

DNA Structure and Function

DNA (deoxyribonucleic acid) is the hereditary material in almost all living organisms. Its structure and organization are fundamental to understanding genetics and cell biology.

• Androgen Receptor Function: A functional androgen receptor is crucial for normal development and phenotype expression. Mutations can lead to disorders such as androgen insensitivity syndrome.

• Frederick Griffith and Oswald Avery Experiments: Griffith's experiment demonstrated transformation in bacteria, suggesting that genetic material could be transferred. Avery's work identified DNA as the transforming principle.

• Key Features of DNA: DNA consists of a double helix structure, base pairing (A-T, G-C), and is organized into chromosomes. It contains genetic information encoded in nucleotide sequences.

• Chargaff’s Rules: The amount of adenine equals thymine, and the amount of guanine equals cytosine in DNA.

• Histones and DNA Packaging: DNA wraps around histone proteins to form nucleosomes, which further coil to create chromatin. This packaging allows efficient storage and regulation of DNA.

• Heterochromatin vs. Euchromatin: Heterochromatin is tightly packed and transcriptionally inactive, while euchromatin is loosely packed and active in transcription.

• Centromeres and Telomeres: Centromeres are regions where sister chromatids attach, essential for proper chromosome segregation. Telomeres protect chromosome ends from degradation.

• Repeated DNA Sequences: Repetitive DNA includes satellite DNA, minisatellites, and microsatellites, which play roles in genome structure and function.

• Nuclear Structure: The nucleus contains chromatin, nucleolus, and is surrounded by a nuclear envelope. It organizes and protects genetic material.

Example: The human genome contains approximately 3 billion base pairs of DNA, organized into 23 pairs of chromosomes.

Lecture 12 – DNA Part 2 (Replication/Repair)

DNA Replication and Repair Mechanisms

DNA replication ensures genetic information is accurately passed to daughter cells. Repair mechanisms maintain genome integrity.

• STRs and CODIS: Short Tandem Repeats (STRs) are repetitive DNA sequences used in forensic analysis. CODIS is a database for DNA profiling.

• Semiconservative Replication: Each new DNA molecule consists of one old and one new strand. Replication involves origin recognition, unwinding, and synthesis.

• Okazaki Fragments and Lagging Strand: The lagging strand is synthesized discontinuously as Okazaki fragments, which are later joined by DNA ligase.

• Endonucleases vs. Exonucleases: Endonucleases cut within DNA strands; exonucleases remove nucleotides from ends.

• DNA Polymerase Proofreading: DNA polymerases have proofreading activity to correct errors during replication.

• Replication Fork: The replication fork is the area where DNA unwinds and new strands are synthesized. Key enzymes include helicase, primase, and DNA polymerase.

• DNA Damage and Mutations: Mutations can arise from replication errors or environmental factors. Types include point mutations, insertions, deletions, and frameshifts.

• DNA Repair Mechanisms: Major types include base excision repair, nucleotide excision repair, and mismatch repair.

Example: Xeroderma pigmentosum is a disorder caused by defects in nucleotide excision repair, leading to increased sensitivity to UV light.

Lecture 13 – DNA Part 3 (Transcription)

Transcription and Gene Expression

Transcription is the process by which RNA is synthesized from a DNA template, enabling gene expression.

• Thermocycler and PCR: A thermocycler is used in Polymerase Chain Reaction (PCR) to amplify DNA. PCR is essential for molecular biology applications such as cloning and diagnostics.

• Central Dogma: The central dogma of molecular biology describes the flow of genetic information: DNA → RNA → Protein.

• Transcription in Prokaryotes vs. Eukaryotes: Prokaryotic transcription occurs in the cytoplasm, while eukaryotic transcription occurs in the nucleus and involves more complex regulation.

• Transcription Factors: Proteins that bind to specific DNA sequences to regulate transcription.

• Promoter and Enhancer Elements: Promoters are DNA sequences where RNA polymerase binds to initiate transcription. Enhancers increase transcription efficiency.

• RNA Polymerases: Eukaryotes have three main RNA polymerases (I, II, III) with distinct functions.

• Transcription Unit: Includes the promoter, coding region, and terminator.

• Alternative Splicing: Allows a single gene to produce multiple mRNA variants, increasing protein diversity.

Example: The TATA box is a common promoter element found in many eukaryotic genes.

Lecture 14 – RNA Part 2 (Translation)

Translation and Protein Synthesis

Translation is the process by which ribosomes synthesize proteins using mRNA as a template.

• Codon and Genetic Code: A codon is a sequence of three nucleotides that specifies an amino acid. The genetic code is nearly universal and unambiguous.

• Ribosome Structure: Ribosomes consist of two subunits (large and small) and are the site of protein synthesis.

• tRNA and Aminoacylation: tRNA molecules carry amino acids to the ribosome. Aminoacyl-tRNA synthetases attach the correct amino acid to each tRNA.

• Translation Initiation: Involves assembly of the ribosome on the mRNA and the first tRNA at the start codon.

• Elongation Factors: EF-Tu, EF-Ts, and EF-G facilitate the addition of amino acids and translocation of the ribosome.

• Termination: Occurs when a stop codon is reached, releasing the newly synthesized protein.

• Posttranslational Processing: Proteins may undergo modifications such as folding, cleavage, and addition of functional groups.

Example: Sickle cell anemia is caused by a single nucleotide change in the beta-globin gene, altering the protein produced during translation.

Lecture 15 – Methods

Molecular Biology Techniques

Modern cell biology relies on a variety of molecular techniques to study and manipulate DNA, RNA, and proteins.

• Restriction Enzymes: Proteins that cut DNA at specific sequences, useful for cloning and genetic engineering.

• Sticky Ends: Overhanging DNA ends created by restriction enzymes, facilitating the joining of DNA fragments.

• Sequencing Methods: Sanger sequencing and next-generation sequencing are used to determine DNA sequences.

• Genetically Modified Organisms (GMOs): Organisms whose genomes have been altered for research, agriculture, or medicine. GMOs raise ethical and safety concerns.

• Insulin Production: Recombinant DNA technology allows bacteria to produce human insulin for diabetes treatment.

• CRISPR System: A genome editing tool that uses Cas9 and guide RNA to target and modify specific DNA sequences.

• DNA Editing: Techniques such as CRISPR enable precise changes to DNA, with potential for treating genetic diseases but also raising ethical debates.

Example: CRISPR-Cas9 has been used to correct genetic mutations in laboratory models of disease.

Table: DNA Repair Mechanisms

Repair Mechanism	Main Function	Example
Base Excision Repair	Removes damaged bases and replaces with correct nucleotide	Repair of deaminated cytosine
Nucleotide Excision Repair	Removes bulky DNA lesions (e.g., thymine dimers)	Repair of UV-induced damage
Mismatch Repair	Corrects errors missed by DNA polymerase proofreading	Repair of replication errors

Key Equations and Concepts

• Chargaff’s Rule:

$[A] = [T]$ $[G] = [C]$

• Central Dogma:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

• DNA Replication (Semiconservative Model):

$\text{Parent DNA} \rightarrow \text{Daughter DNA (1 old + 1 new strand)}$

• Genetic Code:

$\text{Codon} = 3 \text{ nucleotides} \rightarrow 1 \text{ amino acid}$

Additional info: Some explanations and examples have been expanded for clarity and completeness.