Introduction to Genetics

Definition and Relationship to Phenotype

Genetics is the study of genes, heredity, and variation in living organisms. A gene is a segment of DNA that encodes functional products, typically proteins, which influence an organism's traits or phenotype.

• Gene: The basic unit of heredity; a sequence of DNA that codes for a specific product.

• Genome: The complete set of genetic material in an organism.

• Phenotype: Observable characteristics resulting from gene expression and environmental influences.

• Chromosomal Theory of Inheritance: Genes are located on chromosomes, which segregate and assort independently during meiosis.

Subdisciplines of Genetics

Genetics encompasses several subfields, each focusing on different aspects of heredity and gene function.

• Molecular Genetics: Study of gene structure and function at the molecular level (e.g., DNA replication).

• Classical (Mendelian) Genetics: Study of inheritance patterns (e.g., Punnett squares).

• Population Genetics: Study of genetic variation within populations (e.g., allele frequencies).

• Quantitative Genetics: Analysis of traits controlled by multiple genes (e.g., height).

• Genomics: Study of entire genomes using sequencing and bioinformatics.

Purpose and Function of DNA

• Purpose: Store, transmit, and express genetic information.

• Requirements: Must be able to replicate, encode information, mutate, and undergo recombination.

DNA Structure and Nucleic Acids

Nucleic Acids and Nucleotides

• Types of Nucleic Acids:

  1. DNA (Deoxyribonucleic acid)

  2. RNA (Ribonucleic acid)

• Major Groups of a Nucleotide:

  1. Nitrogenous base

  2. Pentose sugar

  3. Phosphate group

• Phosphodiester Bond: Links nucleotides in a DNA or RNA strand, forming the backbone.

• Glycosidic Bond: Connects the base to the sugar in a nucleotide.

• Ribose Ring: Five-carbon sugar; carbons are labeled 1' to 5'.

• Deoxyribonucleotide vs. Ribonucleotide: Deoxyribonucleotides have a hydrogen at the 2' carbon; ribonucleotides have a hydroxyl group.

• Nucleotide in DNA: Deoxyadenosine triphosphate (dATP), dGTP, dCTP, dTTP; each has three phosphate groups, but only one remains in the DNA strand after incorporation.

Bases and Base Pairing

• Bases in DNA: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)

• Bases in RNA: Adenine (A), Uracil (U), Cytosine (C), Guanine (G)

• Purines: Adenine, Guanine

• Pyrimidines: Cytosine, Thymine, Uracil

• Base Pairs:

  • A-T (2 hydrogen bonds)

  • G-C (3 hydrogen bonds)

• Chargaff's Rule: and in double-stranded DNA;

DNA Double Helix Properties

• Phosphodiester Backbone: Covalently bonded; bases held by hydrogen bonds.

• Descriptors: Double-stranded, antiparallel, right-handed helix, contains guanine, not always coding.

• Base Pairs per Turn: ~10.5 base pairs per full turn.

• Semiconservative Replication: Each new DNA molecule contains one old and one new strand.

Organization of DNA

DNA in Prokaryotes and Eukaryotes

• Prokaryotic DNA: Found in the nucleoid; highly compacted to fit inside the cell.

• Supercoiling: The overwinding or underwinding of DNA; negative supercoiling is common in cells and helps compact DNA.

• Topoisomerases: Enzymes that manage DNA supercoiling; Type I cuts one strand, Type II cuts both strands.

• Chromosome Structure:

  • Linear chromosomes: Found in nucleus (eukaryotes)

  • Circular chromosomes: Found in mitochondria and chloroplasts

• C-value: Amount of DNA in a haploid genome; C-value paradox refers to lack of correlation between genome size and organism complexity.

• Chromatin: DNA-protein complex that packages DNA in eukaryotes.

• Histones: Basic proteins that form octamers; subunits are H2A, H2B, H3, H4 (plus H1 for linker DNA).

HTML Table: Chromatin Structures in Eukaryotes

• Euchromatin: Less condensed, transcriptionally active.

• Heterochromatin: Highly condensed, transcriptionally inactive.

• Unique-sequence DNA: Single-copy genes.

• Repetitive DNA: Multiple copies, e.g., satellite DNA.

• Centromere: Region for spindle attachment during cell division.

• Telomere: Protective ends of chromosomes; maintain stability.

DNA Replication

Mechanism and Key Proteins

• Replication Phase: Occurs during S phase of the cell cycle.

• Replication Bubble: Region where DNA is unwound for replication; involves helicase, primase, DNA polymerase, ligase, SSB proteins.

• Replicative Proteins:

  • Helicase: Unwinds DNA

  • Primase: Synthesizes RNA primer

  • DNA Polymerase III: Main enzyme for synthesis

  • DNA Polymerase I: Removes RNA primers, fills gaps

  • Ligase: Seals nicks

• Semi-conservative: Each daughter DNA has one parental and one new strand.

• Semi-discontinuous: Leading strand synthesized continuously; lagging strand in Okazaki fragments.

• Okazaki Fragments: Short DNA segments on lagging strand.

• Fidelity: Accuracy of DNA replication.

• Proofreading: Correction of errors by DNA polymerase.

• Exonuclease vs. Endonuclease: Exonuclease removes nucleotides from ends; endonuclease cuts within strand.

• Processivity: Ability of DNA polymerase to synthesize long stretches without dissociating.

• Telomeres: Repetitive DNA at chromosome ends; maintained by telomerase (a reverse transcriptase enzyme).

• snRNA: Small nuclear RNA; involved in splicing.

PCR (Polymerase Chain Reaction)

• Major Steps:

  1. Denaturation

  2. Annealing

  3. Extension

• PCR Tube Contents: Template DNA, primers, dNTPs, buffer, Taq polymerase.

• Similarity to Replication: Both synthesize DNA; PCR is in vitro, uses heat to denature.

• Difference: PCR is cyclic and uses specific primers; cellular replication is continuous and regulated.

DNA Transcription

Central Dogma and Gene Structure

• Central Dogma: DNA → RNA → Protein

• Other Gene Products: rRNA, tRNA, snRNA, etc.

• Eukaryotic Gene Elements: Promoter, exons, introns, enhancers, terminator.

• Coding vs. Template Strand: Coding strand matches mRNA (except T/U); template strand is read by RNA polymerase (3'→5').

• RNA Polymerase: Reads template strand 3'→5', synthesizes RNA 5'→3'.

Transcription Steps and Regulation

• Major Steps:

  1. Initiation

  2. Elongation

  3. Termination

  4. Processing (in eukaryotes)

• Prokaryotic Initiation: RNA polymerase finds TSS via sigma factor.

• Core vs. Holoenzyme: Core: RNA polymerase subunits; holoenzyme: core + sigma factor.

• Intrinsic vs. Rho-dependent Termination: Intrinsic: Hairpin loop; Rho-dependent: Rho protein.

• Polycistronic vs. Monocistronic RNA: Polycistronic: Multiple genes per mRNA (prokaryotes); monocistronic: one gene per mRNA (eukaryotes).

• RNA Polymerases:

  • Pol I: rRNA

  • Pol II: mRNA

  • Pol III: tRNA, small RNAs

• General Transcription Factors: Bind promoter, recruit RNA polymerase II.

• PIC (Pre-Initiation Complex): Assembly of transcription factors and RNA polymerase at promoter.

mRNA Processing

• Order of Events: 5' capping, splicing, 3' polyadenylation.

• 5' Cap: 7-methylguanosine; protects mRNA, aids translation.

• Splicing: Removal of introns; spliceosome complex catalyzes splicing.

• Splice Sites: 5' GU, branch point A, 3' AG.

• Polyadenylation: Addition of polyA tail at 3' end; enhances stability and export.

Translation

Genetic Code and Protein Synthesis

• Genetic Code: Triplet codons; degenerate (multiple codons per amino acid); universal.

• Codon Usage Bias and Wobble: Preference for certain codons; wobble allows flexibility at third base.

• ORF Mutation: Can change amino acid sequence, affecting protein function.

Protein Structure

• Primary Structure: Amino acid sequence.

• Secondary Structure: Alpha helices, beta sheets.

• Tertiary Structure: 3D folding.

• Quaternary Structure: Multiple polypeptides.

• R Groups: Influence folding and function; mutations can alter structure.

• Polypeptide vs. Protein: Polypeptide is a single chain; protein may be one or more polypeptides.

• Protein Subunit: Individual polypeptide in a multi-subunit protein.

tRNA and Ribosome Structure

• tRNA: Cloverleaf structure; anticodon pairs with mRNA codon; charged tRNA has attached amino acid.

• Aminoacyl-tRNA Synthetase: Enzyme that charges tRNA.

• Ribosome: Large and small subunits; rRNA and proteins.

• tRNA Binding Sites:

  1. A (Aminoacyl)

  2. P (Peptidyl)

  3. E (Exit)

• Peptidyl Transferase: Catalyzes peptide bond formation; located in large ribosomal subunit.

• Translocation: Movement of ribosome along mRNA.

• Polysome: Multiple ribosomes translating one mRNA; increases protein yield.

Practice and Application

• Draw and Label:

  • Cell cycle

  • Central dogma diagram

Additional info: This guide expands on the original questions by providing definitions, explanations, and examples for each major topic, ensuring a self-contained resource for exam preparation.