Cell Cycle, Meiosis, Mendelian Genetics, DNA Replication, Gene Expression, and Regulation: Comprehensive Study Guide

Cell Cycle and Cell Division

Definition and Overview

The cell cycle is the series of events that cells undergo to grow and divide. It consists of interphase (growth and DNA replication) and the M phase (mitosis and cytokinesis).

• Chromosome: A DNA molecule with associated proteins, carrying genetic information.

• Sister chromatids: Identical copies of a chromosome, joined at the centromere.

• M phase: The phase where cell division occurs, including mitosis and cytokinesis.

• Interphase: The period between cell divisions, including G1, S, and G2 phases.

• S phase: DNA synthesis phase; chromosomes are replicated.

• Gap phases (G1 and G2): Periods of cell growth and preparation for division.

Phases of the Cell Cycle

• G1 phase: Cell grows and prepares for DNA replication.

• S phase: DNA is replicated.

• G2 phase: Cell prepares for mitosis.

• M phase: Mitosis and cytokinesis occur.

Mitosis vs. Meiosis

• Mitosis: Produces two genetically identical daughter cells.

• Meiosis: Produces four genetically diverse gametes.

Mitosis Subphases

• Prophase: Chromosomes condense; spindle apparatus forms from microtubules and centrosomes.

• Prometaphase: Nuclear envelope breaks down; kinetochores attach to spindle microtubules.

• Metaphase: Chromosomes align at the metaphase plate; mitotic spindle fully formed.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Chromosomes decondense; nuclear envelope reforms.

Cytokinesis

• Division of cytoplasm, resulting in two daughter cells.

Binary Fission

• Prokaryotic cell division; involves protein filaments for chromosome segregation.

Cell Cycle Control

• MPF (Maturation Promoting Factor): A protein kinase activated by cyclin; regulates entry into M phase.

• Cdk (Cyclin-dependent kinase): Enzyme activated by cyclin; controls cell cycle transitions.

• Cell cycle checkpoints: G1 (p53 protein), G2, and M phase checkpoints ensure proper division.

Cancer and Cell Cycle Regulation

• Cancer: Uncontrolled cell division due to failing checkpoints and mutations in key regulator proteins (e.g., E2F, Rb protein).

• Tumor suppressor: Proteins that prevent uncontrolled growth.

• Types of tumors: Benign (non-spreading) and malignant (spreading).

Meiosis and Genetic Variation

Overview

Meiosis is the process by which gametes (sperm and egg) are produced, reducing chromosome number by half and increasing genetic diversity.

• Sex chromosomes: Chromosomes determining sex (X and Y); homologous in females, non-homologous in males.

• Genes and alleles: Genes are units of heredity; alleles are variants of a gene.

• Ploidy: Number of chromosome sets (haploid, diploid).

• Karyotype: Chromosome composition of an organism.

• Maternal and paternal chromosomes: Chromosomes inherited from each parent.

Meiosis I vs. Meiosis II

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

Genetic Variation Mechanisms

• Independent assortment: Random distribution of homologous chromosomes.

• Crossing over: Exchange of genetic material between homologous chromosomes.

Errors in Meiosis

• Nondisjunction: Failure of chromosomes to separate; leads to aneuploidy (e.g., trisomy 21).

• Oogenesis: Egg formation; susceptible to errors.

Benefits of Meiosis

• Promotes genetic diversity, aiding adaptation to changing environments.

Mendelian Genetics

Chromosome Theory of Inheritance

Genes are located on chromosomes, which segregate during meiosis.

• Heredity: Transmission of traits from parents to offspring.

• Trait: Observable characteristic.

• Blending inheritance: Outdated theory; traits blend in offspring.

• Inheritance of acquired characteristics: Outdated theory; traits acquired during life passed on.

Key Terms in Mendelian Genetics

• Gene: Unit of heredity.

• Allele: Variant form of a gene.

• Genotype: Genetic makeup.

• Phenotype: Observable traits.

• Homozygous: Two identical alleles (RR).

• Heterozygous: Two different alleles (Rr).

• Dominant allele: Expressed trait.

• Recessive allele: Masked trait.

• Pure line: Homozygous for a trait.

• Hybrid: Heterozygous for a trait.

• Reciprocal cross: Crosses with reversed parental traits.

• Testcross: Cross with homozygous recessive to determine genotype.

• X-linked and Y-linked: Genes located on sex chromosomes.

Generations and Crosses

• Parental generation (P): Original parents.

• F1 generation: First filial generation.

• Monohybrid cross: Cross involving one trait.

• Reciprocal cross results: Determines if trait is sex-linked.

Mendel's Principles

• Principle of segregation: Alleles separate during gamete formation.

• Principle of independent assortment: Genes for different traits segregate independently.

Types of Inheritance

• Autosomal inheritance: Traits on non-sex chromosomes.

• Sex-linked inheritance: Traits on sex chromosomes.

• Codominance: Both alleles expressed (e.g., blood type AB).

• Incomplete dominance: Intermediate phenotype (e.g., pink flowers).

• Discrete traits: Distinct categories.

• Quantitative traits: Continuous variation.

Punnett Square

• Tool for predicting genotype and phenotype ratios.

DNA Structure, Replication, and Repair

DNA Structure

• Hershey-Chase experiment: Demonstrated DNA is genetic material.

• Phosphodiester linkage: Covalent bond between nucleotides.

• Backbone: Sugar-phosphate; Bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C).

• Directionality: 3’ to 5’; strands run antiparallel.

• Helix structure: Double helix.

DNA Replication

• Semiconservative replication: Each new DNA has one old and one new strand.

• Conservative replication: Entirely new molecule (not supported).

• Dispersive replication: Mixed old and new segments (not supported).

• DNA polymerase: Enzyme synthesizing DNA; requires dNTPs (deoxynucleotide triphosphates).

• Origin of replication: Starting point; replication forks form and proceed bidirectionally.

• Leading strand: Synthesized continuously.

• Lagging strand: Synthesized in Okazaki fragments.

• Telomerase: Extends telomeres in eukaryotes.

DNA Repair

• Proofreading: DNA polymerase corrects errors.

• Mismatch repair: Fixes incorrect base pairs.

• DNA ligase: Seals nicks in DNA.

Gene Function and Expression

Central Dogma of Molecular Biology

• DNA → RNA → Protein

• Genes: Code for proteins.

• mRNA: Messenger RNA; intermediary between DNA and protein.

• RNA polymerase: Enzyme for transcription.

Transcription and Translation

• Transcription: DNA to mRNA.

• Translation: mRNA to protein.

• Genotype: Genetic code; Phenotype: Protein expression.

Genetic Code

• Codon: Three-nucleotide sequence.

• Triplets: Each codon codes for an amino acid.

• Reading frame: Correct grouping of codons.

• Start codon: AUG; Stop codons: UAA, UAG, UGA.

• Code is redundant, unambiguous, non-overlapping, and conservative.

Mutations

• Point mutations: Single nucleotide changes.

• Chromosome-level mutations: Large-scale changes.

• Missense: Changes amino acid.

• Silent: No effect on protein.

• Frameshift: Alters reading frame.

• Nonsense: Creates stop codon.

• Beneficial, neutral, deleterious: Effects on organism.

• Inversion, translocation, deletion, duplication: Types of chromosome mutations.

Transcription, RNA Processing, and Translation

Transcription

• Occurs 5’ to 3’ direction.

• Initiation in bacteria: RNA polymerase binds to promoter; holoenzyme formation.

• Elongation: RNA strand synthesized.

• Termination: RNA polymerase releases DNA.

RNA Processing in Eukaryotes

• Primary transcript: Initial RNA; contains introns and exons.

• RNA splicing: Removes introns; four steps.

Translation

• mRNA sequence translated to amino acids.

• Polyribosomes: Multiple ribosomes translating one mRNA.

• tRNA: Transfer RNA; carries amino acids; has anticodon.

• rRNA: Ribosomal RNA; forms ribosome structure (two subunits).

• Three functions in translation: decoding, peptide bond formation, translocation.

• Initiation: Ribosome binds to mRNA; three steps.

• Elongation: Ribosome moves down mRNA; elongation factors assist.

• Termination: Release factor binds; polypeptide released.

• Polypeptide folding and chemical modifications: Essential for functional proteins.

Control of Gene Expression in Bacteria

Gene Expression and Regulation

• Regulation occurs at transcriptional, translational, and post-translational levels.

• Genes can be turned on or off.

Lactose Metabolism (lac Operon)

• lacZ and lacY: Genes for lactose metabolism.

• lacI: Repressor gene.

• Negative control: Repressor blocks transcription.

• Positive control: Activator enhances transcription.

• Operon model: Cluster of genes regulated together.

• Glucose prevents lac operon expression via two mechanisms (CAP regulation).

Trp Operon

• Genes for tryptophan synthesis; negative feedback regulation.

Global Gene Regulation

• Genes can be regulated by repressors or activators.

• SOS response: Negative control; system for DNA damage repair.

Control of Gene Expression in Eukaryotes

Overview of Eukaryotic Gene Regulation

• Differential gene expression: Different cells express different genes.

• Control occurs at chromatin, transcriptional, post-transcriptional, translational, and post-translational levels.

Chromatin Structure and Remodeling

• Chromatin: DNA + histones; nucleosome structure.

• Histone H1: Helps condense chromatin.

• Condensation/decondensation affects gene accessibility.

• DNA methylation: CpG methylation by methyltransferases silences genes.

• Histone modifications: HATs (acetylation), HDACs (deacetylation).

• Chromatin-remodeling complexes: Alter chromatin structure.

Epigenetic Inheritance

• Chromatin marks passed to daughter cells.

• Environment can influence epigenetic state.

Transcriptional Regulation

• Core promoter: TATA box, TBP.

• Promoter-proximal elements: Regulatory sequences near promoter.

• Enhancers and silencers: Distant regulatory elements.

• Activators and repressors: Proteins that increase or decrease transcription.

• DNA looping and mediator complex facilitate transcription initiation.

Post-Transcriptional Regulation

• Alternative splicing: Different mRNAs from same gene; spliceosomes mediate.

• mRNA stability: Determines how long mRNA is available for translation.

RNA Interference (RNAi)

• microRNA (miRNA): Small RNA molecules; RISC complex mediates gene silencing.

• siRNA and piRNA: Other small RNAs; block translation or degrade mRNA.

Translational and Post-Translational Control

• Regulation of translation initiation; mRNA lifespan affects protein production.

• Protein modifications: Phosphorylation, cleavage.

• Ubiquitin-proteasome pathway: Protein degradation.

Gene Regulation and Cancer

• Tumor suppressor genes (p53): Prevent uncontrolled growth.

• Proto-oncogenes → oncogenes: Mutations lead to cancer.

• DNA damage responses; regulation failures cause cancer.

Comparison: Bacterial vs. Eukaryotic Gene Expression

• Bacteria: DNA less packaged, operons regulate multiple genes, simpler transcription.

• Eukaryotes: DNA highly packaged, individual gene regulation, complex transcription and post-transcriptional control.

Table: Comparison of Gene Expression Regulation in Bacteria and Eukaryotes

Feature	Bacteria	Eukaryotes
DNA Packaging	Minimal	Chromatin, nucleosomes
Gene Organization	Operons	Individual genes
Transcription Regulation	Simple	Complex (enhancers, silencers, chromatin)
Post-Transcriptional Control	Limited	Alternative splicing, RNAi

Key Equations

• DNA replication direction: $5' \rightarrow 3'$

• Genotype ratio for monohybrid cross: $1:2:1$ (homozygous dominant : heterozygous : homozygous recessive)

• Phenotype ratio for monohybrid cross: $3:1$ (dominant : recessive)