Advanced Cytogenetics and Human Genetics Study Guide

FUNDAMENTALS OF CYTOGENETICS

• Cytogenetics: A discipline established by Sutton that combines cytology and genetics to study chromosomes, their behaviors, and their abnormalities.

• Primary Goals:

  • Diagnosis of chromosomal abnormalities.

  • Localization of specific chromosomal regions or DNA sequences (often abnormal).

  • Providing genetic counseling.

  • Conducting research.

• Chromosomes: Thread-like structures within cells composed of DNA and proteins. They are responsible for carrying inherited traits, organizing cell life, and facilitating heredity.

  • Human Somatic Cells: Contain $23$ pairs of chromosomes ($46$ total).

  • Autosomes: Pairs $1$ through $22$.

  • Sex Chromosomes: $1$ pair (X and Y); Females are $XX$, Males are $XY$.

• Genetics: The study of inherited traits and their variations.

• Heredity: The mechanism causing similarities between individuals.

• Variations: The factors causing differences between individuals.

• Genes: The fundamental units of heredity. They consist of biochemical instructions (segments of DNA) telling cells how to manufacture specific proteins that control characteristics. They include exons (coding regions) and introns (non-coding regions).

• Mendelian Trait: A trait caused predominantly by a single gene, such as polydactyly.

• Multifactorial Traits: Traits determined by the interaction of one or more genes and environmental factors, such as hair color (controlled by $3$ genes plus environmental influence).

• Central Dogma: The principle that DNA makes RNA, which in turn makes proteins.

• Genome: The complete set of genetic instructions characteristic of an organism.

• Genomics: The field of analyzing and comparing genomes.

• Exome: The portion of the genome composed of $20,325$ protein-encoding genes.

• Levels of Genetics:

  1. DNA: The genetic material forming genes; identical in every cell of the body.

  2. Gene: A sequence added to DNA that instructs protein production. Alleles are alternate forms of a gene.

     • Genotype: The underlying instructions/alleles present.

     • Phenotype: The visible trait or biochemical effect/alleles expressed.

     • Dominant Allele: Expressed even if only one copy is present.

     • Recessive Allele: Must be present on both chromosomes to be expressed.

  3. Chromosome: A continuous DNA molecule and its associated proteins. A Karyotype is a chart displaying chromosome pairs from largest to smallest.

  4. Human Genome: The complete genetic instruction set ($23$ chromosome pairs).

  5. Hierarchical Organization: Cell -> Tissue (e.g., cuboidal epithelium) -> Organ (e.g., kidney) -> Organ System (e.g., urinary system) -> Organism -> Family -> Population -> Community.

• Mutation: A change in a gene that can affect the whole-person level, causing disease (e.g., mutation in the $CFTR$ gene leads to cystic fibrosis).

HISTORY OF CYTOGENETICS

• Prehistoric Time and Greek Influence ($8000$ to $1000$ BC):

  • Characterized by the domestication and selective breeding of plants and animals.

  • Hippocrates: Authored On the Seed, proposing that active "humors" in various body parts served as bearers of hereditary traits.

  • Aristotle: Proposed that male semen's generative power resided in "pneuma" (vital heat). He also articulated the theory of spontaneous generation (life arising from nonliving matter).

• $1600$–$1900$ Transition:

  • William Harvey ($1600s$): Proposed Epigenesis, stating an organism derives from substances in the egg that differentiate during development.

  • $17th$ Century: The Theory of Preformation suggested sex cells contained a miniature adult called a homunculus.

  • Matthias Schleiden & Theodor Schwann ($1830$): Developed the Cell Theory, stating all organisms are composed of cells.

  • Karl Wilhelm von Nageli ($1842$): First to describe and discover chromosomes in plant cells, calling them transitory cytoplasts.

  • Gregor Johann Mendel ($1856$–$1863$): Studied inheritance and hereditary factors in germ cells; considered the Father of Genetics.

  • Eduard Strasburger ($1875$): Identified cell division and distinct bodies (chromosomes) within the nucleus.

  • Walther Flemming ($1882$): Discovered chromosome mitosis; considered the Father of Cytogenetics.

  • Heinrich Wilhelm Gottfried von Waldeyer-Harts ($1888$): Coined the term "chromosome" (Chromo: color; Soma: body) and used methylene blue to stain them.

  • August Weismann ($1888$): Observed that chromosomes in sperm and egg cells of silk moths fused during fertilization.

• $1900$–$1970s$ Advancements:

  • Theodor Boveri & Walter Sutton ($1902$): Proposed the Chromosome Theory of Inheritance.

  • Nettie Stevens ($1905$): Identified sex chromosomes.

  • Hans von Winiwarter ($1912$): Introduced aceto-orcein staining; incorrectly estimated human chromosome counts as $47$ for men and $48$ for women.

  • Theophilus Shickel Painter ($1921$): Discovered the Y chromosome in testicular cells and estimated $48$ chromosomes.

  • Albert Levan & Joe Hin Tijo ($1955$–$1956$): Correctly determined the human diploid chromosome number is $46$.

  • Francis Crick & James Watson ($1953$): Constructed the DNA double helical model.

  • Jerome Lejeune: Identified Trisomy $21$ in Down syndrome ($1959$) and described Cri-du-chat syndrome ($1963$).

  • Banding Techniques: Torbjorn Caspersson developed Q-banding ($1960s$); G-banding and R-banding followed in the $1970s$.

• $1980s$–Present:

  • Fluorescent In Situ Hybridization (FISH): Independently introduced by Thomas Ried and Yoshinori Watanabe to detect small deletions/duplications.

  • Chromosome Microdissection: Isolating DNA from recognizable chromosomal regions.

  • Comparative Genomic Hybridization (CGH): Identifying gains/losses of chromosomal regions across the genome.

  • Next-Generation Sequencing (NGS): Genome-wide analysis of abnormalities.

CELL STRUCTURE AND ORGANELLES

• Four Basic Tissue Types:

  1. Epithelial: Tight layers forming protective, secretory, and absorptive linings.

  2. Connective: Supports, binds, and fills spaces (cartilage, bone, blood, fat).

  3. Muscle: Cells that contract to provide movement.

  4. Nervous: Neurons transmit electrochemical impulses; neuroglia support neurons.

• Cell Categories:

  • Somatic Cells: Body cells; diploid ($2n$) with two copies of the genome.

  • Germ Cells: Sperm and egg cells; haploid ($n$) with one copy of the genome.

  • Stem Cells: Diploid cells capable of self-renewal and differentiation into specialized cells.

• Domains of Life:

  1. Archaea: Single-celled prokaryotes.

  2. Bacteria: Single-celled prokaryotes; the most abundant organisms.

  3. Eukarya: Eukaryotes (single or multicellular) possessing nuclei and organelles.

• Major Macromolecules:

  • Carbohydrates: Sugars/starches providing energy and structure.

  • Lipids: Fats/oils for hormones, membranes, insulation, and energy storage.

  • Proteins: Vital for clotting, muscle fibers, connective tissue, antibodies, and enzymes (catalysts).

  • Nucleic Acids: DNA and RNA; carry genetic information for protein synthesis.

• Animal Cell Organelles:

  • Nucleus: Porous, double-membraned sac containing DNA and the nucleolus (ribosome production site). The nucleoplasm is the internal fluid.

  • Nuclear Envelope: Surrounds the nucleus; contains nuclear pores (protein rings) and the nuclear lamina (mechanical support/gene silencing).

  • Ribosomes: Globular subunits of RNA and protein; catalysts for protein synthesis.

  • Endoplasmic Reticulum (ER): The quality control center.

    • Rough ER: Studded with ribosomes; site of protein synthesis and folding.

    • Smooth ER: Lacks ribosomes; site of lipid synthesis.

  • Golgi Apparatus: Stacks of membrane-enclosed sacs; the processing center where sugars are linked to proteins/lipids and secretions are stored.

  • Vesicles: Membrane-bound sacs for transport/storage.

  • Lysosomes: Trash centers containing $43$ types of digestive enzymes; absence causes storage diseases (e.g., Tay-Sachs).

  • Peroxisomes: Sacs that detoxify molecules; abundant in liver/kidney. Absence causes adrenoleukodystrophy.

  • Mitochondria: Energy extractors; provide ATP via cristae folds. A liver cell has $1,700$, while muscle has $10,000$.

  • Cytoskeleton: Protein rods/tubules providing architecture.

    • Microtubules: Composed of tubulin; facilitate movement ($Cilia$ include motile and primary sensory types).

    • Microfilaments: Composed of actin; provide structural support.

    • Intermediate Filaments: Abundant in nerve and skin cells.

• Plasma Membrane: A phospholipid bilayer with hydrophilic (water-loving) phosphate heads and hydrophobic (water-fearing) fatty acid tails. Facilitates signal transduction and cellular adhesion.

THE CELL CYCLE AND CELL DEATH

• Interphase: The non-dividing stage.

  • $G_0$: A quiet phase where cells maintain characteristics but do not divide.

  • $G_1$: Resumption of protein/lipid/carbohydrate synthesis.

  • $S$ Phase: Genome replication; chromosomes form two sister chromatids joined at a centromere.

  • $G_2$: Post-replication protein synthesis and membrane assembly.

• Mitosis: Division of genetic material.

  1. Prophase: Chromosomes coil and thicken; spindle assembles; nuclear envelope breaks down.

  2. Metaphase: Chromosomes align at the equator; spindle fibers attach via kinetochores.

  3. Anaphase: Centromeres split; sister chromatids separate to opposite poles.

  4. Telophase: Spindle falls apart; nucleoli and nuclear membranes reform.

• Cytokinesis: Division of the cytoplasm into two daughter cells.

• Cell Death:

  • Apoptosis: Genetically programmed cell death (e.g., carving digits). Failure leads to syndactyly.

  • Necrosis: Accidental cell death due to inflammation/damage.

• Regulation:

  • Checkpoints: Groups of proteins ensuring sequence order.

  • Telomeres: Chromosome tips that shorten with each division; mitosis ceases after approximately $50$ divisions. Telomerase is the enzyme that maintains them.

  • External Factors: Contact inhibition (crowding), hormones, and growth factors.

MEIOSIS AND REPRODUCTION

• Reproductive System Structures:

  • Male: Gonads (testes) produce sperm in seminiferous tubules. Sperm mature in the epididymis, travel through the ductus deferens, and exit the urethra. Accessory glands (Prostate, Seminal Vesicle, Bulbourethral) provide alkaline seminal fluid.

  • Female: Gonads (ovaries) produce oocytes within follicles. Oocytes travel via fallopian tubes to the uterus.

• Meiosis: Cell division reducing chromosome count to haploid ($n = 23$).

  • Meiosis I (Reduction Division):

    • Prophase I: Divided into Leptotene, Zygotene (synapsis), Pachytene (crossing over), Diplotene (chiasmata), and Diakinesis.

    • Metaphase I: Homologous pairs align at the equator.

    • Anaphase I: Homologous chromosomes separate (disjunction).

  • Meiosis II (Equational Division): Sister chromatids separate, resulting in four haploid cells.

• Gamete Formation:

  • Spermatogenesis: Continuous production of sperm from spermatogonia. Results in four equal-sized sperm cells.

  • Oogenesis: Production of eggs; results in one large ovum and three non-functional polar bodies to conserve cytoplasm.

• Prenatal Development:

  • Fertilization: Sperm undergoes capacitation, penetrates the corona radiata and zona pellucida. Fusion creates a diploid zygote.

  • Cleavage: Rapid divisions forming blastomeres -> Morula ($16+$ cells) -> Blastocyst (hollow ball).

  • Implantation: Blastocyst nestles into the uterine lining at week $1$. Trophoblast cells secrete human chorionic gonadotropin (hCG).

  • Germ Layers: Ectoderm (outer), Mesoderm (middle), Endoderm (inner).

  • Fetus: From week $9$ until birth. Differences in sexes appear at week $6$ (SRY gene).

• Teratogens: Agents causing birth defects (Thalidomide for limbs, Alcohol for FAS, excess Vitamin A, Zika virus for microcephaly).

MENDELIAN GENETICS AND INHERITANCE

• Mendel's Laws:

  • Law of Segregation: Alleles for a gene distribute into separate gametes.

  • Law of Independent Assortment: Inheritance of one gene does not influence another (applies to genes on different chromosomes).

• Genetics Definitions:

  • Wild Type: Most common expression in a population.

  • Mutant Type: Variant resulting from gene change.

  • Monohybrid Cross: Follows one trait; results in a $3:1$ phenotypic ratio in $F_2$.

  • Dihybrid Cross: Follows two traits; results in a $9:3:3:1$ phenotypic ratio in $F_2$.

  • Test Cross: Crossing a dominant phenotype individual with a homozygous recessive to determine the genotype.

• Modes of Inheritance:

  • Autosomal Dominant: Successive generations affected; males and females transmit equally.

  • Autosomal Recessive: Can skip generations; often appears in siblings with unaffected parents (carriers).

  • Sex-Linked:

    • Y-linked: Father-to-son only (e.g., male infertility).

    • X-linked Recessive: Always expressed in males; females are usually asymptomatic carriers (e.g., colorblindness, hemophilia).

    • X-linked Dominant: Passed from male to all daughters, no sons (e.g., Rett syndrome, Incontinentia pigmenti).

• Non-Mendelian Modifications:

  • Lethal Alleles: Combinations causing death (e.g., homozygous dominant for Achondroplastic dwarfism).

  • Multiple Alleles: Genes with more than two alleles (e.g., ABO blood groups - $I^A$, $I^B$, $i$).

  • Dominance Relationships:

    • Complete: One allele masks another.

    • Incomplete: Heterozygote has a blended phenotype (e.g., pink snapdragons; Tay-Sachs enzyme levels).

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

  • Epistasis: One gene masks/modifies another (e.g., Bombay phenotype where $hh$ masks A/B antigens).

  • Penetrance: The percentage of individuals with a genotype who show the phenotype (Complete vs. Incomplete).

  • Expressivity: The severity or extent of the phenotype.

  • Pleiotropy: One gene having multiple unrelated effects (e.g., Porphyria variegata).

  • Genetic Heterogeneity: Different genes causing the same phenotype (e.g., $100+$ genes causing blindness).

  • Mitochondrial Inheritance: Maternally inherited circular DNA ($37$ genes). Hallmark of diseases is "red-ragged fibers."

NUCLEIC ACIDS AND GENE EXPRESSION

• DNA Structure: Double-stranded, antiparallel nucleotide chains. Nucleotides consist of deoxyribose sugar, phosphate, and nitrogenous bases (A, T, G, C). A pairs with T ($2$ bonds), G pairs with C ($3$ bonds).

• RNA Structure: Usually single-stranded; contains ribose sugar and Uracil (U) instead of Thymine (T).

  • Types: mRNA (encodes sequence), rRNA (forms ribosomes), tRNA (transports amino acids with anticodons).

• DNA Replication: Semiconservative process during S phase.

  • Enzymes: Helicase (unwinds), Binding proteins (stabilize), Primase (adds primer), DNA Polymerase (builds new strand), Ligase (seals fragments).

• Transcription: copying DNA into mRNA.

  • Steps: Initiation (RNA polymerase binds promoter), Elongation (adding nucleotides), Termination (crossing stop sequence).

  • Processing: 5' Capping, 3' Polyadenylation (Poly-A tail), and Splicing (removing introns).

• Translation: protein synthesis using mRNA code.

  • Genetic Code: Triplet, unambiguous, degenerate (redundant), nearly universal, and non-overlapping.

  • Steps: Initiation (ribosome complex forms at AUG), Elongation (tRNA brings amino acids; peptide bond formation), Termination (stop codons: UAA, UAG, UGA).

• Protein Folding: Guided by chaperones. Misfolded proteins are tagged with ubiquitin and degraded by proteasomes.

  • Prion Diseases: Caused by misfolded PrP proteins (e.g., Scrapie, Mad Cow/BSE, Kuru, Creutzfeldt-Jakob).

CLINICAL CYTOGENETICS AND ABNORMALITIES

• Chromosome Classification:

  • Metacentric: Equal arms.

  • Submetacentric: One long arm ($q$), one short arm ($p$).

  • Acrocentric: Centromere near one end.

  • Telocentric: Centromere at the tip (not in humans).

• Atypical Chromosome Numbers:

  • Polyploidy: Extra sets of chromosomes (e.g., Triploid $3N$).

  • Aneuploidy: Extra or missing single chromosome. Caused by nondisjunction.

    • Trisomy 21 (Down Syndrome): Most common liveborn autosomal aneuploidy ($46$, $XY$ or $XX$, $+21$).

    • Trisomy 18 (Edwards Syndrome): Features heart defects and oddly clenched fists.

    • Trisomy 13 (Patau Syndrome): Features eye fusion and polydactyly.

    • XO (Turner Syndrome): Only $1$ X chromosome in females; short stature, infertility.

    • XXY (Klinefelter Syndrome): Males with extra X; long limbs, infertility.

    • XYY (Jacobs Syndrome): Tall males, speech delays.

• Atypical Structures:

  • Translocations: Robertsonian (acrocentric fusion), Reciprocal (exchange), Insertional (rare transfer).

  • Inversions: Paracentric (no centromere included), Pericentric (includes centromere).

  • Other: Isochromosomes (wrong plane division), Ring chromosomes (loss of telomeres).

• Uniparental Disomy (UPD): Inheriting both copies of a chromosome from one parent (e.g., Prader-Willi/Angelman syndromes).

CANCER GENETICS

• Characteristics: Loss of cell cycle control, heritability, transplantability, dedifferentiation, lack of contact inhibition, and angiogenesis (blood vessel formation).

• Genes Involved:

  • Oncogenes: Overexpressed or activated proto-oncogenes; lift controls on division.

  • Tumor-Suppressor Genes: Normally inhibit division; cause cancer when deleted or silenced (e.g., $RB1$, $p53$, $BRCA1/2$).

  • DNA Repair Genes: Mutations allows other mutations to persist.

• Mutations:

  • Driver Mutations: Provide selective growth advantage ($3$ strikes: breakthrough, expansion, invasion).

  • Passenger Mutations: Incidental mutations not contributing to tumor progression.

• Diagoses/Treatments: Biopsies, tumormarkers (ctDNA), staging. Treatments include surgery, chemotherapy, radiation, tyrosine kinase inhibitors, and CAR-T technology.

GENETIC TECHNOLOGIES AND THE HUMAN GENOME PROJECT

• Recombinant DNA: Initially produced human insulin in E. coli using restriction enzymes and vectors.

• Transgenic Organisms: Modified with genes from other species (e.g., Golden Rice with beta-carotene).

• Genome Editing: CRISPR-Cas9, ZFNs, and TALENs for somatic or germline editing.

• Human Genome Project (HGP): International collab ($1990$–$2003$) to map all human genes ($3.2 \times 10^9$ base pairs).

• Genetic Counseling: Helping families understand inheritance patterns and evaluate risks via predictive and diagnostic testing.

• Assisted Reproductive Technologies (ARTs): Includes IUI, IVF, ICSI (sperm injection), and PGD (genetic diagnosis before implantation).

MATHEMATICAL DATA SUMMARY

• Cell Cycle Duration: Cultured mammalian cells divide $40$–$60$ times.

• Sperm Statistics: Ejaculation delivers $200 \times 10^6$ to $600 \times 10^6$ sperm; count is low if below $15 \times 10^6$ to $200 \times 10^6$ per $mL$.

• Telomere Loss: $50$ to $200$ bases lost per mitosis.

• Human Gene Count: Approximately $20,500$ human genes identified by HGP.

• Exosome Size: $30$ to $100$ nanometers in diameter.

• Mitochondrial DNA: $16,569$ base pairs with $37$ genes.