Exam One Review: Genetics, Processes, and Cell Division

Genetic Building Block Terminology

  • Fundamental Biological Units and Molecules:

    • Gene: The fundamental physical and functional unit of heredity, composed of a specific sequence of nucleotides in DNA that encodes a functional product such as a protein or RNA molecule.
    • Genome: The complete set of genetic material, including all genes and non-coding sequences, present within an organism or cell.
    • Locus: The precise physical position or location of a gene, sequence, or biomarker on a specific chromosome.
    • Allele: One of two or more alternative functional states or sequence variants of a gene at a given chromosomal locus.
    • DNA (Deoxyribonucleic Acid): A double-stranded nucleic acid polymer composed of deoxyribonucleotide units that stores long-term genetic information.
    • RNA (Ribonucleic Acid): A single-stranded nucleic acid polymer composed of ribonucleotide units involved in gene expression, regulation, and protein translation.
    • Nucleic Acid: Biopolymers made up of nucleotide monomers, essential for the storage, transmission, and execution of genetic instructions.
    • Amino Acid: Organic molecules containing an amine group (-NH2\text{-NH}_2), a carboxyl group (-COOH\text{-COOH}), and a distinct side-chain (R-group\text{R-group}), serving as the monomeric subunits of proteins.
    • Protein: A biologically active macromolecule consisting of one or more folded polypeptide chains of amino acids that perform critical cellular functions.

  • Functional Types of RNA:

    • mRNA (Messenger RNA): Single-stranded RNA transcribed from DNA that carries protein-coding instructions from the nucleus to the cytoplasm for translation.
    • tRNA (Transfer RNA): Small adaptor RNA molecules that deliver specific amino acids to the ribosome corresponding to codons present in the mRNA sequence.

  • Chromosomal Architecture:

    • Chromosome: A structured macromolecule of chromatin (DNA wrapped around histone proteins) carrying genetic information in linear order.
    • Centromere: The specialized, constricted DNA region of a chromosome that links sister chromatids and serves as the assembly site for kinetochores during cell division.
    • p-ARM / q-ARM: The two structural segments of a chromosome divided by the centromere:
      • p-ARM: The short arm of the chromosome.
      • q-ARM: The long arm of the chromosome.

  • Nitrogenous Bases and Complementary Base Pairing:

    • Base Pairs: Pairs of complementary nitrogenous bases connected by hydrogen bonds in nucleic acids.
    • Adenine: A purine nitrogenous base that forms complementary base pairs with Thyamine in DNA and Uracil in RNA.
    • Guanine: A purine nitrogenous base that forms complementary base pairs with Cytosine in both DNA and RNA.
    • Cytosine: A pyrimidine nitrogenous base that forms complementary base pairs with Guanine in both DNA and RNA.
    • Thyamine: A pyrimidine nitrogenous base found in DNA that pairs exclusively with Adenine.
    • Uracil: A pyrimidine nitrogenous base found in RNA that replaces Thyamine and pairs with Adenine.

  • Genomic Initiatives and Heritable Modifications:

    • Human Genome Project: The landmark international research initiative that mapped and sequenced the entire human genetic code.
    • Epigenetics: The study of heritable modifications in gene expression or cellular phenotype that occur without changing the primary DNA sequence.
    • Imprinting: An epigenetic mechanism where gene expression is regulated in a parent-of-origin-specific manner, silencing either the maternal or paternal allele.

Genetic Processes and Molecular Frameworks

  • Core Molecular Framework:

    • Central Dogma: The directional paradigm of molecular biology describing the flow of genetic information: DNA→RNA→Protein\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}.
    • Nucleotide: The molecular monomer consisting of a 5-carbon pentose sugar, a nitrogenous base, and a phosphate group.
    • Directionality (5′→3′5' \rightarrow 3'): The intrinsic polarity of nucleic acid strands, where synthesis and processing proceed from the 5′5' phosphate end toward the 3′3' hydroxyl end.
    • Amino Acids to Proteins: The progressive polymerisation and structural folding of amino acid chains into functional three-dimensional protein structures.

  • Primary Genetic Processes:

    • Replication: The high-fidelity duplication of a double-stranded DNA genome.
    • Transcription: The process of copying a specific genomic DNA template sequence into a complementary RNA strand.
    • Translation: The decoding of an mRNA transcript by cellular machinery to synthesize a specific protein sequence.

Detailed Mechanisms of Replication, Transcription, and Translation

  • Replication Mechanisms:

    • Subcellular Location: Occurs in the nucleus of eukaryotic cells.
    • Basic Process: Completely duplicates double-stranded DNA prior to cell division.
    • Sites of Origin: Designated genomic sites (origins of replication) where DNA unwinding begins and replication bubbles form.
    • Okazaki Fragments: Short, discontinuous DNA fragments synthesized on the lagging strand that are later joined together by DNA ligase.
    • Primer: A short single-stranded RNA sequence synthesized by primase that provides a free 3′-OH3'\text{-OH} group for DNA polymerase initiation.

  • Transcription Mechanisms:

    • Subcellular Location: Occurs inside the nucleus of eukaryotic cells.
    • Basic Process: Synthesizes single-stranded RNA from a complementary DNA strand template.
    • RNA Size: Varies depending on the individual gene transcribed and subsequent pre-mRNA processing/splicing.
    • RNA Base Pairs: Direct complementary base pairing using Adenine-Uracil (A-U\text{A-U}) and Cytosine-Guanine (C-G\text{C-G}).
    • Promoter: A specialized regulatory DNA sequence located upstream of a target gene where RNA polymerase binds to initiate transcription.
    • Termination Site: Specific nucleotide sequences at the end of a gene that signal RNA polymerase to halt transcription and release the transcript.

  • Translation Mechanisms:

    • Subcellular Location: Occurs outside the nucleus, within the cytoplasm.
    • Basic Process: Translates mRNA codons into a sequence of amino acids to synthesize proteins.
    • Ribosome: The ribonucleoprotein complex (comprising large and small subunits) that orchestrates translation by reading mRNA and aligning tRNAs.

Cell Division and Cell Cycle Control

  • Mitosis:

    • Overview: Somatic cell nuclear division producing genetically identical daughter cells.
    • Sequential Phases:
      • Prophase: Chromatin condenses into distinct chromosomes, the nuclear membrane breaks down, and the mitotic spindle apparatus begins assembling.
      • Metaphase: Chromosomes align singly along the equatorial metaphase plate with spindle fibers attached to kinetochores.
      • Anaphase: Centromeres cleave, separating sister chromatids into daughter chromosomes that move toward opposite cellular poles.
      • Telophase: Chromosomes decondense at opposite poles, nuclear membranes reform, and cytokinesis completes physical cell cleavage.
    • Product: Two identical diploid (2n2n) somatic daughter cells.

  • Meiosis:

    • Target Cells: Specialized cell division occurring in Gametes / Germ Cells.
    • Differences from Mitosis:
      • Involves two rounds of division (Meiosis I and Meiosis II) following a single DNA replication phase.
      • Homologous chromosome pairing (synapsis) and genetic recombination (crossing over) occur during Prophase I.
      • Reduces genetic content from diploid (2n2n) to haploid (nn).
    • Product: Four genetically unique haploid (nn) gametes.
    • Male vs Female Gametogenesis:
      • Spermatogenesis (Male): Continuous development producing four functional, equal-sized sperm cells per meiotic cycle.
      • Oogenesis (Female): Discontinuous, asymmetric division producing one large functional ovum (egg) containing most cytoplasm and non-functional polar bodies.

  • Cell Cycle Control Systems:

    • Cyclins: Regulatory proteins whose concentration levels oscillate throughout the cell cycle to activate specific Cyclin-Dependent Kinases (CDKs).
    • Checkpoint Proteins: Surveillance mechanisms (e.g., G1/S, G2/M, and spindle checkpoints) that verify cell size, DNA integrity, and replication completion before permitting cell cycle transition.
    • Apoptosis: Programmed cell death pathways triggered when irremediable cellular or DNA damage is encountered, preventing pathological cell proliferation.

Reproduction, Inheritance, and Non-Mendelian Patterns

  • Processes Generating Genetic Variation:

    • Independent Assortment of Chromosomes: The random orientation and segregation of maternal and paternal homologous chromosome pairs during Metaphase I of meiosis.
    • Crossing Over: The reciprocal exchange of homologous non-sister chromatid DNA segments during Prophase I of meiosis.
    • Fertilization: The random combination of unique haploid paternal and maternal gametes to form a diploid zygote.

  • Mendelian Genetics:

    • Autosomal Dominant / Recessive: Monogenic inheritance patterns located on non-sex chromosomes:
      • Autosomal Dominant: Expressed with a single copy of the dominant allele.
      • Autosomal Recessive: Requires two copies of the recessive allele for phenotypic expression.
    • Sex-Linked: Inheritance patterns determined by genes located on sex chromosomes (X or Y chromosomes).
    • Codominance: Expression pattern where both alleles in a heterozygous individual are fully and distinctively expressed without blending.
    • Complex Trait Rule: Most human traits are not simple Mendelian traits and do not display classical single-gene inheritance patterns.

  • Non-Mendelian Genetics Concepts:

    • Pleiotropy: A single gene influencing multiple, distinct, and seemingly unrelated phenotypic traits.
    • Polygenic Inheritance: A single trait governed by the cumulative additive effect of multiple distinct genes.
    • Penetrance: The percentage or proportion of individuals with a specific disease-associated genotype who express the expected phenotype.
    • Variable Expressivity: The spectrum or variation in severity of phenotypic expression among individuals bearing identical genotypes.
    • Imprinting: Gene silencing according to parent-of-origin inheritance.
    • Epigenetics: Environmental or chemical alterations (such as DNA methylation or histone acetylation) modifying gene expression without changing the DNA sequence.