Biology Notes: Cells, DNA, and Organization

Living Things and the Cell

  • All living organisms are made of cells. If you had to define a cell, you’d say it is the fundamental unit of life with a boundary and internal components that carry out life’s processes.
  • What do cells have?
    • Outer boundary: a membrane (plasma membrane) that encloses the cell.
    • Genetic material: DNA stored inside; some cells have a nucleus, some do not.
    • Membrane + DNA form the core of what makes a cell function.
  • Plasma membrane and DNA as universal features:
    • All cells have a plasma membrane.
    • All cells contain genetic material (DNA).
  • Variation in subtypes:
    • Some cells have nuclei (eukaryotic), some do not (prokaryotic).
    • Prokaryotic cells include bacteria and archaea; eukaryotic cells include plants, animals, fungi, and protists.
  • Energy transformation:
    • Living things convert energy to power biological processes.
    • Plants convert solar energy to chemical energy (photosynthesis).
    • Animals obtain chemical energy by eating plants or other organisms and convert it to usable forms for work.
  • Homeostasis and environmental response:
    • Living systems maintain stable internal conditions (temperature, salinity, pressure, etc.).
    • They respond to environmental cues and interact with other organisms; nonliving things typically do not exhibit such coordinated responses.
  • Reproduction and inheritance:
    • Living things reproduce to produce offspring, passing genetic information to progeny.
  • Evolution and variation:
    • Natural selection is a major mechanism of evolution.
    • For natural selection to operate, there must be variation in a population, differential reproductive success, and heritable traits.
    • Those with advantageous traits leave more progeny and become more common over generations.
  • The five unifying themes (summarized):
    • Molecules are organized into cell parts; cell parts into cells.
    • Information is stored in DNA.
    • Energy is transformed to perform cellular work.
    • Organisms interact with the environment and maintain homeostasis.
    • Evolution occurs, primarily via natural selection (with other mechanisms discussed in broader courses).
  • A nod to broader evolutionary perspectives:
    • Lynn Margulis and collaborators emphasized communities and symbiotic origins of cellular complexity (endosymbiosis), a topic touched on later when studying cells.

Emergence and Organization

  • Emergent properties arise when parts are organized in specific ways; the whole becomes more than the sum of its parts.
  • Examples of emergent organization:
    • Flower: organized tissues/parts that attract pollinators.
    • Seahorse camouflage: integrated traits helping survival and reproduction.
    • Jackrabbit: regulates body temperature via large, highly vascular ears for cooling (surface area to volume considerations).
    • Butterfly: collects sugars from plants (producers) and converts to energy.
    • Growth and development: organisms grow and develop through regulated gene expression and physiology.
    • Reproduction: life begets life.
    • Giraffe: traits that influence survival and reproduction across generations.
  • Key educational slide you’ll see on exams: the levels from atoms to biosphere (foundational for understanding biological organization).

From Atoms to Biosphere: Levels of Organization

  • Hierarchy (from small to large):
    • Atoms → Molecules → Organelles → Cells → Tissues → Organs → Organisms → Population → Community → Ecosystem → Biosphere
  • Definitions and connections:
    • Population: all individuals of the same species in a given area.
    • Community: different species living together in a region.
    • Ecosystem: a community plus the physical environment (climate, soil, water, etc.).
    • Biosphere: the global sum of all ecosystems.
  • The cell-theory context:
    • Cells are the basic unit of life; all living things are made of cells.
    • Prokaryotic cells lack a nucleus and membrane-bound organelles; eukaryotic cells have a nucleus and organelles.
  • Size and compartmentalization:
    • Prokaryotes are tiny and lack internal membrane-bound compartments; diffusion is the primary transport method.
    • Eukaryotes are larger partially because they have membrane-bound compartments (organelles), enabling specialized transport and increased size.
  • An analogy: a city inside a cell
    • Membrane-bound organelles are like roads, houses, and buildings that organize activities and allow complex processes to occur efficiently.
  • The transmission of genetic material:
    • All cells contain DNA; the structure and organization of DNA differ between prokaryotes (often circular chromosomes) and eukaryotes (linear chromosomes in a nucleus).
    • In eukaryotes, mitosis evolved for clean division of duplicated chromosomes; meiosis evolved later for sex cells.
  • The spindle apparatus:
    • The spindle body is involved in chromosome separation during cell division (chromosome movement is a key feature of mitosis and meiosis).
  • Genes and inheritance:
    • Genes are the units of heredity located on chromosomes.
    • DNA directs the information needed to build and operate a cell.

DNA, Genes, and the Central Dogma

  • DNA structure:
    • DNA consists of two complementary strands of nucleotides forming a double helix.
    • The backbone is made of sugar-phosphate groups; crossbars in the middle are hydrogen-bonded base pairs.
    • Base pairing rules: A ext{ pairs with } T and G ext{ pairs with } C.
  • Central dogma of molecular biology:
    • Transcription: DNA is copied into messenger RNA (mRNA).
    • Translation: mRNA is decoded to synthesize proteins.
    • Gene expression is the overall process from DNA to RNA to protein.
  • DNA and chromosomes:
    • Every somatic cell typically contains two complete sets of chromosomes (diploid, denoted 2n).
    • Sex cells (gametes) are haploid (denoted n).
    • Two full genome complements per non-sex cell mean the cell has the full set of genetic information; sex cells contain only one set to combine during fertilization.
  • Transcription and translation details:
    • mRNA nucleotides: A, U, C, G (note that RNA uses uracil instead of thymine).
    • The mRNA sequence is complementary to one strand of the DNA sequence and serves as a template for protein synthesis.
  • Gene expression: regulation and tissue specificity
    • While many genes are required by nearly all cells, different cell types express different subsets of genes to perform specialized functions.
    • Example: lens cells express a crystallin gene for lens protein; skin cells express protective proteins specific to skin; hair follicle cells express keratin/hair proteins.
  • A detailed lens gene example (crystalline):
    • Crystalline gene contains the DNA sequence that encodes crystalline protein.
    • Transcription: a copy of the crystalline gene is produced as mRNA with a sequence matching one DNA strand.
    • Translation: ribosomes read the mRNA to assemble the amino acid sequence of crystalline protein.
    • Folding and packaging: the crystalline protein folds into its functional structure and is packed in lens cells to contribute to focusing light.
  • DNA sequence and protein diversity:
    • The four nucleotides (A, T, C, G) in DNA determine the sequence for protein building blocks; different genes have different sequences, leading to different proteins (e.g., insulin gene vs crystalline gene).
    • Example of a hypothetical insulin DNA segment: CCT, GTG, CGG, CTC (illustrative sequence differing from crystalline).

Gene Expression in Specific Cells: Regulation and Examples

  • Despite having the same genome, cell types express different genes.
  • Gene expression is regulated so that only the proteins needed in a given cell type are produced.
  • Many genes are common to all cells, but the subset expressed in a cell determines its function and identity.

In-Class Exercise: Organizing Biological Hierarchy (Group Activity)

  • Students work in groups of three to write names and discuss organization.
  • Task (from transcript): create a list from small to large including atoms, molecules, organelles, cells, tissues, organisms, population; label each level and determine which term fits which level.
  • Purpose: reinforce understanding of hierarchical organization and the relationships between levels.
  • Note: one term may be missing in a given exercise, prompting discussion about where it fits in the sequence.
  • Activity flow (paraphrased):
    • Start with atoms, add molecules, then organelles, then cells, then tissues, then organs, then organisms, then population.
    • Determine which of these levels correspond to each term and discuss where a term belongs (e.g., molecule vs organelle vs cell).
  • Clarifications from discussion prompts:
    • Some terms (e.g., ecosystem vs population) may appear in the activity to emphasize differences between levels.
    • The exercise may involve confirming or switching terms to fit the proper level.

Modern Biology: Genomics, Proteomics, and Systems Biology

  • Reductionist approach (historical): study a small piece of a problem and build toward the whole.
  • Today’s data-rich era:
    • Genomics: study of complete genome sequences.
    • Proteomics: study of all proteins expressed in a tissue or developmental stage.
    • Transcriptomics: (implied) study of RNA transcripts.
    • Bioinformatics and systems biology: integration and analysis of large-scale data to understand whole-system behavior.
  • Computational power enables new capabilities:
    • Large-scale data comparison reveals evolutionary patterns and functional networks.
    • Real-time data integration guides discovery and hypothesis testing.
  • Applied advances mentioned in the transcript:
    • Synthesis of synthetic chromosomes (e.g., constructing a mouse chromosome from DNA pieces) and the potential to reduce repetitive DNA to avoid unwanted recombination.
    • Development of engineered viruses to deliver genetic material (e.g., genes, antibodies) for therapy; not all viruses are pathogenic (the speaker distinguishes general viruses from pathogens like SARS-CoV-2).

Structure–Function Relationship: Specific Examples

  • Hummingbird (structure–function):
    • Hummingbirds can fly forward, backward, and hover by unique shoulder mechanics, enabling feeding from long-tubed flowers and predator avoidance.
    • Adaptation illustrates how anatomical structure supports ecological function.

Summary: Cell Theory and Key Concepts

  • Cell theory recap:
    • The cell is the basic unit of life and the smallest unit capable of independent life.
    • All living organisms are composed of cells.
    • Cells arise from pre-existing cells by division.
  • Core cellular features:
    • Every cell has a plasma membrane.
    • Genetic material (DNA) is present in all cells (in nuclei for eukaryotes; region of DNA in prokaryotes).
  • Prokaryotic vs. Eukaryotic cells:
    • Prokaryotes (bacteria and archaea): no nucleus, no membrane-bound organelles; very small; simple organization.
    • Eukaryotes: nucleus and membrane-bound organelles; larger and compartmentalized; allows complex intracellular transport and higher-order organization.
  • Importance of compartmentalization:
    • Enables larger cell size and more precise control of cellular processes via specialized compartments.
  • DNA and inheritance:
    • DNA stores genetic information; genes are units of inheritance on chromosomes; cell function depends on regulated gene expression.
  • The Central Dogma and gene expression:
    • DNA → RNA (transcription) → protein (translation).
    • RNA uses uracil (U) instead of thymine (T).
    • Gene expression is regulated so that only necessary genes are expressed in a given cell type.
  • Basic conceptual map of organization:
    • Atoms → Molecules → Organelles → Cells → Tissues → Organs → Organisms → Population → Community → Ecosystem → Biosphere

Quick Reference: Key Notation and Concepts

  • Diploid cells: 2n (two complete chromosome sets in most somatic cells).
  • Haploid cells: n (a single chromosome set, as in gametes).
  • Base pairing rules: A-T and G-C in DNA; in RNA, A-U and G-C.
  • Central Dogma equation (conceptual):
    • ext{DNA}
      ightarrow ext{RNA}
      ightarrow ext{protein}
  • Gene expression definition: the process of transcribing DNA into RNA and translating RNA into protein; regulation ensures appropriate proteins are produced for a given cell type.
  • Examples to remember:
    • Crystalline gene in lens cells → crystalline protein → lens focus.
    • Skin cells expressing skin-specific proteins, not crystalline, despite having crystalline genes present.
  • Special terms to know:
    • Plasma membrane, nucleus, organelles, mitosis, meiosis, spindle apparatus, transcription, translation, gene expression, base pairing, double helix, genome, proteomics, genomics, bioinformatics, systems biology

Ethical and Practical Considerations (brief note)

  • Advances in synthetic biology and virus-based therapies raise ethical questions about safety, governance, and equitable access.
  • Education highlights the importance of understanding both foundational biology and its real-world applications.