Cellular Processes and Genetics

Active Transport

  • Active transport mechanisms move substances against their concentration gradient, requiring energy (ATP).
  • These mechanisms move substances to areas where they are already at higher concentrations.
  • Examples include the sodium-potassium ATPase (sodium-potassium pump) and the movement of amino acids into cells for protein synthesis.

Sodium-Potassium Pump

  • An embedded channel protein that consumes energy to move sodium and potassium ions against their concentration gradients.
  • The pump exchanges three sodium ions (Na+) out of the cell for every two potassium ions (K+) into the cell.
  • This unequal exchange creates an imbalance in charge, maintaining the cell membrane potential, with the inside of the cell negatively charged relative to the outside.
  • The pumping action produces a small amount of heat.
  • These pumps consume 50% of our daily caloric intake due to their abundance and constant activity.
  • The sodium-potassium pump is an antiporter because it exchanges solutes in opposite directions.

Vesicular Transport

  • Cells perform vesicular transport daily. These are active transport mechanisms that consume ATP and accomplish bulk transport.
  • Exocytosis: Bulk transport of substances out of the cell.
  • Endocytosis: Movement of substances into the cell.
    • Phagocytosis: "Cell eating" where the cell takes up large particles and breaks them down using lysosomes, forming a phagolysosome; important for immune cells to clear debris.
    • Pinocytosis: "Cell drinking" where the cell uptakes fluid droplets from the extracellular fluid (ECF); all human cells do this to monitor their environment and prepare for potential threats.
    • Receptor-Mediated Endocytosis: A selective process where receptors on the cell membrane bind to specific substrates, causing the membrane to invaginate and form a clathrin-coated vesicle; used for uptake of insulin and low-density lipids (LDL) by endothelial cells.

Exocytosis

  • The reverse of endocytosis. A vesicle containing substances to be secreted docks beneath the plasma membrane, fuses, and releases its contents.
  • Used by endothelial cells to release insulin, for lactation (mammary gland secretion), and by endocrine glands to release hormones.
  • This process is somewhat messy, and the plasma membrane needs to be regenerated afterward.

Cell Organelles

  • Organelles are categorized as those with membranes and those without.

Organelles with Membranes

  • Nucleus
  • Mitochondria
  • Lysosome
  • Peroxisome
  • Endoplasmic Reticulum
  • Golgi Apparatus

Organelles without Membranes

  • Ribosome
  • Centrosome
  • Cytoskeleton (microfilaments and tubules)
  • Inclusions: Storage sites for excess substances.

Nucleus

  • The largest organelle, typically visible with a light microscope, enclosed by a nuclear envelope with pores for material transmission.
  • It contains DNA associated with histones, forming nucleosomes. Collectively, DNA strands and packing proteins are called chromatin.
  • Nucleolus (or nucleoli): A dense region within the nucleus where ribosomes are made.
  • The nucleus houses about 2 meters (approximately 6 feet) of DNA.
  • Function: To code for protein synthesis. DNA is highly organized within the nucleus.

Nucleotides

  • The monomer of nucleic acids, consisting of a phosphate group, a sugar (deoxyribose in DNA), and a base.
  • There are five possible bases: adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U).
  • Purines: Double-ring bases (A and G).
  • Pyrimidines: Single-ring bases (C, T, and U).
    • DNA contains A, G, C, and T.
    • RNA contains A, G, C, and U (Uracil replaces Thymine).

DNA Helix

  • Composed of two strands with a sugar-phosphate backbone and bases in the center.
  • Bases pair according to the law of complementary base pairing: A with T, and C with G.

Protein Synthesis

  • A two-step process: Transcription and translation.

Transcription

  • Converts the information in DNA genes into a messenger RNA (mRNA) strand.
  • mRNA can leave the nucleus through nuclear pores, unlike DNA.

Translation

  • Translates the information in mRNA into a protein, which occurs via ribosomes.
  • Ribosomes are the "factories" that build proteins.

Information Transmission

  • DNA contains a code of nucleotides (A, T, C, G) that ultimately codes for amino acids, the building blocks of proteins.
  • Genes within DNA are read as single strands in sets of three nucleotides, called a base triplet (e.g., TAC).
  • Each base triplet corresponds to a codon in mRNA (e.g., AUG, which is the mirror image of TAC, considering complementary base pairing).
  • Each codon stands for a particular amino acid.
  • During transcription, DNA is converted into mRNA.
  • The initial mRNA strand (pre-mRNA) is longer and contains intervening sequences called introns.
  • Introns are removed through splicing.
  • The remaining sequences, called exons, are recombined in various orders.
  • Mature mRNA is then translated into a unique protein.

RNA Polymerase

  • The enzyme that carries out transcription by binding to the DNA helix.
  • It adds: Guanine to the messenger RNA if there is a cytosine in the DNA.
  • It adds: Uracil, not thymine, if there is an adenine in the DNA.
  • After transcription, RNA polymerase rewinds the helix, leaving the DNA as it was.
  • Alternative splicing allows for different combinations of exons, meaning one gene can code for more than one type of protein.
  • Humans function with only around 25,000 genes but make far more proteins because of alternative splicing.
  • Messenger RNA can exit through the nuclear pores and move to the cytoplasm.

Translation Process

  • The mRNA strand is read by ribosomal RNA molecules (ribosomes) in the cytoplasm.
  • Ribosomes assemble amino acids into a final protein.
  • Transfer RNA (tRNA) delivers the appropriate amino acid to the complex.
  • The tRNA has an anticodon that must match the codon on the mRNA for the amino acid to be added to the growing protein chain.

Ribosome Structure

  • Consists of a small and large subunit.
  • The small subunit binds to mRNA and tRNA.
  • The large subunit pulls all the pieces along one codon at a time and forms the peptide bond to join the protein with the newly delivered amino acid.
  • Base triplets (DNA) are transcribed into codons (mRNA).
  • If the tRNA anticodon matches the mRNA codon, the amino acid is added to the peptide chain, forming a complete amino acid sequence.
  • Transcription occurs in the nucleus.
  • Translation mostly happens in the cytoplasm.

Cell Growth and Division

  • Cells must copy their DNA before replicating, following the law of complementary base pairing.
  • Enzyme: DNA polymerase makes a copy of the cell's DNA.
  • The DNA helix unwinds, and DNA polymerase incorporates consecutive base pairs based on the original strand.

Cell Cycle

  • Two major phases: interphase and mitotic (M) phase.
    • Interphase (G1, S, G2):
      • G1 (First Gap Phase): Cell accumulates materials needed to copy DNA and performs normal anatomical functions.
      • S (Synthesis Phase): Cell makes a copy of its DNA using DNA polymerase enzymes.
      • G2 (Second Gap Phase): Cell produces centrioles and bulks up, preparing for division.
    • M Phase (Mitotic Phase):
      • Division of DNA and cell cytoplasm.
      • Subcategories: prophase, metaphase, anaphase, telophase (mitosis), overlapping with cytokinesis (division of cytoplasm).
  • Cells may enter G0, leaving the cycle, and either die or remain in senescence.
  • Cycle duration varies between cell types.
  • Mitosis is used for embryonic development, tissue growth, and repair of old or worn-out tissues.

DNA Replication

  • Proceeds through S phase.
  • Cell division splits the DNA into two identical daughter cells.

Mitosis

  • Prophase, metaphase, anaphase, and telophase (PMAT).
  • DNA division.

Prophase

  • The nuclear envelope breaks down, and centrioles develop.

Metaphase

  • All DNA lines up in the center of the cell and attaches to centrioles.

Anaphase

  • Physical separation of DNA.
  • Half is pulled to one pole, and the other half to the opposite pole.

Telophase

  • Reestablishment of the nucleus.
  • DNA is confined to the nuclear envelope, and cells begin to split.
  • DNA reaches its highest level of condensed organization, coiling to form chromosomes.
  • A chromosome is an X-shaped structure consisting of two sister chromatids.

Metaphase

  • Chromatids line up in the center (metaphase equator) attached to spindle fibers from centrioles.

Anaphase

  • Spindle fibers pull sister chromatids apart, with each half moving toward opposite poles.

Telophase

  • The nuclear envelope develops, nucleoli form, and DNA relaxes.
  • Spindle fibers break down with overlapping cytokinesis.

Cytokinesis

  • Division of the cytoplasm into two cells.

M Phase

  • Cells should only divide when they have enough nutrients and cytoplasm, after making a copy of their DNA, in the presence of growth factors, and when there is a vacancy.
  • Division should stop when cells are starved, lack growth factors, or experience contact inhibition.

Summary of Cell Cycle Stages

  • G1: The cell performs its normal function and begins bulking up enzymes for DNA replication.
  • S Phase: The cell actively replicates its DNA.
  • G2: Further preparation with the production of more cytoplasm and enzymes for replication.
  • Most cells are in interphase.
  • Mitosis only occurs when all conditions are met.

Cytokinesis

  • A process of cell division and cytoplasm separation overlapping with the latter part of DNA division.
  • Results in two cells with the exact same DNA, regenerating tissues in a normal and healthy manner.