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.