Cell Structure and Organelles

• The cell is the basic structural and functional unit of all known living organisms and contains a variety of structures and organelles.

Cell Structure

• Major cell components include the cell membrane, cytoplasm, and various organelles.

Cell Membrane

• A thin, semi-permeable layer of protein and fats surrounds the cell.
• Roles: protection, selective permeability/transport, communication, and cellular recognition.

Cytoplasm

• The jelly-like material contains all cell organelles enclosed within the cell membrane.
• Roles: site for biochemical reactions (e.g., glycolysis), contains cellular organelles and the cytoskeleton, facilitates intracellular transport, acts as a storage site (proteins, lipids, ions), and is essential in cell division.

Mitochondrion

• Spherical or rod-shaped organelles with a double membrane act as the powerhouse of the cell, playing a role in energy production.
• Reactions convert energy stored in nutrient molecules, such as glucose, into adenosine triphosphate (ATP).
• Oxygen molecules are required but not always necessary during cellular respiration.
• The Krebs cycle (citric acid cycle), fatty acid oxidation, and the electron transport chain (ETC) leading to oxidative phosphorylation (OXPHOS) are processes that lead to energy production in the mitochondria.

Nucleus

• The largest and most prominent of the cell's organelles serves as the control center by storing all of the genetic instructions for manufacturing proteins.
• Surrounded by a membrane called the nuclear envelope, the nucleus contains a gel-like nucleoplasm with solutes that include the building blocks of nucleic acids.
• The nucleolus is a region of the nucleus responsible for manufacturing the RNA necessary for protein construction.
• Genetic instructions are arranged in an orderly manner in strands of DNA, forming threads called chromatin.

Endoplasmic Reticulum (ER)

• A system of channels continuous with the nuclear membrane.
• Two types of ER exist: Smooth ER and Rough ER.
• Smooth ER is involved in lipid synthesis. It synthesizes phospholipids, the main component of biological membranes, as well as steroid hormones, and regulates the concentration of cellular calcium ions (Ca++).
• Rough ER contains ribosomes and acts as the site for protein synthesis from mRNA.

Golgi Apparatus

• Responsible for sorting, modifying, and shipping products that come from the rough ER, looks like stacked flattened discs, and has two distinct sides.
• One side of the apparatus receives products in vesicles; these products are sorted and then released from the opposite side after being repackaged into new vesicles.

Lysosomes

• Organelles contain enzymes that break down and digest unneeded cellular components, such as damaged organelles.
• Important for breaking down foreign material, when certain immune defense cells play a role in apoptosis (programmed cell death).

Peroxisomes

• Play a role in lipid metabolism and detoxification and clear reactive oxygen species through the action of stored enzymes.

Cytoskeleton

• Helps cells maintain its structural integrity.
• Critical for cell motility, cell reproduction, and transportation of substances within the cell.
• Made up of 3 different types of fibres.
  • Microfilaments: thinnest and important in muscle contraction
  • Intermediate filaments: maintain cell structure and resist pull forces
  • Microtubules: thickest; maintain cell shape and structure, help resist compression of the cell, play a role in positioning organelles within the cell, make up cilia and flagella, and set the paths along which genetic material can be pulled during cell division

The Cell Cycle

• An ordered series of events involving cell growth and division produces two new daughter cells and has four basic steps.

DNA (Deoxyribonucleic Acid) Replication

• Cells replicate themselves by dividing to produce two new daughter cells, each with the full complement of DNA as found in the original cell.
• A DNA molecule is made of two strands that "complement", fit together, and bind to each other, creating a double-stranded molecule.
• Composition: 5-carbon sugar, nitrogenous base (Adenine, Cytosine, Thymine/Uracil, Guanine), and phosphate group.
• DNA replication is the copying of DNA that occurs before cell division can take place and has 3 stages.
  • INITIATION: The two complementary strands are separated. Special enzymes, including helicase, untwist and separate the two strands of DNA.
  • ELONGATION: Each strand becomes a template along which a new complementary strand is built. DNA polymerase brings in the correct bases to complement the template strand, synthesizing a new strand base by base.
  • TERMINATION: Once the two original strands are bound to their separate, finished, complementary strands, DNA replication is stopped, and the two new identical DNA molecules are complete.
• Mistakes during DNA replication (MUTATIONS), such as adding an inappropriate nucleotide, can render a gene dysfunctional or useless.
• A DNA proofreading process enlists special enzymes to scan the newly synthesized molecule for mistakes and corrects them.
• Only after correction can division progress; otherwise, the cell can undergo apoptosis or enter G0.

Ribonucleic Acid (RNA)

• Types: Messenger RNA (mRNA), Ribosomal RNA (rRNA), and Transfer RNA (tRNA).
  • Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes for protein synthesis and serves as a template for translation.
  • Ribosomal RNA (rRNA): A structural component of ribosomes that helps in protein synthesis and facilitates the binding of mRNA and tRNA during translation.
  • Transfer RNA (tRNA): Delivers specific amino acids to ribosomes during protein synthesis.

The Cell Cycle Process

• Includes G1 (Gap Phase 1), S Phase (Synthesis Phase), G2 (Gap Phase 2), and the Mitotic Phase

G1 (Gap Phase 1)

• The cell grows and carries out all normal metabolic functions and processes.
• Accumulation of building blocks of chromosomal DNA and associated proteins, along with sufficient energy reserves to replicate each chromosome in the nucleus. Little change is visible under a microscope.

S Phase (Synthesis Phase)

• The period when a cell replicates its DNA.
• Results in the formation of identical pairs of DNA molecules, known as sister chromatids, that are firmly attached to the centromeric region.
• The centrosome is duplicated, giving rise to the mitotic spindle, which orchestrates the movement of chromosomes during mitosis.

G2 (Gap Phase 2)

• The cell continues to grow and makes the necessary preparations for mitosis.
• Cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase.

Mitotic Phase

• A multistep process during which the duplicated chromosomes are aligned, separated, and moved into two new, identical daughter cells; and has 6 steps.
  • PROPHASE: The first phase of mitosis, where loosely packed chromatin coils and condenses into visible chromosomes. The nucleolus disappears, the nuclear envelope disintegrates, and centrosomes move to opposite poles while microtubules extend. Sister chromatids coil tightly with the aid of condensin proteins and become visible under a light microscope.
  • PROMETAPHASE: Chromosomes continue to condense, kinetochores appear at the centromeres, and mitotic spindle microtubules attach to kinetochores.
  • METAPHASE: Chromosomes are lined up at the metaphase plate, and each sister chromatid is attached to a spindle fiber originating from opposite poles.
  • ANAPHASE: Centromeres split in two, and sister chromatids (now chromosomes) are pulled toward opposite poles while certain spindle fibers begin to elongate the cell.
  • TELOPHASE: Chromosomes arrive at opposite poles and begin to decondense; the nuclear envelope surrounds each set of chromosomes, the mitotic spindle breaks down, and spindle fibers continue to push poles apart.
  • CYTOKINESIS: A cleavage furrow separates the daughter cells