MCATTBiology

• CELL THEORY All living things are composed of cells. Cells are the basic functional unit of life. All cells originated from a pre-existing cell. Cells carry genetic information in the form of deoxyribosonucleic acid (DNA); passed from parent to daughter.
• The nucleus contains all the genetic material necessary for replication of the cell. Surrounded by nuclear membrane or envelope. Nuclear pores allow for selective two-way exchange of material between cytoplasm and the nucleolus; pre-mRNA, ribosomal subunits, transcription factors. The nucleolus: synthesizes the ribosomal RNA (rRNA).
• DNA STRUCTURE Deoxyribonucleic acid (DNA) is a macromolecule that stores genetic information in all living organisms. Nucleosides contain a five-carbon sugar bonded to a nitrogenous base; nucleotides are nucleosides with one to three phosphate groups added. Nucleotides are linked by phosphodiester bonds between the sugar base (3' end; hydroxyl group) of one nucleotide and the phosphate group (5' end) of the adjacent nucleotide. DNA contain deoxyribose; in RNA, they contain ribose. Nucleotides are: adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U).
• Mitochondria, the cell's power plants, contain two layers: the outer and inner membranes, with the inner membrane arranged into cristae to increase surface area for electron transport chain enzymes. Protons pumped from the mitochondrial matrix to the intermembrane space create a proton-motive force, driving ATP synthesis (ATP synthase) during oxidative phosphorylation. Semi-autonomous: contain their own genes and replicate independently via binary fission. Trigger apoptosis: release of enzymes from the ETC.
• The Serial Endosymboisis Theory suggests that membrane-bound organelles were formed by the engulfing of a prokaryote by another creating a symbionic relationship. E.g., mitochondria (aerobic by anaerobic), cloroplasts and organelles of motility (like flagella)
• Lysosomes, containing hydrolytic enzymes, break down various substrates including ingested substances and cellular waste. They collaborate with endosomes, which transport materials within the cell, directing them to the Golgi apparatus, the cell membrane, or the lysosomes for degradation. While lysosomal membranes protect against enzyme damage, their release in autolysis can trigger apoptosis, leading to the direct degradation of cellular components.
• The endoplasmic reticulum (ER) consists of interconnected membranes contiguous with the nuclear envelope, folded into complex structures with a central lumen. Protein folding takes place in the ER, typically coinciding with translation by bound ribosomes in the RER. Rough ER (RER): studded with ribosomes for protein synthesis; synthesis of transmembrane and extra-cellular proteins. Smooth ER (SER): lacks ribosomes, it's involved in lipid synthesis (e.g., cholesterol), (drug metabolism and) detoxification and facilitates the transportation of proteins from the RER to the Golgi apparatus.
• The Golgi apparatus consists of stacked membrane-bound sacs and receives materials from the ER via vesicles. Within the Golgi apparatus, products undergo modification, addition of carbohydrates, phosphates, and sulfates, & the introduction of signal sequences for specific cellular destinations. After modification and sorting, products are repackaged into vesicles and directed to their appropriate locations; secretory vesicles merge with the cell membrane for exocytosis. (Post-translational modifications, carbohydrate & lysosome synthesis, protein trafficking).
• Peroxisomes contain hydrogen peroxide and play a crucial role in breaking down very long chain fatty acids through β-oxidation. They participate in phospholipid synthesis and harbor enzymes involved in the pentose phosphate pathway.
• The cytoskeleton serves to provide structural support to the cell and maintain its shape, while also facilitating the transport of materials within the cell. It comprises three main components: microfilaments, microtubules, and intermediate filaments. These components work together to support cellular structure and enable intracellular transportation. Axoneme: the cytoskeletal core that makes up whip-like appendages in eukaryotic cells.
• Microfilaments are polymerized actin rods that form bundles and networks to provide structural support, resist compression and fracture; microvilli. They facilitate cell movement by interacting with myosin and ATP to generate force (sarcomere; muscle contraction). In cytokinesis, they form the cleavage furrow (contractile ring) during mitosis, organizing into a ring structure that contracts to separate daughter cells.
• Microtubules, hollow polymers of tubulin proteins, form pathways throughout the cell for motor proteins like kinesin and dynein to transport vesicles. They compose motile structures like cilia and flagella, with cilia aiding in material movement along cell surfaces and flagella facilitating cell movement, such as sperm motility. Cilia and flagella share a 9+2 microtubule structure, found exclusively in eukaryotic organelles of motility. Axonome: cytoskeletal core that makes up whip-like appendages in eukaryotic cells.
• Intermediate filaments constitute a diverse group of filamentous proteins like keratin, desmin, vimentin, and lamins, contributing to cell-cell adhesion and cytoskeletal integrity. They provide structural rigidity to the cell and anchor organelles such as the nucleus, with their composition varying according to cell and tissue type. These filaments withstand significant tension, enhancing the overall stability of the cell.
• Epithelial tissues serve as protective barriers covering the body and lining its cavities, while also facilitating absorption, secretion, and sensation in certain organs. These cells are tightly joined to each other and to the basement membrane, forming cohesive units. Epithelial cells (in most organs) constitute the parenchyma (functional parts of the organ). They display polarity, with one side facing a lumen or the outside world, and the other side interacting with underlying structures such as blood vessels. They are classified based on the number of cell layers and cell shape.
• Connective tissue serves to support the body and provide a framework for the functions of epithelial cells. Unlike epithelial cells (contribute to parenchyma), connective tissues mainly form the stroma or support structure of organs. Various types of connective tissues such as bone, cartilage, tendons, ligaments, adipose tissue, and blood produce and secrete materials like collagen and elastin to build the extracellular matrix.
• Archaea are single-celled organisms resembling bacteria but possessing genes and metabolic pathways more akin to eukaryotes. They are present in diverse habitats (human body), extremophiles, etc. Many are chemosynthetic, utilizing inorganic compounds (sulfur and ammonia). They start translation with methionine and possess similar RNA polymerases and histone-associated DNA=> Eukaryote. They posses a single circular chromosome, binary fission or budding for reproduction, and resistance to antibiotics => Bacteria.
• All bacteria possess a cell membrane and cytoplasm, with some having flagella or fimbriae akin to cilia. Some bacteria are beneficial symbionts, others are pathogens causing disease. Pathogenic bacteria may live intracellularly or extracellularly. Cocci: spherical bacteria; Streptococccus pyogenes. Bacilli: rod-shaped bacteria; Escherichia coli. Spirilli: spiral-shaped bacteria-most rare, Treponema pallidum (syphilis)
• Obligate Aerobes: bacteria that require oxygen for metabolism. Obligate Anaerobes: bacteria can only survive in no oxygen environments. Facultative Anaerobes: bacteria that can switch its metabolic processes with oxygen or without oxygen (if not present). Aerotolerant Anaerobes: bacteria that can't use oxygen but is not harmed by its presence.
• Prokaryotes carry out the electron transport chain using the cell membrane. There are two types of cell wall: gram positive and gram negative: Gram Positive (violet stain): thick layer of peptidoglycan, a polymeric substance made from amino acids and sugars. It contains lipoteichoic acid. Gram Negative (pink-red stain): very thin cell walls, they contain peptidoglycan (small amounts).They have outer membranes containing phospholipids and lipopolysaccharides.
• Flagella generate propulsion towards food or away from immune cells. Moving in response to chemical stimuli is called chemotaxis. Eukaryotic: composed of microtubules. Prokaryotic: composed of a long protein called flagellin.
• GENETICS OF PROKARYOTIC CELLS Prokaryotic possess single circular chromosomes. In addition, extrachromosomal material can be carried in plasmids. Plasmids may contain antibiotic resistance genes or virulence factors. Plasmids that can integrate into the genome are called episomes.
• PROKARYOTIC REPRODUCTION Bacterial genetic recombination increases bacterial diversity. Transposons are genetic elements that can insert into or remove themselves from the genome.
• BACTERIAL GROWTH Prokaryotes multiply through binary fission, in which the chromosome replicates while the cell grows in size, until the cell wall begins to grow inward along the midline of the cell and divides it into two identical daughter cells. Bacterial growth follows a predictable pattern: The bacteria adapt to new local conditions during the lag phase. Growth then increases exponentially during the exponential (log) phase. As resources are reduced, growth levels off during the stationary phase. As resources are depleted, bacteria undergo a death phase.
• Viruses contain genetic material, a protein coat (capsid), and sometimes a lipid-containing envelope. Viruses are obligate intracellular parasites; they cannot survive and replicate outside of a host cell. Individual virus particles are called virions.
• Bacteriophages are viruses that target bacteria. They contain a tail sheath, which injects the genetic material into a bacterium, and tail fibers, which allow the bacteriophage to attach to the host cell.
• Viral genomes may be made of various nucleic acids: They may be composed of DNA or RNA and may be single- or double-stranded. Single-stranded RNA viruses may be positive sense (can be translated by the host cell) or negative sense (requires a complementary strand to be synthesized by RNA replicase before translation).Retroviruses contain a single-stranded RNA genome, from which a complementary DNA strand is made using reverse transcriptase. The DNA strand can then be integrated into the genome (e.g., HIV).
• VIRAL LIFE CYCLE Infection; attachment and penetration: Viruses infect cells by attaching to specific receptors, and can then enter the cell by fusing with the plasma membrane, being brought in by endocytosis, or injecting their genome into the cell. Translation and Progeny Assembly: phage DNA replicates using the hosts nucleus to be transcribed into mRNA and phage proteins are made. Maturation and Lysis: new phage particles are assembled, then the cell lyses- releasing the newly made phages.
• Prions are infectious proteins that trigger misfolding of other proteins, usually converting an α-helical structure to a β-pleated sheet. This decreases the solubility of the protein and increases its resistance to degradation. Viroids are plant pathogens that are small circles of complementary RNA that can turn off genes, resulting in metabolic and structural changes and, potentially, cell death.
• Mitosis produces two genetically identical diploid daughter cells; somatic cells. PMAT=> Prophase: chromosomes condense, nuclear membrane dissolves, nucleoli disappear, centrioles migrate to opposite sides, and the spindle apparatus (microtubules) form. The kinetochore is contacted by a spindle fiber (centrioles). Metaphase: chromosomes line up along the metaphase plate. Anaphase: sister chromatids are separated and pulled apart. Telophase: nuclear membrane reforms, spindle apparatus disappears, and cytosol and organelles are split between the two daughter cells (cytokinesis).
• Meiosis occurs in gametocytes; produces up to four nonidentical haploid sex cells (gametes). One round of replication and two rounds of division. Meiosis I => Homologous chromosomes are separated from each other. Homologues: chromosomes that are given the same number, but are of opposite parental origin. Prophase I: similar to mitosis, but homologues intertwine in a process called synapsis. Tetrad, four chromatids, go under crossing over; exchange of genetic material. Metaphase I, Anaphase I, Telophase I and Meiosis II, is similar to mitosis.
• The male reproductive system contains internal and external structures. Sperm develop in the seminiferous tubules in the testes; nourished by Sertoli cells. Interstitial cells of Leydig, in the testes, secrete testosterone and other male sex hormones (androgens). Sperm gain motility in the epididymis and are stored there until ejaculation.
• The female reproductive system primarily contains internal structures. Ova (eggs) are produced in follicles in the ovaries. Once each month, an egg is ovulated into the peritoneal sac and is drawn into the fallopian tube or oviduct. The fallopian tubes are connected to the uterus; the lower end which is the cervix. The vaginal canal lies below the cervix; where sperm are deposited during intercourse and the site of childbirth. The external parts of the female genital organs are collectively known as the vulva.
• SEXUAL DEVELOPMENT Gonadotropin-releasing hormone (GnRH) from the hypothalamus causes the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), the functions of which depend on the sex of the individual. In Male Sexual Development, FSH stimulates the Sertoli cells and triggers spermatogenesis, while LH causes the interstitial cells to produce testosterone; responsible for the maintenance and development of the male reproductive system and male secondary sex characteristics. In Female Sexual Development, FSH stimulates development of the ovarian follicles, while LH causes ...
• Menopause occurs when the ovaries stop producing estrogen and progesterone, usually between ages 45 and 55. Menstruation stops and FSH and LH levels rise. Physical and physiological changes accompanying menopause include flushing, hot flashes, bloating, headaches, and irritability.
• Fertilization is the joining of a sperm and an ovum. It usually occurs in the ampulla of the fallopian tube. The sperm uses acrosomal enzymes to penetrate the corona radiata and zona pellucida. Once it contacts the oocyte’s plasma membrane, the sperm establishes the acrosomal apparatus and injects its pronucleus. When the first sperm penetrates it causes a release of calcium ions, which prevents additional sperm from fertilizing the egg and increases the metabolic rate of the resulting diploid zygote. This is called the cortical reaction.
• Fraternal (dizygotic) twins result from the fertilization of two eggs by two different sperm. Identical (monozygotic) twins result from the splitting of a zygote in two. Monozygotic twins can be classified by the placental structures; mono- vs. diamniotic, mono- vs. dichorionic.
• Cleavage refers to the early divisions of cells in the embryo. These mitotic divisions result in a larger number of smaller cells, as the overall volume does not change. The zygote becomes an embryo after the first cleavage because it is no longer unicellular. Indeterminate cleavage results in cells that are capable of becoming any cell in the organism, while determinate cleavage results in cells that are committed to differentiating into a specific cell type.
• The early stages of embryonic development are: Fertilization Morula Blastula Early Gastrula Gastrula
• The morula is a solid mass of cells seen in early development.
• Teratogens are substances that interfere with development, causing defects or even death of the developing embryo. Teratogens include alcohol, certain prescription drugs, viruses, bacteria, and environmental chemicals. Maternal conditions can affect development, including diabetes (increased fetal size and hypoglycemia after birth) and folic acid deficiency (neural tube defects).