AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

CELL

• Cell is the basic unit of life.
• Animal cells are typical of the eukaryotic cell, enclosed by a plasma membrane and present in different shapes and containing a membrane-bound nucleus and cell organelles.
• Cell contents are called the protoplasm.
• Genes reside in cell nucleus.
• It is necessary, therefore, to study a cell and its role in transmitting genes from generation to generation.
• Most cells, both animals and plants, range in size between 1 and 100 micrometers and are thus visible only with the aid of a microscope.
• Discovery of living cells would have been difficult, if not impossible, before the compound microscope was invented by Zacharias Jansen of Holland in 1590.
• In 1665, Robert Hooke of England discovered “Cell” and applied the term “Cell” to the cavities he saw in sections of cork.
• In 1675, Marcello Malpighi published an "Anatomy of Plants", the first systematic study of cell structure.
• In 1839 Theodor Schwann, an animal anatomist, formulated the Cell Theory which set forth the concept that “the elementary parts of all tissues are formed of cells through much diversified in manner”.
• Animal cell is distinct from other eukaryotes especially plant cells by lack of cell walls and chloroplasts, and have smaller vacuoles.
• The lack of a rigid cell wall allowed animals to develop a greater diversity of cell types, tissues, and organs.
• Most animal tissues are bound together in an extracellular matrix by a triple helix of protein known as collagen.
• Plant and fungal cells are bound together by molecules like pectin.
• In animals, there are many different cell types and have a variety of internal membranes and structures.
• An organelle is a specialized sub-unit within a cell that has a specific function, and is separately enclosed within its own lipid membrane. 

 To view "Animal Cell Animation"

Cell Membrane

• The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer") is a double layer of phospholipids. It physically separates the intracellular components from the extra cellular environment and serves as a protection layer for the interior of the cell.
• The cell membrane is selectively permeable, allowing some substances to pass into the cell and blocking others.

Cytoplasm

• This is a collective term for the cytosol plus the organelles suspended within the cytosol.
• The cytoplasm has three major elements as follows,
  • The cytosol
  • Organelles and
  • Inclusions

Cytosol

• The cytosol is the gel like fluid within which all the other cell organelles reside. It is the internal fluid of the cell.
• In cytosol, a portion of cell metabolism occurs. Proteins in cytosol control cell metabolism including signal transduction pathways, glycolysis, intracellular receptors and transcription factors.

Centrosome/Microtubule Organizing Center (MTOC)

• Centrosome also called the "microtubule organizing center (MTOC)".
• It is a small body located near the nucleus where microtubules are produced. It is a regulator of cell-cycle progression. The animal cell centrosome is actually a pair of small organelles (one is perpendicular to the other) called centrioles but the plant cell centrosome is simpler and does not have centrioles.
• The centrosome is duplicated during cell division resulting in two centrosomes, each with its own pair of centrioles.
• The two centrosomes move to opposite sides of the nucleus of the dividing cell and from each centrosome, microtubules grow into a "spindle" which is responsible for separating replicated chromosomes into two daughter cells. Thus each daughter cell inherits one centrosome.

Centriole

• Centrioles are barrel shaped self-replicating organelles found in most animal eukaryotic cells, though absent in higher plants and fungi.
• Centriole is made up of ring of nine bundles of microtubules and each bundle is composed of three microtubules (protein of the cytoskeleton).
• Centrioles are involved in the organization of the mitotic spindle and in the completion of cytokinesis during cell division, but aren't essential to the process.

Golgi Apparatus

• Golgi apparatus also called the Golgi body, Golgi complex, or dictyosome.
• It is an organelle composed of membrane-bound stacks known as cisternae.
• The Golgi body processes and packages the macromolecules such as proteins and carbohydrates synthesized by the cell into membrane - bound vesicles for cell secretion (exocytosis) or for use within the cell.

Smooth Endoplasmic Reticulum

• Endoplasmic reticulum (ER) is a vast system of interconnected, membranous, infolded and convoluted tubes that are located in the cell's cytoplasm.
• The ER is a continuation of the outer nuclear membrane and provides a pipeline between the nucleus and the cytoplasm. The space within the ER is called the ER lumen which acts as a transport system for the cell.
• The functions of the Endoplasmic Reticulum vary greatly depending on the exact type of endoplasmic reticulum and the type of cell in which it resides.
• The smooth endoplasmic reticulum is so named because it appears smooth by electron microscopy. It is involved in the synthesis of lipids and membrane proteins.
• The SER also divides the cytoplasm of the cell into compartments.

Rough Endoplasmic Reticulum

• The surface of the rough endoplasmic reticulum is studded with protein-manufacturing ribosomes giving it a "rough" appearance.
• Proteins synthesized on these ribosomes are collected in the endoplasmic reticulum for transport throughout the cell.
• The function of the rough endoplasmic reticulum is to synthesize and export proteins and glycoproteins.

Ribosomes

• Ribosomes are packets of RNA and protein that are sites for the assembly of proteins in a process called Translation.
• Ribosomes are classified as being either "free" (anywhere in the cytoplasm) or "membrane-bound" (endoplasmic reticulum).

Mitochondria

• Mitochondria also called as "cellular power plants".
• Are spherical to rod-shaped organelles with a double membrane.
• The inner membrane is infolded many times, forming a series of projections called as cristae.
• The space bounded by the inner membrane is called matrix.
• The most prominent role of the mitochondrion is production of energy stored in glucose by conversion into ATP (adenosine triphosphate) - the primary energy source for the cell.

Lysosome

• Lysosome is nicknamed as “cell vesicles” or "suicide-bags" or "suicide - sacs".
• Lysosomes are organelles that contain digestive enzymes necessary for intracellular digestion.
• They are common in animal cells, but rare in plant cells.
• They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria, which are transferred to the cytoplasm as new cell-building materials.

Peroxisome

• Peroxisomes sometimes called microbodies are membrane-bound packets of oxidative enzymes.
• Peroxisomes break down organic molecules by the process of oxidation to produce hydrogen peroxide and then quickly into water and oxygen.
• They are called peroxisomes because they all produce hydrogen peroxide.
• A major function of the peroxisome is the breakdown of fatty acid molecules.

Secretary vesicle

• It is a membrane bounded vesicle derived from the golgi apparatus and containing cell secretions - e.g. hormones, neurotransmitters that are to be released from the cell.
• The contents may be densely packed, and then transported to the cell surface. The vesicle fuses with the cell membrane at a structure called the Porosome, in a process called Exocytosis, dump its contents out of the cell's environment.

Vacuoles

• Vacuoles are found in the cytoplasm of most plant cells and some animal cells.
• It is a fluid-filled, membrane-surrounded cavity inside a cell.
• The vacuole fills with food being digested and waste material that is on its way out of the cell.

Cytoskeleton (CSK)

• Cytoskeleton is a network of protein filaments and motor proteins in the cytoplasm that give shape to a cell, hold and move organelles, and typically involved in cell movement.
• The cytoskeleton maintains the cell shape.
• The cytoskeleton consists of three types of proteins,
  • Microtubules,
  • Intermediate filaments, and
  • Microfilaments

Nucleus

• The nucleus is the largest cellular organelle that includes the nucleolus.It is enclosed by a nuclear envelope, a double membrane and communicates with the surrounding cytosol via numerous nuclear pores.
• Besides the nucleolus, the nucleus contains a number of other non-membrane delineated bodies like Cajal bodies, Gemini of coiled bodies, polymorphic interphase karyosomal association (PIKA), promyelocytic leukaemia (PML) bodies, paraspeckles and splicing speckles.
• The viscous liquid within the nucleus is called nucleoplasm.
• Nucleus contains most of the cell's genetic material, double helix DNA molecules held in complex with a large variety of proteins, such as histones, to form chromosomes.
• Similar DNA is present in every cell of the body, but depending on the specific cell type, some genes within chromosomes may be turned on or off - that's why a fat cell is different from a liver cell.
• The main function of the nucleus is the coordination of the cell's activities, which include growth, intermediary metabolism, protein synthesis, and reproduction (cell division).

Nuclear Membrane

• Nuclear envelope known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca.
• It is a double membrane surrounds the nucleus and separates the contents of the nucleus (DNA in particular) from the cytosol (cytoplasm).
• The outer membrane is continuous with the rough endoplasmic reticulum.
• The space between the two membranes that make up the nuclear envelope is called the perinuclear space (also called the perinuclear cisterna).

Nuclear Pores

• They are formed at sites where the inner and outer membranes of the nuclear envelope are joined.

Nucleolus

• The nucleolus is a membrane less organelle found in the nucleus, and is sometimes called a sub organelle.
• The main function of the nucleolus is the biogenesis and assembly of ribosome components.
• Some cells have more than one nucleolus, but some cell types do not have any.