Chapter 2. Cell Biology PDF

# Cytoskeleton It is a network of microtubules, thin filaments and intermediate filaments. ## 1- Microtubules: They are tubular structure with variable length and 25 nm in diameter. ### Function: 1. They support the cell and maintain its shape. 2. They help the movement of organelles inside th...

# Cytoskeleton It is a network of microtubules, thin filaments and intermediate filaments. ## 1- Microtubules: They are tubular structure with variable length and 25 nm in diameter. ### Function: 1. They support the cell and maintain its shape. 2. They help the movement of organelles inside the cell. 3. They form the mitotic spindle during cell division. 4. Microtubules provide the basis for several components as centrioles, cilia and flagella. ## 2- Thin filaments (Microfilaments): They are about 7 nm in diameter. They are formed of two identical strands coiled around one another to form filament called actin. ### Function: 1. In muscle cells, sliding of actin over myosin filaments leads to muscle contraction. 2. Help in movement of cytoplasmic components. 3. Formation of cleavage furrow during cell division. ## 3- Intermediate filaments: Their diameter is 10 nm. ### Function: 1. Play a role in contraction of smooth muscles. 2. Support the cells and maintain their shape. ## Centrioles and Centrosome Most cells contain two centrioles that are surrounded by a pale area which called **centrosome**. Each centriole is cylindrical structure and composed of nine sets of triplet microtubules. The two centrioles are arranged perpendicular to each other. Centrosome contains also dense bodies called centriolar satellites. ### Function: 1. Centriolar satellites are the sites where microtubules originate. 2. They are associated with microtubule organizing center and formation of mitotic spindle. 3. Formation of basal bodies of cilia and flagella. ## Cilia and Flagella Cilia are hair like motile processes that extend from the free surface of ciliated cells. Both cilia and flagella have the same core organization. The core contains a complex of regularly arranged microtubules called **axoneme**. The axoneme is formed of nine pairs of microtubules called **doublets** and two central microtubules called **singlet**. At the base of each cilium there is a basal body which has a structure similar to a centriole. ## Stereocilia They are long microvilli containing microfilaments. They are not true cilia. # Inclusions Inclusions are nonliving components of cells and have no metabolic activity. They are stored food, by-products of metabolism and pigments. ## 1- Stored food: 1. **Glycogen:** It is the storage form of glucose and is mainly present in liver cells and muscles. It can be stained by PAS reaction. 2. **Lipids:** Are stored mainly in fat cells. In histological sections lipid appears as round clear vacuoles because it dissolves by solvents. It can be stained by Sudan black or Sudan III. ## 2- Pigments: 1. **Exogenous Pigments** * **Carotene:** it is the orange pigment in carrot. * **Dust and carbon particles from tobacco can blacken the lungs.** 2. **Endogenous pigments** * **Hemoglobin:** it is the most abundant pigment in the body. Its is red in color and present in red blood cells (RBCs). * **Lipofuscin:** It is the indigestible remnants of lysosomal activity. It is present in long-lived cells. * **Crystals:** present in some cells and are the crystalline forms of certain proteins. # Nucleus The nucleus is the largest component of the cell. It is usually spherical but may be oval, flattened, twisted, disc-shaped, kidney-shaped or lobulated. It is usually central but may be peripheral or basal. Usually, it occupies the widest part of the cell. Usually, each cell has a single nucleus but some cells have two nuclei (binucleated) as liver cells, some cells have more than two nuclei (multinucleated) as skeletal muscle cells. Two types of cells have no nuclei: RBCs and blood platelets. The nucleus is basophilic and may be lightly stained (vesicular) as in nerve cells or deeply stained as in lymphocytes. ## Structure: The nucleus is formed of: 1. **Nuclear envelope:** It surrounds the nucleus. It is not seen by LM, but by EM it appears as two parallel membranes separated by space celled perinuclear cisterna. The nuclear envelope is perforated at intervals by nuclear pores that permit communication between the nucleus and cytoplasm. 2. **Chromatin:** The genetic material of the cell (DNA) is present in the nucleus in the form of chromosomes that are seen during cell division as thread-like structures. Metaphase chromosome is formed of two sister chromatids. In interphase nucleus, the chromosomes are present in the form of chromatin. There are two types of chromatin: heterochromatin and euchromatin. 1. **Heterochromatin:** It is a condensed inactive form of chromatin that stains deeply. According to its position in the nucleus it may be peripheral chromatin (associated with nuclear envelope), nucleolus – associated chromatin (surrounding the nucleolus) or chromatin islands (scattered in nuclear matrix). 2. **Euchromatin:** It is scattered in the nucleus and represents the active form of chromatin in which the genes are functioning. The nucleus having much euchromatin appear pale and are more active in protein synthesis than those with move heterochromatin. 3. **Nucleolus:** It is a spherical structure with no limiting membrane. It stains basophilic because it is rich in rRNA and its nucleolus - associated chromatin. There are one or two nucleoli in each nucleus. Nucleolus is formed of: * Pars granulose. * Pars fibrosa. * Nucleolar organizer regions. 4. **Nuclear matrix:** It is the component that fills the spaces between the chromatin and the nucleolus and the nuclear envelope. # Nucleic Acids There are two types of nuclei acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). ## DNA: Each DNA molecule consists of two long polynucleotide chains wound together into a double helix. Each nucleotide consists of phosphate group, pentose sugar (deoxyribose) and nitrogenous base. The bases are adenine (A), guanine (G), cytosine (C) and thymine (T). ## RNA: It is similar to DNA but differs in 3 ways: 1. It is a single strand. 2. The sugar is ribose. 3. The base uracil (U) is present instead of thymine (T). There are three types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA). # Function of the nucleus: 1. It directs the cell division. 2. It carries the genetic information and the hereditary characters of the living organisms. 3. It directs the processes of protein synthesis and other activities of the cell. 4. Formation of RNA. # Cell Division There are two types of cell division: **mitosis** which occurs in somatic cells, and **meiosis** that occurs in ovary and testis to produce germ cell (ova and spermatozoa).