Chapter 3 BOOK NOTES PART 4

Role of Centrioles in Cilia and Flagella Formation

• Centrioles play a crucial role in the formation of cilia and flagella by serving as the base or anchor for these cellular extensions.

• Before cilia formation, centrioles multiply and align beneath the cell's plasma membrane at the exposed surface.

• Microtubules then extend from each centriole, exerting pressure on the plasma membrane to form ciliary projections.

• The base of a cilium is attached to a basal body, which is formed by centrioles.

• This process enables the movement of substances across cell surfaces through ciliary action.

Structure and Function of Cilia and Flagella

• Cilia are whiplike, motile cellular extensions that move substances in one direction across cell surfaces.

• They consist of microtubules arranged in a 9+2 pattern, with dynein arms, radial spokes, and cross-linking proteins.

• Flagella, longer than cilia, are also formed by microtubules and propel the cell itself (e.g., sperm cell).

• Centrioles forming the bases of cilia and flagella are known as basal bodies.

• The microtubule arrangement in cilia/flagella differs slightly from that of centrioles, with additional structural components like cross-linking proteins and dynein arms.

Ciliary and Flagellar Movement

• Cilia exhibit a rhythmic movement with power and recovery strokes, producing a pushing motion in a single direction.

• The bending action of microtubule doublets, powered by dynein arms and ATP, causes ciliary movement.

• This movement creates a current at the cell surface, resembling traveling waves.

• Flagella, on the other hand, propel the cell itself, such as the tail of a sperm cell.

• The coordinated bending of cilia creates a wave-like motion that propels substances across cell surfaces.

Microvilli

Structure and Function of Microvilli

• Microvilli are tiny fingerlike extensions of the plasma membrane that increase surface area.

• They are commonly found on absorptive cells like those in the intestines and kidney tubules.

• Microvilli contain bundled actin filaments that extend into a terminal web near the cell surface.

• Actin filaments in microvilli act as a mechanical stiffener, shaping the cell and increasing absorption efficiency.

• Unlike cilia, microvilli do not exhibit movement but significantly enhance surface area for absorption.

Nucleus

Structure and Function of the Nucleus

• The nucleus serves as the control center of the cell, containing genetic information and instructions for protein synthesis.

• It is larger than other organelles and typically spherical or oval in shape.

• The nucleus comprises the nuclear envelope, nucleoli, and chromatin.

• The nuclear envelope is a double membrane barrier with outer membrane continuous with the rough ER.

• Nuclear pores puncture the nuclear envelope, facilitating the exchange of materials between the nucleus and cytoplasm.

Components of the Nucleus

• Nucleoli are involved in ribosomal RNA synthesis and ribosome assembly.

• Chromatin consists of DNA and proteins, organizing genetic material within the nucleus.

• The nuclear lamina, composed of intermediate filaments, maintains nuclear shape and organizes DNA.

• The nucleus regulates protein synthesis based on cellular signals and requirements.

• Nuclei in different cell types may vary in size and shape, reflecting their specialized functions.

Cell Nucleus

Nuclear Envelope and Nuclear Pores

• The nuclear envelope maintains the shape of the nucleus and organizes DNA within the nucleus.

• Nuclear pores puncture the nuclear envelope, allowing the passage of molecules such as mRNAs and large particles.

• A nuclear pore complex, made of proteins, forms an aqueous transport channel that regulates the entry and exit of molecules.

• Small molecules can pass through nuclear pore complexes easily, while protein and RNA molecules are guided through by soluble transport proteins.

• The nuclear envelope is selectively permeable, allowing substances to pass more freely compared to other cell membranes.

Nucleoplasm and Nucleoli

• The nucleoplasm is a jelly-like fluid within the nucleus that contains dissolved salts, nutrients, and essential solutes.

• Nucleoli are dark-staining spherical bodies where ribosomal subunits are assembled.

• Nucleoli are not membrane-bounded and are rich in components necessary for synthesizing and assembling ribosomal subunits.

• Ribosomal RNA (rRNA) molecules combine with proteins in nucleoli to form ribosomal subunits.

• Finished ribosomal subunits leave the nucleus through nuclear pores and enter the cytoplasm to form functional ribosomes.

Chromatin Structure

Composition of Chromatin

• Chromatin is composed of DNA, histone proteins, and RNA chains.

• Nucleosomes are the fundamental units of chromatin, consisting of histone proteins and DNA wrapped around them.

• DNA winds around nucleosomes and continues to the next cluster via linker DNA segments.

• Chromatin can be visualized at increasing levels of structural complexity, from nucleosomes to chromosomes.

• Histones play a crucial role in packing DNA molecules in a compact manner and also regulate gene expression.

Structural Complexity of Chromatin

• Chromatin appears as a fine, unevenly stained network under a light microscope.

• Special techniques reveal chromatin as bumpy threads weaving through the nucleoplasm.

• Chromatin structure progresses from nucleosomes to higher levels of complexity like tight helical fibers and chromosomes.

• During cell division, chromatin threads coil and condense to form chromosomes, preventing tangling and breakage.

• Histone modifications, such as methylation and acetylation, play a role in gene regulation and chromatin condensation.

Cell Organelles

Smooth Endoplasmic Reticulum

• Membranous system of sacs and tubules without ribosomes

• Site of lipid and steroid cholesterol synthesis, lipid metabolism, drug detoxification, and calcium ion storage

• Located in the cytoplasm

• Functions in lipid and steroid synthesis, drug detoxification, and calcium storage

Golgi Apparatus

• Stack of flattened membranes and vesicles near the nucleus

• Packages, modifies, and segregates proteins for secretion, lysosomal inclusion, and plasma membrane incorporation

• Modifies carbohydrates on proteins

• Located in the cytoplasm

Peroxisomes

• Membranous sacs containing catalase and oxidase enzymes

• Detoxify various toxic substances, particularly hydrogen peroxide

• Located in the cytoplasm

• Contains enzymes for detoxification

Lysosomes

• Membranous sacs with acid hydrolases for intracellular digestion

• Sites of intracellular digestion

• Located in the cytoplasm

• Contains digestive enzymes for intracellular digestion

Microtubules

• Cylindrical structures made of tubulin proteins

• Provide support, shape to the cell, involved in intracellular and cellular movements

• Form centrioles, cilia, and flagella

• Located in the cytoplasm

Cell Extensions and Inclusions

Centrioles

• Paired cylindrical bodies composed of nine triplets of microtubules

• Organize the microtubule network, form spindle and asters during mitosis

• Serve as basal bodies for cilia and flagella

• Located in the cytoplasm

Cilia

• Short cell-surface projections with microtubule arrangement

• Create a unidirectional current for moving substances across cell surfaces

• Located as cellular extensions

• Involved in substance movement across cell surfaces

Nucleus

• Largest organelle surrounded by a nuclear envelope with pores

• Contains nucleolus, nucleoplasm, and chromatin

• Acts as the control center for genetic information and protein synthesis instructions

• Responsible for genetic information transmission

Microvilli

• Tubular extensions of the plasma membrane containing actin filaments

• Increase surface area for absorption

• Located as cellular extensions

• Enhance surface area for absorption processes

The Cell Cycle

Interphase

• Period from cell formation to division

• Cell carries out routine activities and prepares for division

• Divided into G1, S, and G2 subphases

• G1: Metabolically active phase with protein synthesis and growth

• S: DNA replication phase ensuring genetic material duplication

• G2: Final phase with enzyme synthesis for division preparation

Mitotic Phase

• Consists of mitosis and cytokinesis

• Mitosis phases: prophase, metaphase, anaphase, telophase

• Checkpoints ensure DNA replication and repair before division

• Essential for cell division and genetic material distribution

DNA Replication

• Process ensuring identical genetic material in daughter cells

• Involves replication of DNA strands and histone assembly

• Critical for accurate genetic information transmission

• Precedes cell division to maintain genetic integrity

DNA Replication

DNA Replication Process

• Before cell division, DNA must be replicated to pass identical copies of genes to daughter cells.

• Replication starts in the S phase, where chromatin threads replicate simultaneously.

• DNA replication begins at multiple origins of replication along the DNA molecule.

• Steps involved in replication include uncoiling, separation, and assembly of new complementary strands.

• Semiconservative replication results in two daughter DNA molecules from one parental molecule.

• The final step involves splicing new DNA segments using DNA ligases and re-coiling the DNA.

DNA Replication and Cell Division

• Progression from DNA replication to cell division is smooth when DNA is undamaged.

• Damage halts the cycle at checkpoints until DNA repair mechanisms fix the issue.

• Histones associate with DNA during replication, forming new chromatin strands and sister chromatids.

• Sister chromatids are held together by a centromere until anaphase of mitotic cell division.

• Sister chromatids are distributed to daughter cells to ensure identical genetic information.

Cell Division

Importance of Cell Division

• Essential for body growth and tissue repair.

• Cells like skin and intestinal lining cells reproduce continuously.

• Liver cells divide slowly to maintain organ size but can reproduce quickly if needed.

• Nervous tissue, skeletal muscle, and heart muscle cells lose division ability when mature.

M Phase of Cell Cycle

• M phase involves mitosis and cytokinesis.

• Mitosis is the division of the nucleus into two daughter cells.

• Mitosis consists of prophase, metaphase, anaphase, and telophase.

• Cytokinesis is the division of the cytoplasm, completing cell division.

• Meiosis produces sex cells with half the genetic material, discussed in Chapter 27.

Mitosis Process

• Mitosis is the process of nuclear division distributing chromosomes to daughter cells.

• Four phases of mitosis: prophase, metaphase, anaphase, and telophase.

• Each phase smoothly transitions into the next, with varying durations in different cell types.

• Cytokinesis begins in late anaphase, forming two genetically identical daughter cells.

• Interphase precedes mitosis, where the cell carries out metabolic activities and grows.

Prophase of Mitosis

• Chromatin condenses into chromosomes with sister chromatids held at the centromere.

• Centrosomes separate, forming asters and a mitotic spindle.

• Nuclear envelope breaks, allowing spindle-chromosome interaction.

• Kinetochore microtubules attach to kinetochores, pulling chromosomes to the equator.

Metaphase and Anaphase of Mitosis

• Metaphase sees chromosomes align at the metaphase plate with centromeres at the spindle equator.

• Enzymes trigger chromatid separation at the end of metaphase.

• Anaphase is the shortest phase where centromeres split, initiating chromosome separation.

Mitosis and Cell Division

Phases of Mitosis

• Metaphase: The second phase of mitosis where the centrosomes are at opposite poles of the cell. Chromosomes align at the metaphase plate, ensuring precise alignment of centromeres. Enzymes triggered at the end of metaphase prepare to separate chromatids.

• Anaphase: The third phase of mitosis characterized by the abrupt splitting of centromeres, leading each chromatid to become an individual chromosome. Kinetochore microtubules shorten, pulling chromosomes towards opposite poles, while nonkinetochore microtubules lengthen and push the poles apart.

• Telophase: The final phase of mitosis, opposite to prophase. Chromosomes uncoil, forming chromatin. New nuclear envelopes develop around each chromatin mass, nucleoli reappear, and the spindle apparatus disassembles.

• Cytokinesis: The division of cytoplasm that begins in late anaphase and continues through telophase. A contractile ring of actin microfilaments forms a cleavage furrow, separating the cell into two.

• Regulation of Cell Division: Controlled by internal and external factors such as cell surface area to volume ratio, chemical signals like growth factors, and space availability. Checkpoints like the restriction point regulate cell growth and division.

Regulation of Cell Division

• Cell Surface Area to Volume Ratio: As cells grow, volume increases faster than surface area, limiting cell size and explaining why most cells are microscopic.

• Chemical Signals: Growth factors and hormones released by neighboring cells influence cell division.

• Contact Inhibition: Normal cells stop dividing upon contact with other cells.

• Cell Cycle Checkpoints: Key points like the restriction point regulate cell growth by halting further progression.

• Internal and External Factors: Influence the decision of cells to divide or cease division.

Protein Synthesis

Role of DNA in Protein Synthesis

• Genes and Genetic Code: DNA serves as the master blueprint for protein synthesis, specifying the structure of protein molecules and enzymes essential for various biological processes.

• Gene Structure: A gene is a segment of DNA carrying instructions for creating a polypeptide chain. Humans have approximately 20,000 protein-encoding genes.

• Genetic Alphabet: DNA bases (adenine, thymine, cytosine, guanine) form triplets that specify amino acids. The sequence of triplets in a gene determines the order of amino acids in a polypeptide.

• Exons and Introns: Coding regions in genes are exons, separated by non-coding introns. Introns can act as control elements, allowing for the production of different proteins from a single gene.

• Variability in Proteins: DNA variations allow cells to produce a wide range of proteins necessary for various cellular functions.

RNA's Role in Protein Synthesis

• Types of RNA: Messenger RNA (mRNA) carries genetic information to the cytoplasm, Ribosomal RNA (rRNA) forms ribosomes, and Transfer RNA (tRNA) ferries amino acids to ribosomes.

• Transcription and Translation: Transcription encodes DNA information into mRNA, while translation decodes mRNA to assemble polypeptides.

• RNA Processing: Introns are removed from mRNA before it moves to the cytoplasm for protein synthesis.

• Location of Genes: Genes are located on a small portion of nuclear DNA, coding for short-lived mRNA, stable rRNA, and tRNA.

• Polypeptide Synthesis: Involves transcription and translation processes, where DNA information is transcribed into mRNA and decoded to form polypeptides.