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What is the primary function of bound ribosomes?
Free ribosomes are attached to the endoplasmic reticulum.
False
What marks proteins for destruction before they are degraded by proteasomes?
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The centrosome is near the nucleus and organizes ______ during cell division.
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Match the following structures with their primary functions:
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Which of the following statements about proteasomes is true?
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What is the primary function of proteasomes?
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The cytoskeleton is composed of two main types of protein filaments.
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What are the three main types of protein filaments found in the cytoskeleton?
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Proteasomes help control protein quality through the process of __________.
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Match the type of filament to its characteristic:
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Which of the following is NOT a function of the cytoskeleton?
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Cytoskeletal structures do not play a role in muscle contraction.
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What is the primary function of cilia?
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Microvilli are longer than cilia.
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What type of junctions prevent leakage between cells?
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The primary role of __________ is to strengthen cells and provide resistance to mechanical stress.
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Match the structures with their primary function:
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Which structure is typically found as a single per cell?
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Microvilli have powered mobility like cilia.
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What role do gap junctions serve in cells?
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What is the role of histone proteins in relation to DNA?
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During transcription, RNA is synthesized using RNA polymerase with DNA as a template.
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Define chromatin.
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The four nitrogenous bases of DNA are adenine, cytosine, guanine, and _____ .
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Match the following steps of protein synthesis with their descriptions:
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Which type of membrane junction is primarily responsible for sealing off intercellular spaces?
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Desmosomes provide direct passageways for ions and small molecules between adjacent cells.
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What is the primary structural component of gap junctions?
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Tight junctions are important for maintaining the ______ of membrane proteins and lipids.
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Match the following junctions with their functions:
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Which junction is specifically important for cells exposed to mechanical stress?
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Gap junctions are made up of proteins that form small, individual connections between cells.
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What are the two main functions of tight junctions?
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Desmosomes are structured with ______ filaments that extend from the plaque.
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Which of these substances can pass through gap junctions?
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What is the average diameter of a nucleus?
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Erythrocytes contain a nucleus.
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What is produced in the nucleolus?
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The nuclear envelope separates the cytoplasm from the ______.
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Match the following terms with their descriptions:
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Which of the following is NOT a component of DNA?
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DNA forms a triple helix structure.
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What pairs with adenine in DNA base pairing?
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The ______ of the nucleus regulates the movement of materials between the nucleus and cytoplasm.
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How does chromatin condense during cell division?
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Study Notes
Ribosomes
- Bound ribosomes are attached to the endoplasmic reticulum (rough ER) and synthesize proteins destined for export, plasma membrane, or lysosomes.
- Free ribosomes are suspended in the cytosol and synthesize proteins for use within the cell.
Centrosomes
- Located near the nucleus, containing a pair of centrioles arranged perpendicularly.
- Each centriole is composed of triplets of microtubules arranged in a circle.
- Organizes microtubules (proteins of the cytoskeleton) and supports their growth in nondividing cells.
- Directly forms spindle fibers during cell division.
Proteasomes
- Barrel-shaped protein complexes that degrade proteins, located within the cytosol and the nucleus.
- Degrade damaged proteins, incorrectly folded proteins, and proteins no longer needed by the cell, using ATP-dependent pathway.
- Control the quality of exported cell proteins.
The Cytoskeleton
- A framework of proteins throughout the cell interior and underneath the plasma membrane, supporting the cell and organizing organelles.
- Composed of three main types of protein filaments: microfilaments, microtubules, and intermediate filaments.
- Microfilaments are microscopic, 7 nanometers in diameter, composed of actin protein, and appear as two interwoven filaments.
- Microtubules are larger protein filaments.
- Intermediate filaments are intermediate in size between microfilaments and microtubules, forming a stable framework.
- Functions include structural support, maintaining cell shape, organizing organelles, providing internal structure for the plasma membrane and microvilli, stabilizing cell junctions, separating chromosomes during mitosis and cytokinesis, facilitating cytoplasmic streaming, helping move organelles and vesicles, and participating in muscle contraction and movement of cilia and flagella.
Cilia and Flagella
- Cilia are small, hairlike projections, typically found in large numbers, and function in movement, such as in the respiratory passageways.
- Flagella are similar in structure to cilia but typically longer and usually only present as a single flagellum per cell.
- Movement of flagella is driven by microtubules within the core, requiring ATP energy.
Microvilli
- Thin, microscopic plasma membrane extensions, shorter and narrower than cilia.
- Densely packed and lack powered mobility.
- Supported by microfilaments and increase surface area for more efficient membrane transport.
Types of Membrane Junctions
- Tight junctions prevent leakage between cells, located at the apical surfaces of cells, and keep substances from passing between cells.
- Desmosomes are anchoring junctions, strengthening the cells, and are important for cells that experience mechanical stress.
- Gap junctions provide communication between cells, allowing substances and signals to pass directly between cells.
Tight Junctions
- Composed of plasma membrane proteins forming strands/rows.
- Positioned like spot welds at the apical surfaces of adjacent cells.
- Functions include sealing off intercellular spaces, preventing unregulated substance passage between epithelial cells, forcing most materials to move through, not between, epithelial cells, maintaining polarity, and controlling permeability of substances passing through.
Desmosomes (Macula Adherens)
- Called "adhering spots".
- Composed of several proteins binding neighboring cells.
- Thickened structure (protein plaque) on the inner surface of the plasma membrane of adjoining cells.
- Protein filaments extend from the plaque across the intercellular space, anchoring to the plaque of the other cell.
- Intermediate filaments from the cytoskeleton extend from the plaques, providing strength and support.
- Important for cells exposed to stress.
Gap Junctions
- Composed of six integral plasma membrane proteins forming a fluid-filled tunnel/pore.
- Functions include providing a direct passageway for ions, glucose, amino acids, etc.between cells, and allowing the spread of electrical activity.
The Nucleus
- The largest cell structure, often called the cell's control center.
- Usually has one; erythrocytes (red blood cells) have none, and skeletal muscle cells have many.
- The shape of the nucleus generally matches the shape of the cell.
Nuclear Envelope and Nucleolus
- The nucleus is enclosed by a double membrane called the nuclear envelope.
- This membrane separates the cytoplasm from the nucleoplasm and controls material movement between the nucleus and the cytoplasm.
- Functions of the nucleus include cellular regulation (houses DNA molecules which are genetic instructions for protein synthesis) and production (produces ribosomal subunits in the nucleolus, ribosomal subunits are exported to the cytoplasm to assemble ribosomes).
DNA, Chromatin, and Chromosomes
- The nucleus houses nuclear DNA.
- DNA is formed by repeating nucleotide monomers, which consist of a five-carbon sugar, one of four nitrogenous bases, and phosphate groups.
- Adenine pairs with thymine, and guanine pairs with cytosine.
- DNA forms a double helix.
- Chromatin is formed by DNA associated with histone proteins, tightly packaged to form chromosomes.
Transcription:
- Occurs in the nucleus, where a segment of DNA is "read" to produce a new RNA strand using RNA polymerase enzyme.
- Required structures include DNA, ribonucleotides, and RNA polymerase.
- Steps include initiation, elongation, and termination.
- Initiation: DNA is unwound to access the information, specific enzymes unwind the DNA and RNA polymerase attaches to the DNA strand, moving along to the promoter region.
- Elongation: Free ribonucleotides pair with the DNA template strand, RNA polymerase forms hydrogen bonds between the bases, the sequence of the DNA template determines the sequence of the mRNA strand, and RNA polymerase continues until the entire gene is transcribed.
- Termination: RNA polymerase is released, hydrogen bonds between DNA and mRNA break, and the DNA rewinds into a double helix.
- Several modifications are made to pre-mRNA before it leaves the nucleus including splicing, capping, and polyA tail formation.
Translation: Synthesizing Protein
- Occurs in the cytoplasm, ribosomes use mRNA to assemble amino acids into a protein.
- Required structures for translation include ribosomes, mRNA, tRNA, and amino acids.
- Three types of RNA are produced: mRNA, tRNA, and rRNA.
- Translation steps include initiation, elongation, and termination.
- Initiation: The small ribosomal subunit attaches to the mRNA.
- Elongation: The tRNA carrying the corresponding amino acid binds to the mRNA codon, and the ribosome moves along the mRNA, adding amino acids to the growing polypeptide chain.
- Termination: The ribosome reaches the stop codon, the polypeptide chain is released, and the ribosome dissociates.
- mRNA carries the recipe to synthesize a specific protein, it is then translated into a protein.
Learning Strategy
- Transcription is analogous to a chef cooking a meal. The ribosome is the kitchen, mRNA is the recipe, tRNA brings ingredients (amino acids), and the protein is the final dish.
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Description
Test your understanding of vital cellular structures like ribosomes, centrosomes, and proteasomes. This quiz covers their functions, locations, and significance in cell biology. Perfect for students studying cell structure and organization.
