Chapter 7 - Cell Structure and Function

Prokaryotic vs. Eukaryotic Cell Structure

Cells are the basic units of life, and they can be classified as prokaryotic or eukaryotic based on their structural features.

• Prokaryotic cells lack a nucleus and membrane-bound organelles; their DNA is located in a region called the nucleoid.

• Eukaryotic cells have a true nucleus enclosed by a nuclear envelope and possess various membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

• Example: Bacteria are prokaryotes; plants and animals are eukaryotes.

Adaptive Significance of Organelles

Organelles allow eukaryotic cells to compartmentalize functions, increasing efficiency and specialization.

• Advantages: Separation of incompatible chemical reactions, increased surface area for metabolic processes.

• Disadvantages: Greater energy requirement for maintenance and transport between compartments.

• Example: Mitochondria perform cellular respiration; chloroplasts carry out photosynthesis in plants.

Plant vs. Animal Cell Structures

Plant and animal cells share many organelles but also have unique structures.

• Plant cells: Have cell walls, chloroplasts, and large central vacuoles.

• Animal cells: Lack cell walls and chloroplasts but have centrioles and lysosomes.

• Example: The presence of chloroplasts allows plants to photosynthesize.

Organelle Structure and Function

Each organelle has a unique structure and function within the cell.

• Nucleus: Stores genetic material and coordinates cell activities.

• Mitochondria: Site of ATP production via cellular respiration.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Contain digestive enzymes for waste breakdown.

Endomembrane System and Protein Synthesis

The endomembrane system includes the nuclear envelope, ER, Golgi apparatus, lysosomes, and vesicles, working together to synthesize, modify, and transport proteins.

• Protein synthesis: DNA → mRNA (nucleus) → ribosome (cytoplasm/ER) → protein modification (ER, Golgi) → transport (vesicles).

• Malfunction: Defects in any component can disrupt protein processing and cell function.

Cytoskeleton: Microfilaments, Microtubules, and Intermediate Filaments

The cytoskeleton provides structural support, facilitates cell movement, and organizes organelles.

• Microfilaments (actin): Involved in cell movement and shape.

• Microtubules: Maintain cell shape, form spindle fibers during mitosis, and serve as tracks for organelle movement.

• Intermediate filaments: Provide mechanical strength.

Chapter 12 - Cell Cycle and Mitosis

Chromosomes, Chromatids, and Sister Chromatids

During cell division, genetic material is organized and distributed to ensure each daughter cell receives a complete set.

• Chromosome: A DNA molecule with associated proteins.

• Chromatid: One of two identical halves of a duplicated chromosome.

• Sister chromatids: Paired chromatids joined at the centromere.

Stages of Mitosis

Mitosis is the process by which a eukaryotic cell divides its nucleus and genetic material.

• Prophase: Chromosomes condense, spindle forms.

• Metaphase: Chromosomes align at the cell equator.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

• Cytokinesis: Division of the cytoplasm.

Cell Cycle Phases and Checkpoints

The cell cycle consists of interphase (G1, S, G2) and mitotic phase (M). Checkpoints regulate progression to ensure fidelity.

• G1 phase: Cell growth.

• S phase: DNA replication.

• G2 phase: Preparation for mitosis.

• M phase: Mitosis and cytokinesis.

• Checkpoints: Ensure DNA integrity and proper chromosome alignment.

Chapter 15 - DNA Replication

Chemical Components and Structure of DNA

DNA is composed of nucleotides, each containing a phosphate group, deoxyribose sugar, and a nitrogenous base (adenine, thymine, cytosine, guanine).

• Double helix: Two antiparallel strands held by hydrogen bonds between complementary bases (A-T, C-G).

Semiconservative Replication

DNA replication is semiconservative: each new DNA molecule consists of one parental and one new strand.

• Meselson-Stahl experiment: Demonstrated semiconservative mechanism.

Replication Bubble and Key Enzymes

Replication begins at origins, forming bubbles where DNA is unwound and copied.

• Helicase: Unwinds DNA.

• Primase: Synthesizes RNA primers.

• DNA polymerase: Adds nucleotides in 5' to 3' direction.

• Ligase: Joins Okazaki fragments on lagging strand.

Leading and Lagging Strands

DNA polymerase synthesizes the leading strand continuously and the lagging strand in short Okazaki fragments.

• Directionality: DNA is synthesized 5' to 3'.

DNA Repair and Proofreading

DNA polymerases proofread and repair mismatches to maintain genetic fidelity.

Chapter 17 - Gene Expression: Transcription and Translation

Genetic Code and Gene Expression

The genetic code translates DNA sequences into proteins via transcription and translation.

• Transcription: DNA → mRNA.

• Translation: mRNA → protein.

• Codon: Three-nucleotide sequence on mRNA specifying an amino acid.

Transcription: Initiation, Elongation, Termination

Transcription involves several steps and key components.

• Initiation: RNA polymerase binds promoter, unwinds DNA.

• Elongation: RNA polymerase synthesizes mRNA.

• Termination: RNA polymerase releases mRNA and detaches.

RNA Processing in Eukaryotes

Pre-mRNA undergoes modifications before translation.

• 5' cap and 3' poly-A tail: Protect mRNA from degradation.

• Splicing: Removal of introns, joining of exons.

Translation: Initiation, Elongation, Termination

Translation converts mRNA into a polypeptide chain using ribosomes and tRNAs.

• Initiation: Ribosome assembles at start codon (AUG).

• Elongation: tRNAs bring amino acids, peptide bonds form.

• Termination: Stop codon reached, polypeptide released.

Mutations and the Genetic Code

Mutations are changes in DNA sequence that can affect protein function.

• Missense: Change in one amino acid.

• Nonsense: Introduces a stop codon.

• Frameshift: Insertion/deletion alters reading frame.

• Silent: No change in amino acid sequence.

Chapters 12 and 16 - Mutations and Cancer

Types of Mutations and Their Effects

Mutations can be classified by their impact on gene function and protein structure.

• Missense: Alters one amino acid in the protein.

• Nonsense: Creates a premature stop codon.

• Frameshift: Shifts the reading frame, often resulting in nonfunctional protein.

• Silent: No effect on protein sequence.

Mutations and Cancer

Cancer is often caused by mutations in genes that regulate the cell cycle.

• Cell cycle regulation: Mutations in tumor suppressor genes or proto-oncogenes can lead to uncontrolled cell division.

• Age factor: Cancer is more common in older individuals due to accumulation of mutations over time.