DNA Replication, Cell Division, Protein Synthesis, Cell Transport, and Metabolism: Study Guide

DNA Replication and Cell Division

Somatic Cells vs. Gametes and Cell Division Mechanisms

• Somatic cells are all body cells except gametes (sperm and egg), and they divide by mitosis.

• Gametes are reproductive cells and are produced by meiosis.

• Cell division ensures growth, repair, and reproduction in organisms.

Purpose of Mitosis

• Mitosis allows for the production of genetically identical daughter cells for growth and tissue repair.

DNA Replication

• Helicase unwinds the DNA double helix.

• DNA Polymerase adds nucleotides to the growing DNA strand, synthesizing new DNA in the 5' to 3' direction.

• Nucleotides are the building blocks of DNA, each consisting of a phosphate group, deoxyribose sugar, and a nitrogenous base.

Cell Cycle

• The cell cycle consists of interphase (G1, S, G2 phases) and mitotic phase (mitosis and cytokinesis).

• Most cells in a population are typically in interphase.

Stages of Mitosis

• Prophase: Chromosomes condense, nuclear envelope breaks down.

• Metaphase: Chromosomes align at the cell's equator.

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

• Telophase: Nuclear envelopes reform around chromosomes.

Cytokinesis

• Cytokinesis is the division of the cytoplasm, occurring after mitosis, resulting in two separate daughter cells.

Protein Synthesis

Central Dogma

• The central dogma of molecular biology describes the flow of genetic information: DNA → RNA → Protein.

DNA vs. RNA Nucleotides

• DNA nucleotides: deoxyribose sugar, bases A, T, C, G.

• RNA nucleotides: ribose sugar, bases A, U, C, G.

Genes

• A gene is a segment of DNA that codes for a specific RNA and, usually, a protein.

Transcription (DNA to mRNA)

• RNA Polymerase binds to DNA and synthesizes a complementary mRNA strand.

• Transcription factors help RNA polymerase bind to the promoter region.

• mRNA is processed (splicing, capping, poly-A tail) before leaving the nucleus.

Structure of RNA Molecule

• Single-stranded, contains ribose sugar, bases A, U, C, G.

• Has an amino acid end and an anticodon end (in tRNA).

Translation (mRNA to Protein)

• Occurs at the ribosome, where mRNA codons are read and matched with tRNA anticodons.

• Ribosome has E, P, and A sites for tRNA binding.

• Polypeptide chains are synthesized and fold into functional proteins.

• Genetic code table is used to translate mRNA codons into amino acids.

Mutations

• Mutations are changes in DNA sequence that can lead to dysfunctional or non-functional proteins.

Cell Transport

Solute vs. Solvent

• Solute: Substance dissolved in a solution.

• Solvent: Substance that dissolves the solute (e.g., water).

Selective Permeability of Cell Membrane

• Cell membranes allow some substances to pass while restricting others, maintaining homeostasis.

Structure of Cell Membrane

• Phospholipid bilayer forms the basic structure.

• Proteins serve various functions:

  • Carrier proteins

  • Channels

  • Receptor proteins

  • Anchoring proteins

  • Recognition proteins

Transport Mechanisms

• Simple diffusion: Movement of molecules from high to low concentration without energy input.

• Facilitated diffusion: Movement via transport proteins, still passive.

• Osmosis: Diffusion of water across a semipermeable membrane.

• Active transport: Movement against concentration gradient, requires energy (e.g., sodium-potassium pump).

• Endocytosis: Cell engulfs material (e.g., phagocytosis).

• Exocytosis: Cell expels material.

Osmosis and Tonicity

• Water moves from low solute concentration to high solute concentration.

• Hypertonic: Higher solute outside cell; cell shrinks.

• Hypotonic: Lower solute outside cell; cell swells.

• Isotonic: Equal solute; no net water movement.

Sodium-Potassium Pump

• Active transport mechanism exchanging 3 Na+ out for 2 K+ in, using ATP.

Metabolism

Anabolic vs. Catabolic Reactions

• Anabolic reactions: Build complex molecules from simpler ones (require energy).

• Catabolic reactions: Break down complex molecules into simpler ones (release energy).

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

ATP and Energy Release

• ATP stores energy in high-energy phosphate bonds.

• Energy is released when the terminal phosphate is removed: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• ATPases catalyze this reaction.

• Phosphorylation is the addition of a phosphate group to a molecule, important in cellular signaling and metabolism.

Glycogen and Carbohydrate Storage

• Carbohydrates, fatty acids, and amino acids can be stored as glycogen or fat.

• Glycogen is stored in liver and muscle cells.

Glycolysis

• Occurs in the cytoplasm.

• Breaks down 1 glucose into 2 pyruvate, producing 2 ATP and 2 NADH.

• Under anaerobic conditions, pyruvate is converted to lactate.

Pyruvate to Acetyl CoA

• In mitochondria, pyruvate is converted to acetyl CoA, producing NADH and CO2.

Krebs Cycle (Citric Acid Cycle)

• Occurs in mitochondria when oxygen is present.

• Each acetyl CoA produces:

  • 3 NADH

  • 1 FADH2

  • 1 GTP (or ATP)

  • 2 CO2

Electron Transport Chain (ETC)

• Located in the inner mitochondrial membrane.

• NADH and FADH2 donate electrons, which move through complexes, pumping protons and generating ATP via ATP synthase.

• Oxygen is the final electron acceptor, forming water.

• Each NADH yields about 2.5 ATP; each FADH2 yields about 1.5 ATP.

Gluconeogenesis, Glycogenesis, Glycogenolysis

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources.

• Glycogenesis: Formation of glycogen from glucose.

• Glycogenolysis: Breakdown of glycogen to glucose.

Lipid Catabolism

• Lipids store more energy than carbohydrates due to more C-H bonds.

• Triglycerides are hydrolyzed to glycerol and fatty acids.

• Glycerol enters glycolysis; fatty acids undergo beta-oxidation to form acetyl CoA.

Protein Catabolism

• Proteins are broken down into amino acids.

• Amino acids can enter the Krebs cycle at various points after deamination (removal of amino group).

• Transamination is the transfer of an amino group to another molecule.

• The urea cycle converts toxic ammonia to urea for excretion.

Summary Table: Major Metabolic Pathways

Pathway	Main Location	Key Products
Glycolysis	Cytoplasm	2 ATP, 2 NADH, 2 Pyruvate
Krebs Cycle	Mitochondrial Matrix	3 NADH, 1 FADH2, 1 GTP, 2 CO2 (per acetyl CoA)
Electron Transport Chain	Inner Mitochondrial Membrane	~28 ATP, H2O
Beta-Oxidation	Mitochondria	Acetyl CoA, NADH, FADH2
Transamination/Deamination	Liver	Urea, Keto acids