AP Biology Vocabulary

Key Exam Details

• The AP Biology exam is a 3-hour end-of-course test.
• It consists of 60 multiple-choice questions and 6 free-response questions.
• Both sections are weighted equally, accounting for 50% of the final score.
• Multiple-choice: 1 hour and 30 minutes.
• Free-response: 1 hour and 30 minutes.
• Exam covers the following course content categories:
  • Chemistry of Life: 8–11%
  • Cell Structure and Function: 10–13%
  • Cellular Energetics: 12–16%
  • Cell Communication and Cell Cycle: 10–15%
  • Heredity: 8–11%
  • Gene Expression and Regulation: 12–16%
  • Natural Selection: 13–20%
  • Ecology: 10–15%

Chemistry of Life

• 8–11% of the AP Biology exam will cover the topic Chemistry of Life.

Water and the Elements of Life

• Water is composed of two hydrogen molecules covalently bonded to an oxygen molecule.
• The oxygen atom attracts electrons more strongly, resulting in a slightly negative charge on the oxygen and slightly positive charges on the hydrogen atoms.
• Molecules with distinct regions of charge are called polar compounds.
• The polar nature and shape of water molecules allow them to form hydrogen bonds with each other.

Properties of Water

• Solvent: Water's polarity makes it an excellent solvent for many molecules.
• Cohesion: Water molecules stick together due to hydrogen bonds.
  • Cohesion leads to surface tension, minimizing surface area.
• Adhesion: Water is attracted to dissimilar molecules.
  • Adhesion is responsible for capillary action, which allows water to move through small spaces, even against gravity.
  • Capillary action is crucial for water transport in plants.
• Carbon, hydrogen, nitrogen, and oxygen make up 99% of living matter.

Organic Molecules

• Organic molecules, which contain carbon, are the basis of life.
• Carbon can form four bonds with other elements, making it ideal for creating complex biological molecules.
• Carbon can form single, double, or triple bonds, leading to various molecular shapes (rings, branches, long chains).
• Carbon is the backbone of carbohydrates, proteins, lipids, and nucleic acids.
• Nitrogen and phosphorus are also crucial for nucleic acids and proteins.

The Makeup and Properties of Macromolecules

• Large biological molecules are vital for life.
• You should be familiar with carbohydrates, lipids, proteins, and nucleic acids.
• Carbohydrates, proteins, and nucleic acids are polymers, made of repeating monomer units.
  • DNA: nucleotides.
  • Proteins: amino acids.
  • Carbohydrates: sugars.
• Lipids are generally not polymers.
• The composition and order of monomers influence the function of macromolecules.

Formation and Breakdown of Macromolecules

• Macromolecules form through dehydration synthesis, where a covalent bond forms between two monomers, releasing water.
• Hydrolysis is the reverse process, breaking down polymers into monomers using water.
• Synthesis reactions require energy, stored in the macromolecule's covalent bonds.
• Hydrolysis releases this stored energy for the cell to use.

Proteins

• Proteins are the most abundant organic molecules in organisms.
• They have diverse structures and functions.
• Proteins are made of amino acid strings connected by covalent bonds.
• There are 20 types of amino acids in biological organisms, but they all share similar structural features.

Protein Structure

• Amino acid structure: a central carbon atom with:
  • An amino group $(NH_2)$
  • A carboxyl group $(COOH)$
  • A hydrogen atom.
  • An R group (determines the amino acid's identity).
• Peptide bonds link amino acids via dehydration synthesis between the carboxyl terminus of one amino acid and the amino terminus of the next.
• Proteins have directionality: the beginning has an amine group, and the end has a carboxyl group.

Primary Structure

• The composition and sequence of amino acids determine the protein's properties and shape.
• Amino acids can be charged, uncharged, hydrophobic, or cause changes in 3D structure.

Secondary Structure

• Secondary structure arises from interactions between amino acid backbone elements (excluding R groups).
  • α helixes: Helical structures formed by hydrogen bonds between the carbonyl group of one amino acid and the amino group of another four amino acids down the line. R groups point outward.
  • β sheets: Formed when polypeptide chain sections run parallel. R groups point outward on top and bottom.

Tertiary Structure

• Tertiary structure is due to interactions between R groups, including:
  • Non-covalent bonds.
  • Di-sulfide bonds.
• Tertiary structures minimize free energy by adopting the most energetically stable position.

Quaternary Structure

• Quaternary structure results from interactions between amino acids on different polypeptide chains.
• Protein structures can be denatured (lose higher-order structures) due to pH or temperature changes but generally return to their proper structures when conditions normalize.
• The information needed for structure formation is retained within the protein's polypeptide sequence.

Carbohydrates

• Carbohydrates provide an immediate energy source and form structural components.
• The formula for carbohydrates is $(CH<em>2O)</em>n$ where $n$ is the number of repeating units.

Monosaccharides

• Monosaccharides contain 3–7 monomers of $CH_2O$ in a chain or ring.
• Common monosaccharides: glucose, fructose, and galactose.

Disaccharides

• Disaccharides form when two monosaccharides undergo dehydration synthesis, forming a covalent (glycosidic) bond.
• Common disaccharides: sucrose, lactose, and maltose.

Polysaccharides

• Polysaccharides are long chains of monosaccharides that are linear or branched.
• Common polysaccharides: starch, chitin, cellulose, and glycogen.
• Disaccharides and polysaccharides can be made of identical or different monosaccharide monomers.
• The composition affects the properties of the macromolecule.

Lipids

• Lipids are nonpolar macromolecules made of hydrophobic hydrocarbon chains.
• They include fats, waxes, phospholipids, and steroids.
• Lipids form structural units of membranes and store energy.

Fats (Triglycerides)

• Fats consist of a glycerol backbone and three fatty acid chains.
• Fatty acid chains are hydrocarbons bonded to a carboxyl group.
• Fatty acid chains vary in length and saturation.

Saturated Fats

• Saturated fats have only single bonds in the hydrocarbon chain.
• They have straight fatty acid chains that pack tightly and are solid at room temperature.

Unsaturated Fats

• Unsaturated fats have double bonds in the hydrocarbon chain.
  • Cis fatty acids: Hydrogen atoms are on the same side of the double bond, causing a bend in the chain, resulting in liquid oils at room temperature.
  • Trans fatty acids: Hydrogen atoms are on opposite sides of the double bond, retaining the normal chain shape. They behave like saturated fats at room temperature but are difficult for the body to metabolize.

The Structure of DNA and RNA

• DNA (deoxyribonucleic acid) is a polymer made of nucleotides, carrying the genetic information.
• Nucleotides consist of a sugar (deoxyribose), a phosphate group, and a nitrogenous base.

Nucleotide Structure

• Deoxyribose is a ring structure with an oxygen and four carbons. Carbon atoms are numbered 1' to 5'.
• The phosphate group bonds to the 5' carbon.
• Nucleotides are linked by covalent bonds between the sugar and phosphate groups to form a DNA strand.
• The connection occurs between the 3' carbon of one nucleotide and the phosphate group of the next, resulting in a 3' to 5' orientation.

DNA Structure

• DNA nucleotides come in four varieties: adenine (A), thymine (T), guanine (G), and cytosine (C).
• In RNA, uracil (U) replaces thymine.
• G and A are purines (double nitrogenous rings).
• C, T, and U are pyrimidines (single nitrogenous rings).
• Two DNA strands with complementary base pairs are linked by hydrogen bonds to form a double helix.
• C-G pairs via three hydrogen bonds.
• A-T (or A-U in RNA) pairs via two hydrogen bonds.
• The DNA backbone is made of sugar and phosphate groups linked together by phosphodiester bonds.
• The two DNA strands run antiparallel—one strand runs 3'-5', and the other runs 5'-3'.

RNA Structure

• RNA (ribonucleic acid) is similar to DNA but has ribose as the sugar group and uracil (U) instead of thymine.
• RNA is usually single-stranded.

Cell Structure and Function

• Around 10–13% of questions on your exam will cover the topic Cell Structure and Function.

Cellular Components and Functions

• Cells are the smallest functional unit of life.
• There are two categories of cells: prokaryotes and eukaryotes.

Prokaryotes

• Unicellular.
• Lack cell walls and membrane-bound organelles.
• Generally have a single circular chromosome.
• Include bacteria and archaea.

Eukaryotes

• Have multiple chromosomes in a membrane-bound nucleus and other organelles.
• Include single-celled organisms like protozoans and yeast, as well as multicellular organisms.

Animal vs. Plant Cells

Animal Cells

• Have a thin plasma membrane made of a phospholipid bilayer.
• Membrane proteins regulate molecule entry/exit and facilitate cell communication.

Plant Cells

• Have a cell wall made of cellulose and lignin providing strength, rigidity, and protection.
• The cell wall helps store water and regulate diffusion without rupture.
• Also have a central vacuole that stores water and nutrients.
• Plant cells contain chloroplasts and mitochondria, while animal cells only have mitochondria.

Eukaryotic Cell Organelles

Cell Size

• Cells exchange molecules with their environment (nutrients, heat, oxygen, waste products).
• Cells must maintain a membrane surface area that supports metabolic needs.
• The volume of the cell increases faster than the surface area.
• Mathematically, cell volume increases faster than surface area, which limits cell growth. Highly metabolically active cells are either small or have specializations to increase surface area for exchange (e.g., membrane folds).

Surface Area to Volume Ratio Equations

The Cell Membrane Structure and Function

Fluid Mosaic Model

• The plasma membrane is composed of a phospholipid bilayer with cholesterol, proteins, glycolipids, and glycoproteins embedded within it.
• The membrane components are in constant motion.

Components of the Cell Membrane

• Phospholipids: The majority of the plasma membrane is made of a hydrophilic head (glycerol and phosphate group) and hydrophobic tails (non-polar fatty acids).
  • They form a bilayer with hydrophilic heads facing the aqueous internal and external environments, and hydrophobic tails in the middle.
• Membrane Proteins: They can traverse the entire membrane, partially extend on one side, or loosely attach to one side, depending on hydrophobicity.
  • Integral membrane proteins integrate into the membrane with hydrophobic portions anchoring them; transmembrane proteins traverse both ends of the membrane.
  • Peripheral membrane proteins loosely attach to the membrane, either to other integral proteins or phospholipids.
• Glycolipids and Glycoproteins: They attach to the outer surface of the cell and are used as signals for the cell type.
  • The immune system uses glycoproteins to identify cells that belong to the organism versus invaders.
• Cholesterol: Embedded within the hydrophobic bilayer to buffer fluidity at low and high temperatures.

Selective Permeability

• The phospholipid bilayer creates a semipermeable barrier due to the hydrophobic interface.
• The cell membrane is selectively permeable, allowing only certain molecules to pass through.
  • Small nonpolar molecules (N2, O2, CO2) pass easily.
  • Uncharged polar molecules (water) can pass through in small amounts.
  • Hydrophilic substances (large polar molecules and ions) require membrane transporters.
• Plants and bacteria have cell walls made of complex carbohydrates for support and protection against mechanical and osmotic stress.

Cell Regulatory Mechanisms

Membrane Transport

• Passive transport: Movement across a membrane without energy use (diffusion from high to low concentration areas).
• Active transport: Movement across a membrane requiring energy (molecules move from low to high concentration areas).
• Endocytosis: Vesicle formation in the plasma membrane taking in molecules from the external environment into the cell.
• Exocytosis: Vesicle fusion with the membrane dumping contents into the extracellular space.

Types of Passive Transport

• Simple diffusion: Molecules pass across the membrane freely, driven by concentration gradients (e.g., oxygen and carbon dioxide).
• Facilitated diffusion: Channels or carrier molecules assist molecules across the membrane (e.g., aquaporins for water).
• Osmosis: Water moves across a semipermeable membrane from an area of low solute concentration to high solute concentration, to equalize concentrations.

Osmosis and Tonicity

• Osmolarity: Total solute concentration in a solution.
• Osmotic pressure: Pressure that pulls water from one side of a membrane to the other.
• Tonicity: Ability of water to move across a membrane by osmosis, considering osmolarity differences and water movement.

Cell Environments

• Isotonic: Solute concentration is the same in the internal and external environments.
• Hypertonic: The external environment has more solute than the internal environment.
• Hypotonic: The external environment has less solute than the internal environment.
• Cells in a hypotonic solution take on too much water and can burst.
• Cells in a hypertonic solution are at risk of shriveling.

Active Transport

• Active transport mechanisms use metabolic energy (often ATP) to transport molecules and maintain concentration gradients.
• For example, neurons maintain high potassium and low sodium concentrations internally using the $Na^+/K^+$ ATPase.

Cellular Compartmentalization

• Membranes compartmentalize processes, separating them from the rest of the environment.
• Cells create internal environments different from the external environment (e.g., neurons and electrochemical gradients).
• Internal membranes increase reaction surface areas and keep molecules in the necessary locations for function (e.g., the internal mitochondrial membrane and ATP synthesis).

Endosymbiosis

• Membrane-bound organelles evolved from prokaryotic cells through endosymbiosis.
• A eukaryotic cell engulfed a prokaryote (likely a photosynthetic autotroph) but did not digest it, establishing a mutually beneficial relationship.
• Over time, this relationship became permanent as the prokaryotic cell stopped carrying all genes needed for survival and the eukaryotic cell depended on the energy it provided.
• Mitochondria and chloroplasts share features with prokaryotic cells, such as circular DNA genomes.

Cellular Energetics

• Around 12–16% of the questions on your AP exam will cover Cellular Energetics.

The Structure and Function of Enzymes

• Energy releasing reactions have an energy barrier (activation energy) that must be overcome.
• Catalysts reduce activation energy and increase reaction rate.
• Enzymes are biological catalysts that change the conformation of molecules to optimize reactions.
• The active site of an enzyme interacts with the molecule.
• The substrate is the molecule that the enzyme binds to.

Environmental Factors Affecting Enzyme Activity

• Heat: Increases reaction rate but excessive heat can denature proteins.
• pH: Affects hydrogen bonds, altering enzyme efficiency.
• Substrate/Enzyme Concentration: More substrate or enzyme generally speeds up reaction rate.

Enzyme Inhibitors

• Competitive inhibitors compete for the active site, blocking substrate binding.
• Noncompetitive inhibitors bind to other sites on the enzyme, changing enzyme conformation and reducing substrate binding efficiency.

The Role of Energy in Living Systems

• Living things require energy for life.
• Bioenergetics is the study of energy transformation in living organisms.
• Metabolism encompasses all energy transformations, including photosynthesis, respiration, and movement.
• Energy input constantly exceeds loss.
• Energy is stored in chemical bonds of molecules (sugars or fats).
• Anabolism stores energy in chemical bonds.
• Catabolism releases energy.
• Energy release mechanisms are often linked to processes for immediate cell use.
• ATP is the primary energy source for cells.
• ATP is broken down into ADP (adenosine diphosphate) and AMP (adenosine monophosphate) through fermentation and respiration, with energy released at both steps.

The Processes of Photosynthesis

• Photosynthesis converts light energy to cellular energy (glucose).
• Equation: $6CO<em>2 + 6H</em>2O + Light → C<em>6H</em>{12}O<em>6 + 6O</em>2$
• Photosynthesis evolved in cyanobacteria, oxygenating Earth's atmosphere.
• Pathways that evolved in prokaryotes are the foundations for eukaryotic photosynthesis in plants.

Phases of Photosynthesis

• Photolysis (light-dependent reaction): takes place in the thylakoid membranes of chloroplasts.
  • Light energy is absorbed by chlorophylls, generating ATP and NADPH.
  • Water is used and oxygen is released.
• Calvin cycle (light-independent reaction): takes place in the stroma of chloroplasts.
  • Carbon dioxide reacts with ATP and NADPH to create small 3-carbon sugars.
  • Sugars are joined to form glucose, which provides energy and fixed carbon.
• Fixed carbon refers to organic carbon molecules, like sugars.

Light Dependent Reactions

• Light dependent reaction relies on photosystems to harvest light energy.
• During this process, called non-cyclic phosphorylation, light is absorbed by two photosystems that harness the electrons from water to make NADPH and ATP from NADP+ and ADP.

Photosystems

• PSI and PSII are embedded in the thylakoid membrane.
• Contain a light harvesting complex (proteins, chlorophylls, and pigments) and a special pair of chlorophyll a at the core.

Steps of Non-Cyclic Phosphorylation

• Light energy is harnessed by PSII through the light harvesting complex.
• Energy is passed via PSII’s special pair of chlorophyll a (called P680), which boosts an electron to a higher energy level.
• The electron is passed to an acceptor molecule and replaced with an electron taken from water, releasing oxygen.
• The high energy electron is then passed along the membrane by a series of transporters, called the electron transport chain (ETC).
• The electron loses a little energy at each step of the ETC; some of this energy is used to pump $H^+$ ions from the stroma into the thylakoid membrane—this pumping creates a proton gradient that is linked to formation of ATP from ADP through a process called chemiosmosis via an enzyme called ATP synthase.
• Phosphorylation of ADP to generate ATP in photosynthesis is called photophosphorylation.
• PSI receives the excited electron, which is passed along the ETC.
• Light energy received by PSI is then used to excite the electron through its special chlorophyll pair (called P700).
• The electron, now at a very high energy level, is then passed by an acceptor molecule which passes it along another electron transport chain that results in the creation of NADPH from NADP+.
• ATP and NADPH generated in during the light cycle are then used in the stroma of chloroplasts to generate sugar from carbon dioxide through the Calvin cycle.

The Processes of Cellular Respiration

• All forms of life use respiration and fermentation to break down biological macromolecules (sugars and fats) into ATP.
• Cellular respiration breaks down glucose into carbon dioxide and water to generate ATP.
• As in photosynthesis, cellular respiration takes place in a series of steps where energy is passed along an electron transport chain.

Steps of Cellular Respiration

1. Glycolysis: The 6-carbon glucose is broken down into two 3-carbon pyruvates, two ATP molecules, and two NADH molecules. ADP is converted to ATP, and NAD+ is converted to NADH. Occurs in the cell’s cytosol.
2. Pyruvate Oxidation: Pyruvate molecules are transported into mitochondria, where they are broken down into two carbon molecules (acetyl groups) that are bound to coenzyme-A, in a molecule called acetyl CoA. This step reduces NAD+ to NADH but does not generate ATP.
3. Citric Acid Cycle (Krebs Cycle): Acetyl CoA is metabolized to produce one NADH, one FADH2, two carbon dioxides, and either one ATP or one GTP.
4. Oxidative Phosphorylation: NADH and FADH2 are then used to make more ATPs through an electron transport chain in the membrane of mitochondria. The electron transport chain uses electrons donated by carrier molecules, in this case, NADH and FADH2, which are then passed along the chain. As electrons are passed along, they lose some energy, which is used to pump $H^+$ ions from the mitochondrial matrix to the intermembrane space. This creates a chemical gradient, which is used to convert ADP to ATP through chemiosmosis by ATP synthase. The final electron acceptor in oxidative phosphorylation is an $O_2$ molecule, which generates water. In prokaryotes, the first three steps of respiration take place in the cytosol and oxidative phosphorylation takes place in the cell’s membrane. In some anerobic prokaryotes other proton acceptors are used at the end of the electron transport chain in a process called anaerobic cellular respiration.

• In some cells, the electron transport chains are decoupled from the generation of ATP used to generate heat instead of energy storage.
• If oxygen is not available at the end of the electron transport chain to accept electrons, cells use fermentation, continuing glycolysis but converting pyruvates to other organic molecules like lactic acid and alcohol to regenerate NAD+ from NADH, which allows glycolysis to continue.

Molecular Diversity and Cellular Response to Environmental Changes

• Overlapping mechanisms exist for generating ATP, such as fermentation in the absence of oxygen.
• Plants have different chlorophylls to harness energy from different wavelengths of light.
• These different molecules help organisms adapt to changing environments and optimize species survival.

Cell Communication and Cell Cycle

• Around 10–15% of the questions on your AP exam will cover the topic Cell Communication and Cell Cycle.

The Mechanisms of Cell Communication

• Cells communicate through long-range and short-range signals.
• This often involves a cell releasing a ligand (molecule) that is received by another cell with a receptor for that ligand.

Methods of Cell Communication

• Paracrine Signaling: One cell releases ligand to signal nearby cells (e.g., neuronal synapses).
• Autocrine Signaling: A cell releases a signal to itself to regulate growth and intracellular processes.
• Endocrine Signaling: A cell releases a ligand (hormone) into the bloodstream to affect cells far away.
• Signaling through Cell-Cell Contact: Two cells physically contact each other, passing on a signal through gap junctions or binding of ligands and receptors, used by the immune system to recognize pathogens.

Signal Transduction

• Signal transduction pathways link a signal to the appropriate response.
• It begins when a ligand is recognized by a receptor.
• Ligands can be small chemicals, small peptides, or large proteins.
• Receptors generally recognize one or a few ligands.

Types of Receptors

• Intracellular receptors: Reside within the cell (e.g., hormone receptors that change shape and enter the nucleus to induce gene expression).
• Cell surface receptors: Reside within the plasma membrane and respond to signals from outside (e.g., ligand-gated ion channels, G-protein coupled receptors, and enzyme-linked receptors).
  • Ligand-gated ion channels: Open or close in response to ligand binding, allowing or stopping the flow of ions.
  • G-protein coupled receptors: Respond to a signal by activating their coupled G-protein, that then interacts with other proteins within the cell, relying on G-protein coupled receptors (e.g., olfactory system).
  • Enzyme-linked receptors: Coupled to an enzyme which, when activated, induces reactions within the cell.
• Once a receptor is activated by a ligand, it initiates a signaling cascade that causes changes within the cell (e.g., inducing gene expression, secreting a molecule, changing cell growth, or changing cell identity).

Cellular Responses and Feedback Mechanisms

• Feedback mechanisms maintain internal environments and respond to changes.
• Negative Feedback Mechanisms: Return a system to its set point after a disruption, maintaining homeostasis.
• Positive Feedback Mechanisms: Amplify a response for a quick change, moving an organism farther away from equilibrium (e.g., fruit ripening releasing ethylene).

The Events in a Cell Cycle

• The cell cycle is the process where cells grow, duplicate their DNA, and divide to make two cells with identical DNA.
• In eukaryotic cells, it includes interphase, mitosis, and cytokinesis.
• During mitosis, the cell divides to make two cells with identical DNA.
• Interphase is the stage of the cell cycle where cells grow and copy their DNA.

Stages of Interphase

1. Gap I (G1): The cell grows, copies organelles, and synthesizes molecules needed for division.
2. S Phase: The cell duplicates its DNA and makes a second centrosome, creating sister chromatids.
3. Gap II (G2): The cell continues to grow and prepare for cell division.

Stages of Mitosis

1. Prophase: The chromatin condenses, the nucleolus disappears, and the mitotic spindle begins to form.
2. Prometaphase: The nucleus disappears, chromosomes become compact and attach to the mitotic spindle at their kinetochore, and the protein structure forms at the centromere of chromosomes.
3. Metaphase: Chromosomes line up at the center of the cell (metaphase plate) with each sister chromatid attached to microtubules at different centrosomes, ensuring daughter cells get one copy of each chromatid. The cell will not divide until the chromosomes are properly attached.
4. Anaphase: Microtubules pull one copy of each chromatid toward their centrosomes.
5. Telophase: The mitotic spindle breaks down, a nucleus forms around each set of chromosomes, the nucleolus reappears, and chromosomes unwind to form chromatin.

• Cells split through cytokinesis which begins around anaphase and ends after telophase is over.
  • In animal cells, a cleavage furrow forms between the two cells.
  • In plant cells, a stiff cell plate forms between the two cells to split them.
• After cell division, daughter cells can re-enter the cell cycle or go into the resting phase (G0).
• A cell can remain in G0 forever, or reenter the cell cycle when it receives a cue.

Cell Cycle Regulation

• The cell cycle is tightly regulated.
• If conditions are not ideal, resulting daughter cells could be damaged and undergo apoptosis (regulated cell death) or result in abnormal cell growth (cancer).
• To keep this from happening, the cell has positive and negative regulators to guide the cell cycle.

Cell Cycle Checkpoints

• Checkpoints are points within the cell cycle where progression can be stopped until conditions are deemed favorable to proceed.
• There are three checkpoints in the cell cycle:
  • G1 Checkpoint: Occurs during the G1 phase of the cell cycle. At this point, the cell decides whether to irreversibly commit to replicating by looking for growth, cell size, and DNA damage before it decides to enter the S phase. If damage is detected or conditions are not favorable, the cell will try to repair or correct problems, or wait in G0 until conditions are better.
  • G2 Checkpoint: occurs in the G2 phase - the cell checks for cell size, protein abundance, and integrity of synthesized chromosomes. If problems are found, the cell will try to repair them before advancing.
  • M Checkpoint: The cell checks to see if the kinetochores of sister chromatids are attached to the metaphase plate. The cell cycle will not proceed until this occurs.
• Molecules called cyclins and cyclin dependent kinases (CDKs) are positive regulators of the cell cycle; that is, they help the cell cycle proceed. Different cyclins increase expression at different checkpoints in the cell cycle. Without this interaction, the cell cycle will not proceed past a checkpoint. Cyclins bind their partnerCDKs and this interaction causes the CDK to phosphorylate other target proteins. These phosphorylation events activate proteins that are needed for the cell cycle to move forward; thus, until a critical level of cyclin/CDKs are present, the cell cycle will not proceed.
• Tumor suppressor proteins negatively regulate the cell cycle by checking for DNA damage and halting the cell cycle. In some cases, they halt progression through inhibiting cyclin/CDKs.

Heredity

• Around 8–11% of the questions on your AP exam will cover Heredity.

The Process and Function of Meiosis

• Meiosis is the form of cell division that creates gametes (sperm and eggs).
• Meiosis forms four daughter cells with half the amount of DNA as a normal cell.
• Normal cells are diploid (two non-identical copies of each chromosome).
• Non-identical copies of each chromosome are homologous chromosomes.
• Gametes are haploid (single copy of each chromosome).
• Meiosis has two rounds of cell division: meiosis 1 and meiosis 2.

Distinct Features of Meiosis

• During prophase 1, homologous pairs of chromosomes align and exchange DNA in a process called crossing over or recombination, resulting in unique chromosomes being distributed to gametes.
• In metaphase 1, homologous chromosome pairs are passed to daughter cells instead of individual chromosomes. Sister chromatids stay attached during this phase.
• In telophase 1, the nucleus may or may not form depending on the species.
• Meiosis 1 ends with two non-identical cells that each carry sister chromatids from one chromosome of a homologous pair. These chromatids are then split in meiosis 2.
• Meiosis 2 proceeds much like mitosis, but ends in four haploid cells, each carrying a single copy of each chromosome.

The Concepts of Genetic Diversity

• Diversity allows populations to adapt to their environments.
• Meiosis generates diversity through independent assortment, crossing over, and random fertilization.

Diversity in Meiosis

• Independent Assortment ensures that gametes only receive a haploid set of chromosomes from each parent. This creates diversity in offspring by allowing for an exponential number of combinations of chromosomes to be passed along.
• Crossing Over in meiosis I causes recombination between homologous chromosomes, so that gametes receive unique combinations of genetic material from the parent cells. Note that chromosome structure is maintained, so that gametes receive all the necessary genes for life to continue but will have different allele combinations compared to the parental chromosomes. This causes an additional layer of diversity in genetic material that is passed on by creating unique chromosomes in each generation.
• Gametes that undergo fertilization can contain any of the millionsof unique combinations of chromosomes produced in meiosis from both parents doubling the number of unique combinationsthat can be produced.