BIO101: Introduction to Molecular and Cellular Biology

Read this discussion about the nature of science, scientific inquiry, and hypothesis testing.

Watch this lecture, which discusses the unifying themes of biology. We will cover the last portion of the lecture which describes the chemistry and properties of water in Units 2 and 3. Note that the lecturer later apologizes for saying the "thymus" enlarges when she had meant to refer to the "thyroid" when describing a goiter in the neck at the 30-minute mark of the video.

The characteristics of living organisms (evolutionary adaptation, growth, reproduction, etc.) differentiate living and non-living things. The fundamental difference between a living thing and a dead thing is metabolism. Metabolism is the chemistry of life. It includes all of the chemical reactions occurring in all of the cells that make up an individual organism. Life ceases when the chemical reactions of metabolism cease. Read this chapter to explore and identify the common characteristics of living organisms.

These characteristics of life include adaptation, cell structure, growth and development, homeostasis, metabolism, reproduction, and responsiveness to the environment.

Watch this lecture, which introduces the atom and discusses the relationship between atoms and elements. What do you think the periodic table represents?

Watch this video, which discusses the subatomic particles of the atom.

Watch these short videos for an overview of the elements within an atom: protons, neutrons, and electrons. Make sure you understand the definitions for the proton, neutron, and electron. You should also be able to define atomic measurements, such as atomic number and mass, and calculate the number of subatomic particles based on this number.

Atoms have a specific structure that determines their behavior in an element or compound. Electrons occupy spaces around the nucleus. These spaces have a hierarchical arrangement. An orbital is a space that can be occupied by electrons. Each orbital can contain up to two electrons.

There are different types and shapes of orbitals: s, p, d, and f. There is only one kind of s orbital, but there are three kinds of p orbital, five d orbitals, and seven f orbitals. A collection of orbitals of the same type makes up a subshell, and a collection of subshells makes up a shell (also called an energy level). The first shell includes only one subshell (the s subshell), which is made up of only one s orbital. The second shell is made up of two subshells (an s and a p subshell), with the s subshell being made up of one s orbital and the p subshell being made up of three p orbitals. Since different shells contain different numbers of orbitals, each shell has a different maximum number of electrons it can hold.

Watch this lecture to learn about orbitals and review the structure of an atom.

Living structures are three-dimensional (3D), and electrons fly in three-dimensional orbitals. This feature is critical because it explains how elements join to form 3D molecules that build the 3D tissues and organs of life. Watch this lecture, which describes how the electrons fly in different orbitals.

Watch this lecture to learn about electron configuration and valence electrons.

Read this section, which describes how atoms and elements form different types of bonds between atoms.

After you read, see if you can answer these questions: What makes ionic bonds different from covalent bonds? Why are hydrogen bonds and van der Waals interactions necessary for cells? How do buffers help prevent drastic swings in pH? Why can some insects walk on water? What property of carbon makes it essential for organic life? Compare and contrast saturated and unsaturated triglycerides.

Watch this lecture to learn about the details of ionic, covalent, and metallic bonds, and how to balance chemical equations.

Now that you have learned about atoms and their structures, watch this overview of how to write and draw atoms, molecules, and chemicals correctly.

Now, let's learn how to balance chemical reactions. Elements come together to form molecules called products. It is important to make sure the same amount of elements are found on both sides of the chemical reaction.

Energy is a basic process common among all living organisms. We define energy as the capacity to do work, which refers to some sort of change. For example, moving an object from one place to another requires work, and energy is required for that work. Heat is energy in the form of the movement of particles (atoms, ions, or molecules) within a substance.

Heat is energy that is unavailable for performing work. Temperature is a measure of the average speed of the particles in an object. Temperature and heat are not the same thing. Temperature does not depend on how much matter is present, whereas heat does.

For example, a swimming pool has the same temperature as a cup of water from that swimming pool, but the swimming pool contains much more heat than the cup of water because it contains much more matter.

Read this section to learn how energy flows through a living system and how enzymes catalyze chemical reactions.

Energy can be readily converted between forms. For example, a book that falls from a shelf converts potential energy into kinetic energy. When a person moves the book back to the shelf, they convert kinetic energy into potential energy. The metabolism of life involves countless interactions between matter and energy and countless conversions between energy forms, so it is important to understand the distinction between matter and energy.

Watch this video, which offers an overview of the concept of energy and discusses how it relates to atoms and molecules. All atoms and molecules have kinetic energy or movement. We measure kinetic energy as temperature.

Watch this lecture, which explains the importance of the first law of thermodynamics to living organisms. Many scientists call the first law of thermodynamics the law of conservation of energy since It states that energy can be neither created nor destroyed, but it may change form. For example, imagine a campfire: the energy is stored in chemical bonds in the wood and is released as light and heat.

After watching the video, make sure you can identify the different forms of energy and how they are transferred and transformed.

Watch this video for a discussion of the Gibbs Law of Free Energy. After you have read this section, you should be able to write and define each component (G, H, S, T) of the equation for free-energy change, distinguish between exergonic and endergonic reactions in terms of free energy change, predict which reactions are spontaneous, and explain why organisms do not violate the second law of thermodynamics.

Read this chapter to learn about the four unique properties of water. After you read, you should be able to explain why water is an excellent solvent, describe water molecules that are polar and capable of hydrogen bonding with four neighboring water molecules, and distinguish between a solute, solvent, and solution.

Watch this lecture, which reviews the special chemical properties of the water molecule.

Watch these short videos, which give an overview of acids, bases, and the pH scale.

Watch this lecture to learn how the pH scale affects biological systems. Remember that acids are proton donors, while bases are proton acceptors.

As you review this section, notice the difference between a strong and weak acid and base. How do the pH measurements of acidic and basic solutions affect living organisms? Different foods have different amounts of hydrogen ions, which is also known as proton concentration.

Watch this lecture, which describes how acids and bases relate to the concentration of hydrogen ions in a solution. You should be able to explain how acids and bases alter the hydrogen ion concentration of a solution directly and indirectly.

Watch this lecture to review biological macromolecules and their role in biological organisms. Identify their building blocks and how they form polymers. Also, understand the structures and how they relate to the function of each macromolecule's role in organisms. After watching, you should be able to list the four major classes of macromolecules, distinguish between monomers and polymers, and define the terms dehydration synthesis and hydrolysis.

Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides. Some polysaccharides (like cellulose and chitin) are important for their structural strength, whereas other polysaccharides (like starch and glycogen) are important for storing energy. Polysaccharides also serve as important identity markers on the surfaces of cells, so they play a role in immunity.

A carbohydrate is an organic compound such as sugar or starch, which plants use to store energy. Read this text, which discusses the role of carbohydrates in cells and in the extracellular materials of animals and plants. When you finish, make sure you can describe the function and structure of carbohydrates.

Lipids are important for storing energy, thermal insulation, and providing protective padding. Phospholipids form the infrastructure of all cell membranes. Lipids also make up natural waxes and oils and many hormones. Read this section to learn more about lipids. When you have finished reading this section, make sure you can describe the function and structure of lipids.

Proteins perform an impressively long list of biological functions. They function as enzymes, structural elements, chemical signals, transporters, and receptors. They also play important roles in cell-to-cell adhesion and immunity. As you read this section, make sure you can describe the four levels of protein structure.

Nucleic acids include various DNA and RNA molecules. They serve informational purposes. DNA stores the genetic code, and various types of RNA help in the process of interpreting that code to build proteins. Certain RNAs can also function as catalysts.

Read this chapter to learn about important organelles. Most of these organelles are membrane-bounded and only appear in eukaryotic cells, which are structurally more complex than prokaryotic cells from which they evolved. Pay close attention to Figure 1 and to the differences in animal cells, plant cells, and bacterial cells.

Watch this lecture for a visual tour of the organelles of the cell.

Watch these short videos for another look at the different types of cells (prokaryotes and eukaryotes), intracellular and extracellular fluid, the nucleolus which builds ribosomes, and the other organelles, such as the mitochondria which create ATP from glucose, ribosomes, and chloroplasts, that inhabit them. Again, it is important to know the structures and functions of each organelle. The final video in the series discusses the cytoskeleton which is made out of protein filaments and helps with cell movement.

After you have watched the series of videos, you should be able to describe the structure and function of all organelles in the cell, explain how organelles control protein synthesis in the cytoplasm, discuss the difference in organelles between plant and animal cells, list the components of the endomembrane system, describe the energy conversions carried out by mitochondria and chloroplasts, and describe the functions of the cytoskeleton.

Watch this short video for a visual overview of the molecular structure and function of the cell or plasma membrane. Note that the membrane is composed mainly of phospholipids and proteins. It is selectively permeable, allowing certain organic molecules to pass into and out of the interior.

The chief components of the cell or plasma membrane are phospholipids. Each phospholipid molecule is an amphipathic molecule (polar at one end and non-polar at the other end). This explains why plasma membranes form.

In the presence of water, phospholipids self-assemble into a bilayer, with the non-polar tails in each monolayer pointing toward the non-polar tails of the other monolayer, and the polar heads of each monolayer pointing toward the watery solution on its side of the membrane (the water interior of the cell for one monolayer, and the water exterior of the cell for the other monolayer). In addition to the phospholipid bilayer, a plasma membrane features various other macromolecules, including proteins, sterols, and polysaccharides.

As you read this chapter on the cell membrane, pay attention to how certain molecules "transport" across the cell membrane. We will examine these mechanisms in more detail later in the unit.

After you read, you should be able to describe how phospholipids and most other membrane constituents are amphipathic molecules, list the major functions of membrane proteins, and define the three types of transport: selective permeability, active transport, and passive transport.

Watch this lecture that reviews cell membranes and details their central role in cell function and survival. Learn the components on the plasma membrane and reflect on their macromolecule characteristics.

After you have watched the videos in this section, you should be able to explain how molecules cross cell membranes, discuss the channel proteins, carrier proteins, pumps, and aquaporins involved in transport, distinguish between osmosis, facilitated diffusion, and active transport, and describe how large molecules are transported across a cell membrane.

Watch these videos, which describe the properties of lipids that form a major component of the plasma membrane. As you review these characteristics, keep the material you learned in the previous resources in mind. Pay attention to the difference between unsaturated and saturated fatty acids of phospholipids. You should also be able to explain how cholesterol and steroids resist changes in membrane fluidity.

This video gives an additional explanation of the chemical and visual structure of lipids.

Read this text, which explains how the Fluid Mosaic model describes the structure of the plasma membrane as a mosaic of components – including phospholipids, cholesterol, proteins, and carbohydrates – which gives the membrane a fluid character. These macromolecules have special characteristics that relate to the functionality of the plasma membrane.

After reading, you should be able to define the fluid mosaic model, explain why membranes with different functions have different types of membrane proteins, describe the fluidity of the components of a cell membrane, and distinguish between peripheral and integral membrane proteins and their major functions.

Watch this video to see how hydrophobic molecules move from a high to a lower concentration across the plasma membrane via simple diffusion. Notice that proteins are not required for this process.

This video discusses simple diffusion and introduces the concept of osmosis, the movement of water across the plasma membrane to a place where there is a higher concentration of solutes. Make sure you can define hypertonic, hypotonic, and isotonic solutions.

After you have watched this video, you should be able to define diffusion and explain why it is a passive process that does not require energy, define terms hypertonic, hypotonic, and isotonic solutions, and define osmosis and predict the direction of water movement based on differences in solute concentrations in the cell or solutions.

This video provides some animations of osmosis and diffusion. Notice that molecules on both sides of the membrane will continue to move until they reach equilibrium.

Watch this video to review what we have learned about passive transport and osmosis.

Facilitated diffusion is the movement of hydrophilic molecules from a high concentration to a lower concentration across the plasma membrane. Due to the partially hydrophobic nature of the membrane, facilitated diffusion requires a protein. As you watch, pay attention to how transport proteins facilitate diffusion. Make sure you can distinguish between osmosis, simple diffusion, and facilitated diffusion.

Active transport is the movement of molecules from an area of low concentration to a higher concentration across the plasma membrane. The movement of substances against the concentration gradient is non-spontaneous and requires energy and a protein. This process explains how signal molecules are transported across the cell membrane. As you watch, pay attention to the definitions of the types of proteins and energy used in active transport. You should be able to describe the process of cotransport.

As you read this text, be sure you understand the functional differences between the three classes of receptors (ion channel linked, g-protein-linked, and enzyme-linked), even though all three operate by binding to a signal molecule at the exterior surface. It helps to look at diagrams to make sense of the differences.

Watch these videos, which present the basic concepts of bioenergetics. Make sure you can distinguish between kinetic and potential energy and explain how energy is transformed from one form to another.

Watch this lab experiment demonstrating how the human body breaks down the sugar in a gummy bear into carbon dioxide, water, and energy.

Watch this video for an overview of the metabolic process. It describes the two major kinds of metabolic pathways, linear and cyclical. Pay attention to the descriptions for catabolic and anabolic pathways in cellular metabolism.

Note that catabolism breaks down molecules (creates energy), while anabolism assembles molecules (requires energy). You should also be able to define the components of a metabolic pathway: reactants (substrate molecules), intermediates (intermediate substrates), and the final products.

Watch this video, which reviews and builds on what we have learned so far about energy, including the first law of thermodynamics, the second law of thermodynamics, exergonic and endergonic chemical reactions, and how humans get energy. The presenter discusses how cells use ATP as an energy source.

An enzyme is a protein that serves as a biological catalyst. A catalyst is a substance that accelerates a chemical reaction without actually being a reactant in that reaction. In other words, a catalyst (and therefore an enzyme) does not get changed into another substance (a product). An enzyme interacts with reactants in such a way as to make it much more likely for those reactants to chemically react, turning them into products.

We call the reactants of a catalyzed reaction substrates. An enzyme operates by temporarily binding to substrates. The rate of the reaction (its speed) when an enzyme catalyzes it is typically at least one million times the rate without the help of an enzyme. This is why enzymes are absolutely vital. Without enzymes, the biochemical reactions of metabolism would occur much too slowly to support life. Importantly, since an enzyme does not get altered in a reaction in which it participates, it is reusable, and it can continue to catalyze the same sort of reaction if more substrate is present.

Watch this video, which discusses the characteristics and functions of enzymes. The presenter describes various factors that promote and affect enzyme activity, such as temperature, pH, concentrations, competitive inhibition, and allosteric regulation. Pay attention to the discussion on feedback inhibition and the regulatory mechanisms of catabolic pathways. Think about how this type of regulation benefits cells in terms of energy conservation.

After watching this video, you should be able to describe how inhibitors regulate the activity of enzymes of metabolic pathways and explain how excess products inhibit pathways to prevent a cell from wasting chemical resources.

Read this section to review enzyme nomenclature. Notice how the names of the enzymes often relate to actions on substrates.

Any living cell is able to extract energy from fuel and temporarily store that energy in the form of ATP, or a similar energy currency. The primary processing of fuel is called glycolysis. Only certain cells under the right conditions are able to continue where glycolysis leaves off, allowing much more usable energy to be extracted from the fuel.

That additional processing of energy is called cellular respiration. Glycolysis and cellular respiration both extract usable energy from fuel by undergoing oxidation/reduction (or redox) reactions.

Oxidation refers to the loss of electrons from a particle (such as a fuel molecule), whereas reduction is the gain of electrons. Since electrons are not destroyed in chemical reactions, oxidation occurs only if reduction also occurs. When something is oxidized (loses electrons), something else gets reduced (gains electrons).

Organisms extract energy from fuel molecules by oxidizing these fuel molecules. In cellular respiration, there is a substance (external to the process) that ultimately accepts the electrons that have been removed from the fuel. For aerobic organisms, that substance is oxygen, and when oxygen accepts those electrons (along with protons) from the fuel molecules, the oxygen gets reduced into water. Cellular respiration is important because it allows for maximal oxidation of fuels, which maximizes the amount of energy that can be extracted and stored as ATP.

Watch this video for an overview of cellular respiration. After you watch, you should be able to explain the role of cellular respiration.

Watch this video, which explores the concept of energy chemical systems. The presenter explains how molecules move back and forth between a low energy state (more stable) to a high energy state (more unstable). Pay attention to the description of endergonic and exergonic reactions. The presenter explains the movement of energy between ADP and ATP, and NAD+ and NADH, FAD, and FAD2.

Watch this lecture to review the role of the ATP molecule and its importance for cells. It will help you understand the phosphate functional group and how the hydrolysis of ATP releases energy.

At this point, you should be able to explain the catabolic and anabolic pathways in cellular metabolism, describe the structure and function of ATP, explain how ATP performs cellular work, and distinguish between consumers, producers, and decomposers of nutrition.

Watch this overview of the process of glycolysis and its ingredients. Glycolysis has two main steps: energy-requiring and energy-releasing.

Watch this video, which provides more details about glycolysis.

At this point, you should be able to describe how the carbon skeleton of glucose changes as it proceeds through glycolysis, the structure of ATP, the investment phase that requires ATP, the payoff phase that produces ATP, and the products of glycolysis.

Watch this lecture for another overview of the process of glycolysis.

In this section, we look at the chemical reactions involved in this cycle in greater detail. Notice that there are multiple names for this cycle in biology: scientists use the three names (the Krebs cycle, citric acid cycle, and TCA cycle) interchangeably since they all refer to the same process.

Watch this lecture for a tour of the Krebs/citric acid cycle.

After you watch, you should be able to identify where the Krebs Cycle takes place in the mitochondria, describe the process of the Krebs Cycle, list the products of the citric acid cycle, and explain why we call it a cycle.

Read this section to review the next steps in the breakdown of glucose in aerobic cellular respiration. The citric acid cycle and oxidative phosphorylation are the two processes that lead to the complete oxidation of glucose.

After you read, you should be able to describe where pyruvate is oxidized to acetyl CoA, how the oxidation of pyruvate links glycolysis to the citric acid cycle, and what a cycle is and the products of the citric acid cycle.

Watch this video, which outlines the final steps in cellular respiration. Pay close attention to the creation of ATP by the specialized protein complexes found in the mitochondria’s inner membrane.

After you watch, you should be able to describe the proteins that are a part of the electron transport chain, explain the movement of electrons down the electron transport chain is coupled production of ATP by chemiosmosis, explain where and how the respiratory electron transport chain creates a proton gradient, and distinguish the electron transport chain from the ATP synthase.

Watch this video, which tabulates the amount of energy cellular respiration generates.

Watch this lecture for another tour of the electron transport chain.

Read this text, which reviews what we have learned about oxidative phosphorylation and chemiosmosis. This process involves the coupling of the proton motive force initiated by the oxidation of the electron carriers NADH and FADH₂. Hydrogen ions flow from the intermembrane space back to the matrix of the mitochondria through the ATP synthase. This chemiosmosis of hydrogen ions is coupled to the endergonic formation of ATP.

After you read, you should be able to distinguish between substrate-level phosphorylation and oxidative phosphorylation, explain the endergonic production of ATP by chemiosmosis through ATP synthase, and illustrate the production of ATP yield from the oxidation of a glucose molecule.

Watch this lecture to review the entire process of cellular respiration, from glycolysis to ATP generation, and the role of oxidation-reduction reactions in biological systems.

Read this text to review the process of fermentation.

After you read, you should be able to describe the purpose of fermentation, distinguish alcohol fermentation and lactic acid fermentation, and compare the processes of fermentation and anaerobic cellular respiration.

Watch this video, which offers another explanation of anaerobic cellular respiration and its byproducts lactic acid and ethanol.

Read this overview of the process of photosynthesis. The text discusses basic photosynthetic structures and the two cycles in photosynthesis: the light and dark cycles, which we will explore later in the unit.

Read this review of the light-dependent reactions of photosynthesis. Light dependent reactions require light energy and water to create products ATM, NADPH, and oxygen.

Read this review of the Calvin cycle, which is the light-independent reactions in photosynthesis. Light independent reactions require ATP, NADPH from the light reactions. and carbon dioxide to create sugar for the plant.

Watch this overview of photosynthesis, which gives a visual demonstration of how and where this process works inside the plant.

Incorporating carbon from an inorganic source like carbon dioxide into organic compounds like glucose is called carbon fixation, and that is an extremely important function of photosynthesis. Carbon fixation results in products (organic compounds) that contain more chemical energy than the reactants (carbon dioxide molecules). Doing so requires an input of energy. The input of energy for the carbon fixation that occurs during photosynthesis is energy in the form of sunlight. Powered by that light energy, water molecules are split into oxygen and hydrogen atoms, and the hydrogen atoms from the water end up bonded to carbon atoms from carbon dioxide molecules to form high-energy carbohydrates. This occurs in two major pathways that comprise photosynthesis.

Notice that the speaker explains why he also calls light-independent reactions "carbon reactions". The light reactions take energy from light and convert it into this chemical form of ATP and NADPH. They also produce oxygen. The carbon reactions are powered by the ATP and NADPH to do the work of photosynthesis.

Watch these videos, which review the chemical processes that occur during the light-dependent and light-independent reactions of photosynthesis. Make sure you can describe how photosynthesis converts low-energy molecules into energy-rich carbohydrates. You should be able to explain how energy is transformed and transferred during photosynthesis.

Watch this video, which reviews the structures and cycles of photosynthesis. Pay attention to the chemical reactions that occur throughout the process, where they take place, and the products of these reactions. You should be familiar with the two stages of photosynthesis and the products of each stage of the process.

Watch this video about the details of the light reactions of photosynthesis. After you watch, you should be able to list the components of the chloroplasts, describe the role of chlorophyll in the light reactions, describe the photosystems and their role in the light reactions, and trace the movement of electrons in noncyclic electron flow.

Watch this video about photophosphorylation of light reactions. After you watch, you should be able to describe the role of the cytochrome complex in the light reactions, trace the movement of electrons in cyclic vs. non-cyclic electron flow, and describe the role of ATP synthase in light reactions.

Watch this video about the light-independent Calvin cycle. You should be able to describe the role of light-dependent byproducts ATP and NADPH in the Calvin cycle, illustrate and describe the function of each of the three phases of the Calvin cycle, and discuss the products of the Calvin Cycle.

Watch this lecture, which discusses an alternate mechanism of the Calvin cycle called photorespiration. You should be able to describe what happens when ribulose-bisphosphate carboxylase-oxygenase (RuBisCO) interacts with O₂ instead of CO₂ because of concentration differences.

Watch these videos to learn about C-4 and CAM photosynthesis. Make sure you are able to explain the differences between these unique types of photosynthesis processes.

Watch this video for an overview of the carbon cycle. Note that the carbon cycle has serious implications for the regulation of our atmosphere and climate, such as the production and accumulation of greenhouse gasses. We will not discuss this phenomenon in detail in this course, but it is an important aspect of the study of environmental science.

As their names indicate, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are both nucleic acids. Any nucleic acid is a polymer made up of monomers called nucleotides.

Any nucleotide consists of three components:

1. A pentose (five-carbon sugar)
2. A nitrogenous base attached to the pentose
3. A phosphate group also attached to the pentose

For DNA, the specific pentose in each of its nucleotides is deoxyribose, whereas RNA features ribose as the pentose in each of its nucleotides. There are normally four kinds of DNA nucleotides because there are four normal nitrogenous bases used in DNA: adenine, guanine, cytosine, and thymine.

RNA also features four kinds of nucleotides, and the nitrogenous bases are nearly the same as for DNA, except that RNA uses uracil instead of thymine. DNA functions for self-replication (before a cell divides into two cells) and for transcription, a process that produces RNA. There are different functional categories of RNA including mRNA, tRNA, rRNA, and others.

Watch this overview of DNA and RNA that explains how these important nucleic acids form the blueprint for life's structures. After you watch, you should be able to explain the difference between RNA and DNA, identify the three types of RNA, and discuss the central dogma of how information flows from gene to protein.

Read this section to learn about the phases of the cell cycle. This includes interphase, mitosis, and cytokinesis. As you review the cell cycle, pay particular attention to the diagram in Figure 1. After you read, you should be able to list the phases of the cell cycle, describe the events that occur during each phase, and describe the roles of checkpoints in the cell cycle control system.

Watch this video to review the particular events of each mitotic phase. Make sure you can illustrate the phases and describe the events that characterize each phase.

Watch this video, which discusses the cell cycle. Make sure you can recognize the phases of mitosis from the diagrams.

Watch this video for a guided tour of mitosis as seen on a microscopy slide of an onion root tip. The video gives examples of cells in four stages of mitosis: prophase, metaphase, anaphase, and telophase.

Prokaryotic cells typically reproduce by binary fission, which is much simpler than eukaryotic cell reproduction. Read this section and notice the differences observed in prokaryotic cell reproduction.

Read this introduction to sexual reproduction. Organisms give half of their chromosomes to their offspring. Pay attention to the definition of haploid and diploid cells. As you read, think about the ways sexual reproduction benefits a species by creating diversity.

Another important purpose of meiosis is to increase the genetic variability of the gametes produced. The increase in genetic variability comes in the forms of crossing over (during prophase of meiosis I) and independent assortment (during metaphase of meiosis I). As you study the phases of meiosis, appreciate that crossing over and independent assortment produce new genetic combinations, and separation of homologs reduces the ploidy.

Read this section to learn about the events that occur during meiosis.

Watch these videos to review the steps of mitosis and meiosis. Notice the additional division in meiosis that leads to genetic variability. How would you describe stages of meiosis and how cells are produced? Can you explain why meiosis is needed for sexual reproduction? What cells are involved in fertilization? How does fertilization occur?

After you have watched the videos, you should be able to distinguish between somatic cell and gamete, distinguish between haploid and diploid cells, list the phases of meiosis I and meiosis II, describe the events of each phase, and illustrate the phases of meiosis.

Watch this video for a summary of the differences between mitosis and meiosis.

Watch this video for a guided tour of meiosis as seen on a microscopy slide of lily flower buds. The speaker presents examples of cells in four stages of meiosis: late prophase I, telophase I, anaphase II, and telophase II.

Read this text, which discusses how nondisjunction leads to disorders in chromosome number and how errors in chromosome structure occur through inversions and translocations.

Watch this video for an overview of chromosomes.

Watch this video, which discusses the structure of chromosomes and their composition. At this point, you should be able to differentiate between the concepts of chromosomal structure, chromatin, and chromatids. You should also be able to define the role of the centromere.

Many consider Gregor Mendel to be the father of genetics because his experiments in simple genetics provided a foundation for modern genetics. Read this section to review Mendel's laws. Be sure to spend some time reviewing the figures. Read this brief introduction to genetics to learn about one of the pioneers of the genetic laws of inheritance.

Genes are sequences of DNA. These genes serve as instructions for how to make corresponding sequences of RNA. The sequences of RNA, in turn, serve as instructions for how to build proteins. Proteins are the final product of gene expression, and the particular proteins that are built in the cells of an organism are what give that organism its traits. The traits are directly determined by the proteins, but the traits are indirectly determined by the genes because the genes indirectly instruct the cells how to build the proteins. For any given characteristic (like hair color, for example), there might be multiple possible traits (such as black hair and brown hair).

The particular trait exhibited by an individual for a particular characteristic (like the black hair trait for the hair-color characteristic) is known as that individual's phenotype. The phenotype depends on the proteins produced, which depends on the version of the corresponding gene (DNA) that an individual possesses.

We use the term "genotype" to refer to that individual's particular DNA sequence for that particular gene. Therefore, genotype indirectly determines phenotype.

The laws of inheritance can be used to predict traits in offspring. Read this section to review these laws, and take time to review the figures to review the movement of traits to offspring.

Read this section to explore the laws of inheritance in more detail. After you read, you should be able to describe the exceptions of Mendelian genetics, define incomplete dominance and codominance, and explain the inheritance of the ABO blood system.

A mutation is an accidental change in the DNA sequence of an organism. As you know, your DNA is arranged in chromosomes, which is a long sequence of DNA nucleotides. A gene is a subsequence of DNA nucleotides within the longer sequence making up the chromosome. A mutation arises when the sequence gets changed either by an error during DNA replication or by some other accident (including exposure to certain chemicals or forms of radiation, for instance). The reason a gene is able to serve as a code (stored information) is because of its particular sequence of DNA nucleotides.

Mutation changes that sequence. Therefore the code is changed, and when that altered code is used to make RNA, the RNA will also be altered. Similarly, when the code in that altered (mutated) version of RNA is used to build a protein, that protein might be different from the unmutated version.

Since proteins directly determine phenotypes, a mutant protein (resulting from a mutant form of DNA for the corresponding gene) might result in an altered phenotype. Mutation is the original source of genetic variation, and it is the reason why there are different species and why there are differences between individuals of the same species. As you continue to study genetics, try to keep in mind that differences in genotypes and phenotypes are ultimately due to mutations that have accidentally occurred.

After you watch these videos, make sure you can define mutation and explain how mutations, such as that in sickle cell, can result in a changed phenotype.

Watch these lectures for an explanation of inheritance and genetics. Punnett squares are named after the geneticist Reginald Punnett, who developed the method for predicting probabilities. Work the examples to practice Punnett squares.

After you watch, you should be able to distinguish between dominant and recessive, distinguish between heterozygous and homozygous, distinguish between genotype and phenotype, use a Punnett square to predict the results of a monohybrid cross, use a Punnett square to predict the results of a dihybrid cross, and state the phenotypic and genotypic ratios of Punnett crosses.

Watch this brief video on pedigrees, which are how we learn more about genetics in human populations. Pedigrees are useful in populations like these, where we cannot test in a controlled environment.

Watch this video to review the unique inheritance patterns linked to sex-determining chromosomes.

Watch this lecture to review the structure of DNA. All of the nucleotides in DNA are made of the same basic parts: deoxyribose sugar molecules and nitrogenous bases (the purines: adenine and guanine, and the pyrimidines: thymine and cytosine). Adenine pairs with thymine, and guanine pairs with cytosine. After you finish this section, you should be able to create complementary base pairs on a DNA strand.

Review this 3D model of the double helix structure of a DNA molecule.

Read this text, which explains what occurs during DNA replication. Pay attention to how different enzymes catalyze the production of leading and lagging strands of DNA.

After you have read the text, you should be able to explain how DNA replicates in a semiconservative manner, describe the process of DNA replication, describe the functions for the variety of enzymes that catalyze this process, and define Okazaki fragments.

Watch this video animation of DNA replication. It shows how both strands of the DNA helix are unzipped and copied to produce two identical DNA molecules.

First, let's review these five lectures which discuss protein synthesis. After you have watched these lectures and read the text that follows them, you should be able to describe the steps and catalysts of transcription, and describe the structure and function of mRNA.

Read this text, which discusses this process of transcription. Pay attention to the discussion on alternative splicing and mutations found in organisms in the "Evolution Connection" box.

Watch these videos. After you watch, you should be able to describe the structure and function of mRNA, rRNA, and tRNA, the structures and function of the ribosome, and the steps and catalysts of transcription.

Read this text, which discusses the process of transcription.

Read this section, which explores the regulation of genes and how gene regulation is used in cells. After you read, you should be able to explain how genes are modified before or after transcription and translation and explain the differences of regulation between prokaryotic and eukaryotic gene regulation.

As you watch this video, pay attention to the ethical concerns that these genetic altering technologies present. There is a grave risk that scientists could inadvertently create damaging genetic mutations that could seriously harm humans, animals, and our very biosphere.

This article discusses the CRISPR technique in more detail.

Watch this video. What disease are the scientists studying? What is the hypothesis that the scientists hope to support? What causes human genetic variation? How do scientists determine ancestry? In terms of asthma, what can scientists determine when they compare ancestral patterns? How might these findings be used in the future?