Learn about the gene as a stretch of DNA on a chromosome. Also find out about alleles as versions or sequence variants of a gene. [8:17]

Khan Academy learning modules include a Community space where users can ask questions and seek help from community members. Educators should consult with their Technology administrators to determine the use of Khan Academy learning modules in their classroom. Please review materials from external sites before sharing with students.

This narrated animation allows the learner to understand how six feet of DNA can be tightly packed to fit inside the nucleus of every human cell. [1:43]

Biology 2e is designed to cover the scope and sequence requirements of a typical two-semester biology course for science majors. The text provides comprehensive coverage of foundational research and core biology concepts through an evolutionary lens. Biology includes rich features that engage students in scientific inquiry, highlight careers in the biological sciences, and offer everyday applications. The book also includes various types of practice and homework questions that help students understand—and apply—key concepts. The 2nd edition has been revised to incorporate clearer, more current, and more dynamic explanations, while maintaining the same organization as the first edition. Art and illustrations have been substantially improved, and the textbook features additional assessments and related resources.

By the end of this section, you will be able to do the following:

Describe the behavior of chromosomes during meiosis, and the differences between the first and second meiotic divisions
Describe the cellular events that take place during meiosis
Explain the differences between meiosis and mitosis
Explain the mechanisms within the meiotic process that produce genetic variation among the haploid gametes

By the end of this section, you will be able to do the following:

Describe the structure of prokaryotic and eukaryotic genomes
Distinguish between chromosomes, genes, and traits
Describe the mechanisms of chromosome compaction

By the end of this section, you will be able to do the following:

Describe the structure of eukaryotic cells
Compare animal cells with plant cells
State the role of the plasma membrane
Summarize the functions of the major cell organelles

This comprehensive study guide covers the main terms and concepts needed for a unit on cell division and the cell life cycle.

A free CK-12 account is required to view all materials.

College-level course focusing on the principles of genetics. Course topics include structure and function of genes, chromosomes and genomes, biological variation, population genetics, use of genetic methods to analyze protein function, gene regulation, and inherited disease. Course features include lecture notes, assignments and solutions, and exams and solutions.

Covers the four unifying principles of biology.

A free CK-12 account is required to view all materials.

This pathway provides an introduction to the cell cycle and how its regulation is important to normal cell division. Cancer is often caused by unregulated cell division. For a deeper look at this topic, we recommend the pathway Cell Reproduction from the OpenStax textbook Biology for AP® Courses.

This pathway explores division of cells through mitosis and meiosis and how disregulation of these processes can lead to disease.

Students reinforce their knowledge that DNA is the genetic material for all living things by modeling it using toothpicks and gumdrops that represent the four biochemicals (adenine, thiamine, guanine, and cytosine) that pair with each other in a specific pattern, making a double helix. They investigate specific DNA sequences that code for certain physical characteristics such as eye and hair color. Student teams trade DNA "strands" and de-code the genetic sequences to determine the physical characteristics (phenotype) displayed by the strands (genotype) from other groups. Students extend their knowledge to learn about DNA fingerprinting and recognizing DNA alterations that may result in genetic disorders.

At DNA from the Beginning, choose from classical genetics, molecules of genetics, genetic organization and control. Each concept is explained through animation, an image gallery, video interviews, biographies and links.

Thomas Hunt Morgan will explain the research he did to determine that some traits are linked to the sex of an individual.