Chromosomes are coiled structures containing genes. They are the form in which DNA goes through cell division. Cell division in eukaryotic cells includes mitosis, in which the nucleus divides, and cytokinesis, in which the cytoplasm divides and daughter cells form. Mitosis has four phases: prophase, metaphase, anaphase, and telophase.
- CA.9–12.IE.1.d, CA.9–12.LS.1.c
- NSES.9–12.A.1.4; NSES.9–12.C.1.3, 4; NSES.9–12.C.2.1, 2
- AAAS.9–12.5.B.4; AAAS.9–12.5.C.4
- Describe chromosomes and their role in mitosis.
- Outline the phases of mitosis.
anaphase: third phase of mitosis, during which sister chromatids separate and move to opposite poles of the cell
centromere: region of sister chromatids where they are joined together
chromatid: one of two identical copies of a chromosome that are joined together at a centromere before a cell divides
chromatin: grainy material that DNA forms when it is not coiled into chromosomes
chromosome: coiled structure made of DNA and proteins containing sister chromatids that is the form in which the genetic material of a cell goes through cell division
gene: unit of DNA on a chromosome that is encoded with the instructions for a single protein
homologous chromosomes: pair of chromosomes that have the same size and shape and contain the same genes
metaphase: second phase of mitosis, during which chromosomes line up at the equator of the cell
prophase: first phase of mitosis, during which chromatin condense into chromosomes, the nuclear envelope breaks down, centrioles separate, and a spindle begins to form
telophase: last stage of mitosis, during which chromosomes uncoil to form chromatin, the spindle breaks down, and new nuclear membranes form
Check Your Understanding
Did you know that:
- The bacterium E. coli has one chromosome, a human has 46 chromosomes, and a species of field horsetail (a plant) has 216 chromosomes!
- If all of the nucleotide bases in all your chromosomes were spaced about 1 mm apart, they would spread from Memphis, Tennessee to Los Angeles, California (a distance of over 1600 miles)!
Introducing the Lesson
Introduce chromosomes by showing students a colorful human karyotype (see URLs below for examples). Call on volunteers to explain what the karyotype represents (a complete set of human chromosomes). Say that two copies of each of the 23 different human chromosomes must be present in each cell.
Ask: How do all those chromosomes divide properly during cell division so that each daughter cell gets a complete set? (Answers may vary.) Tell students they will learn how when they read this lesson.
Have students work through the phases of mitosis with the interactive animation at the URL below.
Visual learners and English language learners may benefit from the excellent video animation of mitosis at the URL below. They will be whisked inside a cell to see the colorful, dynamic interactions of chromosomes and other structures. The eerie, sci-fi music adds to the dramatic nature of the presentation. ELL
Ask a small group of students to create a three-dimensional model of mitosis. If they need inspiration, they can get a wide range of ideas at the URLs below. Give the group a chance to present the model to the class.
Assign the activity “The Process of Mitosis” at the URL below. Students will actively model the mitosis in the classroom with a few readily available materials.
Students may hold one of the following common misconceptions about chromosomes. Make sure they are aware of the correct information.
- The X-shaped structures seen during mitosis are “basic” (unreplicated) chromosomes. (No; they are condensed, replicated chromosomes consisting of two identical DNA double helices.)
- The X-shaped structures seen during mitosis are homologous chromosome pairs. (No; they are condensed, replicated chromosomes. Pairing of homologous chromosomes does not occur during mitosis.)
Reinforce and Review
Copy and distribute the lesson worksheets in the CK-12 Biology Workbook. Ask students to complete the worksheets alone or in pairs as a review of lesson content.
Have students answer the Review Questions listed at the end of the lesson in the FlexBook®.
Points to Consider
Cell division occurs not only as organisms grow. It also occurs when they reproduce.
- What role do you think cell division plays when prokaryotes such as bacteria reproduce?
- How do you think cell division is involved in the reproduction of eukaryotes such as humans?