Mitosis, Meiosis and Fertilization

–Major Concepts, Common Misconceptions and Learning Activities[1]

Major Concepts and Common Misconceptions

Major Concepts from Next Generation Science Standards[2]:

  • LS1.A: Structure and Function – "All cells contain genetic information in the form of DNA molecules. Genes are regions in the DNA that contain the instructions that code for the formation of proteins."
  • LS1.B: Growth and Development of Organisms – "In multicellular organisms individual cells grow and then divide by a process called mitosis, thereby allowing the organism to grow. The organism begins as a single cell (fertilized egg) that divides successively to produce many cells, with each parent cell passing identical genetic material (two variants of each chromosome pair) to both daughter cells. Cellular division and differentiation produce and maintain a complex organism, composed of systems of tissues and organs that work together to meet the needs of the whole organism."
  • LS3.A: Inheritance of Traits – "Each chromosome consists of a single very long DNA molecule, and each gene on the chromosome is a particular segment of that DNA. The instructions for forming species' characteristics are carried in DNA. All cells in an organism have the same genetic content …"
  • LS3.B: Variation of Traits – "In sexual reproduction, chromosomes can sometimes swap sections during the process of meiosis (cell division), thereby creating new genetic combinations and thus more genetic variation."

More Detailed Major Concepts:

  • Eachcell contains chromosomes and each chromosome contains a long DNA molecule. Each DNA molecule has many genes. A gene provides the instructions for making a protein. Different versions of a gene are called alleles, and different alleles give the instructions for making different versions of a protein. These different versions of a protein can result in different phenotypic characteristics.
  • Chromosomes come in pairs of homologous chromosomes. In each pair of homologous chromosomes, both chromosomes havethe same genes at the same locations, but a gene may have different alleles in the two chromosomes of a homologous pair.
  • The cell cycle includes two growth phases, DNA replication, mitosis and cytokinesis. DNA replication and mitosis ensurethat each daughter cell receives a complete copy of the DNA in the parent cell. The cell cycle produces new cells for growth and repair. Many repeats of the cell cycle produce the trillions of cells in a human body (except not the sperm and eggs).
  • At the beginning of mitosis, the two copies of the DNA in each chromosome are condensed into compact sister chromatids which are attached at a centromere. During mitosis, spindle fibers line up the chromosomes in the middle of the cell and then separate the sister chromatids of each chromosome, resulting in two complete sets of chromosomes at opposite ends of the cell.
  • At the end of mitosis, cytokinesis separates the two halves of the cell to form two genetically identical daughter cells.
  • A different type of cell division, called meiosis, produces haploid gametes (sperm and eggs).Each haploid gamete contains one from each pair of homologous chromosomes. Thus, when a sperm fertilizes an egg, the resulting zygote has the normal diploid number of chromosomes.
  • The DNA is replicated before meiosis begins. Then,Meiosis Iseparates pairs of homologous chromosomes and Meiosis II separates sister chromatids.
  • Different gametes produced bythe same person have different genetic makeup due to the separation of homologous chromosomes with different alleles into different gametes, independent assortment, and crossing over.
  • Some differences between mitosis and meiosis are:

Characteristic / Mitosis / Meiosis
# of daughter cells / 2 / 4
Produces: / Diploid body cells / Haploid gametes
# and type of cell divisions / 1; separates sister chromatids / 2; the first division separates pairs of homologous chromosomes and the second division separates sister chromatids
Genetic makeup of daughter cells / Identical with each other and the original cell / Different from original cell and
from each other
  • When a haploid sperm fertilizesa haploid egg, the resulting diploid zygote receives one copy of each gene from the mother and one from the father. The zygote undergoes repeated rounds of mitosis to produce trillions of genetically identical cells in the body of the offspring.In this way, each person receives half of his/her genes from his/her mother and half from his/her father.As a result, children tend to resemble their parents and their siblings. However, the genetic diversity of the sperm and eggs produced by each parent results in genetic diversity of the different offspring produced by the same mother and father.
  • Understanding how gene-carrying chromosomes behave duringmeiosis and fertilization provides the basis for understandinginheritance.

It is important to begin by focusing on these major concepts to ensure that students understand the basics and have a meaningful context for understanding the specifics ofmitosis and meiosis.The approach to teaching mitosis and meiosis suggested below is designed to accomplish this goal and to help students overcome the following common misconceptions: [3]

  • Students have difficulty distinguishing between mitosis and meiosis and between somatic and germ lines.
  • Students don't understand the role that meiosis plays in heredity (e.g. why offspring resemble their parents and why there are genetic differences between siblings).
  • Students do not understand the role of chance in producing new heritable characteristics by forming new combinations of existing genes… Sexual reproduction is not recognized as a source of variation.
  • In general students do not appreciate the chemical basis of inheritance.

RecommendedSequence of Learning Activities

All of the recommended activities are aligned with the Next Generation Science Standards ( The first two listed activities cover all the basic concepts in the previous section. These activities are part of an integrated sequence of learning activities for teaching genetics, presented in"Genetics – Major Concepts and Learning Activities" (available at

CoreRecommended Activities

  • Mitosis and the Cell Cycle – How a Single Cell Develops into the Trillions of Cells in a Human Body(Student Handout and Teacher Preparation Notes available at

In this hands-on, minds-on activity students use model chromosomes and answer analysis and discussion questions to learnhow the cell cycle produces genetically identical daughter cells. Students learn how DNA replication and mitosis ensure that each new cell gets a complete set of chromosomes with a complete set of genes.Students learnwhy each cell needs a complete set of genes and how genes influence phenotypic characteristics. Finally, students analyze exponential growth to understand how a single cell develops into the trillions of cells in a human body. This activity can be used as an introduction to mitosis or to reinforce understanding of mitosis.

  • Meiosis and Fertilization–Understanding How Genes Are Inherited(Student Handout and Teacher Preparation Notes available at

In this hands-on, minds-on activity, students use model chromosomes and answer analysis and discussion questions to learn about the processes of meiosis and fertilization.As they model meiosis and fertilization, students follow the alleles of a human gene from the parents' body cells through gametes to zygotes; thus, students learn how a person inherits one copy of each gene from each of his/her parents. To learn how meiosis contributes to genetic variation, students analyze the results of crossing over and independent assortment. Students also compare and contrast meiosis and mitosis, and they learn how a mistake in meiosis can result in Down syndrome or death of an embryo. This activity can be used to introduce meiosis and fertilization or to review these processes.

Recommended Supplementary and Follow-Up Activities

  • Cell Differentiation and Epigenetics (Student Handout and Teacher Notes available at

In this analysis and discussion activity, students answer minds-on questions as they learn about the differentiation of specialized cell types, including the role of changes in epigenetic control of gene expression during cell differentiation. Students also learn about environmental influences on epigenetic control of gene expression and the need for cell division and differentiation even in a fully grown adult.

  • Mitosis and Meiosis Card Sort (Cards available at

This activity is designed to help students review the processes of mitosis and meiosis and to ensure that students understand how chromosomes move during mitosis vs. meiosis.Students arrange the cards from a shuffled deck of the stages of mitosis and meiosis in the sequence of steps that occur during cell division by mitosis and another sequence of steps that occur during cell division by meiosis.

  • Genetics (Student Handout and Teacher Preparation Notes available at

This activity helps students to understand basic genetics concepts, including how genotype influences phenotype and how understanding meiosis and fertilization provides the basis for understanding inheritance. Teachers can choose various combinations of the modules in the Genetics Student Handout and the Genetics Supplement. These modules include (1) an introductory module that uses the example of albinism to help students understand the basic concepts and introduces students to the Punnett square as a model of how genes are transmitted from parents to offspring through the processes of meiosis and fertilization, (2) a Coin Toss Genetics activity and an analysis of the genetics of sex determination, which help students understand the probabilistic nature of inheritance and the strengths and limitations of Punnett squares as models of inheritance, (3) an analysis of the inheritance of sickle cell anemia that reinforces basic concepts and introduces the important points that a single gene often has multiple phenotypic effects and alleles often are neither completely dominant nor completely recessive, and (4) pedigree analyses for recessive and dominant alleles, including challenge questions that introduce the role of new mutations and engage students in evaluating the relative advantages and disadvantages of Punnett squares and pedigrees as models of inheritance.

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[1]By Dr. Ingrid Waldron, Department of Biology, University of Pennsylvania, 2018. These Teacher Notes and multiple activities for teaching biology are available at

[2] Quotations from

[3]These misconceptions are paraphrased from a useful discussion of key concepts, common misconceptions, and learning activities for meiosis and variation in Chapter 3 of Hard to Teach Biology Concepts by Susan Koba with Ann Tweed, 2009, NSTA Press.