Publication No. 10754
Allele’s Crossing Over to the Other Side
Student Laboratory Kit
Materials Included In Kit
Isopropyl alcohol, 70%, 100 mL
Additional Materials Required
Isopropyl alcohol is a moderate fire risk; flammable liquid; slightly toxic by ingestion and inhalation. Please wear eye protection when handling isopropyl alcohol. Dispose of isopropyl alcohol using Flinn Suggested Disposal Method #18a. Remind students to wash hands with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Paper towels may be disposed of in regular trash, Flinn Biological Waste Disposal Type VI. All other materials can be reused.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering PracticesAsking questions and defining problems
Engaging in argument from evidence
Disciplinary Core IdeasMS-LS3.B: Variation of Traits
HS-LS3.B: Variation of Traits
Cause and effect
Stability and change
HS-LS3-2. Make and defend a claim based on evidence that inheritable genetic variations may result from (1) new genetic combinations through meiosis, (2) viable errors occurring during replication, and/or (3) mutations caused by environmental factors.
Answers to Prelab Questions
No. Sister chromatids are identical to each other and joined at a centromere whereas a homologous chromosome is composed of pairs of sister chromatids.
Answers to Questions
Student answers may include different alleles of the same gene, the parent’s chromosomes have different alleles, heterozygous form.
Alleles may be capital or lower-case at each position but the two halves must match.
Bead color indicates which parent supplied that allele. Beads of the same color but on different chenille wires originated on the same chromosome but separated during crossing over.
What does chenille wire color indicate?
Wire color indicates which parent supplied the DNA.
I. Meiosis I
When two homologous chromosomes lineup next to each other they form a structure called a tetrad.
The two chromosomes should have three changes from Figure 14.
How is this different from metaphase in mitosis?
In mitosis each chromosome is split into two halves so that each daughter cell contains one complete copy of DNA from each parent.
How much genetic material does each new cell have as compared to the original cell (before any genetic material was copied)? Explain.
Each cell has two copies of one parent’s DNA rather than one copy of each parent’s DNA
II. Meiosis II
In interphase II, no replication occurs. Why not?
The purpose of meiosis is to reduce to a haploid number so that two haploid cells can unite to form a new diploid offspring.
Identical to Figure 15, each chromatid is in its own box.
Are the four resulting cells identical or different from one another and why?
The four cells are different from each other because the alleles on each chromatid may differ from each other. They do each have one copy of each chromatid.
After. Homologous chromosomes separate during anaphase I while sister chromatids separate during anaphase II.
Sister chromatids are identical to each other.
Meiosis is more complex because it involves genetic recombination and also twice as many steps to completion.
No. Cells created for reproduction undergo crossing over to ensure genetic variation in the next generation.
The daughter cells of mitosis are diploid with two copies of each gene whereas the daughter cells of meiosis are haploid with one copy of each gene. The haploid cell will combine with another haploid cell during sexual fertilization to create a diploid offspring.
Special thanks to Romena Holbert, Xenia High School, Xenia, OH, for providing the idea and the instructions for this activity to Flinn Scientific.
Allele’s Crossing Over to the Other Side
The sequence of events occurring during meiosis is explained in this simple kinesthetic activity.
Two processes must be accomplished when cells, called gametes, are created for reproduction. First, the amount of genetic information must be cut in half so that chromosome numbers do not double in the next generation. Second, genetic variation must be added to the next generation of organisms. Events that occur during meiosis address both of these issues.
Interphase occurs just before meiosis I begins. In this stage, the chromosomes are in the chromatin or thread-like form. This loose form is needed so that the DNA can replicate itself in preparation for cell division. In humans this means that the two versions of a gene, one from the mother and one from the father, are both replicated, creating two identical copies of each version. The result is that there are a grand total of four copies of each gene.
Meiosis I begins with prophase I when the duplicated threads of chromatin condense to form two identical, or sister, chromatids. These sister chromatids attach to each other at a special point called the centromere. This whole structure is called a chromosome. There are two chromosomes, one with two copies of the mother’s genes and one with two copies of the father’s genes. Also during prophase, the centrioles are copied. Centrioles control the migration of the chromosomes to the opposite ends of the cell during cell division.
In prometaphase I, two homologous chromosomes—that is, the chromosomes that contain the same genes—move adjacent to each other to form a structure called a tetrad (Tetra = four in Greek). While these two homologous chromosomes or homologues are aligned as a tetrad, they may exchange sections of similar genetic code with each other. This process is called crossing over because the homologues appear to “cross” each other as DNA strands are exchanged. It is this exchange of genetic information that creates new genetic variation in living organisms. Crossing over does not occur between sister chromatids because they are identical and no genetic change would occur if identical pieces of DNA switched places. Also during prometaphase I, the spindle fibers start to attach to the centromeres and the nuclear membrane breaks apart.
In metaphase I, the nuclear membrane completely disappears and the genetically altered and attached chromosomes align in the middle of the cell. The orientation is random, with the homologue from either parent on a side. This means that there is a 50–50 chance for the daughter cells to receive either the mother’s or the father’s genetically altered chromosome.
During anaphase I, the altered homologous chromosomes separate and migrate to opposite ends of the cell. The chromosomes migrate when they are pulled toward opposite centrioles by spindle fibers that are attached between the centrioles and the centromere on each homologue.
Telophase I occurs when the chromosomes reach the centrioles on the opposite sides of the cell. Cytokinesis occurs at the same time. In animal cell, the cell membrane pinches in to divide the cytoplasm and organelles into two cells. In plant cells, new cell walls form along the center of the cell creating two cells.
Interphase II is the period between meiosis I and meiosis II. Interphase II is very different from interphase I because no DNA replication occurs. The nuclear membrane reforms in many organisms. In some organisms the chromatids separate and unravel while in others they do not separate or unravel.
Meiosis II begins with prophase II. During prophase II, the chromatin condenses (if it unraveled during interphase II), the centrioles are duplicated and spindle fibers begin to reform.
In prometaphase II, the nuclear membrane begins to break apart and the new spindle fibers attach to the centromeres of the chromosomes. One spindle fiber from each centriole attaches to the centromere on each chromosome. Recall that there is just one copy of each chromosome in each cell but that each chromosome is composed of two sister chromatids.
Metaphase II is characterized by the alignment of the chromosomes along the center of the cell in preparation for the separation of the sister chromatids. Keep in mind, the sister chromatids are no longer identical because of the crossover events that occurred in prometaphase I.
In anaphase II, the centromere is split in half as the sister chromatids are separated, moving to opposite sides of the cell.
In telophase II each sister chromatid moves toward a centriole located on the opposite side of the cell. At the same time cytokinesis occurs, splitting the cell in half again. A total of four new haploid cells have been produced from the original cell. Each haploid cell contains one sister chromatid, which includes a single complete set of genes.
Isopropyl alcohol, 70%
Beads, pink, 16
Beads, white, 16
Chenille wire, green, 2
Chenille wire, white, 2
Isopropyl alcohol is a moderate fire risk; flammable liquid; slightly toxic by ingestion and inhalation. Please wear eye protection and avoid sources of ignition when handling isopropyl alcohol. Wash hands thoroughly with soap and water before leaving the laboratory.
What does the potential for differences in capitalization on the alleles indicate?
What does bead color indicate? How are the beads of the same color but on different chenille wires related to one another?
I. Meiosis I
Place the models on the sheet of paper. The paper represents one cell.
Use the chenille wire models to simulate three events of crossing over by exchanging pink and white beads with the same letter on them.
Line up the chenille wire models in the center of the paper with the green chromosome on the left-hand side of the paper and the white chromosome on the right-hand side of the paper.
Separate the chromosome models by color, moving the green chromosome to the left and the white chromosome to the right.
Tear the paper “cell” in half so that each chromosome will be on its own section of the paper. Each chromosome is contained in its own cell.
Remember, no DNA is replicated for the next cell division. The cell prepares for the next cell division by increasing the amount of intercellular organelles and cytoplasm.
If the chromosome had unraveled during interphase II, it would condense again during this phase.
New spindle fibers attach to the centromeres.
Line up the chromosome models in the center of each “cell” again, but 90° from their positions in metaphase I.
Untwist the chenille wires. This represents the splitting of the centromere.
Tear the paper in half again so that each chenille wire is on its own quarter sheet of paper. Each chromatid is now contained in its own haploid cell.
Are the four resulting cells identical or different from one another? Explain.
Student Worksheet PDF