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Tetrad in Meiosis: Definition & Explanation

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  • 0:00 What is a Tetrad?
  • 0:25 Basic Meiosis Reminder
  • 1:40 Forming the Tetrad
  • 2:45 What Does the Tetrad Do?
  • 3:05 Finishing Meiosis
  • 3:45 Lesson Summary
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Lesson Transcript
Instructor
Katie Chamberlain

Katie has a PhD in Microbiology and has experience preparing online education content in Biology and Earth Science.

Expert Contributor
Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

A tetrad is a special foursome of DNA that is formed during meiosis. Get the scoop on what it is, when it is formed, and why it is necessary in this lesson.

What Is a Tetrad?

Imagine two pairs of identical twins standing next to each other to make a group of four. Now, imagine that the people are strands of DNA during meiosis. Bingo, a tetrad.

Okay, it is not a perfect example, but it is a good start.

A tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.

If that is confusing, don't worry. The nitty gritty details are coming right up.

Basic Meiosis Reminder

Humans have 23 different chromosomes (numbered 1-23), but they have two versions of each one. This means that all cells in the human body have 46 chromosomes. Well, except for the sex cells.

If the sperm and egg each had 46 chromosomes, then their offspring would end up with 92 chromosomes. Instead, humans make their sperm and egg cells with only 23 chromosomes each. Then, when they join to make a baby it will have 46 chromosomes, the magic number.

The process of making cells with only half the DNA is called meiosis.

When an egg is fertilized by a sperm, it receives one version of each chromosome from the mother and one from the father. So, two versions of chromosome #1, two versions of chromosome #2, all the way up until two versions of chromosome #23. The two versions of each chromosome are called homologous chromosomes.

Think of your chromosomes like a deck of cards from 1-23. Your father gave you 1-23 of clubs and your mother gave you 1-23 of diamonds. The five of clubs and the five of diamonds are homologous chromosomes. Similar, because they are both number 5…but slightly different.

Forming the Tetrad

Before meiosis can begin, a regular cell replicates its 46 chromosomes. The original version and the new copy remain attached together like conjoined twins and are called sister chromatids. So, just before meiosis, a cell has 46 chromosomes (23 pairs of homologous chromosomes), and each one consists of two sister chromatids. If we think back to our cards, now imagine that you copied each card and stapled the copy to the original. In other words, your five of clubs is stapled to an identical five of clubs.

Meiosis. The long and short chromosomes are different chromosomes. The red and grey are different versions of each. Step 1 replicated them. Step 2 is alignment and formation of the tetrad. Step 3 is crossing over. Step 4 is separation of the homologous chromosomes. Step 5 is separation of the sister chromatids.
Meiosis Process

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Additional Activities

Pipe Cleaner Meiosis

In this activity, students will use pipe cleaners to model the chromosome rearrangements that occur during meiosis, including the formation of the tetrad. To do this, you'll need at least two different colored pipe cleaners, each cut into two-inch pieces, scissors, and paper and pencils.

Student Instructions

Now that you've read about the tetrad in meiosis, it's time to put your knowledge to work. Here, we're going to be using pipe cleaners to model meiosis. Follow the steps and write down your answers to the questions as you go.

  1. Start by cutting the pipe cleaners into 2-inch pieces. You will need two pipe cleaner pieces of each color, for a total of four pieces.
  2. Next, draw a circle on a piece of paper. This will be your cell.
  3. Place one pipe cleaner of each color in the center of your cell. These are the starting chromosomes. What do the chromosomes do before meiosis starts?
  4. Now, replicate your chromosomes by adding in a second piece of the same color and forming an 'X' shape with the matching pieces. What are the two pipe cleaners of the same color called?
  5. . Next, carry out tetrad formation with your pipe cleaners. Align the two 'X's in the center of the cell.
  6. Now, carry out crossing over. Before you follow the instructions, write down how you think you might do this.
  7. Cut off a small piece of each color pipe cleaner, then attach it to the other color. Why does this represent crossing over?
  8. Next, carry out the following steps of meiosis with your pipe cleaners according to the lesson.

Expected Results:

Students should understand that each different colored pipe cleaner is a homologous chromosome. Before meiosis starts, the homologous chromosomes must duplicate. After the chromosomes duplicate, each part of the 'X' is called a sister chromatid. These sister chromatids line up to form a tetrad during prophase I. Then, crossing over occurs. The cutting and reattaching of the pipe cleaners represent crossing over because, inside the cell, the pieces of the chromosome are cut and pasted back together too.

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