Tetrad in Meiosis: Definition & Explanation
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.
![]() |
The first stage of meiosis is called prophase I, and this is when the tetrad forms. The two homologous chromosomes will align next to each other. But, since each is made up of two sister chromatids it will look like a group of four. In card version: the two fives of clubs stapled together are aligned with the two fives of diamonds that are also stapled together.
What Does the Tetrad Do?
While these two pairs of conjoined twins are hanging out next to each other, they swap some DNA in an act called crossing over or recombination. This is done to mix some of the DNA around and increase genetic variation.
Imagine the fives of clubs swapping a few of their club pictures with some of the diamond pictures. The result is some clubs with a touch of diamond and some diamonds with a hint of club.
Finishing Meiosis
As meiosis I continues, the homologous chromosomes are pulled apart into separate cells. In other words, the version from the father (clubs) and the version from the mother (diamonds) separate. The sister chromatids remain stuck (stapled) together until the next phase of meiosis (meiosis II), and then they are also pulled apart. By the end of meiosis, four cells will be created, each with one of the chromatids from the tetrad. Also, don't forget that there are 23 different tetrads formed during meiosis because each chromosome 1-23 will have a tetrad.
Lesson Summary
The tetrad occurs during the first phase of meiosis. It is the foursome of chromatids that forms when replicated homologous chromosomes align. It must be formed for crossing over to occur. It is broken apart when the homologous chromosomes separate in meiosis I.
To unlock this lesson you must be a Study.com Member.
Create your account
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.
- 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.
- Next, draw a circle on a piece of paper. This will be your cell.
- 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?
- 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?
- . Next, carry out tetrad formation with your pipe cleaners. Align the two 'X's in the center of the cell.
- Now, carry out crossing over. Before you follow the instructions, write down how you think you might do this.
- Cut off a small piece of each color pipe cleaner, then attach it to the other color. Why does this represent crossing over?
- 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.
Register to view this lesson
Unlock Your Education
See for yourself why 30 million people use Study.com
Become a Study.com member and start learning now.
Become a MemberAlready a member? Log In
Back