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Tetrad in Meiosis: Definition & Explanation

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.

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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