Metaphase I: Stages of Meiosis

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  • 0:00 Your Genes
  • 0:45 Doing The Math
  • 1:15 How It Works
  • 2:40 Why It Matters
  • 3:30 Lesson Summary
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Lesson Transcript
Instructor: Dominic Corsini
This lesson addresses metaphase 1 and the process of meiosis. It includes key concepts such as cell division, chromosome number, and nondisjunction. Illustrations, explanations, and real-world examples are included.

Your Genes

How many chromosomes do people have? Chromosomes are tightly coiled segments of DNA that contain genetic information. Most people on Earth have 46 chromosomes, which are divided into 23 pairs. These 46 chromosomes contain all the instructions to make you. They are why you have a certain hair color, eye color, height, weight, skin tone, etc. Essentially, these chromosomes and the information they contain make us who we are.

Human Karyotype
Human Karyotype

This picture shows the chromosome arrangement for most people. The final paring of chromosomes determines gender. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

Doing the Math

When sperm fertilizes an egg, the resulting cell should contain 46 chromosomes. Therefore, it is important people reduce their chromosome number by half prior to reproducing. This assures the newly fertilized egg cell contains the required 46 chromosomes, with 23 donated by each parent. This reduction occurs through meiosis. Meiosis is a cellular process that produces sperm or eggs cells (depending on gender) and cuts chromosome counts in half.

How it Works

Meiosis is a multistep process, and one of the phases within this process is called metaphase I. Metaphase I is when homologous chromosomes line up along the equator of the cell. Homologous chromosomes contain the same genes in the same locations and are often distinguishable based on size. In our original image, the two chromosome 1's are homologous to one another, as are chromosomes 2, 3, 4, and so on. Recognizing that homologous chromosomes are paired is important for understanding meiosis.

Here is a graphic representation of meiosis. Notice that metaphase I occurs early in the progression.


The red and blue structures in this graphic represent your chromosomes. For simplicity, only two homologous pairs are shown. In reality, there are 23. Again, chromosome pairings are based on size. In metaphase I, homologous chromosomes are grouped together. These pairs are then pulled apart in the following phase (Anaphase I).

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