This and the lesser-known Patau syndrome. Both conditions

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.

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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

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).

Meiosis eventually produces four new cells. These cells each contain 23 chromosomes and represent sperm or egg. Note that the 23 chromosomes inside each sperm or egg cell is half the original number of 46. This assures that fertilization results in the correct compliment of 46 chromosomes for a developing embryo.

Why it Matters

Chromosome separation is important for different reasons. One reason is because failure of chromosomes to separate can lead to severe, or fatal, genetic conditions.

Two examples are Down syndrome and the lesser-known Patau syndrome. Both conditions result from nondisjunction, or the failure of chromosomes to separate.In the case of Down syndrome, an affected individual receives an extra copy of chromosome 21. While survivable, the condition often results in developmental delays and mental disability. On the other hand, Patau syndrome is caused by the inheritance of an extra chromosome 13.

This chromosome is larger and contains more genetic information. As a result, afflicted individuals rarely survive beyond a few weeks.

Lesson Summary

Humans normally contain 46 chromosomes. Chromosomes are tightly coiled segments of DNA. These chromosomes are grouped into 23 homologous pairs. Homologous chromosomes contain the same gene sequences as one another.

Prior to fertilization, chromosome number is halved by meiosis. One of the early phases in meiosis is metaphase I. Metaphase I is when homologous chromosomes line up along the equator of the cell. Eventually, these homologous chromosomes separate. Failure to separate is called nondisjunction.

Nondisjunction can result in genetic conditions, such as Down syndrome or Patau syndrome.


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