CLINICAL DIAGNOSIS

CHROMOSOME SPECIFIC

Background

HOW DOES TRISOMY ARISE?
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Introduction

Some background in normal reproduction... 

The first step in reproduction is the creation of the reproductive cells or gametes.  For women, the gametes are eggs.  For men, the gametes are sperm.   The egg and the sperm cells contain half of the total chromosome complement, that is, 23 chromosomes instead of 46 chromosomes.  This reduction to 23 chromosomes is necessary so that when the egg and the sperm unite at conception, to create a baby, the proper number of chromosomes, 46, is restored in that child.  The process of reducing the number of chromosomes from 46 chromosomes to 23 chromosomes in the reproductive cells is called meiosis.  

Chromosomes

Chromosome changes

> How does trisomy arise?

What is mosaicism?

How does mosaicism occur?

Meiosis is a two-step process of cell division.  The first stage is referred to as meiosis I and the second stage is called meiosis II.  Normal meiosis is illustrated in the diagram on the left.  The diagram is looking at only one chromosome pair in order to simplify the illustration.  However, it is important to remember that all 23 chromosome pairs are involved in this process simultaneously.  Let's look at the diagram.  

Say this is a pair of chromosome number 5.  These are homologous chromosomes.  We can call the yellow chromosome the maternal chromosome, if it was originally inherited from this person's mother.  And we can call the blue chromosome the paternal chromosome, if it was originally inherited from this person's father.  

In the first step each chromosome makes an identical copy of itself.  The copy is attached at the centromere which make the "x-like" shape of the replicated chromosomes.  The replicated copies are called sister chromatids.

During meiosis I, the first division, the homologous pairs separate into two new daughter cells.  

During meiosis II, the second division, the replicated pair of sister chromatids separate into two new daughter cells.  

The final result is four gametes or reproductive cells, each which contain 23 chromosomes.

Spermatogenesis is the making of sperm.  In spermatogenesis, the result of meiosis is four sperm cells which can develop into mature sperm.  Spermatogenesis is an ongoing process throughout life.

The making of eggs or ova is called oogenesis.  Oogenesis is different from spermatogenesis.  In oogenesis the result of meiosis is one mature egg or ovum and two polar bodies.  Polar bodies are smaller cells which are the bi-products of the creation of the mature ovum.  During meiosis I the primary oocytes undergoes a very unequal division.  The larger cell, which receives most of the cells contents (cytoplasm and organelles), becomes the secondary oocyte, and the other smaller cell becomes the first polar body.  In meiosis II, a second unequal cell division occurs in the secondary oocyte.  The result is the ovum and the second polar body.  The polar bodies die off.  The ovum is much larger than a sperm cell and it contains the materials required for development.  Human males produce 200,000,000 sperm per day, whereas females produce one egg (usually) each menstrual cycle.

Show me a diagram of the production of sperm and eggs.

There is a difference between the outcome of errors which occur during meiosis I when compared to the outcome of those which occur during meiosis II.  In the diagrams above, the colour of the chromosomes illustrate the difference.  

i) Meiosis I errors: When the trisomic zygote is caused by an error in meiosis I, the zygote receives three different chromosomes.  Although both the yellow and the blue chromosomes were both contributed by the egg, they are homologous chromosomes where the actual genetic information contained within the chromosome is different.  Originally the yellow chromosome was the maternal copy and the blue chromosome was the paternal copy.

ii) Meiosis II errors: When the trisomy is caused by an error in meiosis II, two alike chromosomes are inherited.  Both of the blue chromosomes inherited from the egg are sister chromatids, which contain very similar genetic material.  However, there is a slight difference in the two blue chromosomes.  This is due to a process called recombination which occurs prior to the first cell division in meiosis I.  Refer back to the first diagram on this page to recall this time in cell division.  During recombination the homologous pairs align themselves very closely and the two chromatids that are closest to one another exchange pieces of genetic material through a process called crossing over.  This is illustrated in the diagram on the right.  The purpose of this process is to ensure variation in the gametes.  Due to the process of recombination each of the four gametes are unique.  Show me an animation of crossing over.

A non-disjunction error during meiosis I or II leads to an individual with an extra chromosome in every cell of his/her body.  

Looking at the diagrams above which are illustrating non-disjunction, what happens with the reproductive cells which appear to be empty, that is, the sperm or egg which are lacking a chromosome?  If one of these gametes is fertilized by a normal gamete the result is monosomy of the chromosome involved.  Monosomy is a deficiency in number of chromosomes and is defined as only one copy of a chromosome that is normally present in two copies.  These eggs and sperm, which contain one less chromosome, have 22 chromosomes.  When fertilized, the outcome is 45 chromosomes in total.  In general, monosomies are less likely to survive when compared to trisomies. 

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