A Closer Look at Chromosomes

Overview

It takes just one egg and one sperm to initiate the complex process of building a human being. Trillions of cells are eventually copied from a single fertilized egg. Genetic information in the form of deoxyribonucleic acid, or DNA, is duplicated and passed on as cells divide and multiply. DNA, the blueprint that tells our bodies how to function, is replicated and preserved within just about every cell in the body.

Chromo-Zoom

Cell
The 100 trillion cells that comprise the human body come in many different shapes and sizes, but have one thing in common. Except for red blood cells, each cell contains a nucleus, and within that nucleus vital genetic material provides a blueprint for how the entire body functions.

Nucleus
The nucleus is separated from the rest of the cell by a membrane. The nucleus functions as the cell’s control center, regulating growth, reproduction, and metabolism. The instructions for how all cells should function are embedded within the nucleus. However, each cell “turns on” only those instructions it needs to complete its required tasks.

Chromosomes
Forty-six chromosomes reside within the nucleus. Twenty-three of them have been copied from the original chromosomes in the mother’s egg cell. The other twenty-three have been copied from the original chromosomes in the father’s sperm cell. Corresponding chromosomes copied from the mother and father exist side by side as pairs.

DNA
Each chromosome consists of a tightly coiled thread of DNA. Stretched out end to end, the forty-six DNA threads from a single tiny cell would extend longer than six feet! Sections of DNA are made up of coded instructions called genes. These genes determine specific human characteristics.

Double Helix
When you get really close up, you arrive at DNA’s structure, the double helix. When a cell is ready to divide, the DNA double helix “unzips” during the first of a complex series of steps to copy itself. Once the DNA is copied, a cell can divide into two new cells. Both new cells will have all of the genetic information that the original cell had.

Copying Chromosomes

Cell Duplication
As a cell prepares to divide, its chromosomes bunch up as the DNA within copies itself. Then the chromosomes split in two. The two new cells that form are identical to the original cell, complete with forty-six chromosomes and the same DNA information. Sometimes errors occur when a cell duplicates, and the new cells are not identical to the original cell.

When Something Goes Wrong
In one type of error called a translocation, a part of a chromosome breaks off and reattaches to a different chromosome or is lost. The genetic instructions included within the broken portion of the chromosome end up in the wrong location or entirely missing. As these cells duplicate, the scrambled genetic instructions are duplicated as well. Translocated chromosomes may be inherited or may occur spontaneously because of environmental factors such as exposure to high radiation or cancer-causing chemicals such as those in tobacco smoke.

Balanced and Unbalanced Translocation
A translocation that generates no net gain or loss of genetic material in the cell is called a balanced translocation. Even though the genetic instructions are in the wrong place, the affected cell may be able to function properly.

However, if genetic material gets lost or added in the shuffle, the translocation is considered to be unbalanced. The cell either has extra genetic instructions, or not enough and it is more likely that the cell will function abnormally. Sometimes the abnormal functioning is not even noticeable. Other times, it can cause major diseases such as some forms of leukemia.

Spot the Mismatch

Chromosome #21 and #14
The translocation where an extra copy of chromosome 21 attaches itself to chromosome 14, is one cause of Down syndrome, a condition that affects physical and mental development.

Chromosome #8 and #14
The translocation between these two chromosomes can cause Burkitt's lymphoma, a cancer that affects the immune system.

Chromosome #9 and #22
The translocation between these two chromosomes can cause chronic myelogenous leukemia, a type of cancer that causes a specific kind of white blood cell to replicate in an uncontrolled way.

A Doctor Making a Difference

In the early 1970s, Dr. Janet Rowley developed a new staining technique to identify chromosomes. She proved that translocation—the process where portions of chromosomes break off and shift from one chromosome to another—had occurred in several patients with chronic myeloid leukemia. Dr. Rowley argued that translocation could cause cancer, going against the established view that chromosome abnormalities were an effect of disease. Despite initial resistance to her ideas, Dr. Rowley's work has proven immensely influential. Today, the translocation in chronic myeloid leukemia is treated with a specific drug which induces remissions in the vast majority of patients.