Chromosome

Chromosomes are small structures found within cells. They store compressed DNA, the genetic blueprints of the structure of an organism. They are found solely within the nucleus and mitochondria of a cell. Some disorders are caused by flawed chromosomes, including chromosomes that imperfectly copy themselves.

Background

The nucleus, the central part of the cell, was discovered in the nineteenth century. Scientists of the time knew that nuclei contained a set of rod-like structures. These structures were first observed during mitosis, the process of cell division, by Carl Nägeli. They also knew that when a cell divided, these structures duplicated with the cell. However, scientists of the time did not understand the function of these structures. They named them chromosomes because the structures seemed to absorb colored stains in the cell.

In the early 1900s, Thomas Hunt Morgan published a revolutionary theory about chromosomes. At the time, Morgan was working with fruit flies. Through selectively mating fruit flies, he was able to discern that all male fruit flies had the XY chromosome combination, while all female fruit flies had the XX chromosome combination. He was also able to discern that some traits, which would later be called alleles, were distinctively tied to a chromosome combination. Morgan had discovered that some alleles were tied to particular chromosomes. This led Morgan to theorize that chromosomes played an important part in the process of offspring inheriting physical traits from their parents. He called this discovery the chromosomal theory of inheritance.

The chromosomal theory of inheritance helped start the scientific field of genetics. Genetics is the study of genes and DNA. DNA is a blueprint for an organism and tells the body how to develop. In sexually reproducing organisms, genes are located in chromosomes.

Anatomy of Chromosomes

The anatomy of the chromosome begins with DNA. The proteins that make up DNA are arranged in a long, twisted shape called a double helix. These shapes are wrapped many times around another group of proteins, called histones. After being wrapped and layered around the histones, these structures are called nucleosomes. The nucleosomes bind themselves to each other, coiling into a long string called chromatin loops. The chromatin loops then wrap themselves tightly around each other, expanding the new structure until they form a chromosome.

All chromosomes have a pair of long arms and a pair of short arms. The long arms are called the q arms, and the short arms are called the p arms. These arms are held together by a central structure called a centromere. The final shape of the chromosome is similar to the letter "X."

The ends of the p and q arms are capped by special proteins called telomeres. Telomeres protect the ends of the chromosome and stop them from fraying, tearing, or breaking down.

Each human cell contains forty-six chromosomes, twenty-three chromosomes from each parent. Two chromosomes in each cell are sex chromosomes. They determine the sex of the organism. The rest, called autosomes, determine the various traits of the organism. While sex chromosomes differ between males and females, both male and female autosomes are identical.

Chromosomes play important roles in mitosis, the process of cell division. In the first phase of cell division, interphase, the chromosomes are still hidden inside the nucleus. As the chromatin begins to condense and the chromosomes become visible, the cell enters prophase. The chromosomes then begin migrating, marking the beginning of prometaphase. They line up along the middle of the cell, aligning themselves to long fibers called spindles. After aligning, the chromosomes split in half, and each half travels toward opposite sides of the cell. The splitting of the chromosomes marks anaphase. Following that, the body of the cell itself divides in two, creating two new cells.

When chromosomes replicate incorrectly during mitosis or are damaged in some other process, the results can cause a variety of disorders. The four most common chromosomal abnormalities are aneuploidy, deletions, inversions, and translocations. Aneuploidy refers to any abnormality in which an organism's DNA contains an incorrect number of a specific chromosome. For example, women who have Turner syndrome have one fewer sex chromosome. Individuals with Turner syndrome exhibit short stature, an early loss of ovarian function, and may not enter puberty. Down syndrome is also the result of an aneuploidy.

Deletion occurs when a portion of the chromosome is destroyed. In many cases, the body will attempt to repair the structure of the chromosome itself, but cannot restore the missing genetic material. Cri du chat syndrome, a disorder best known for larynx deformations that cause young children to make high-pitched cries, is the result of a deletion abnormality.

In an inversion abnormality, a piece of chromosome breaks off. It is then reattached upside down, reversing the placement of the genes on the chromosome. Because all the genetic material is intact, inversions may not always cause disorders. However, inversions can be passed down from parent to child.

A translocation abnormality involves a section of chromosome breaking off, then reattaching in a different place. These pieces may reattach to the same chromosome, or may attach to a new chromosome. In some cases, two chromosomes may break in the same place, then exchange pieces. This is called a balanced translocation. Balanced translocation abnormalities do not cause disorders. However, parents with balanced translocation abnormalities may have an increased risk of having children with a chromosomal abnormality.

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