CHROMOSOME

What is a chromosome?

In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

Chromosomes are not visible in the cell's nucleus-not even under a microscope-when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope. Most of what researchers know about chromosomes was learned by observing chromosomes during cell division.

Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or "arms". The short arm of the chromosome is labeled the "p arm". The long arm of the chromosome is labeled the"q arm". The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.

Types of chromosome

Metacentric chromosome

A chromosome with a centrally placed centromere that divides the chromosome into two arms of approximately equal length.

Submetacentric chromosome:

A chromosome in which the centromere lies to one side of the center, producing arms of unequal length.

Acrocentric

If the p (short) arm is so short that is hard to observe, but still present, then the chromosome is acrocentric

Telocentric

A telocentric chromosome's centromere is located at the terminal end of the chromosome. Telomeres may extend from both ends of the chromosome. For example, all mouse chromosomes are telocentric.Humans do not possess telocentric chromosomes. Some authors denote extreme acrocentric chromosomes as telocentric- 21, 22, Y.

Centromere

During mitosis in a typical plant or animal cell, each chromosome divides longitudinally into two sister chromosomes that eventually separate and travel to opposite poles of the mitotic spindle. At the beginning of mitosis, when the sister chromosomes have split but are still paired, every chromosome attaches to the spindle at a specific point along its length. That point is referred to as the centromere or spindle attachment region.

Images from an electron microscope show that each sister is attached to fibers emanating from only one pole of the spindle. This allows the sisters to be pulled to opposite poles during mitosis. The electron microscope images also show that the spindle fibers do not terminate on the chromosomes themselves but rather on separate structures, known as kinetochores. Kinetochores are trilaminar bodies that assemble at the centromeres during the early stages of mitosis and disappear after the chromosomes have separated.

The budding yeast Saccharomyces cerevisiae has the simplest known centromeres consisting of a DNA segment only 110 bases in length. The DNA segment in the yeast centromere binds to specific proteins, which, like the kinetochores in higher organisms, link the chromosome to spindle fibers during mitosis. Centromeres of higher plants and animals are much larger, consisting of thousands or millions of bases of DNA and numerous proteins. For reasons that are unknown, centromeres are often flanked by long segments of DNA that do not contain functional genes. These nonfunctional DNA segments, called pericentric heterochromatin, vary in length in different organisms. In some cases they constitute more than half of the whole chromosome.

The crucial role of centromeres in the orderly behavior of chromosomes can be demonstrated by using X rays or other treatments that cause chromosomes to fragment. Pieces of chromosomes that lack centromeres (acentric fragments) do not attach to the spindle and are not pulled to the poles during mitosis. They generally are not included in the nuclei that are newly formed after cell division, and they usually degenerate in the cytoplasm.

Conversely, two fragments that each contains a centromere sometimes fuse, producing a dicentric chromosome. If the two centromeres happen to attach to the same pole at mitosis, the chromosome may move intact to that pole. However, if the centromeres attach to opposite poles, the chromosome will be stretched during mitosis and will eventually break. In general, therefore, only chromosomes with one centromere are stable.

Some organisms, including hemipteran insects and nematode worms, have holocentric or holokinetic chromosomes. In these organisms, spindle fibers attach all along one side of each sister chromosome, and the chromosomes are pulled more or less sideways to the pole.

Centromeres also play an important role during meiosis, in which the number of chromosomes is halved. The first meiotic division differs from a typical mitotic division in two respects:

1. In the first meiotic division, chromosomes derived from the organism's maternal and paternal parents pair at the beginning of meiosis. As in a typical mitosis, each of these chromosomes has split into two sisters, so after pairing there are four chromosomes in a group.


2. When these four chromosomes attach to the spindle, sister chromosomes attach to the same pole, not to opposite poles as occurs in mitosis.

As a result, both maternal chromosomes move to one pole, while both paternal chromosomes move to the opposite pole. It is this unique behavior of the centromeres at meiosis that accounts for the separation of maternal and paternal genes during formation of sperm and eggs, which in turn is the basis of Mendelian genetics.

Karyotyping

Karyotyping is the process of preparing chromosomes for analysis. The tissues used vary but the most common tissues sampled are lymphocytes (white blood cells), skin fibroblasts, amniotic fluid cells, gonadal tissue, chorionic villi (CVS) and bone marrow. Direct preparations without the use of a mitogen are made on bone marrow, lymphocytes, gonadal tissue, CVS, and tumors. When cultured, cells are placed in a nutrient solution containing amino acids and other nutrients, a pH indicator, antibiotics, with or without mitogens (phytohemaglutinin,PHA, or pokeweed mitogen) and then grown at 37°C in CO₂ atmosphere. Lymphocytes are cultured three days and amniotic cells, fibroblasts, CVS, and other fetal tissues are cultured ten days. Colcemid, which inhibits the assembly of microtubules of the spindle, is added prior to harvest to stop mitosis and to collect sufficient cells with prophase-prometaphase chromosomes. Cells may be grown on microscope slides in sterile chambers or they may be attached to the flask in which they are grown. If release of cells from substrate is necessary, trypsin and EDTA are added. The cells are then centrifuged to remove the culture medium and/or the trypsin and EDTA. A hypotonic solution is added to swell the cells followed by centrifugation and fixation in methanol:acetic acid. The cells are dropped onto slides if they were not grown on slides. Most banding procedures require pretreatment before staining. Trypsin and Giemsa produce G-banded chromosomes (GTG).

Karyotype:46,XY,t(1;3)(p21;q21),del(9)(q22),t(1;18)

 

References:

homepages.uel.ac.uk
www.wrongdiagnosis.com
www.webref.org
www.answers.com
carolguze.com
www.ncbi.nlm.nih.gov

Editorial Team, Mindfiesta