Chromosome

A chromosome is, minimally, a very long, continuous piece of DNA, which contains many genes, regulatory elementss and other intervening nucleotide sequences. In the chromosomes of eukaryotes, the uncondensed DNA exists in a quasi-ordered structure inside the nucleus, where it wraps around histones (structural proteins, Fig. 1), and where this composite material is called chromatin. During mitosis (nuclear division), the chromosomes are condensed and called metaphasic chromosomes. This is the only natural context in which DNA is visible with an optical microscope. Prokaryotes do not possess histones or nuclei. In its relaxed state, the DNA can be accessed for transcription, regulation, and replication. Chromosomes were first observed by Karl Wilhelm von Nägeli in 1842 and their behavior later described in detail by Walther Flemming in 1882. In 1910, Thomas Hunt Morgan proved chromosomes to be the carriers of genes.

Table of contents
1 Chromosomes in eukaryotes
2 Chromatin
3 Chromosomes in different species
4 Karyogram
5 Chromosomal aberrations

Chromosomes in eukaryotes


Figure 1: Different stages of DNA condensation.
(1) Single DNA strand.
(2) Chromatin strand (DNA with histones).
(3) Condensed chromatin during interphase with centromere.
(4) Condensed chromatin during prophase.
(5) Chromosome during metaphase.

Chromatin

Two types of chromatin can be distinguished:
  • Euchromatin, which consists of DNA that is active, e.g., expressed as protein.
  • Heterochromatin, which consists of mostly inactive DNA. It seems to serve structural purposes during the chromosomal stages. Heterochromatin can be further distinguished into two types:
    • Constitutive heterochromatin, which is never expressed. It is located around the centromere and usually contains repetitive sequences.
    • Facultative heterochromatin, which is sometimes expressed.

In the early stages of mitosis, the chromatin strands become more and more condensed. They cease to function as accessible genetic material and become a compact transport form. Eventually, the two matching chromatids (condensed chromatin strands) become a chromosome, linked at the centromere. Long microtubules are attached at the centromere and two opposite ends of the cell. During mitosis, the microtubules pull the chromatids apart, so that each daughter cell inherits one set of chromatids. Once the cells have divided, the chromatids are uncoiled and can function again as chromatin. In spite of their appearance, chromosomes are well structured (Fig. 2). For example, genes with similar functions are often close together in the chromosome, but not in the linear DNA. The short arm of a chromosome can be extended by a satellite chromosome that contains codes for ribosomal RNA.

Figure 2: Chromosome.
  1. Chromatid. One of the two identical parts of the chromosome.
  2. Centromere. The point where the two chromatids touch, and where the microtubules attach.
  3. Short arm.
  4. Long arm.

Chromosomes in different species

Species

# of chromosomes

Species

# of chromosomes
Fruit fly 8 Human 46
Rye 14 Ape 48
Guinea Pig 16 Sheep 54
Dove 16 Horse 64
edible snail 24 Chicken 78
Earthworm 32 Carp 104
Pig 40 Butterflies ~380
Wheat 42 Fern ~1200

Table 1: Examples of chromosome numbers (diploid).

Within a species, the number of chromosomes is the same (Table 1). Asexually reproducing species have one set of chromosomes, which is the same in all body cells. Sexually reproducing species have somatic cells (body cells), which are diploid [2n] (they have two sets of chromosomes, one from the mother, one from the father) or polyploid [Xn] (more than two sets of chromosomes), and germ line cells (reproductive cells) which are haploid [n] (they have only one set of chromosomes). When a male and a female germ line cell merge (fertilization), the (now diploid) cell undergoes meiosis (maturation of the fertilized egg). During meiosis, the matching chromosomes of father and mother can exchange small parts of themselves (crossover), and thus create new chromosomes that are not inherited solely from either parent.

Karyogram

To determine the (diploid) number of chromosomes of an organism, cells can be locked in metaphase in vitro (in a reaction vial) with colchicine. These cells are then stained (the name chromosome was given because of their ability to be stained), photographed and arranged into a karyogram (an ordered set of chromosomes, Fig. 3), also called karyotype. Like many sexually reproducing species, humans have special gonosomes (sex chromosomes, in contrast to autosomes for body functions). These are XX in females and XY in males. In females, one of the two X chromosomes is inactive and can be seen under a microscope as Barr bodies.


Figure 3 : Karyogram of a human female.
(copyright 1995 Department of Pathology, University of Washington, Cytogenetics Gallery. Reproduced with permission.)

Chromosomal aberrations

The malfunction of either the chromosomal segregation or the crossover can lead to severe diseases. They can be divided into two classes:
  • Chromosomal aberration or Partial chromosomal dysplasia (malformation), which are usually the result of a defective crossover. Examples are:
    • Cri du chat syndrome, which is caused by the deletion of part of the short arm of chromosome 5. Victims make high-pitched cries that sounds like a cat. They have wide-set eyes, a small head and jaw and are mentally retarded.
    • Wolf-Hirschhorn syndrome, which is caused by partial deletion of the short arm of chromosome 4. It is characterized by severe growth retardation and mental defect.
  • Missing or additional chromosomes, called aneuploidy, which are the result of an incomplete chromosomal segregation, many of which are associated with cancer (see Duesberg hypothesis). Examples are:
    • Down syndrome (extra chromosome 21). This is also know as mongolism or trisomy 21. Symptoms are decreased muscle tone, asymmetrical skull, slanting eyes and mental retardation.
    • Klinefelters Syndrome (XXY). Men with Klinefelter syndrome are usually sterile. They tend to have longer arms and legs and tend to be taller than their peers. Other common symptoms are lack of emotion, fatigue, apathy and an increased tendency to develop psychiatric disorders.
    • Turner syndrome (X instead of XX or XY). In Turner syndrome, female sexual characteristics are present but underdeveloped. People with Turner syndrome often have a short stature, low hairline, abnormal eye features and bone development and a "caved-in" appearance to the chest.
You can find a detailed graphical display of all human chromosomes and the diseases annotated at the correct spot at [1].

See also:



In the News

Portable Sampling Cart Monitors Emissions From Wood-burning Cookstoves
A new method of measuring emissions from cookstoves could help improve human health and enhance the accuracy of global climate models.

Intel Body Count: 10,500 Jobs
The struggling chipmaker will eliminate around 10 percent of its workforce -- most cuts coming in management, marketing and IT -- as part of its biggest restructuring since the mid-1980s.

[Ironic] An Italian pensioner committed suicide after his wife fell in
Recalling the end of Romeo and Juliet, the 70-year-old man, Ettore, who had sat by his wife's bedside for four months after she slipped into a coma following a heart attack, finally gave up hope and gassed himself in the garage of his family home.Less than a day later, his wife, Rossana, woke up in her hospital bed in Padua and immediately asked for him.

Are Women Being Scared Away From Math, Science, And Engineering Fields
Have you ever felt outnumbered? Like there are just not that many people like you around? One group that may experience this kind of threat is women who participate in math, science and engineering settings -- settings in which the gender ratio is approximately three men to every one woman. New research shows that when women feel outnumbered, their academic performance expectations and actual performance decreases.

Combating Infectious Disease With Probiotics
Scientists have discovered that probiotic bacteria can protect against bacterial infection. The research group examined a range of beneficial bacteria and found one specific probiotic bacterium which was able to kill Listeria monocytogenes, an often lethal pathogen in pregnant women.

New Device Will Help Stroke Victims' Recovery
Engineers at Cardiff University, UK, are using their expertise to help create a device which could greatly increase the rate of recovery for stroke victims. The device, known as Alladin, will diagnose the precise state of a victim from the time of the attack throughout their recovery, enabling medical staff to determine the correct treatment at each stage.

Magnetic Nanoparticles Assembled Into Long Chains
Chains of 1 million magnetic nanoparticles have been assembled and disassembled in a solution of suspended particles in a controlled way, scientists at the National Institute of Standards and Technology (NIST) report. Such particles and structures, once their properties are more fully understood and can be manipulated reliably, may be useful in applications such as medical imaging and information storage.

[Ironic] An Italian pensioner committed suicide after his wife fell in
Recalling the end of Romeo and Juliet, the 70-year-old man, Ettore, who had sat by his wife's bedside for four months after she slipped into a coma following a heart attack, finally gave up hope and gassed himself in the garage of his family home.Less than a day later, his wife, Rossana, woke up in her hospital bed in Padua and immediately asked for him.

[Ironic] An Italian pensioner committed suicide after his wife fell in
Recalling the end of Romeo and Juliet, the 70-year-old man, Ettore, who had sat by his wife's bedside for four months after she slipped into a coma following a heart attack, finally gave up hope and gassed himself in the garage of his family home.Less than a day later, his wife, Rossana, woke up in her hospital bed in Padua and immediately asked for him.

Marine Team Finds Surprising Evidence Supporting A Great Biblical Floo
Did the great flood of Noah's generation really occur thousands of years ago? Was the Roman city of Caesarea destroyed by an ancient tsunami? Will pollution levels in our deep seas remain forever a mystery? These are just a few of the questions that are being addressed by a new environmental marine research team. Scientists and Swedish environmental philanthropist Andreas Weil are collaborating on an adventure in research aboard "The Mediterranean Explorer."The floating laboratory and educational facility is taking on some of the deepest mysteries of the Mediterranean waters and is bringing together scientists from diverse cultures and backgrounds. Issues of global warming, climate change, and even biblical history are being probed in this exciting initiative.


MP3 Music Downloads

Preview songs, Download Free Music,Burn CDs at ITunes.com
iTunes_RGB_9mm

 

Google




InformationQuickFind.com - Find Information Fast

Links