Human Chromosome Map

DNA
Deoxyribonucleic acid – DNA – is a nucleic acid, usually in the form of a double helix – that contains the genetic instructions monitoring the biological development of all cellular forms of life, and many viruses. DNA is a long polymer of nucleotides (a polynucleotide) and encodes the sequence of the amino acid residues in proteins using the genetic code, a triplet code of nucleotides.
DNA is often referred to as the molecule of heredity as it is responsible for the genetic propagation of most inherited traits (hair colour, disease susceptibility etc.).

The DNA, which carries genetic information in cells, is normally packed in the form of one or more large macromolecules called chromosomes.

Chromosome
A chromosome, from the Greek “chroma” = color, and “soma” = body, is a very long continuous piece of DNA (a single DNA molecule), which contains many genes, regulatory elements and other intervening nucleotide sequences.

Human genetic information is stored in 23 pairs of chromosomes that vary widely in size and shape.
Chromosome 1 is the largest and is over three times bigger than chromosome 22.
The 23rd pair of chromosomes is two special chromosomes - X and Y - that determine our sex. Females have a pair of X chromosomes (46, XX) – mitochondrial DNA, whereas males have one X and one Y chromosome (46,XY). In females, one of the two X chromosomes is inactive and can be seen under a microscope as Barr bodies.

Chromosomes are made of DNA, and genes are special units of chromosomal DNA.
Each chromosome is a very long molecule, so it needs to be wrapped tightly around proteins for efficient packaging.

All humans have the same set of genes, arranged in the same order, but the information carried in genes differs slightly from person to person. This is what makes each of us unique. Only identical twins share the exact same combination of genes. The child receives exactly half of its genetic information from the mother and exactly half from the father.

The structure of chromosome
Near the center of each chromosome is its centromere, a narrow region that divides the chromosomes into a long arm (q) and a short arm (p). We can further divide the chromosomes using special stains that produce stripes known as a banding pattern. Each chromosome has a distinct banding pattern, and each band is numbered to help identify a particular region of a chromosome. This method of mapping a gene to a particular band of the chromosome is called cytogenetic mapping.

1 – chromosome
2 – centromere
3 – short arm
4 – long arm

Chromosome	Genes	Bases
1	2968	245,203,898
2	2288	243,315,028
3	2032	199,411,731
4	1297	191,610,523
5	1643	180,967,295
6	1963	170,740,541
7	1443	158,431,299
8	1127	145,908,738
9	1299	134,505,819
10	1440	135,480,874
11	2093	134,978,784
12	1652	133,464,434
13	748	114,151,656
14	1098	105,311,216
15	1122	100,114,055
16	1098	89,995,999
17	1576	81,691,216
18	766	77,753,510
19	1454	63,790,860
20	927	63,644,868
21	303	46,976,537
22	288	49,476,972
X (sex chromosome)	1184	152,634,166	147,686,664
Y (sex chromosome)	231	50,961,097	22,761,097

Source: www.wikipedia.org

Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies.

Karyotype
To determine the number of chromosomes of an organism, cells can be locked in metaphase in vitro with colchicines. These cells are then stained, photographed and arranged into a karyotype, also called karyogram. Like many sexually reproducing species, humans have special gonosomes (sex chromosomes, in contrast to autosomes).

Nucleotide sequence
Within a gene, the sequence of nucleotides along a DNA strand defines a messenger RNA sequence, which then defines a protein that an organism is liable to manufacture or “express” at one or several points in its life using the information of the sequence. The relationship between the nucleotide sequence and the amino-acid sequence of the protein is determined by simple cellular rules of translation, know collectively as the genetic code. The genetic code consists of three-letter “words” (terms a codon) formed from a sequence of three nucleotides (e.g. ACT, CAG, TTT) – adenine, thymine, guanine, cytosine. These codons can then be translated with messenger RNA and then transfer RNA, with a codon corresponding to a particular amino acid. There are 64 possible codons (4 bases in 3 places 43) that encode 20 amino acids. More amino acids, therefore, have more than one possible codon. There are also three “stop” or “nonsense” codons signifying the end of the coding region, namely the UAA, UGA a UAG codons. Only a small fraction (1,5%) of the total sequence of the genome appears to encode protein. The function of the rest is a matter of speculation. It is known that certain nucleotide sequences specify affinity for DNA binding proteins, which play a wide variety of vital roles, in particular through control of replication and transcription. These sequences are frequently called regulatory sequences, and researchers assume that so far they have identified only a tiny fraction of the total that exists. “Junk DNA” represents sequences that do not yet appear to contain genes or to have a function. The reasons for the presence of so much non-coding DNA in eukaryotic genomes and the extraordinary differences in genome size (“C-value”) among species represent a long-standing puzzle in DNA research known as the “C-value enigma”.
Some DNA sequences play structural roles in chromosomes.
Sequences also determine a DNA segment’s susceptibility to cleavage by restriction enzymes, the quintessential tools of genetic engineering. The position of cleavage sites throughout an individual’s genome determines one kind of an individual’s “DNA fingerprint”.

Genetic disorders
Scientist has noticed that some disorders (called genetic disorders) runs in families, and there are genes responsible for it.
A genetic disorder is a disease caused in whole or in part by a “variation” (an unusual form) or “mutation” (alteration) of a gene. Genetic disorders can be passed on to family members who inherit the genetic abnormality. A small number of rare disorders are caused by a mistake in a single gene. But most disorders involving genetic factors arise from a complex interplay or multiple genetic changes and environmental influences.
Some chromosome abnormalities do not cause diseases in carriers, such as translocations, or chromosomal inversions, although they may lead to a higher chance of having a child with a chromosome disorder. Abnormal numbers of chromosomes or chromosome sets – aneuploidy – may be lethal or give rise to genetic disorders.

Genetic disorders may be grouped into 3 categories:

1. Single gene disorders – caused by a mistake in a single gene. The mutation may be present on one or both chromosomes of a pair. E.g. sickle cell disease (HbS – haemoglobin), cystic fibrosis (CFTR gene) and Tay-Sachs disease (HEXA gene on chromosome 15)
2. Chromosome disorders – caused by an excess or deficiency of the genes. E.g. Down syndrome (chromosome 21), Edward’s syndrome (chromosome 18), Patau Syndrome (chromosome 13), Wolf-Hirschhorn syndrome (chromosome 4)
3. Multifactorial inheritance disorders – caused by a combination of small variations in genes, often in concert with environmental factors. E.g. heart disease, cancers, Alzheimer’s disease

DNA in use
DNA in crime – Forensic scientist can use DNA located in blood, semen, skin, saliva or hair left at the scene of a crime to identify a possible suspect, a process called genetic fingerprinting of DNA profiling.
DNA in computation – DNA plays an important role in computer science, bioinformatics and computational biology, both as a motivation research problem and as a method of computation in itself (string searching algorithms, text editors, database theory, cryptography).
DNA in historical and anthropological study – evolutionary biology, population genetics, ecological genetics.

Research
The history of DNA research begin in 19th century, when Friedrich Miescher discovered a substance he called “nuclein” in 1869. Late he isolated a pure sample of the material now known as DNA from the sperm of salmon, and in 1889 his pupil, Richard Altmann, named it “nucleic acid”. This substance was found to exist only in the chromosomes.
Chromosomes were first observed in plant cells by Swiss botanist Karl Wilhelm von Nägeli in 1842, and independently, in Ascaris worms, by the Belgian scientist Edouard Van Beneden. The name was invented later during late years of 19th century by German anatomist, Heinrich von Waldeyer.
With the advent of new techniques in DNA analysis, we are able to look at the chromosome in much greater detail.