A Chromosome is an organized structure of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.

Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing. DNA that makes up chromosomes becomes more tightly packed during cell division and forms the Chromosomes.

Discovery of Chromosomes and Their Functions

Heinrich Wilhelm Gottfried von Waldeyer-Hartz Waldeyer is credited with the discovery of chromosomes in

the year 1888 within the nucleus.

Walter Sutton & Theodor Boveri

Sutton and Boveri formulated the Chromosomal theory

of inheritance, independently in 1902.

Function Each chromosome carries several thousand genes

(inherited units of DNA). These provide information which is the “master plan,” or design, of a person. Genes determine features such as hair, skin, and eye color, the shape of facial features, height, and, it is thought, even some personality traits. Tiny differences in the information they provide make each person unique.

In humans and most other complex organisms, one copy of each chromosome is inherited from the female parent and the other from the male parent. This explains why children inherit some of their traits from their mother and others from their father.

Chromosomes are the vectors of heredity.

Chromosomes vary widely between different organisms.

Eukaryotic cells (cells with nuclei) have large Linear Chromosomes and prokaryotic cells (cells without defined nuclei) have smaller Circular Chromosomes.

Cells may contain more than one type of chromosome; for example, mitochondria in most eukaryotes and chloroplasts in plants have their own small chromosomes

Definitions of Chromosomes:

1. A threadlike linear strand of DNA and associated proteins in the nucleus of animal and plant cells that carries the genes and functions in the transmission of hereditary information.

2. A circular strand of DNA in bacteria and cyanobacteria that contains the hereditary information necessary for cell life.

Chromosome Number The chromosomes are present in pairs. The pairs are

called the Homologous Pairs. A species will always have the same number of

chromosomes. This is called the Chromosome Number and it will always be an even number. This number is called the Diploid Number (2n).

During gamete formation, the homologous chromosomes separate and the gametes will have only half the number of chromosomes. This number is called the Haploid Number (n).

The chromosomes can be numbered and studied. The pictorial representation of the entire set of chromosomes is called the Karyogram.

The Chromosomes are classified into two types, Autosomes and Sex Chromosomes.

The autosomes have homologous chromosomes as pairs whereas the sex chromosomes are of two different types - X and Y. A female has two X-chromosomes and a male has an X and a Y chromosome.

The locations of genes on chromosomes are called the Loci (Sing. Locus).

Human Karyogram

The Chromosome Numbers Of Some Plants And Animals

Structure of Eukaryotic Chromosome: Each chromosome is made up of a coil of chromatin. This is composed of deoxyribonucleic acid (DNA) and protein. The chromosomes are cylindrical structures with one or more constrictions. The shape of the chromosome is specific and constant. Eukaryotic chromosome has DNA, RNA, histones, non histones proteins and

metallic ions. A typical chromosome has narrow zones called Constrictions The Centromere is associated with the Primary Constriction. In some chromosomes there occurs another constriction called the Secondary

Constriction associated with the presence of nucleolus. The sides of the chromosomes on either side of the centromere are called the

arms. Centromere divides the chromosome into two sections, Chromatids or “arms.”

The short arm of the chromosome is labeled the “p ARM.” The long arm of the chromosome is labeled the “q ARM.”

Each chromatid has a Double Helical DNA Molecule. The two chromatids are held together by the centromere. During cell division, the spindle fibres are attached to the centromeres. During anaphase, when the chromosomes or chromatids move apart, they form different shapes based on the position of the centromere.

Telomere: It is the terminal part of chromosome. Telomeres are repetitive stretches of DNA located at the ends of linear chromosomes. They protect the ends of chromosomes in a manner similar to the way the tips of shoelaces keep them from unraveling. .

Types of Chromosomes

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. The different types of chromosomes based on the position of the centromere are: Metacentric : The centromere is at the centre and the chromosome is V-shaped during anaphase. Sub-metacentric: The centromere is a little away from the centre and the chromosome gets an L shape. Acrocentric: The centromere is nearer to one end and the chromosome is J-shaped. Telocentric: The centromere is at the end and the chromosome is l-shaped.

A typical chromosome has narrow zones called Constrictions. Primary Constrictions: It is made up of centromere and kinetochore. Centromere contains a complex system of fibres called kinetochores and is essential for the movement of chromosome. If the centromere of chromosome is damaged, such chromosome fails to move at anaphase Secondary Constrictions: Only one or two chromosomes have secondary constrictions and such secondary constrictions are called as Nucleolar Organism. Satellite is a short chromosomal segment and separated from the main chromosome by a relatively elongated secondary constriction.

Chromosome Banding The treatment of chromosomes with Dyes to reveal characteristic patterns of horizontal bands like bar codes revolutionized the ability to identify specific chromosomes. Certain dyes produce reproducible patterns of bands when used to stain

chromosomes this technique is called Chromosome Banding. Chromosome banding has become a standard and indispensible tool

for cytogenetic analysis, and several banding techniques have been developed:

* Q banding: chromosomes are stained with a fluorescent dye such as quinacrine * G banding: produced by staining with Giemsa after digesting the chromosomes with trypsin * C banding: chromosomes are treated with acid and base, then stained with Giesma stain

Each of these techniques produces a pattern of dark and light (or

fluorescent versus non-fluorescent) bands along the length of the chromosomes. Importantly, each chromosome displays a unique banding pattern, analagous to a "bar code", which allows it to be reliably differentiated from other chromosomes of the same size and centromeric position.

What are the minimum and maximum number of chromosomes?

The record for minimum number of chromosomes belongs to a subspecies of the ant Myrmecia pilosula, in which females have a single pair of chromosomes. This species reproduces by a process called haplodiploidy, in which fertilized eggs (diploid) become females, while unfertilized eggs (haploid) develop into males. Hence, the males of this group of ants have, in each of their cells, a single chromosome.

The record for maximum number of chromosomes is found in found in the fern family. Polyploidy is a common conduction in plants, but seemingly taken to its limits in the Ophioglossum reticulatum. This fern has roughly 630 pairs of chromosomes or 1260 chromosomes per cell. The fact that these cells can accurately segregate these enormous numbers of chromosomes during mitosis is truly remarkable.

HUMAN CHROMOSOMES Humans have 23 pairs of

chromosomes, for a total of 46 chromosomes.

Twenty-two of these pairs, called autosomes, look the same in both males and females.

The 23rd pair, the sex chromosomes, differ between males and females. Females have two copies of the X chromosome, while males have one X and one Y chromosome.

When the sperm and egg unite in fertilization, they create a single cell, or zygote, with 46 chromosomes. When cell division occurs (called mitosis), these 46 chromosomes are duplicated. This process is repeated billions of times over, with each of the cells containing the identical set of chromosomes. Only the gametes, or sex cells, are different. In their cell division (called meiosis), the members of each pair of chromosomes are separated and distributed to different cells. Each gamete has only 23 chromosomes.

Chromosomal abnormalities Chromosomal abnormalities usually result from an error that occurs when an egg or sperm cell develops.

These errors can be: a. in the number of Chromosomes b. Or in the structure of chromosomes.

In most cases, an embryo with the wrong number of chromosomes or severe abnormalities in the Chromosome Structure does not survive.

Changes in the number or structure of chromosomes in new cells may lead to serious problems. For example, in humans, one type of leukemia and some other cancers are caused by defective chromosomes made up of joined pieces of broken chromosomes

Trisomy is an abnormality in number of chromosomes where an individual has three copies of a specific chromosome, instead of two. Most individuals with Down syndrome generally have three copies of chromosome 21.

Other errors can occur before fertilization and alter the structure of one or more chromosomes. Individuals with structural chromosomal abnormalities usually have the normal number of chromosomes. However, small pieces of a chromosome (or chromosomes) may be deleted, duplicated, inverted, misplaced or exchanged with part of another chromosome. These structural rearrangements may have no effect on a person if all of the chromosome is there but just rearranged. In other cases, the rearrangements may result in pregnancy loss or birth defects.

Errors in cell division can occur soon after fertilization. This can result in Mosaicism, a condition in which an individual has cells with different genetic makeups. For example, individuals with the mosaic form of Turner syndrome are missing an X chromosome in some, but not all, of their cells. Some individuals with chromosomal mosaicism may be mildly affected, but the severity of the condition depends largely on the percentage of abnormal cells.

The Most Common Chromosomal Abnormalities- Abnormalities in Number

Downs Syndrome: most common chromosomal abnormality, affecting about 1 in 800 babies and these individuals have three copies of chromosome 21 . Individuals with Down syndrome have varying degrees of intellectual disability, characteristic facial features and, often, heart defects and other problems. 45 XX or 45XY = Total 47

Trisomies 13 (Patau Syndrome)and 18 (Edwards Syndrome): have severe mental retardation and many physical birth defects and most affected babies die before their first birthday 45 XX or 45XY= Total 47

Turner syndrome: Girls with Turner syndrome have one X chromosome and are missing all or part of the other X chromosome. They usually are infertile and do not undergo normal puberty changes unless they are treated with sex hormones. ave other health problems, including heart and kidney defects. 44X= Total 45

Triple X: About 1 in 1,000 females has an extra X chromosome. affected girls tend to be tall, have normal intelligence and normal puberty and are fertile 44XXX= Total 47

Klinefelter Syndrome: Boys with Klinefelter syndrome have two, or occasionally more, X chromosomes along with their Y chromosome. Affected boys usually have normal intelligence, though many have learning problems. As adults, they produce lower-than-normal amounts of the male hormone testosterone (and often are treated with this hormone) and are infertile.

XYY: About 1 in 1,000 males is born with one or more extra Y chromosomes. Affected males are sometimes taller than average, have normal sexual development and are fertile. Most have normal intelligence, though some have learning, behavioral and speech/language problems.

Less Common Chromosomal Abnormalities- Structural abnormalities

* Deletions: A small section of a chromosome is missing. * Microdeletions: An extremely small amount of a

chromosome is missing, possibly only a single gene. * Translocations: A section of a chromosome is attached

to another chromosome. * Inversions: A section of chromosome is snipped out and

reinserted upside down. * Duplications: A section of a chromosome is duplicated,

so there is extra genetic material. * Ring chromosome: Material is deleted at both ends of a

chromosome, and the new ends join together to form a ring.

Less Common Chromosomal Abnormalities- Structural abnormalities

• Cri-du-chat (Cat Cry) Syndrome (Deletion On Chromosome 5): Affected children have a cat-like, high-pitched cry during infancy, mental retardation and physical abnormalities. About 1 in 20,000 to 50,000 babies is born with this disorder.

• Prader-Willi syndrome (deletion on chromosome 15): Affected children usually have mental retardation or learning disabilities, behavioral problems and short stature. They also may develop extreme obesity. 1 in 10,000 to 25,000 babies is affected.

• 22q11 deletion syndrome (deletion on chromosome 22): About 1 in 4,000 babies is born with deletions in a specific region of chromosome 22. These deletions cause a variety of problems that can include heart defects, cleft lip/palate, immune system abnormalities, characteristic facial features and learning disabilities. Certain combinations of these features are sometimes called DiGeorge or velocardiofacial syndrome. Individuals with this disorder have a 50-percent chance of passing the chromosomal abnormality on to their offspring with each pregnancy.

• Wolf-hirschhorn Syndrome (Deletion On Chromosome 4): This disorder is characterized by mental retardation, heart defects, poor muscle tone, seizures and other problems. It affects about 1 in 50,000 babies .

With the exception of individuals with 22q11 deletion syndrome, individuals with these disorders generally do not reproduce.