Chromosome Structure and Nomenclature

Cytogenetic: The study of chromosomes and their abnormalities and cell division. Chromosomes: is threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.

Picture 1. Chromosomes as they appear under a Microscope.

Picture 2. The chromosomes arranged according to size; chromosome 1 is the largest. The last two chromosomes are the sex chromosomes .

Structure: chromosomes are not uniform in width through out their length ,with each Chromosomes having a construction, centromere it is responsible for the movement of the chromosome at the cell division. The centromere divide the chromosome in to short and long arms, and by international Convention, the short arm is termed the “p arm” while the long arm of the chromosome is termed the “q arm”.

The tip of each end is called telomere.

Chromosomes are classified according to the position of centromere: Metacentric: centromere in the middle of chromosome. Submetacentric: centromere divides the chromosome into 1\3 and 2\3. Acrocentric: centromere near the end

Chromosomal Abnormalities: Chromosome abnormalities are responsible of genetic diseases, occurring in approximately 1 of every 150 live births. They are the leading known cause of both mental retardation and pregnancy loss. Chromosome abnormalities are seen in 50% of first-trimester and 20% of second-trimester spontaneous abortions.

There are 2 types of chromosomal abnormalities Numerical Aneuploidy (Trisomy& Monosomy). Polyploidy (Mosaic aneuploidy). 2. Structural Deletion. Duplication. Translocation . Inversion.

Numerical Anomalies:”chr abnormality” Aneuploidy: involve the gain or loss of one or more chromosome. Trisomy: presence (i.e. gain ) of an extra chromosome. Ex. Trisomy 21, this trisomy produces Down syndrome that its caused by the presence of an extra copy of chromosome 21. we write “ 47,+ 21” .

Monosomy: absence (i.e. loss ) of a single chromosome. Ex. sterile female with Turner syndrome. We write “ 45 , x” Note: Chromosome number in Normal male is “ 46 , XY “ Normal female is “ 46 , XX”

Polyploidy: Chromosome abnormality in which the number of chromosomes in a cell is a multiple of 23 but is greater than the diploid number. Examples triploidy [69 chromosomes in the human] caused by fertilization of an egg by two sperm. 1n + 2n=3n tetraploidy[92 chromosomes in the human] caused by failure maturation of division in ovum\sperm. 2n+2n=4n

Picture show Triploidy

Structural Abnormalities: structural abnormalities result from chromosomal breakage with subsequent reunion in a different configuration, and they may be balanced or un balanced. In balanced rearrangement the chromosome complement is complete which no gain or loss of genetic material, i.e. they are harmless. However, the carriers of balanced rearrangements are often at risk of producing children with unbalanced chromosomal complement. .

In unbalanced rearrangements the chromosomal complement contains an incorrect amount of chromosomal material. Translocation: involves two non homologous chromosomes (e.g., chromosome 2 and chromosome 6). Following a break in each of the chromosomes, and subsequent reunion, a segment of chromosome 2 becomes attached to chromosome 6 and vice versa.

Reciprocal translocation: A break occurs in each of two chromosomes with segment being exchanged. Ex. balanced translocation involve the long arms of chromosome 11 and 22. The resulting chromosomes are called derivative chromosomes. The carrier of a reciprocal translocation is usually unaffected.

Robertsonian translocation: The short arms of two non homologous chromosomes are lost and the long arms fuse at the centromere to form a single chromosome.

Deletion: Caused by a chromosome break and subsequent loss of genetic material. A single break leading to a loss that includes the chromosome's tip is called a terminal deletion. When two breaks occur and the material between the breaks is lost, an interstitial deletion.

Inversion: involve only one chromosome in which two breaks occur and in the process of repair, the intervening segment is rejoined in an inverted or opposite manner. An inversion is paracentric if the inverted segment is on the long arm or the short arm and does not include the centromere.

The inversion is pericentric if breaks occur on both the short arm and the long arm and the inverted segment contains the centromere.

Duplications: refers to an extra chromosomal segment within the same homologous chromosome or an extra chromosomal segment on another non-homologous chromosome.