Chromosome-1

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Clinical cytogenetics is the study of chromosomes, their structure and their inheritance. Chromosome abnormalities are responsible for at least half of spontaneous abortions or miscarriages and are important cause of congenital malformations. More than 0.5% of newborns are born with significant abnormalities of autosomes or sex chromosomes.

Chromosomes are comprised primarily of DNA in association with various proteins and RNA; are located in the nucleus; are not usually visible in non-dividing cells but condense into visible structures as the cell prepares to divide; occur in pairs, one member derived from the female parent and the other from the male parent; members of a chromosome pair are called homologous; number 46 in humans, with 22 pairs of autosomes and a pair of sex chromosomes (2 X's in females; an X and a partially homologous Y in males);

in cells that contain pairs of homologous chromosomes are designated as diploid cells and the 46 chromosomes are referred to as 2n; in gametic cells—eggs (females) and sperm (males)— there is only one copy of each chromosome, and these cells are referred to as haploid with n number of chromosomes (n = 23 in humans); have a specialized region called the centromere to which fibers attach during cell division; the location of the centromere gives a chromosome its characteristic shape; may be stained to bring out distinctive features that allow the identification of each chromosome.

Karyotyping

•Mapping of the full chromosome set of the nucleus of a cell. The chromosome characteristics of an individual or a cell line are usually presented as a systematized array of metaphase chromosomes from a photomicrograph of a single cell nucleus arranged in pairs in descending order of size and according to the position of the centromere

Centromere

Each chromosome has a primary constriction called a centromere. The centromere can be found almost anywhere along the chromosome—near the top, near the middle, or in-between, but not at the very ends of the chromosome. The centromere divides the chromosome into two "arms," and unless the centromere is right in the middle, there will be a short arm, labeled p, and a long arm, labeled q.

Centromere Locations The placement of the chromosome does not always has to be in the middle of the chromosome. The centromere can be in the middle (metacentric), between the middle and the end (submetacentric), close to the end (acrocentric), or at the end (telocentric). See pictures below.

The Location of the Centromere

Numbering of Chromosome Bands

Metacentric Chromosome

Submetacentric Chromosome

Acrocentric Chromosom

G-Banding This method of staining devised by Dr. Giemsa ,

Method of staining chromosomes that produces light and dark bands characteristic for each chromosome. Bands are produced by staining with Giemsa stain after pretreating chromosomes with trypsin. We uses typsin to partially digest the proteins holding the chromosomes together (histones). This lets the chromosome relax, letting the stain bind to the exposed DNA. Each homologous chromosome pair has a unique pattern of gbands, enabling recognition of particular chromosomes. We can use the G-bande to facilitate the identification of structural abnormalities.

Giemsa banding, or G-banding Under specific preparation conditions, commonly known as Giemsa banding, or G-banding, chromosomes will appear to have between 400 or more alternating light and dark bands, depending on the technique used (the most common techniques yield 400, 550, or 850 bands). These bands, which can vary in intensity from very light to almost solid black, have been numbered by scientists and provide location markers with which breakpoints and small structural aberrations can be identified.

G-Banded Karyotype

The standard nomenclature for banding of human chromosomes includes a minimum of four items of information: the chromosome number; the arm symbol, p (short arm) or q (long arm); the "region" number along that arm; and the band number within that region.

For example, the designation 1p32 stands for chromosome 1, short arm, region 3, band 2. Sometimes, sub-bands, separated by a decimal point from the band designation, are also noted. For example, 9q34.1

Chromosome Staining and Banding  Techniques Convential Staining  C-Banding the isochromosomes can be selectively

identified because of the presence of heterochromatin  G-Banding  Q-Banding  R-Banding Giemsa Reverse Banding (RHG) , R Banding by Fluorescence Using Acridine Orange (RHG)  T-Banding  NOR-Staining Silver Nucleolar Organizing Region Staining

Gene Dosage Compensation 1.

2. c.

d. e.

How does an organism compensate for the fact that some individuals have a double dosage of sex-linked genes while others have only one? In female mammals, most diploid cells have only one fully functional X chromosome. The explanation for this process is known as the Lyon hypothesis, proposed by the British geneticist Mary F. Lyon. In females, each of the embryonic cells inactivates one of the two X chromosomes. The inactive X chromosome contracts into a dense object called a Barr body. Barr body = A densely staining object inside the nuclear envelope, that is an inactivated X chromosome in female mammalian cells.

Lyon hypothesis 1.

b. c. d.

Female mammals are a mosaic of two types of cells those with an active maternal X and those with an active paternal X. Which of the two Xs will be inactivated is determined randomly in embryonic cells. After an X is inactivated, all mitotic descendants will have the same inactive X. As a consequence, if a female is heterozygous for a sex-linked trait, about half of her cells will express one allele and the other cells will express the alternate allele.

The number of sex chromatin bodies is equal to the number of X chromosomes minus one.

Chromosome Abnormalities When mutations involve large parts of the chromosome such that they are visible under the light microscope, they are called chromosome aberrations. With routine light microscopy the smallest visible addition or deletion from a chromosome is about 4Mb. Chromosome abnormalities are classified into numerical and structural abnormalities. They can involve either the sex chromosomes or the autosomes.

Chromosome aberration 1. Numerical aberration 2. Structural aberration

1. Numerical aberration 1) Polyploidy 2) Aneuploidy 3) Mosaic

Polyploidy Polyploidy is the presence of whole sets of chromosomes in excess of the normal, or euploid, number; the number of chromosomes is a multiple of the normal diploid number of 46 human chromosomes. A complete extra set of chromosomes raises the total number to 69 (triploidy). This usually arises from: Fertilisation by two sperm (dispermy) Or from failure of one of the maturation divisions of either the egg or the sperm, producing a diploid gamete.

A triploid fetus would be 69,XXY (most common), 69,XXX or 69,XYY depending on the origin of the extra set of chromosomes. Four times the haploid number (tetraploidy) is usually due to failure to complete the first zygotic division.

A proportion of polyploid cells occurs normally in human bone marrow, as megakaryocytes usually have 8-16 times the haploid number. Tetraploid cells are also a normal feature of regenerating liver and other tissues.

Polyploidy

Aneuploidy Aneuploidy is the gain or loss of individual chromosomes from the normal diploid set of 46. It is most often caused by nondisjunction, the failure of chromosomes to separate properly during the early stages of mitosis or meiosis(paired chromosomes or sister chromatids), most often the latter. or may be due to delayed movement of a chromosome at anaphase (anaphase lag).

Aneuploidy Monosomy ,The loss of a single chromosome,is rarely seen in live births, for the vast majority of monosomic embryos and fetuses are probably lost to spontaneous abortion at very early stages of pregnancy. An exception is the loss of an X chromosome that produces Turner syndrome. Trisomy, the gain of an extra copy of a chromosome, resulting in 47 chromosomes , most often involveing X chromosome( multiple copies ).

Trisomies, the gain of a single chromosome, are more common than polyploidies and have been associated with various cancers. Over 20 broad categories of cancer with hundreds of subtypes have been associated with .

Trisomy 18

Trisomy 21

Aneuploidy

Monosomy

Trisomy

Causes of Aneuploid Nondisjunction Chromosome lost

Non-disjunction

Nondisjunction (gametegenesis)

in

1.one pair homologous chromosome 2.one pair sibling chromatid

meiosis

Chromosome lost

Mosaic Chromosomal mosaicism is when different cells within an individual, who has developed from a single fertilized egg, have a different chromosomal makeup( karyotype). 

In this illustration, the green cell represents a cell with an abnormal chromosome make-up.  All cells that come from the green cell will share the same chromosome change.  We say that all cells originating from that cell are in the same cell line.  The baby that develops from this embryo will have some cells in his/her body which have the typical number of chromosomes and some that have the chromosome change.  

a) Normal and abnormal cells are found in most tissues

Skin cells

Blood cells

b) Normal and abnormal cells are confined to specific tissues

Skin cells

Intestine cells

Causes of Mosaic 1.Nondisjunction in in zygote division

46

46 45 45

46

47

45 47

47

2n-1 or 2n+1 Nondisjunction in First zygote division

46

46 46 45

47

2n,2n-1 or 2n+1 Nondisjunction in Second zygote division

2. Chromosome lose in zygote division

46

46 45

46

46

45 45 46 46

45

46 46 46 46