Chromosomes.pptx(Nursing) M.Sc [ Advance health Nurisng]

Chromosome and its impact in health

1. Chromosomes KARISHMA SINGH RAJPUT

2. DEFINITION OF CHROMOSOME Chromosome: A visible carrier of the genetic information. The 3 billion bp (base pairs) in the human genome are organized into 24 distinct, physically separate microscopic units called chromosomes. All genes are arranged linearly along the chromosomes. The nucleus of most human cells contains two sets of chromosomes, one set given by each parent. Each set has 23 single chromosomes--22 autosomes and an X or Y sex chromosome. (A normal female will have a pair of X chromosomes; a male will have an X and Y pair.) Chromosomes contain roughly equal parts of protein and DNA; chromosomal DNA contains an average of 150 million bases.

3. Cont … DNA molecules are among the largest molecules now known. Chromosomes can be seen under a light microscope and, when stained with certain dyes, reveal a pattern of light and dark bands reflecting regional variations in the amounts of A and T vs G and C. Differences in size and banding pattern allow the 24 chromosomes to be distinguished from each other, an analysis called a karyotype. A few types of major chromosomal abnormalities, including missing or extra copies or gross breaks and rejoinings (translocations), can be detected by microscopic examination; Down's syndrome, in which an individual's cells contain a third copy of chromosome 21, is diagnosed by karyotype analysis.

4. Chromosomes Composition: In eukaryotes, chromosomes consist of a single molecule of DNA [Link to visual proof] associated with:  Many copies of 5 kinds of histones. Histones are proteins rich in lysine and arginine residues and thus positively-charged. For this reason they bind tightly to the negatively-charged phosphates in DNA.  a small number of copies of many different kinds of non-histone proteins. Most of these are transcription factors that regulate which parts of the DNA will be transcribed into RNA.

5. STRUCTURE  For most of the life of the cell, chromosomes are too elongated and tenuous to be seen under a microscope.  Before a cell gets ready to divide by mitosis, each chromosome is duplicated (during S phase of the cell cycle).  As mitosis begins, the duplicated chromosomes condense into short (~ 5 μm ) structures which can be stained and easily observed under the light microscope.  These duplicated chromosomes are called dyads.  When first seen, the duplicates are held together at their centromeres. In humans, the centromere contains ~1 million base pairs of DNA. Most of this is repetitive DNA: short sequences (e.g., 171 bp) repeated over and over in tandem arrays.  While they are still attached, it is common to call the duplicated chromosomes sister chromatids, but this should not obscure the fact that each is a bona fide chromosome with a full complement of genes.  The kinetochore is a complex of proteins that forms at each centromere and serves as the attachment point for the spindle fibers that will separate the sister chromatids as mitosis proceeds into anaphase.

6. Cont …  The shorter of the two arms extending from the centromere is called the p arm; the longer is the q arm.  Staining with the trypsin- giemsa  For most of the life of the cell, chromosomes are too elongated and tenuous to be seen under a microscope.  Before a cell gets ready to divide by mitosis, each chromosome is duplicated (during S phase of the cell cycle).  As mitosis begins, the duplicated chromosomes condense into short (~ 5 μm ) structures which can be stained and easily observed under the light microscope.  These duplicated chromosomes are called dyads.  When first seen, the duplicates are held together at their centromeres. In humans, the centromere contains ~1 million base pairs of DNA. Most of this is repetitive DNA: short sequences (e.g., 171 bp) repeated over and over in tandem arrays.  While they are still attached, it is common to call the duplicated chromosomes sister chromatids, but this should not obscure the fact that each is a bona fide chromosome with a full complement of genes.  The kinetochore is a complex of proteins that forms at each centromere and serves as the attachment point for the spindle fibers that will separate the sister chromatids as mitosis proceeds into anaphase.  The shorter of the two arms extending from the centromere is called the p arm; the longer is the q arm.  Staining with the trypsin- giemsa

7. Chromosome Numbers:  All animals have a characteristic number of chromosomes in their body cells called the diploid (or 2n) number.  These occur as homologous pairs, one member of each pair having been acquired from the gamete of one of the two parents of the individual whose cells are being examined.  The gametes contain the haploid number (n) of chromosomes.

8. Karyotypes: The complete set of chromosomes in the cells of an organism is its karyotype. It is most often studied when the cell is at metaphase of mitosis and all the chromosomes are present as dyads. The karyotype of the human female contains 23 pairs of homologous chromosomes:  22 pairs of autosomes  1 pair of X chromosomes The karyotype of the human male contains:  the same 22 pairs of autosomes  one X chromosome  one Y chromosome

9. Cont … Above is a human karyotype (of which sex?). It differs from a normal human karyotype in having an extra #21 dyad. As a result, this individual suffered from a developmental disorder called Down syndrome. The inheritance of an extra chromosome is called trisomy, in this case trisomy 21. It is an example of aneuploidy Translocations Karyotype analysis can also reveal translocations between chromosomes. A number of these cause cancer, for example  the Philadelphia chromosome (Ph1) formed by a translocation between chromosomes 9 and 22 and a cause of Chronic Myelogenous Leukemia (CML)

10. DNA Content: The molecule of DNA in a single human chromosome ranges in size from 50 x 106 nucleotide pairs in the smallest chromosome (stretched full-length this molecule would extend 1.7 cm) up to 250 x 106nucleotide pairs in the largest (which would extend 8.5 cm). Stretched end-to-end, the DNA in a single human diploid cell would extend over 2 meters.

11. References: 1. ^ Paux E, Sourdille P, Salse J, et al. (2008). "A Physical Map of the 1-Gigabase Bread Wheat Chromosome 3B". Science 322 (5898): 101–104. Doi: 10.1126/science.1161847. PMID 18832645. 2. ^ Haeckel E. 1866. Generelle Morphologie der Organismen : Allgemeine Gründzuge der organischen Formen -Wissenschaft. 2 vols, Reimer, Berlin. 3. ^ Mayr E. 1982. The growth of biological thought. Harvard. p749