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Cell Biology
NEW TECHNIQUES FOR IDIOGRAM PREPARATION
Modern techniques used in karyotype preparation are ISH, FISH (Fluorescence in Situ Hybridisation), Mc FISH (Multicolour fluorescence in situ Hybridisation) and flow cytometry.
USE OF KARYOTYPING OR IDIOGRAM
(i) It suggests primitive or advanced features of an organism. If karyotype shows a large size difference between the smallest and the largest chromosome of the set and having fewer metacentric chromosomes then it is calledasymmetric karyotype, which is a relatively advance feature. Symmetric karyotype is primitive feature.
(ii) The karyotype of different species are compared and similarities in them represent the evolutionary relationships.
(iii) Karyotype is helpful in detection of chromosomal abberrations and polyploidy.
(iv) In research of medical genetics Forensic science cytogenetics and Anthropogenetics.
1. In Situ Hybridization : Using DNA probe labelled with radioactive molecule to locate the position of DNA sequence on chromosome.
2. Fluorescence in Situ Hybridization (FISH) : DNA may also be labelled with fluorochrome to locate the position of DNA sequence on chromosome.
3. Multicolour Fluorescence in Situ Hybridization (Mc FISH) : More fluorochrome colour to locate the position of DNA sequence on chromosome.
4. Flow cytometry : This is recent technique. In this technique a suspension of many thousands of chromosome is made and the suspended chromosome are stained with a DNA binding flurochrome.
STRUCTURE OF CHROMOSOME (Parts which appears in metaphase chromosome)
1. Pellicle – This is outermost, thin proteinaceous covering or sheath of chromosome.
2. Matrix – This is a liquid nongenetic achromatic ground substance of chromosome, which has different type of enzymes, minerals, water, proteins.
3. Chromonema (singular Chromonemata) →Term by Vejdovsky. This is an important, genetical, highly coiled thread, throughout the length of a chromosome or chromatid. It was called chromonema.
Types of coiling in chromonema –
(i) Plectonemic-coiling :- When both the chromonema are inter twined and can not be seperated easily. (in mitotic prophase chromosomes)
(ii) Paranemic coiling :- When both chromonema can be easily seperable. (In meiotic prophase)
4. Centromere/Kinetochore :- (Primary constriction)
Centromeric DNA is called as alphoid DNA.
5. Chromatid – At metaphase stage each chromosome is consist of two cylindrical structures - called chromatids.
Both sister chromatids or longitudinal half chromosome are joined together by a common centromere. A chromosome, may have single chromatid (in Anaphase or Telophase) or two chromatid. (as in metaphase)
6. Secondary constriction : Besides primary constrictions one or two, other constriction may also occurs on some chromosome, which are known as secondary constriction.
7. Satellite : part of chromosome remains after the NOR is known as chromosomes satellite/ Trabent.
8. Telomere : Chromosomes have polarity and polar ends of chromosomes is known as Telomere.
TYPES OF CHROMOSOMES ON THE BASIS OF POSITION OF CENTROMERE
(i) Telocentric :- When centromere is terminal or located at the tip of chromosome.
(ii) Acrocentric :- When the centromere is sub-terminal or located near the tip.
(iii) Metacentric :- When the centromere is located at mid of the chromosome.
(iv) Sub metacentric :- When the centromere located near centre or mid point of chromosome.
TYPES OF CHROMOSOMES ON THE BASIS OF NUMBER OF CENTROMERE
(i) Acentric :- Chromosome without centromere.
(ii) Monocentric :- Chromosome with one centromere.
(iii) Dicentric :- When the number of centromere is two.
(iv) Polycentric chromosome :- When the number of centromere is more than two & diffused in throughout chromosome length.
ULTRA STRUCTURE OR FINE STRUCTURE OF THE CHROMOSOME
Nucleosome model :- Bead like structure in chromatin was first observed by Olin's etal. This model was proposed by Kornberg & Thomas in 1974, which is most important and universally accepted model for the structure of chromosome. This model explain that how giant DNA molecule & histone (Chromatin) packaged in to a chromosome. Term nucleosome was given by P.Oudet in 1975. "Nucleosome is a unit of chromatin (chromosome) which is composed of about 200 base pairs of the
DNA and an Octamer (Core particle) of four types ( H2A, H2B, H3& H4) of histone proteins". Nucleosome is also known as Nu-body or g-particle.
Nucleosome = Binding DNA (146 bp) + Octamer core (H2A, H2B, H3, H4 × 2) + Linker DNA + H1 Histone
SPECIAL TYPE OF CHROMOSOMES
1. Salivary gland chromosome :- This type of chromosome was discovered by E.G. Balbiani, in Chironomous larva of Drosophila . Size of this chromosome may upto 2000 micron (2mm) and number of chromatids may be 512 to many thousands. Thus, this type of chromosome also known as Giant chromosome.
2. Lamp brush chromosome :- Discovered by Flemming and Ruckert from oocytes of vertebrates (Amphibia) during diplotene stage of cell division. These chromosomes look like lamp - brush, thus called as lamp brush chromosomes.
Size of these chromosomes may upto 5900 micron, and also called as giant chromosome.
3. B-Chromosome/Accessory chromosome/Supernuemerary chromosome :-
4. Mega chromosomes :
5. Isochromosomes :
6. Ring chromosome :- Prokaryotic chromosome are ring chromosome or consists of circular folded DNA without histone.
7. Sex chromosome :- May be XX or XY 8. HACs, MACs, YACs, BACs, etc.
HUMAN CHROMOSOMES
Group (A) : 1–3 chromosomes of largest size and submetacentric or metacentric centromere.
Group (B) : 4–5 chromosomes with less larger size, submetacentric
Group (C) : 6–12 chromosomes with medium sized and submetacentric centromere.
Group (D) : 13–15 chromosomes, shorter than group ‘C’ with centromere near the end (Acrocentric). They are SAT chromosomes or satellite.
Group (E) : 16–18 chromosomes, short sized, with median (Metacenteric) or submedian centromere (Submetaceteric).
Group (F) : 19–20 chromosomes, short sized with median centromere.
Group (G) : 21–22 chromosomes, smallest in size, acrocentric and are also posess satellites.
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