CHROMOSOMES SMG
- 1. Dr. Saji Mariam George Associate Professor (Retired) Assumption College Autonomous Changanacherry CHROMOSOMES
- 2. CHROMOSOMES ( Chromo = colour ; Soma = body) • The term chromosome – coined by Waldeyer (1888). • At the time of cell division, chromatin become condensed into chromosomes. • Chromosome number – constant for a species. .
- 3. CHROMOSOME MORPHOLOGY • Studied during Metaphase and Anaphase - periods of maximum condensation - chromosomes become thick and filamentous. • Each chromosome has two chromatids – attached to each other by a clear zone – Centromere (Kinetochore or primary constriction).
- 4. • Each chromatid – a single linear DNA molecule with its associated proteins known as chromonema (Plural- Chromonemata). • Chromonemata of thin chromosomes of mitotic and meiotic prophase have alternating thick and thin regions
- 5. • The thick, bead-like structures – Chromomeres – accumulation of chromatin material – tightly folded regions of DNA correspond to the units of genetic function in the chromosome. • The thin region in between the chromomeres – interchromomeres.
- 7. CENTROMERE (KINETOCHORE OR PRIMARY CONSTRICTION) • Region of chromosomes that becomes attached to the mitotic spindle • Contain specific DNA sequences with special proteins bound to them forming a disc-shaped structure. • Function – to provide a centre of assembly for microtubules for chromosomal movement during mitosis.
- 9. • Monocentric - one centromere - most of the chromosomes. • Diffuse centromere (Holocentric chromosomes) – microtubules attached along the length of the chromosomes. • E.g. In species of Juncaceae and Cyperaceae.
- 10. Fig. a : Monocentric chromosomes (in the upper part) Fig.b : Chromosomes with diffuse centromeres (in the lower part)
- 11. Acentric and Dicentric chromosomes • In some chromosomal abnormalities, chromosomes breaks and fuse with other ones, producing acentric (chromosomes without centromere) or Dicentric chromosomes (with two centromeres).
- 12. Dicentric chromosome Image:Steven M. Carr (2013) https://www.mun.ca
- 13. Types of Chromosomes based on shape Shape - determined by the position of the centromere. 4 Types : i. Telocentric: Centromere is terminal – rod shaped ii. Acrocentric: Centromere near one end – a very short arm and a very long arm iii. Submetacentric: Centromere near the centre – two unequal arms – ‘J’ or ‘L’ shaped. iv. Metacentric - Centromere in the centre – 2 equal arms – ‘V’ shaped . Image:https://www.embibe.com
- 14. Secondary constriction • Some chromosomes may have a secondary constriction at any part of the chromosomes. • Certain secondary chromosomes play a vital role in the formation of the nucleolus – Nucleolar organizers or Nucleolar Organizing Regions (NOR) Nucleoli form around the chromosomal regions that contain 5.8 S, 18 S and 28 S rRNA genes. They contain the genes for the pre-RNA.
- 16. TELOMERE • Extreme tip of the chromosome. • It protects the end of the chromosome from deterioration and prevents other chromosomal segments to be fused with it. • A region of repetitive nucleotide sequences at each end of a chromosome . • In most organisms, these repeats are maintained by the enzyme, telomerase. • Telomeres assure the physical integrity of chromosome ends.
- 17. Human telomeres on metaphase chromosomes using digital fluorescence microscopy. Image:https://www.utsouthwestern.edu
- 18. SATELLITES • Present in some chromosomes. • A round body separated from the rest of the chromosomes by a delicate chromatin filament , which may be long or short. • Chromosome with satellite - SAT chromosomes. • For a particular chromosome, the size and shape of the satellite and the filament are constant.
- 20. KARYOTYPE • It is the characteristic chromosome set of a species. • This include the characters by which a set of the chromosomes of a species is identified such as the number of chromosomes, relative size, position of the centromere, length of arms, secondary constriction, satellite etc. • Diagrammatic representation of the karyotype is known as ideogram.
- 21. Human Karyotype (Male)2n = 46 (23 pairs of homologous chromosomes ; 22 pairs of autosomes & 1 pair of allosomes (sex chromosomes –XY) Image: https://www.mun.ca
- 22. Ideogram of human chromosome Image :https://commons.wikimedia.org
- 23. Chemical composition/ Chemical organization of Nucleosome / Nucleoproteins Chromatin - a viscous , gelatinous substance that contains DNA, RNA, basic proteins – Histones and non- histone (more acidic) proteins. • Histones and DNA are present in a 1:1 ratio. • The content of RNA and non – histone proteins is variable in different chromatin. • The non- histone proteins are heterogeneous – vary in different tissues - include RNA and DNA polymerases and other regulatory proteins.
- 24. i) DNA • The most important chemical component of chromatin – measured in picogram. • Each species has a characteristic content of DNA which is constant in all individuals of that species – C- value. • Nuclei contain a constant amount of DNA – all the cells in a diploid organism contain the same DNA content , 2C.
- 25. • Since gametes are haploid, they have half the DNA content, 1C. C – value Paradox (Gall, 1981): There is no correlation between the C values of species and their evolutionary complexity. Eg. : The Salamander , Ambystoma has 84 picogram (pg) of DNA where as man has only about 3pg of DNA per haploid genome.
- 26. ii) Histones • Small proteins having high content (10 to 20 %) of basic amino acids Arginine and Lysine. • Bind tightly to DNA • Five histones –H2A, H2B, H3, H4. – very similar in different species and are present twice every 200 base pairs of DNA. • The fifth histone, H1 is present only once per 200 base pairs of DNA and varies between species and within tissues of the same species.
- 27. • Histones play an important role in maintaining the structure and functional integrity of chromatin. They also protect DNA from the action of nucleases. iii) Non- histone proteins • Include all the enzymes and factors that are involved in DNA replication, transcription and in the regulation of transcription. e.g. DNA polymerase
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