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Special type of chromosomes

Chromosomes are known as hereditary vehicles
They are formed of strands of DNA molecules which contain information for the development of different characteristics and performance of various metabolic activities of the cells
The coordination of various function is brought about through the formation of enzymes which are complex protein molecules

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Special type of chromosomes

  2. 2. WHAT IS A CHROMOSOME DNA packaged tightly in thread like structures Mainly seen in decondensed form but during cell division in condensed form DNA wrapped around proteins like histone results in nucleosome , thus the complex of DNA plus histones and other structural proteins is called chromatin Chromosome have p or two short arms , q or two large arms These arms are joined together by centromeres , they help to keep chromosomes aligned during the complex process of cell division
  3. 3. The two copies of a chromosome are called sister chromatids The sister chromatids are identical to one another and are attached to each other by proteins called cohesins As long as the sister chromatids are connected they are still considered to be one chromosome . However , as soon as they are pulled apart during cell division , each is considered a separate chromosome Telomeres are repetitive stretches of DNA located at the ends of linear chromosomes They protect the ends of chromosomes in a manner similar to the way the tips of shoelaces keep them from unravelling
  4. 4. In many types of cells , telomeres lose a bit of their DNA every time a cells divides . Eventually , when all of the telomere DNA is gone , the cell replicate and dies
  6. 6. CHROMOSOME  Chromosomes are known as hereditary vehicles They are formed of strands of DNA molecules which contain information for the development of different characteristics and performance of various metabolic activities of the cells The coordination of various function is brought about through the formation of enzymes which are complex protein molecules
  7. 7. The information for the synthesis of these protein molecules is contained in the DNA molecules in the sequence of nitrogen bases The sequences of three nitrogen bases that codes for one molecule of a particular amino acid constitutes a triplet codon DNA of a particular cistron tanscribes its m RNA
  8. 8. It carries genetic code similar to its DNA It moves out of the nucleus and get attached to the ribosome The t RNA molecule pick specific activated amino acids and recognize the codon on the m RNA by their specific anticodons The amino acids are thus linked together forming a polypeptide chain , which is finally changed into functional protein either singly or by the association of several polypeptide chains
  9. 9. SPECIAL TYPES OF CHROMOSOMES In some organisms there are special tissues in which chromosomes undergo structural specializations Such specialized chromosomes includes Giant chromosomes 1. Polytene chromosomes 2. Lampbrush chromosomes B chromosomes/supernumerary chromosomes
  10. 10. GIANT CHROMOSOMES Exceptionally enlarged and highly specialized Found during some stages of life cycle of certain cells Two kinds of giant chromosomes
  11. 11. ENDOMITOSIS The enormous size is the result of peculiar kind of chromosomal re- duplication called endomitosis or endoduplication It is repeated duplication of chromatids in geometrical progression , without accompanying cell division In this process the DNA and chromatids keep on multiplying in geometrical progression Thus , endomitosis makes the chromosomes multistranded and increases its DNA content nearly 1000 times or more It is common in certain types of cells of plants or animals Examples are human liver cells , the giant abdominal neuron of Aplysia , and macronucleus of ciliates
  12. 12. POLYTENE CHROMOSOMES Special types of interphase chromosomes that are large These giant chromosomes are found in the cells of salivary glands of Drosophila and Chironomus larvae and in the cells of fat bodies of larval stages of certain dipterans They were first observed in the salivary glands of Chironomus and hence are called salivary gland chromosomes They are cable like multi stranded giant chromosomes
  13. 13. They are also seen in synergids and antipodal cells of ovules of certain angiosperms , intestinal epithelium , malphigian tubules etc. They were first observed by Balbani
  14. 14. STRUCTURE The polytene chromosomes presents a distinct pattern of transverse banding It consist of dark coloured bands alternating with light coloured interbands Nearly 85% of the chromosomal DNA is in the dark bands and the remaining 15 % is in the interbands (light) Bands are mostly heterochromatic, and the interbands are euchromatic
  15. 15. It is assumed that the band region is formed by the merging of chromomeres of adjoining chromatids through condensation and localized super coiling of chromatin filaments The bands are specific for a particular chromosome and banding pattern is almost similar in both the homologous chromosomes of a pair Therefore during meiosis the pairing between homologous chromosomes is band to band Each band consists of genes that can code for more than one protein hence polygenic
  16. 16. Each band is believed to contain tissue specific genes Each gene in the band may have multiple copies which are not transcriptionally active all the time On the other hand , interbands are believed to contain house keeping genes , which are constantly transcribed and translated into products necessary for the cell The structure of giant chromosomes is modified in certain regions such that one or more specific bands enormously enlarge and bulge out , forming local swellings , called chromosome puffs They are formed by the uncoiling of chromatin filaments
  17. 17. Puffs are transcriptionally active expansions or extensions of chromatin So they are active centres of the synthesis and accumulation of RNAs In some cases , in addition to RNA , extra DNA is also producing during puffing Such puffs are the sites of specific gene amplification and are responsible for differentiation and development In some puffs , chromatin filaments are drawn out as a series of lateral loops , known as BALBIANI rings
  18. 18. There are four categories of chromosome puffs Stage specific puffs Tissue specific puffs Constitutive puffs Environmentally induced puffs Puffing is a cyclic and reversible process At definite times , puffs appear , grow and finally disappear to repeat the whole cycle over and over again
  19. 19. Puffing involves the regular succession of four major events Excessive accumulation of acidic proteins Detachment of the DNA bound basic proteins to expose the DNA for transcription Despiralisation and decondensation of chromatin Active synthesis and storage of RNAs Puffing is mainly initiated during accumulation of acidic proteins ,it is also shown that insect moulting hormone called ecdysone induces puffing
  20. 20. Puffs and Balbiani rings are regarded as the active centres for DNA duplication and RNA synthesis Hence puffing is interpreted as a manifestation of gene expression Since active DNA duplication occurs in puffs , they may contain heavy accumulation of redundant and repetitive DNA sequences These are the main characteristics of polytene chromosome present in dipteran
  21. 21. In case of polytene chromosomes in ciliates they doesn't have neither contain puffs , and Balbiani rings nor do they show any sign of transcriptional activity In them fully formed chromosomes first get transected by ‘membranous’ partitions , which appear in interband regions These membranes consist of only a single diffuse layer of proteins , which accompanies polytenization of chromosomes Then , each band and the adjacent interband get completely enclosed within a membrane , forming a vesicle
  22. 22. Thus a series of vesicles are formed all along the chromosome The vesicle stage would be followed by the shrinkage of chromosome The transection of chromosome , the formation of vesicles and the shrinkage of the chromosome are associated with a progressive reduction in DNA content Nearly 85-90-% of the DNA is degraded to acid soluble products , that are permanently lost from the chromosome Soon after degradation and elimination of DNA , vesicles disappear The small amount of DNA that still remains and undergoes
  23. 23. LAMPBRUSH CHROMOSOMES Longest chromosomes of all In oocytic nuclei of those animal which have large yolky eggs , the prophase of first meiotic division is extremely extended During this phase the oocytes grow and synthesis nutrition for the future embryo In them , the chromosomes become greatly enlarged and assume unusual configuration A large number of loops projects out from the chromatid axis , giving it a lampbrush appearance . Hence are called lampbrush chromosomes
  24. 24. The lampbrush chromosomes are bivalents , each consisting of two chromatids These persist during the prolonged diplotene phase of first meiotic prophase First observed by Flemming (1882) and described by Ruckert (1892) They are seen in diplotene phase of insects , sharks , amphibians , reptiles and birds The lampbrush chromosomes in the oocytes of salamanders are the largest of all known chromosomes
  25. 25. STRUCTURE A lampbrush chromosome consists of a central axis and a paired series of loop like lateral extensions The central axis consists of two bivalent homologous chromosomes , each with two chromatids , so altogether 4 chromatids are present They are held together through contact points or chiasmata The chromatids are drawn out into paired lateral loops ,so central axis extend to the lateral loops as loop axis Loop axis is only a single chromatid of one of the
  26. 26. Lateral loops contain bundles of sub-microscopic fibrils Both the central axis and the loop axis are formed of deoxyribonucleoprotein The loop axis may be coated with a matrix of RNA and proteins Lateral loop contain a fully extended DNA duplex ,they are believed to be formed of uncoiling of sister chromatids , making their DNA available for transcription Lateral loops are the active centres for RNA synthesis
  27. 27. Each loop may contain one or more transcription unit or transcriptons Most of the newly synthesized RNA molecules associate with proteins and form a ribonucleoprotein (RNP) matrix  m RNA is also associated development of zygote The chromosomal axis may contain paired series of granular chromomeres , from were loop axes extend Usually 1-9 loops may arise from each chromomere Small swellings , without loops , may be present towards the end of the central axis ,they represent the telomeres Each bivalent may contain a loopless swelling also it represents the centromere
  28. 28. There are to main hypothesis regarding lampbrush chromosomes According to Callen and Lyod - master and slave hypothesis , each loop axis is polygenic , with repetitive DNA sequences and multiple copies of genes The centromere is believed to contain the master copy of each gene (master gene) , and the loop axis contains its duplicate copies (slave genes ). Only master gene take part in transcriptional activity
  29. 29. According to spinning out and retraction hypothesis , a chromomere is a fully transcribed gene from end to end by spinning out a transient loop The new loop material spins out on one side of a chromomere at the thin end of loop and returns to a condensed stage on the other side after completing the synthesis of RNA These are associated with the rapid synthesis of yolk (deutoplasmic) and RNA , protein in the maturing ovum These disappear by the end of first prophase when chromosomes become thick and more condensed
  30. 30. B CHROMOSOMES OR SUPERNUMERARY CHROMOSOMES Accessory , aberrant or redundant chromosomes Extra copies of normal chromosomes Not homologous to normal chromosomes and are not essential Normally they are highly heterochromatic They are present in many species of animals and flowering plants like maize , rye ,some ants ,some grasshoppers etc. Their increased presence in individual organisms produce no marked phenotypic effects
  31. 31. They are often lost or gained in an individual organisms Some features include; Smaller in size , heterochromatic ,allocyclic and telocentric or acrocentric Exhibit structural polymorphism among cells and tissues of same organism Genetic effects is less compared to other chromosome Present in large amount affect fertility and vigour Relatively unstable , get eliminated due to lagging and non – disjunction Non – mendelian inheritance No chiasmata pairing
  32. 32. Appreciable quantity of highly repetitive DNA with very low cytosine and guanine content More amount of histone and lesser protein content Two main groups ; Mitotically stable B chromosome whose number is same in the cells of an individual organism Mitotically unstable B chromosome whose number is different in different cells of an organism
  33. 33. Effects of B chromosome Delay the flowering time of plants Adversely affect the plant character like height , weight etc. Suppress or promote meiotic pairing The current concept is that B chromosome might have formed from small fragments through the clustering of highly repetitive and heteropycnotic satellite DNA and other structural re-arrangements They eventually become as large as other chromosomes by repeated duplication
  34. 34. Thank you