special type of chromosomes
TRANSCRIPT
SPECIAL TYPE OF CHROMOSOMES
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
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
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
CHROMOSOME STRUCTURE
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
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
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
SPECIAL TYPES OF CHROMOSOMES
In some organisms there are special tissues in which chromosomes undergo structural specializations
Such specialized chromosomes includesGiant chromosomes
1. Polytene chromosomes
2. Lampbrush chromosomes
B chromosomes/supernumerary chromosomes
GIANT CHROMOSOMES
Exceptionally enlarged and highly specialized
Found during some stages of life cycle of certain cells
Two kinds of giant chromosomes
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
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
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
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
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
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
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
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
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
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
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
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 replication
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
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
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 chromosomes
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
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
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
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
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
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
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
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
Thank you