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

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