Chromosomes

 Chromosomes

Karl Nageli in 1842 observed darkly stained thread like structures in the nucleus. In 1876, Balbiani described rod-like structures that were formed in the nucleus before cell division. In 1879 Walter Flemming used the word “Chromatin” (Gr. Chroma-colour) to describe the substance that stained intensely with basic dyes in interphase nuclei.

He suggested that the affinity of chromatin for basic dyes was due to their content of “nuclein”, a phosphorous –containing compound isolated from pus cells by Meischer in 1871, a substance that is now called DNA.

In 1888 Waldeyer used the word Chromosome of interphase nuclei and rod-like objects observed during mitosis.

Chromosomes are concerned with the transfer of genetic traits from one generation to next and hence referred to as ‘Physical basis of heredity’. Sutton and Boveri proposed the chromosome theory which states that chromosomes are the vehicles of hereditary.

Chromosome number

The number of chromosomes varies from species to species, but is constant for a given species. Therefore, they are of great importance in the determination of phylogeny and taxonomy of the species. 

The somatic cells contain two sets of chromosomes. This number is called diploid number which is represented by 2n. The gametes contain only one set of chromosomes. This number is called haploid number and it is represented by n. 

The lowest number of chromosomes is 2 (2n=2) found in roundworm Ascaris megacephala. The maximum number of chromosomes is 1700 found in Radiolarian (Aulacantha, protozoan).

Among plants, chromosome number varies from 2n=4 in Haplopappus gracilis (compositae) to 2n = 1200 in some pteridophytes. 

Size:

The size of mitotic metaphase chromosomes generally varies from 0.5µ to 30 µ in length and between 0.2 µ to 30 µ in diameter. Trillium contains the largest chromosomes that may attain 30-32 µ in length.

In general plants have longer chromosomes than animals. Species having lower chromosome number possess longer chromosomes than those having higher chromosome number.

Structure

Mitotic metaphase chromosomes are the most suitable for studies on chromosome morphology. Chromosomes shows 1. Chromatids, 2. Centromere, 3. Secondary constriction, 4. Satellite,   5. Telomere.

Chromatids:

The chromosome contains two identical, spirally coiled filaments or arms called chromatids. Each chromatid contains a single DNA molecule.

The two chromatids of a chromosome are held together at a point called centromere. The DNA of each telophase chromosome (composed of a single chromatid) replicates during the synthesis (S) phase of interphase. This produces an identical copy of the chromatid so that during prophase and metaphase the chromosome is made up of two chromatids.

Since the two chromatids of a prophase chromosome are produced through replication, they are referred to as sister chromatids. 

The two chromatids separate from each other during anaphase and move to opposite poles. As a consequence, each chromosome is represented by a single chromatid during telophase.

Centromere

The region where two sister chromatids of a chromosome are held together during mitotic metaphase is known as centromere. Under light microscope, the centromere generally appears as a constriction in the chromosome. Therefore, it is also termed as primary constriction.

Centromere divides the chromosome into two parts called arms. In most cases, one are of a chromosome is longer than the other, hence they are termed as long arm (q) and short arm (p), respectively.

There are two cup like discs in the centromere which are called kinetochores. They contain highly repetitive DNA with special proteins attached. 

Under electron microscope, the kinetochore shows a trilaminar structure. It shows a 10nm thick dense proteinaceous layer, a middle layer of low density and a dense inner layer tightly bound to the centromere. 

During mitosis, 4 to 40 microtubules of mitotic spindle become attached to the kinetochore.  Thus, they function to provide a centre of assembly for microtubules.i.e., a nucleation centre for polymerization of tubulin protein into microtubules. Thus,centromere is the region of attachment of microtubules of spindle fibres during cell division. As a result, centromeres are the first parts of chromosomes to be seen moving towards the opposite during anaphase.

The position of centromere varies from chromosome to chromosome and it provides different shapes to the chromosome. Based on the position of the centromere, the chromosomes are classified as follows:

1. Metacentric:

The centromere is located in the middle of the chromosome, forming two arms of equal length. During anaphase they appear in the shape of ‘V’.

2. Sub-metacentric:

The centromere is located slighted away from the middle of the chromosomes, forming two unequal arms. During anaphase they appear as J or L shaped.

3. Acrocentric:

The centromere is located close to one end of chromosome, forming two unequal arms in which one arm is very long and one arm is very short. 

4. Telocentric:

The centromere is located at the end of the arm. Only one arm is present. During anaphase they appear I shaped. 

The chromosomes of most organisms contain only one centromere and are known as monocentric chromosomes. In some chromosomal abnormalities, chromosomes break and fuse with other, producing chromosomes without centromere called acentric chromosomes or with two centromeres called dicentric chromosomes or with more than two centromeres called polycentric chromosomes. 

Sometimes a chromosome may undergo a break into two, so that only one part has the centromere while the other is without the centromere. The part lacking the centromere is called acentric chromosome. It doesnot take part in mitosis, as spindle fibre cannot be attached to it. It loss usually results in lethality.

Dicentric chromosomes with two chromosomes are sometimes produces, as for example during pairing of structurally different homologous chromosomes (produced as a result of translocation). If there two centromeres move to opposite poles during anaphase the chromosomes breaks. 

Some species have diffuse centromeres i.e  the centromere is not located in one position but lies in a diffused condition along the length of the chromosome, so that the  microtubules attached along the length of the 

Telomere:

The two ends of a chromosome are known as telomeres (Gr telo- far, meros- part). The individuality of each chromosome is related to the fact that it is terminated at either end by a telomere, a term coined by H.J. Muller to indicate the uniqueness and stability of this portion of the chromosome.

The telomeres show polarity which prevents the other segments of chromosomes joining with them. Thus, they provide stability to chromosomes and protect their individuality. If a chromosome breaks, the broken ends can fuse with each other due to lack of telomeres. 

The telomere is rich in G repeats. The replication of telomere is brought about by telomerase. The repeated division of chromosome during cell division results in shortening of telomere. The aging of man is said to be 

Secondary Constriction:

In addition to primary constriction, the arms of chromosome may show one or more additional constrictions called the secondary constrictions. They can be distinguished from the primary constriction by the absence of bending of chromosomes during the anaphase.

Satellite:

The chromosome region lying between the secondary constriction and telomere is known as satellite. Chromosomes bearing secondary constriction and satellite are called as SAT chromosomes. The prefix SAT – stands for Sine Acid Thymonucleinico – (without thymonucleic acid or DNA). 

Nucleolus is always associated with the secondary constriction of sat-chromosomes. Therefore, secondary constrictions are also called Nucleolus Organizer Regions (NOR). NOR contains several hundred copies of the gene coding for ribosomal RNA (rRNA).

Heterochromatin and Euchromatin

When a chromosome in a higher organism is observed during the mitotic cycle under light microscope, with conventional fixation and staining procedures, different parts of it show different condensation and therefore different staining cycles.

In 1928, Heitz defined heterochromatin as those regions of chromosomes that remain condensed during interphase and early prophase and form the so-called chromocentres. The rest of the chromosome, which is in a non-condensed state, was called euchromatin.

The heterochromatin regions can be visualized in condensed chromosomes as regions that stain more strongly or more weakly than the euchromatin regions, showing what is called a positive or a negative heteropyknosis of the chromosomes (hetero – different, pyknosis – staining)

It is thought that in heterochromatin DNA remains tightly packed in the 30nm fiber, which probably represents the configuration of transcriptionally inactive chromatin.

Two types of heterochromatin are generally recognized: constitutive heterochromatin, which is permanently condensed in all types of cells, it is present in the same position on two homologous chromosomes and is inherited.

Facultative heterochromatin develops later during the course of development of the organism and is condensed only in certain cell types or at special stages of development. Constitutive heterochromatin in both paternal and maternal chromosomes responds directly and similarly to development since it is inherited.

Facultative heterochromatin is related to development but it occurs in one of two genetically alike complements so that two chromosomes behave differently. Its significance is due to its capacity for shutting off normal gene function for relatively longer or shorter periods during the lifetime of an organism.

Euchromatin: portion of chromosomes that stain lightly and are only partially condensed

Nucleosome- Solenoid Model

Proposed by Korneber and Thomas in 1974

The length of the DNA is far greater than the size of the nucleus in which it is contained. To fit into the nucleus, the DNA has to be condensed in some manner. 

When eukaryotic nuclei were observed on EM, it was found that chromatin has a repeating structure of beads about 10 nm in diameter connected by a string of DNA. 

The existence of a repeating unit of chromatin – called nucleosome- was predicted from the biochemical studies. 

It was found that the DNA molecule in a eukaryotic chromosome is not free but is bound to basic proteins called the histones, in a structure called chromatin. Histones are small proteins that are basic because they have a high content ( 10 to 20%) of the basic amino acids arginine and lysine There are five major classes of histones – H1, H2A, H2B, H3 and H4. 

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The DNA-histone complex is the basic unit of structure in the chromosome. The two components are present roughly in equal amounts by weight, and they generally account for  60-90% of the chromatin mass. Both are formed at the same stage of cell division – S (synthesis).

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DNA is a polyanion by virtue of the continuous sequence of acidic phosphate (PO4--) groups, and these are neutralized and stabilized by the histones, which because of their basic nature act as polycations. Being basic, histones bind tightly to DNA, which is acid

By electron microscopy chromatin looks like a regularly beaded thread. The bead units in chromatin are called nucleosomes. It is an oblate particle about 50b-55 A0 high and 110 A0 in diameter. 

Each unit has a definite composition – namely, one molecule of H1, two molecules each of H2A, H2B, H3 and H4, and one segment of DNA containing about 200 nucleotides pairs. Prolonged treatment of the nucleosomes with the nuclease removes some of the DNA and causes loss of H1.

The resulting structure, called the core particle, consists of an octomer of pairs of H2A, H2B, H3 and H4

, around which the remaining 145 base pair length of DNA is wound in about 13/4 turns. Thus, histones are located internally in the beads, with the DNA wrapped around the outside of the clustered proteins.

Thus, a nucleosome consists of a core particle and linker DNA to which H1 is attached. Possibly, H1 also binds to adjacent core particles to make a more compact structure. The H1 is not located internally within beads; it is external in the inter bead stretches of DNA, and it may be added after the beaded structure is formed.

DNA lining the H1 particle is termed as linker DNA. Its length is 15 – 100 bp, depending on the cell type. The linker DNA is coiled or folded in the normal state of chromatin.

Assembly of DNA and histones is first stage -A sevenfold reduction in the length of DNA.

The beaded flexible fibre of 11nm wide is roughly five times the width of free DNA

The second level folding is the shortening of 11 nm fibre to form a solenoidal supercoil with six nucleosome per turn, called the 30 nm fibre – the supercoiled nucleosome is called solenoid.

The further folding of the 30 nm fibre is less well understood.

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Chromatin can be isolated from nuclei in various stages of cellular activity, either in an extended state or visibly compacted as chromosome.

In extended state, the chromatin exists in the form of long strands having a diameter ranging from 10nm to 100nm

As a cell passes through its growth cycle, the structure of its chromatin changes. In a resting cell the chromatin is dispersed and fills the entire nucleus. Later, after DNA replication, the chromatin condenses about 100-fold and chromosomes form. 

Functions

The main function of chromosomes is to carry the genetic information from one cell generation to another.

Another important function is to protect the genetic material i.e., DNA from being damaged during cell division.

Gene action in eukaryotes is regulated through the histones and non-histone proteins associated with chromosomes.

The specialized structures of telomeres prevents end to end interactions between chromosomes. This ensures stability of the different chromosomes.

Karyotype

The sequential arrangement of all chromosomes of the individual is called karyotype. A karyotype describes the chromosomes of a cell in terms of Number, their relative size, position of centromere, length of the arms, secondary constrictions and satellites.

A diagrammatic representation of a karyotype of a species – idiogram (Gr. Idios = distinctive, gramma –something written)

Genrally, in an ideogram, the homologous chromosomes are arranged in a series of decreasing size. That is, largest chromosome is placed in the first position and the smallest one is placed at last. The sex chromosomes are usually placed in their appropriate positions according to their size and are marked as X and Y. 

Karyotypes suggest primitive or advanced nature of an organism.

Heterochromatin

The darkly stained condensed region of the chromatin

Characterized by high content of repetitive DNA sequences and very few structural genes

DNA is packed in 30 nm fibre

Constitutive heterochromatin :  DNA is permanently inactive and remains condensed state throughout the cell cycle. – around centromere, in telomere

Facultative: it is essentially euchromatin that has undergone heterochromatization

Euchromatin:

Portion of chromosomes that stain lightly are only partially condensed