Morphology of chromosome

 Chromosome morphology can be well studied at mitotic metaphase. Under the light

microscope, the following structural features can be seen:

• Chromatid

o Chromomere

o Chromonema

• Centromere

• Secondary constriction

• Satellite

• Telomere

• Pellicle and matrix

Chromatid





Chromatid

Chromatid is the structural and functional unit of chromosomes. At metaphase, each chromosome consists of two longitudinal parts called chromatids. These chromatids are held together at a point called the centromere. The chromatids separate from each other during mitotic anaphase (and during anaphase II of meiosis) and move to the opposite poles during anaphase.

During telophase and the G, phase of interphase each chromosome is composed of a single chromatid. Since the two chromatids making up a chromosome are produced through replication of a single chromatid during the synthesis (S) phase of interphase, they are referred to as sister chromatids. In contrast, the chromatids of homologous chromosomes are known as non-sister chromatids.

Chromomere

During the early prophase stage of mitosis or meiosis (particularly during pachytene), small beadlike, deep staining compact structures are observable on the chromonemata; these structures are called chromomeres. During mitosis, chromomeres are not quite distinct, while they are very clear during the pachytene stage of meiotic prophase I. Giant polytene chromosomes of Diptera show the chromomeres as dark staining bands.




The size of chromomeres varies within the same chromosome. Chromomeres proximal to the centromere are large, and they become progressively smaller towards the chromosomes distal ends. Chromomeres were considered to be the constant expressions of coiling of chromonema.

It is now accepted that the chromomeres are the locally coiled structures of DNA and they represent a unit of DNA replication (replication unit), RNA synthesis (genetic transcription), and RNA processing. Generally, larger chromomeres contain more DNA, replicate later and require more time for replication than smaller chromomeres.

Chromonemata

Under the light microscope, a number of longitudinal strands can be observed in the chromosomes; these strands are called chromonemata (singular, chromonema). The localized coiling of chromonema forms the chromomeres. Highly condensed regions of chromonema during interphase are called heterochromatin, while the less condensed regions are called euchromatin. It is now widely accepted that a single DNA double helix (20 Å diameter) is complexed with histones to form a strand or “chromonema”, commonly known as chromatin fiber, which has a diameter of about 250Å (about X10 the diameter of DNA double helix).

Centromere

It is also called the primary constriction or kinetochore. The two sister chromatids of a chromosome are held together in this region. The parts of a chromosome on the two sides of its centromere are called arms. Depending on their length, one of them is called the long arm (L) or q arm, while the other is called the short arm (S) or p arm. The DNA of the centromere are mostly repetitive short sequences of DNA, the sequences are repeated over and over in tandem arrays. Spindle fibers attach to the centromere and thus it is responsible for chromosome movement during cell division. The centromere is made up of 4 or 5 kinetochore granules in addition to thin fibrils. Kinetochores are the attachment point for spindle fibers which helps to pull apart the sister chromatids as the mitosis process proceeds to the anaphase stage. The kinetochores are a complex of about 80 different proteins.

Secondary constriction

Certain chromosomes have one or more non-centromeric ‘secondary constrictions. It may be nucleolar or non-nucleolar secondary constriction. The nucleolar constriction is also called the nucleolar organizer region (NOR) since nucleolus is formed and attached to this region.

However, all the secondary constrictions do not form nucleoli. Nucleoli are formed during telophase, persist throughout the interphase, and disappear in the middle or late prophase. The nucleolus is the site of ribosomal RNA (rRNA) synthesis. The NOR of the chromosomes contains several hundred copies of the gene coding for rRNA. Non-nucleolar secondary constrictions are often called tertiary constrictions and may represent regions of differential spiralization, lower nucleic acid content, or structural weakness.

Satellite

The chromosomal region between the secondary constriction and nearest telomere is called a satellite and chromosomes that possess this region called a satellite chromosome or sat chromosome. It varies in size according to the position of the secondary constriction. If the secondary constriction is very close to an end of the chromosome, the satellite may be a barely perceptible dot. Ordinarily, one pair of chromosomes in each genome possesses this satellite body and in some cases, there may be two or more satellite bodies in a single chromosome.

One chromosome of the genome may possess a larger satellite, while another (non-homologous) chromosome may possess a smaller satellite. They play a vital role in the formation of the nucleolus after cell division is completed.

Telomere

The term telomere was coined by Muller in 1938 to denote the natural unipolar chromosome ends. Telomeres do not have any specific morphological features observable under light or electron microscope. Telomere protects the chromosomes from fusion with other chromosomes or chromosome fragments and thereby provides stability to the chromosomes. A loss of the telomere results in an instability of the broken chromosome end which can fuse with broken ends of other chromosomes. During the leptotene stage of meiosis, chromosome ends are attached to the nuclear membrane. In some cases, the telomeres are also involved in chromosome movement, they are then called neo-centromeres.

Pellicle and matrix

Each chromosome is bounded by a membrane called a pellicle. It is very thin and is formed of an achromatic substance. This membrane encloses a jelly-like substance which is usually called matrix. The function and structure of the matrix are not known. Presumably, it aids in keeping the chromonemata within bounds so that the maneuvers of the chromosome during cell division can take place unhindered. It may also serve as an insulating sheath for the genes during cell division.

Euchromatin and heterochromatin

Based on the packaging of chromatin, the regions in chromosomes are divided. Some regions of chromatin are densely packed and are called heterochromatin. This region takes more stain and appears dark. The heterochromatin is genetically inert as transcription proteins are unable to gain access to the genes. But most regions of chromosomes are less densely packed and stain lightly and are called euchromatin. This region is genetically active and occupies most part of the nucleus.

Heterochromatin is further classified into i) constitutive heterochromatin and ii) facultative
heterochromatin

Constitutive heterochromatin: It consists of short repeated sequences of DNA which is
permanently inactive and remain in the condensed state throughout the cell cycle. This type
of heterochromatin occurs around the centromere and in the telomere. In many species, the
entire chromosome becomes heterochromatic and is called B- chromosome or accessory
chromosome.
Facultative heterochromatin: It is not condensed permanently, it undergoes de-condensation
periodically and involve in transcription. Frequently, in facultative heterochromatin one
chromosome of the pair becomes either totally or partially heterochromatin. For example, one
of the X chromosome in mammalian female remain heterochromatin and forms Barr body.


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