Chromosomes: Meaning, Number and Morphology | Genetics

In this article we will discuss about:- 1. Meaning of Chromosomes 2. Chromosome Number 3. General Morphology.

Meaning of Chromosomes:

In 1875, Strasburger discovered thread-like structures which appeared during cell division. Later, these structures were termed as chromosomes (chroma = colour; soma = body) by Waldeyer in 1888 since they showed affinity for basic dyes and could be stained deeply, while the cytoplasm took little or no stain.

Chromosomes are invisible (through light microscope) during interphase but they are easily observable during cell division. Under the electron microscope, only chromatin fibres of about 300 Å diameter are observed during interphase. Chromosomes assume various shapes and sizes during the different stages of cell division.

There occurs a cycle of coiling de-coiling (condensation-de-condensation) of chromosomes during the cell cycle. In interphase, the chromosomes are in a de-coiled (decondensed) state but in metaphase, the maximum coiling is achieved.

Therefore, the general morphology of chromosomes can be studied easily at metaphase. For the purposes of identification and distinction, chromosomes can be well studied using specialized staining techniques that yield specific banding patterns.

Chromosome Number:

Chromosome number in every species is generally constant. In higher organisms, the number of chromosomes in somatic cell is called somatic number irrespective of the ploidy level and is represented by 2n. In the gametes, the chromosome number is reduced to half, it is known as gametic number or haploid number and is represented by “n”.

The basic chromosome number is denoted by “x”, so that the chromosome number of a diploid cell or individual is expressed as “2x”.

However, this designation is generally used in polyploid species. In normal diploid (2n = 2x), the chromosomes are present in duplicate, i.e., each chromosome is represented by two homologues, one member of which is contributed by the male gamete, when ­the other is derived from the female gamete.

Chromosomes that have similar morphology and identical in genetic content are known as homologous chromosomes. In polyploids (allopolyploids), there are more than two homologues of each chromome. Somatic and gametic chromosome numbers of some organisms are given in Table 6.1.

There is wide variation in the chromosome numbers of different plant and animal species. Haplopappus gracilis (family Compositae) contains only two pairs of chromosomes (2n = 4), while the plant Ophioglossum reticulatum (a fern) contains 630 pairs (2/i = 1260) of chromosomes in its somatic cells.

In animals, the lowest chromosome number has been reported in the round worm (Ascaris megalocephala) and Geratrix hermaphroditus (both 2n = 2 ; n = 1). On the other extreme, some protozoa possess up to 1600 chromosomes.

In certain differentiated tissues, the chromosome number may differ from the normal somatic number of the same individual. For example, the tapetal cells of anther become highly polyploid by the process of endomitosis. Mammalian liver cells also become polyploid by the same process.

Chromosome Size:

During the cell cycle, the chromosome length varies due to coiling and de-coiling. The maximum chromosome condensation occurs at metaphase therefore, chromosome size is determined at mitotic metaphase. In general plant chromosomes are larger than animal chromosomes.

Among the higher plants, monocots generally possess larger chromosomes than dicots. Usually, the mitotic chromosomes range in length from 0.25 µm in fungi and birds to 30 µm in some plant species such as Trillium.

Their diameter varies form 0.2 µm to 3.0 µm. On the other extreme, the polytene giant chromosomes of Diptera may be about 300 µm in length and about 10 µm in diameter. Thus there is a great variation in the size of chromosomes.

However, Stebbins has noted the following regularities in the variation in chromosome size:

(1) Among the spore bearing vascular plants, the heterosporous genera and families (Salviniaceae, Marsiliaceae) tend to have smaller chromosomes than the homosporous ones.

(2) Gymnosperms (e.g., Ginkgo) have the largest mean and modal chromosome sizes. However, certain genera and families (e.g., Tradescantia, Lilium, Loranthaceae) of Angiosperms may have larger chromosomes than Gymnosperms.

(3) Woody angiosperms mostly have small chromosomes.

(4) There exist great variation in size of chromosomes between different genera of the same family in the herbaceous angiosperms.

The distribution of chromosome sizes within a family and its phylogenetic position have no obvious relationship. Further, there is no relationship between the number of chromosomes and their size. For example, mean chromosome length in Lotus tenuis (n = 6) and Viciafabci (n = 6) belonging to the family Leguminosae are 1.8 µm, and 14.8 µm, respectively.

In Gramineae family, Chloris barbata (n = 10) and Secale cereale (n = 7) have average lengths of chromosomes as 1.5 µm, and 7.5 µm, respectively. In family Liliaceae, mean chromosome length in the species Tofieldia nuda (n = 15) is 1.3 µm, while that in the species Lilium pardalinum (n = 12) is much more (20.0 µm).

General Morphology of Chromosomes:

Chromosome morphology can be well studied at mitotic metaphase (Fig. 6.1). However, certain structures are more prominent during the pachytene stage of meiotic cell division.

Under the light microscope, the following structural features can be seen:

(a) Chromatid

(b) Centromere,

(c) Telomere,

(d) Secondary constriction and satellite,

(e) Knob and

(f) Chromomeric.

Chromomeres and knobs are more clearly seen during the pachytene.

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 centromere. The chromatids separate and move to the opposite poles during anaphase so that during telophase and the G, phase of interphase each chromosome is composed of a single chromatid.

During the S-phase of interphase, DNA replication occurs; the single chromatid of each chromosome replicates to produce two sister chromatids. Therefore, each chromosome is seen to be made up of two chromatids during prophase and metaphase. The two chromatids of the same chromosome are called sister chromatids, while chromatids of homologous chromosomes are called non-sister chromatids.

Centromere:

It is also called the primary constriction or kinetochore. The two sister chromatids of a chromosome are held together at this region (Fig. 6.1). 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), while the other is called short arm (S).

Spindle fibres attach to the centromere and thus it is responsible for chromosome movement during cell division. The chromosome fragment lacking a centromere (acentric fragment) fails to orient itself on the metaphase plate and is unable to move to any of the poles during anaphase and is ultimately eliminated.

At the end of metaphase and beginning of anaphase, the centromere divides longitudinally. Sometimes the centromere may divide at right angle to the longitudinal axis or transversely, producing two telocentric chromosomes.

Condensation of the chromatin is less at the centromere than in the other parts of the chromosome: therefore, the centromere looks thinner and strains weakly during late prophase, metaphase and anaphase of mitosis.

When treated with oxiquinoline or colchicine, the centromere looks to be a compound structure under the light microscope. Lima-de-Faria in 1956 showed that the centromere is made up of 4 or 5 kinetochore granules in addition to thin fibrils (Fig. 6.2).

Electron microscopic studies have shown that centromere is composed of 300 A chromatin fibres and one spindle attachment granule in each of the two sister chromatids.

The centromere may be classified into the following four types:

(i) Localized centromere

(ii) Neo-centromere or secondary centromere

(iii)Semi-localized centromere

(iv) Non-localized centromeres

(a) Multiple or polycentric condition

(b) Diffuse or holocentric (holokinelic) condition.

Localized centromere:

In majority of the organisms, each chromosome possesses a single permanently localized centromeric region to which the spindle microtubules are attached during the movement of chromosome. About 50 to 100 microtubules are attached to a single centromere.

Such chromosomes are called mono-centric or eucentric. In case of certain structural changes a chromosome may possess two centromeres and then it is called dicentric (aneucentric) chromosome/chromatid. Such chromosomes produce bridge at anaphase.

Neo-centromere or Secondary centromere:

Normally each chromosome has one centromere but in some species, such as, maize neo-centromeres or secondary centromeres are also present. The telomere shows the movement on the spindle during meiotic cell division thus acting as the secondary centre of movement. When maize chromosome 10 possesses a terminal heterochromatic knob, there occurs a preferential segregation of this chromosome due to neo-centromeric activity.

Semi-localized centromere:

These were discovered by Vaarama in 1954. Such centromeres are found in Pleurozium and are regarded as intermediate between the multiple and the localized centromere types. During mitosis, the chromosome movement activity is related to a localized centromere but in meiosis the activity is shifted to another localized site. Thus the position of spindle activity undergoes a regular shift; this is called the centromere shift.

Non-localized centromere:

In such cases, spindle fibres are not attached to a particular localized centromere on the chromosome but they are attached to the entire length of the chromosome.

Such centromeres are classified into the following two types:

(a) Multiple or polycentric (poly-kinetic) condition:

The chromosome possesses many centromeres which are separated from each other by small non-centric segments. Such chromosomes are actually compound or multiple structures and are found in the germ line of ascarid nematodes, e.g., Ascaris.

(b) Diffuse or holocentric (Holo-kinetic) condition:

They were discovered in 1941 by Hughes-Schrader and Ris in coccids. The kinetic activity is distributed on the entire chromosome. Such chromosomes have been found in Hemiptera, Homoptera and few Protista of the animal kingdom and in the genus Luzula in plants. Based in the position of the localized centromere, the chromosomes are designated as follows (Table 6.2).

(i) Metacentric:

In chromosomes, the centromere position is either exactly at the middle (median) or near the middle region (median region) of the chromosome. Both the chromosome arms are nearly or quite equal in length (Fig. 6.3). At anaphase, the chromosomes take the typical V-shape, the centromere being at the apex of V.

(ii) Sub-metacentric:

The centromere position in sub-metacentric chromosomes is sub-median so that their two arms are distinctly unequal (Fig. 6.3). The arm ratio (long arm/short arm) is about 3.0 (Table 6.2). The chromosomes take a J- or L-shape at anaphase.

(iii) Sub-telocentric:

Centromere position in such cases is sub-terminal so that they consist of one long and one short arm, the arm ratio being about 7.0 (Table 6.2). Such chromosomes become J- or rod-shaped during anaphase (Fig. 6.3).

(iv) Acrocentric:

Centromere position is near the end. One arm is long while the other is very short. The arm ratio is much greater than 7.0 (Fig. 6.3).

(v) Telocentric:

Centromere of such chromosomes is located at one end of the chromosome, i.e., it has a terminal position; such chromosomes consist of a single arm. Telocentric chromosomes appear rod-shaped during anaphase (Fig. 6.3).

The position of the centromere can be altered due to pericentric inversion or unequal translocation. A transverse breakage in the centromeric region of a chromosome produces two telocentric chromosomes equivalent to the two arms of the concerned chromosome.

Arm ratio and centromeric index:

The length of the short arm (S) of a chromosome is designated by the letter “p”, while that of its long arm (L) is denoted by the letter “q”. The arm ratio (A) is calculated by the formula,

A=p/q …(6.1)

(A= q/p is also used for chromosome designation as shown in Table 6.2). Similarly, the centromeric index (C) of the chromosome is calculated by the following formula.

The arm length ratio, centromeric index, total chromosome length, nucleolar organizer position, heteropycnotic segments at metaphase of mitosis are used to prepare an idiogram of the chromosome (section 6.4.2). In an idiogram, chromosomes can be classified according to the centromere position as shown in Table 6.2.

Secondary Constriction:

Certain chromosomes have one or more non-centromeric ‘secondary constrictions”, in addition to their primary constrictions (Fig. 6.4). It may be nucleolar or non-nucleolar secondary constriction. The nucleolar constriction is also called 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 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.

This region is negatively heteropycnotic. Non-nucleolar secondary constrictions are often called tertiary constrictions and may represent regions of differential spiralization, lower nucleic acid content or structural weakness.

The chromosome region distal (toward the telomere) to the secondary constriction is called satellite, while a chromosome bearing the satellite is called satellited chromosome or SAT- chromosome. Satellites show considerable variation in their size.

One chromosome of the genome may possess larger satellite, while another (non-homologous) chromosome may possess smaller satellite. For example, the chromosomes 6 and 7 of barely are SAT-chromosomes; chromosome 6 has a larger satellite than chromosome 7. In some cases, satellites are very small and are not manifested in every cell, e.g., the satellites of D (13, 14, 15) and G (21, 22) groups of human chromosomes.

Every species has at least one homologous pair of nucleolar chromosomes, but the number varies with species. Barley (Hordeunt vulgare) contains 2 pairs of SAT-chromosomes, whereas human somatic cells contain 5 pairs. There occurs a competition or suppression effect of the satellites.

When certain species of Crepis were crossed with each other, satellites of one parent des-appeared in the F₁ hybrid. Satellites are mostly heterochromatic. But certain genes are located in these regions, e.g., the polymitotic (po) gene is located in the satellite of chromosome 6 in maize.

Knobs:

In some species, chromosomes possess spherical heterochromatic bodies which are called knobs. They are generally observed during pachytene stage of meiosis. Knobs vary in size from slightly wider than the chromosome to many times its diameter.

They mostly occupy the terminal position on the chromosome, but sub-terminal and interstitial knobs are also found. Knobs have been found in many plants but they are best known in maize. The knobs may be used as markers for the identification of particular chromosomes.

Within any genotype, the number and position of the knobs is constant but their size may vary during the different stages of development. Chromosome 9 of maize contains a large terminal knob which makes its identification rather easy.

Telomere:

The term telomere was coined by Muller in 1938 to denote the natural unipolar chromosome ends eukaryotes (Fig. 6.4). 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 now fuse with broken ends of other chromosomes. During the leptotene stage of meiosis, chromosome ends are attached with the nuclear membrane. In some cases, the telomeres are also involved in chromosome movement, they are then called neo-centromeres. Telomeres do not have any specific morphological features observable under light or electron microscope.

In 1958, Lima-de-Faria and Servella recognized the following two distinct regions of telomere:

(i) Proto-telomere:

It is composed of one to three large chromomeres and deep staining fibrils. It occupies the terminal position of the chromosome.

(ii) Eutelomere:

It is the sub-terminal segment adjacent to the proto-telomere. Eutelomere is composed of two minute chromomeres and transparent fibrils. Eutelomere is weak in staining reactions. Under the electron microscope, the telomere is observed to the made up of irregularly folded chromatin fibres which are about 23 nm in diameter. The chromatin fibres loop back into the chromatid body and they rarely terminate at the telomeres.

Chromomere:

During the early prophase stage of mitosis or meiosis (particularly during pachytene), small bead­like, deep staining compact structures are observable on the chromosomes; 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.

Chromomeres have the following different kinds of structure:

(i) The usual type found in Lilium,

(ii) Chromomeres forming the bands of polytene giant chromosomes

(iii) The large chromomeres of lampbrush chromosomes,

(iv) The telo-chromomeres observed on meiotic chromosomes of tomato and

(v) The centromeric chromomeres or kinetomeres localized within the centromeric region.

Some chromomeres may be heterochromatic; they are generally larger than the euchromatic chromomeres. The size of chromomeres varies within the same chromosome. Chromomeres proximal to the centromere are large, and they become progressively smaller towards the chromosome ends. This phenomenon of chromomere size gradient was described in rye and Agapanthus by Lima-de-Faria in 1952.

The number, size and position of chromomeres is constant on a particular chromosome of a given species at any stage of development. On the other hand, the number of chromomeres per chromosome at one stage of development differs from that at another stage.

During meiotic prophase, particularly during pachytene, the number of chromomeres is several times more than that observed during mitotic prophase. However, the distance between adjacent chromomeres is remarkably constant in the same species during different developmental stages and in the different species possessing chromosomes of different sizes.

For example, mean chromomere distances in Solatium lycopersicum (Family Solanaceae) are 0.83µ at pachytene and 0.96 µm at root tip mitotic prophase, reported by Lima-de-Faria and associates in 1959. The plant species

Agapanthus umbellatum (Liliaceae) shows the mean chromomere distance at different developmental stages as 0.84 µm (pachytene), 0.95 µm (prophase II) and 1.09 µm (root tip mitotic prophase). In several organisms, such as, maize, tomato and others, the chromomere pattern at pachytene can be used to identify specific chromosomes or specific chromosome segments.

Chromomeres were considered to be the constant expressions of coiling of chromonema. It is now accepted that the cromomeres 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. Chromomeres were once believed to be genetic loci. The bands of salivary gland chromosomes were also considered as the sites of genes and the one-band-one-gene hypothesis illustrates this belief.

However, inter-band regions also contain genes and therefore, the one-band-one-gene hypothesis is not entirely accurate. It has been suggested that the DNA content of a chromomere is sufficient to specify 10 to 100 proteins.

Chromonema:

Under the light microscope, a number of longitudinal strands can be observed in the chromosomes; these strands are called chromonemata (singular, chromonema) (Fig. 6.4). Chromonema is composed of chromatin and contains gene; it is not uniform in thickens. Its localized coiling forms the chromomeres. Hightly condensed regions of chromonema during interphase are called heterochromatin, while the less condensed regions are called euchromatin.

The number of chromonemata (strands) present in a single chromosome has been a matter of controversy for a long time. But recent electron microscopic, auto-radiographic and biochemical studies have revealed the eukaryotic chromosomes to contain a single (unineme) continuous molecule of DNA double helix.

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 Fibre, which has a diameter of about 250Å (about X10 the diameter of DNA double helix).

Early cytologists supposed that chromonemata are surrounded by a mass of achromatic substance called the matrix, and that the matrix in turn, was enclosed within an outer membrane called pellicle or sheath (Fig. 6.4). The matrix was thought not to be stainable by the dyes which stain the chromosomes. However, electron micrographs of metaphase chromosomes do not reveal either the matrix or the pellicle.