Definition: “Changes in the structure and number of chromosomes are called chromosomal aberration.” These are two types of chromosomal aberrations as follows –
- Numerical aberrations
- Structural aberrations
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Numerical aberrations –
The change in the number of chromosomes is called numerical aberration. The change may result in either an increase or decrease in the number of chromosomes, that condition is called ploidy. The ploidy can occur either in the complete set of chromosomes or in the individual chromosomes. It is of two types-
- Aneuploidy is a condition in which there is the addition or deletion of one or more chromosomes in a diploid set of chromosomes.
- It causes due to non-disjunction of chromosomes e.g. in human, 46 chromosome is a diploid condition. If there is 47 or 45 instead of 46, this will be the Aneuploidy.
- It was first discovered by Bridges in Drosophila in 1916. Aneuploidy may be hypoploidy (loss of one chromosome from a genome) or hyperdiploid (one or more chromosomes extra in a set of chromosomes).
- Hypoploidy is of the following types.
- Monosomy – In this condition, there is one chromosome less in one homologous pair i.e. 2n – 1 condition. The monosomic parent produces two types of gametes like (n) and (n – 1) during gametogenesis. Normally (n – 1) types of gametes die. But if they survive and take part in fertilization, the resulting offspring will have genetic imbalance which leads to reduced fertility of high mortality. e.g. 44A + XO i.e. Turner’s syndrome.
- Nullisomy – In this condition, a pair of chromosomes is less in the genome i.e. 2n – 2. This condition arises due to the union of two monosomic gametes or from the non-disjunction of chromosomes. Generally, nullisomic conditions do not survive. But their polyploid forms survive which are weak and sterile.
- Trisomy – It is a type of hyperdiploid in which there is an increase in one chromosome inset of chromosome i.e. 2n + 1.
Examples of Trisomy,
i) Trisomy of 21st chromosome – Down’s syndrome,
ii) Trisomy of 13th chromosome – Patau’s syndrome,
iii) Trisomy of 18th chromosome – Edward’s syndrome.
iv) Trisomy of sex chromosomes -XXY (Klinefelter’s syndrome), XXX (Super female).
The term euploidy is derived from the Greek word. (EU = even or true; ploidy = set). Euploidy is a condition in which an organism possesses one or more full sets of chromosomes. Euploidy is classified into haploidy, diploidy, triploidy, and polyploidy.
- A condition in which an organism contains only one set of chromosomes in its somatic cell is called haploidy or monoploidy.
- Haploidy is denoted by ‘n’. It is rare in animals and found in insects like honey bees and wasps and also in rotifers.
- The haploidy may be normal or abnormal in a particular species.
- E.g. males in honey bees and wasps are haploid with normal features, while in amphibians if it occurs then causes abnormality.
- It can be produced in plants but these plants remain small with reducing leaves, weak and sterile e.g. Fungi, Sorghum Datura, etc.
- It is a condition in which an organism contains 3 sets (3n) of chromosomes in the nucleus of the body cell.
- Triploidy originates from the union of a haploid (n) and diploid (2n) gametes of different strains of the same species.
- Such organisms are always sterile. It is studied in some animals like reptiles, man, some cells of birds, and other invertebrates.
- However in plants triploidy has economic value and it can be propagated by asexual methods like grafting, budding, etc. It gives fruits of superior quality.
- It is a condition in which an organism contains more than usual two sets of chromosomes. Such an organism is called polyploidy.
- They may be triploid, tetraploid, pentaploid, and so on. Polyploidy is rare in animals and it results in the sterility of an individual.
Chromosomes have a definite structure and organization. However, sometimes chromosomes undergo certain structural modifications which are known as chromosomal aberrations or chromosomal mutations, or structural anomalies. It leads to change in
arrangement of the genes on chromosomes.
Various kinds of chromosomal aberrations are as follows,
Deletion or Deficiency
- When there is a loss of some part of a chromosome, it is called deletion or deficiency.
- In this case a chromosome breaks at two places, the broken part gets separated while the two ends of the chromosome join together and give rise to a mutated chromosome.
- There are two types of deletion,
- Intercalary or interstitial deletion – When there is breakage of the middle part of a chromosome with a particular gene, this type of deletion is called an intercalary or interstitial deletion.
- Terminal deletion – When the deletion is at the terminal end of the chromosome, it is called a terminal deletion. The loss of part of chromosome with genes affects the organism adversely. If deletion takes place in homozygous chromosomes, some characters are completely lost resulting in the death of an organism.
Examples of Deletion,
In a human being deletion in the 5th pair of homologous chromosome results in a condition called cri – du – chat syndrome. A person is physically retarded and produces a sound like the cry of a cat, hence the name cat’s cry or cri – du – chat.
- Some time deleted portion of the chromosome attaches to another chromosome at the centromere resulting in the duplication of a part of the chromosome.
- This condition is called duplication or addition. This extra part of the chromosome behaves like independent chromosome.
- A gamete with such a duplicated chromosome receives extra genes in duplicated form which results in formation of new species.
- Hence it is important in evolution.
- In this process chromosome breaks at two points and the central piece get detached. The broken piece then re-attaches at its original position with the two ends reversed.
- Fusion of the gamete carrying inversion with normal gamete does not show a visible phenotypic change in offspring.
- But during a subsequent generation, it leads to cytological abnormalities. Hence inversion is important in evolution. Inversion is of two types,
- Pericentric inversion – When the centromere is in the region of inversion, it is called pericentric inversion. Such inversion results in the dicentric chromosome.
- Paracentric inversion – It is an inversion in which the centromere is not involved.
- These are changes in the arrangement of genes in the chromosome.
- There is no change in the quality or quantity of genes, only the rearrangement of genes occurs.
- There is no change in phenotypic characters of an individual due to translocation, only change occurs in the position of genes.
There is shifting of a part of one chromosome to another nonhomologous chromosome. Translocation is of two types,
- Homozygous Translocation – Exchange of linkage group of the genes with homologous counterpart is called homozygous translocation.
- Heterozygous Translocation – It is a change of linkage groups due to the exchange of genes between different chromosomes. A segment of a chromosome is shifted to a non-homologous chromosome.
Morphology of Chromosome
The term chromosome (Chrome = Color; Soma = body) was first coined by W. Waldeyar (1888).
It means a darkly stained body. It is a carrier of hereditary characters which are passed from one generation to the next. The genetic information is stored in the form of genes.
In prokaryotes, there is a single chromosome either in the form of circular DNA or RNA. In eukaryotic cells, chromosomes are present in the nucleus.
The chromosomes are distinct, highly organized, condensed, rod-like structures visible under a light microscope at the metaphase and anaphase stages of cell division.
It can be studied properly at metaphase or anaphase of cell division.
Number in Chromosomes –
The chromosome number of every species of plant or animal is constant.
The somatic cells of an organism generally contain two sets of chromosomes, forming homologous pairs and are called diploid (2n).
While their gametes contain one set of chromosomes and are called haploid (n). This haploid set of chromosomes is known as the genome.
The chromosome number of some species is given below,
- Man Homo sapiens 46
- Rhesus monkey Mucaca mulatt 42
- Horse Equus calibus 64
- Rat Rattus norvegicus 42
- Fruit fly Drosophila melanogaster 08
- Housefly Musca domestica 12
Size of Chromosomes –
The size of the chromosome is normally measured at mitotic metaphase. It ranges between 0.25 μ (fungi and birds) to 30 μ (Trillium).
In man, it is about 5 μ. The size of the chromosome is inversely proportional to its number in the organism. Plant cells generally have larger chromosomes than animal cells.
The chromosomes in the cell are never alike in size. The largest chromosomes are the lampbrush chromosome of certain vertebrate oocytes and the polytene chromosome of certain dipteran insects.
The shape of Chromosomes –
The shape of the chromosome changes from phase to phase of cell division. In the resting phase or interphase stage, the chromosomes occur thin, coiled, elastic, threadlike structures called chromatin threads. In metaphase and anaphase, they become thick and filamentous.
Depending on the position of the centromere on the chromosome, it has different shapes as follows,
- Metacentric – The centromere is present nearly in the middle of the chromosome. The two arms of a chromosome are approximately equal. They appear ‘V’ shaped during anaphase movements.
- Submetacentric – Centromere is some distance away from the middle. One arm of the chromosome is shorter than the other. Such chromosomes are called submetacentric. They appear ‘L’ shaped during anaphase movements.
- Acrocentric – The centromere is a little away from the end of a chromosome. One arm of the chromosome is very short and the other arm is very long. These chromosomes are called acrocentric. They appear ‘J’ shaped during anaphase movements.
- Telocentric – When the centromere is located at the tip of the chromosome then it is called telocentric. There is no division of chromosomes into arms. These chromosomes appear ‘i’ shaped during anaphase movements. Telocentric chromosomes are very rare.
Structure of chromosome
The chromosome or chromatid consists of chromonema, centromere, secondary constrictions, nucleolar organizers, telomere, and satellite.
- A metaphysic chromosome shows two subunits called chromatids hold together at the point called the centromere. Each chromatid is formed of two fibrils called chromonemata.
- The number of chromonemata is not fixed in each chromatid. It varies from 2 to 32 in number. During prophase, the chromosome becomes visible and filamentous called chromonemata.
- Cromonemata form gene bearing portion of the chromosomes. The bead-like appearance of chromatin material on chromonemata is called chromomeres.
- At metaphase chromosomes are tightly coiled and cromomeres are no longer visible.
- Cromomeres are regions of tightly folded DNA and they are believed to correspond to units of genetic function in the chromosomes.
- A specialized constricted region on the chromosome is called primary constriction or centromere. The position of the centromere is constant for a particular chromosome.
- It becomes functional during cell division. The principal function of the centromere is the attachment of the chromosome to the spindle apparatus during cell division.
- Generally, there is only one centromere on the chromosome. However, in some animals and plants, it is not organized and lies diffused along the length of the chromosome. This centromere is called diffused centromere and the chromosome is called polycentric.
- Sometimes a chromosome may break into two so that only one part gets a centromere while the other remains without centromere. This part of the chromosome without centromere is called an acentric fragment.
- If a new centromere appears on this fragment, then it is called neo-centromere.
- There may be one or more secondary constrictions on chromosomes. These are nonstaining gaps on the chromosomes.
- Their location is constant for a particular chromosome; hence it is useful for the identification of chromosomes. In man, secondary constriction II is formed on the long arms of chromosomes 1, 10, 13, 16, and Y.
- Nucleolar organizer (Secondary constriction I): There is an additional construction on two homologous chromosomes of a diploid set, called nucleolar organizer.
- These are necessary for the formation of the nucleolus. It appears as a constriction near one end of the chromosome.
- The nucleolar organizer may be attached to the nucleolus. It represents about 0.3% of the total amount of nuclear DNA. It is believed to be concerned with the formation of 28 S rRNA.
- The part of the chromosome beyond the nucleolar organizer is very short and appears like a satellite.
- In humans, chromosomes 13, 14, 15, 21, 22, and Y have nucleolar organizers and satellites.
- Chromosomes bearing satellites are called SAT chromosomes (SAT stands for Sine Acid Thymonuclenico. i.e. without thymonuclic acid or DNA).
- The tips of chromosomes are called telomeres.
- It differs in structure and composition from the rest of the chromosome.
- They prevent the ends of chromosomes from sticking together.
- These are specially modified regions of chromosomes for attachment to the nuclear envelope.
Heterochromatin and Euchromatin
The chromatin material is of two types,
- Heterochromatin is a part of a chromosome that remains highly condensed throughout the cell cycle.
- It stains deeply.
- It replicates late at the end of the ‘S’ phase.
- Heterochromatin is unstable and affected by temperature, sex, age of parents, the proximity of centromere, and the presence of an additional Y chromosome.
- It is metabolically inert. DNA of heterochromatin does not transcribe in RNA for protein synthesis.
- Cross-over frequency is less in heterochromatin.
Types of Heterochromatin
- Constitutive Heterochromatin – It is originally called satellite DNA, it is mostly inactive during protein synthesis with highly repeated sequences. The genes in this region can replicate but do not transcribe into mRNA.
- Facultative Heterochromatin – It comprises about 2.5% of the genome. It is metabolically inactive. This heterochromatin results from the inactivation of one of the two X- chromosomes in females.
- The regions of a chromosome which never show condensation are called euchromatin.
- Euchromatin stains less deeply.
- Euchromatin contains diffused or less tightly coiled regions.
- Euchromatin replicates during the early stage of the ‘S’ phase.
- Euchromatin is genetically active. Its DNA synthesizes mRNA during interphase.
- Cross-over frequency is more in euchromatin.
Molecular organization of eukaryotic chromosomes
In resting nondividing eukaryotic cells, the chromosomal material is called chromatin.
It is amorphous and spread in the nucleus but when the cell prepares for mitosis or meiosis’ the chromatin condenses to form a species-specific number of well-defined chromosomes.
A chemically eukaryotic chromosome consists of DNA (35%), proteins (60%), and less amount of RNA (5%). The histone to DNA ratio is 1:1. The proteins are of two types viz. i) histones and ii) non-histone chromosomal proteins (NHCP).
Histones are basic proteins associated with the DNA of eukaryotic cells.
There are five major types of histones as H1, H2A, H2B, H3, and H4. Histones are rich in the basic amino acids arginine and lysine.
These histones combine with a single 20A0 DNA duplex and form a bead called a nucleosome. Nucleosomes are basic structural subunits of chromatin.
The non-histone chromosomal proteins include the other proteins of chromatin e.g. RNA polymerase, actin, myosin.
According to Paulson (1977), non-histones form a backbone that provides the basic shape to the metaphase chromosome.
Structure of nucleosome
Electron microscopic view reveals that chromatin consists of a series of beads called nucleosomes connected by thread-like structures (linkers).
Each nucleosome consists of histone core or octamer formed of two units of each H2A, H2B, H3, and H4 histone wrapped by 200 base pairs of DNA.
This DNA forms about 1¾ turn around the octamer. The rest of the segment of DNA connects two adjacent nucleosomes is called linker DNA.
It contains about 15 to 100 base pairs. The linker DNA is coiled or folded in a normal state of chromatin. The H1 histone is associated with linker DNA.
Coiling of linker DNA makes nucleosomes closely packed to produce a chromatin fiber of 100A0 diameter.
Supercoiling of 100A0 chromatin fiber forms a solenoid with a diameter of 300A0 fiber.
Types of chromosomes
Autosomes control the determination of the somatic characters of an organism. The
number of autosomes is always more in the nucleus. e.g. in a human being out of a total of 46 chromosomes, 44 chromosomes are autosomes and 2 are the sex chromosomes.
2. Sex chromosomes
Sex chromosomes are responsible for the determination of the sex of an individual.
These are of two types i.e. X and Y chromosomes.
The X chromosome is longer than the Y chromosome.
It is straight, rod-like, and sub-metacentric. It contains a large amount of euchromatin with DNA and a small amount of heterochromatin therefore X chromosomes are genetically active.
The y chromosome is short slightly curved and acrocentric.
It contains less DNA and a large amount of heterochromatin; therefore Y chromosome is genetically inactive.
Special types of chromosomes
Polytene chromosomes (Salivary gland chromosome) –
Normally chromosomes are not visible during interphase but polytene chromosomes are the exception.
They are visible during the interphase.
They were first observed by Balbiani (1881) in salivary glands of Chironomus tetanus; hence the name salivary gland chromosome.
These chromosomes are also found in some other organs like the foregut, midgut, trachea, Malpighian tubules, and fat body cells of dipteran insects.
These chromosomes have a large number of chromonemata hence they are called polytene chromosomes.
In Drosophila larvae, the polytene chromosome is about 100 times longer and 300 times big than the normal somatic chromosome.
This enlargement of the chromosome is due to endomitosis or polyteny. Endomitosis is a process in which chromosomal constituents undergo several extra duplications without cell division.
As a result, chromosome gets enlarged very much in length and width. Due to extra duplications number of chromatids increases in the polytene chromosome between the range of 500 to 1000.
After staining, polytene chromosomes show alternate dark and light bands (interband). The number and arrangement of the bands are species-specific.
The bands are possibly composed of the chromomeres of the chromonemata. According to Bridges (1935), the linear order of bands corresponds with the linear arrangement of genes on the chromosome. However, each band does not represent a gene.
The interband are also found to be genetically active.
During larval development of some dipterans, some of the bands become expanded, called ‘puff’. When a puff becomes very much enlarged, it is called a ‘Balbiani ring’.
The pattern of puffing is a characteristic of the phenotype of a cell. Pelling (1964) showed that the puffs are sites of RNA synthesis. Puffing activities increase during larval development due to the release of the hormone ecdysone (molting hormone).
The formation of puff and Balbiani rings are reversible.
The lampbrush chromosomes were first observed in Salamander (amphibian) oocytes in 1882.
These are also found in immature eggs or oocytes of amphibians, birds, mammals and spermatocytes of certain insects, birds, and other vertebrates.
These chromosomes in shark oocytes were first described by Ruckert (1892). These are named lampbrush chromosomes due to their appearance like a brush used for cleaning the chimneys of kerosene lamps.
The size of the lampbrush chromosome may be more than 1000 microns in length. However, it contracts greatly and reduces in size.
These chromosomes are very elastic and can be stretched up to about two times before they break.
During the early prophase, lampbrush chromosomes are in the form of a pair of homologous chromosomes with few points of contact between them.
Each chromosome consists of two chromatids or axial filaments.
Along the axial filaments, there is a row of dense granules or chromomeres. These are tightly coiled points on the axial filament. From each chromomere arises a pair of lateral loops.
The axial filaments and the chromomeres consist of DNA. The loops represent a lateral extension of the axial filaments. Loops are the active sites for RNA synthesis.
It has been suggested that each loop corresponds to a single functional unit (gene) that codes for a single primary polypeptide product. So, a number of loops may decide the number of functional units (genes).
The function of the lampbrush chromosome is a synthesis of RNA and proteins by the loops. They are also concerned with the production of yolk in the egg.