Thursday, July 18, 2024
General Agriculture

Chromosome Morphology and Cytological Principles of Plant Breeding

E. Strasburger in 1875 first discovered thread-like structures which appeared during cell division. These thread like structures were called chromosomes due to their affinity for basic dyes. The term chromosome is derived from two Greek words; chrom = colour, soma = body. This term was first used by Waldeyer in 1888.

Chromosomes

Of all components of cell, the chromosomes have been studied most extensively and perhaps more is known about them than any other cell organelle.

The chromosome has greater constancy than any other cell component and it maintains it special qualities from one cell generation to another.

Chromosomes contributed to the division of cells and they are of prime importance as they carry the genes which are the hereditary material.

Chromosome Number

The number of chromosomes in a given species is generally constant. All the members of the species ordinarily have definite and generally a constant somatic and gametic chromosome number.

Somatic chromosome number is the number of chromosomes found in somatic cells of a species and is represented by 2n.

Generally somatic cells contain two copies of each chromosome except the sex chromosomes. Both the copies are ordinarily identical in morphology, gene content and gene order and hence known as homologous chromosomes.

Gametic chromosome number is exactly half of somatic chromosome number and is represented by n. it denotes the number of chromosomes found in gametes of a species. The number of chromosomes varies greatly from 2n = 4 (n = 2) in Haplopappusgracilis (Compositae) to 2n = > 1200 in some Pteridophytes.

Name of the organismChromosome number (2n)
Rice24
Tomato24
Wheat42
Onion16
Maize20
Garden pea14
Cotton52
Humans46
Drosophila8

Chromosome Size

The size of the chromosome shows a remarkable variation depending upon the stage of cell division. The chromosomes are the longest and thinnest during interphase (resting stage) and hence not visible under light microscope.

Chromosomes are the smallest and thickest during mitotic metaphase. In general, plants have longer chromosomes than animals and species having lower chromosome number have longer chromosomes than those having a higher chromosome number.

Among plants, dicots in general have shorter and higher number of chromosomes than monocots. Among the higher plants, the longest mitotic chromosomes are those of Trillium spp., which may reach 32 μ in size. In most fungi all chromosomes are extremely minute. Chromosome size is not proportional to the number of genes present on the chromosome.

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Chromosome Morphology

Chromosome Morphology and Cytological Principles of Plant Breeding

The outer covering or sheath of a chromosome is known as pellicle, which encloses the matrix. Within the matrix lies the chromatin. Flemming introduced the term chromatin in 1879.

The term chromatin refers to the Feulgen positive materials observed in interphase nucleus and later during nuclear division. Chromatin readily stains with basic dyes especially Basic Fuchsin, which is specific for DNA which in turn is a major constituent of chromosomes.

The chromosome morphology changes during cell division and mitotic metaphase is the most suitable stage for studies on chromosome morphology. In mitotic metaphase chromosomes, the following structural features can be seen under the light microscope.

Chromatid: Each metaphase chromosome appears to be longitudinally divided into two identical parts each of which is called chromatid. Both the chromatids of a chromosome appear to be joined together at a point known as centromere.

The two chromatids of chromosome separate from each other during mitotic anaphase (and during anaphase II of meiosis) and move towards opposite poles.

Since the two chromatids making up a chromosome are produced through replication of a single chromatid during 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.

Centromere: Centromere and telomere are the most stable parts of chromosomes. The region where two sister chromatids appear to be joined during mitotic metaphase is known as centromere.

It generally appears as constriction and hence called primary constriction. Centromere is a localized and easily detectable morphological region of the chromosomes which helps in the movement of the chromosomes to opposite poles during anaphase of cell division.

The centromere divides the chromosomes into two transverse parts called arms. The centromere consists of two disk shaped bodies called kinetochores.

The kinetochores do not form part of the chromatid but lie one on each side of the chromosome such that each chromatid is having its own kinetochore.

Chromosome Morphology and Cytological Principles of Plant Breeding

One kinetochore is attached to the spindle fibres towards one pole and the other similarly towards the other pole. Depending on position of the centromeres, chromosomes can be grouped as:

Metacentric: Centromere is located exactly at the centre of chromosome, i.e. both arms are equal in size. Such chromosomes assume ‘V’ shape at anaphase.

Submetacentric: The centromere is located on one side of the centre point such that one arm is longer than the other. These chromosomes become ‘J’ or ‘L’ shaped at anaphase.

Acrocentric: Centromere is located close to one end of the chromosome and thus giving a very chort arm and a very long arm. These chromosomes acquire ‘J’ shape or rod shape during anaphase.

Telocentric: Centromere is located at one end of the chromosome so that the chromosome has only one arm. These chromosomes are ‘I” shaped or rod shaped. Normally chromosomes are monocentric having one centromere each.

Acentric (without centromere) and dicentric (with two centromeres) chromosomes, if produced due to chromosomal aberrations, cannot orient properly on the equatorial plate and lag behind other chromosomes during anaphase movements.

In certain organisms, centromere does not occupy a specific position, but is diffused trough out the body of chromosome. Such chromosomes, which do not have a localized centromere, are found in Luzula spp. and insects belonging to the order Hemiptera.

Telomere: The two ends of chromosomes are known as telomeres. They are highly stable and do not fuse or unite with telomeres of other chromosomes due to polarity effect.

Any broken end of a chromosome is unstable and can join with a piece of any other chromosome. But the telomeres impart stability to the chromosome, which retains its identity and individuality through cell cycle and for many cell generations.

Secondary constriction: The constricted or narrow region other than that of centromere is called secondary constriction and the chromosomes having secondary constriction are known as satellite chromosomes or sat chromosomes.

Chromosome may possess secondary constriction in one or both arms of it. Chromosomal end distal to the secondary constriction is known as satellite.

Production of nucleolus is associated with secondary constriction and therefore it is also called nucleolus organizer region and satellite chromosomes are often referred to as nucleolus organizer chromosomes.

Chromomere: In some species like maize, rye etc. chromosomes in pachytene stage of meiosis show small bead like structures called chromomeres. Chromomeres are visible during meiotic prophase (pachytene) and invisible in mitotic metaphase chromosomes.

The distribution of chromomeres in chromosomes is highly characteristic and constant. The pattern of distribution being different for different chromosomes.

They are clearly visible as dark staining bands in the giant salivary gland chromosomes. Chromomeres are regions of tightly folded DNA. Chromomeres of single chromosome show considerable variation in size. They may differ in size as in the case of maize or they may be of uniform size as in the case of rye.

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Chromonema: A chromosome consists of two chromatids and each chromatid consists of thread like coiled structures called chromonema (plural chromonemata). The term chromonema was coined by Vejdovsky in 1912. The chromonemata form the gene bearing portion of chromosomes.

Matrix: The mass of acromatic material which surrounds the chromonemata is called matrix. The matrix is enclosed in a sheath which is known as pellicle. Both matrix and pellicle are non-genetic materials and appear only at metaphase, when the nucleolus disappears.

Composition of Chromosomes

Chromosome Morphology and Cytological Principles of Plant Breeding

The material of which chromosomes are composed is called chromatin. N.Fleming introduced the term chromatin in 1879. Chromatin was classified into two groups by cytologists on the basis of its affinity to basic dyes like acetocarmine or feulgen (basic fuchsin) reagent at prophase.

The darkly stained regions were called heterochromatin, while lightly stained regions were called euchromatin. This differential staining capacity of different parts of a chromosomes is known as ‘heteropycnosis’.

In general heterochromatin is found in centromeric and telomeric regions and these regions of chromosome generally replicate later than the euchromatic regions of chromosomes.

The genes within the heterochromatic regions are usually inactive. Most of the genome of an active cell is euchromatic and the genes with in this euchromatic region are expressed. Heterochromatin is further classified into two groups: a) Constitutive and b) Facultative

Constitutive heterochromatin: It is present in all cells at identical positions on both homologous chromosomes of a pair.

Facultative heterochromatin: It varies in state in different cell types, at different stages or sometimes, from one homologous chromosome to another. A well-known example of facultative heterochromatin is the Barr body, an inactivated X chromosome in somatic cells of mammalian female(XX).

Differences between Heterochromatin and Euchromatin


HeterochromatinEuchromatin
1Represent regionsarklystainedLightly stained regions
2Contains few inactive genesContains lot of active genes
3Covers small region of chromosomeLarger region of chromosome
4Usually found near centromere and telomereFound in the middle of chromosome between centromere and telomere
5Two types – Constitutive and facultativeOnly one type
6Late replicatingNormal replicating
7Usually no active part in transcriptionPlays active transcriptionrolein

30 nm fibre3-8nm fibre

Karyotype and Ideogram: The general morphology (size of chromosomes, position of centromere, and presence of secondary constriction and size of satellite bodies) of somatic chromosomal complement of an individual constitutes its karyotype.

It can be defined as “the characteristic features by which a set of chromosomes of a species is identified”. Generally, karyotype is represented by arranging the chromosomes in descending order of size, keeping their centromeres in the same line.

Thus the largest chromosome is placed on extreme left and the shortest on extreme right. The karyotype of a species can be represented diagrammatically showing all the morphological features of chromosomes. Such a diagram is known as ideogram or ideotype.

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Agric4Profits

Benadine Nonye is an agricultural consultant and a writer with several years of professional experience in the agriculture industry. - National Diploma in Agricultural Technology - Bachelor's Degree in Agricultural Science - Master's Degree in Science Education - PhD Student in Agricultural Economics and Environmental Policy... Visit My Websites On: 1. Agric4Profits.com - Your Comprehensive Practical Agricultural Knowledge and Farmer’s Guide Website! 2. WealthinWastes.com - For Effective Environmental Management through Proper Waste Management and Recycling Practices! Join Me On: Twitter: @benadinenonye - Instagram: benadinenonye - LinkedIn: benadinenonye - YouTube: Agric4Profits TV and WealthInWastes TV - Pinterest: BenadineNonye4u - Facebook: BenadineNonye

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