Chromosomes it’s Structure and Function

Key Concepts

• Chromosome – definition

• Discovery of chromosomes

• Structure of chromosomes

• Functional significance of chromosomes

• Types of chromosomes

Introduction

Chromosomes are thread-like structures present in the nucleus of plant and animal cells. They are complexes of DNA and protein. Chromosomes carry genetic material that is transferred from one generation to the other. They are clearly visible during cell division.  

The term chromosome is derived from two Greek words, Chroma – color; soma – body. Due to their strong affinity towards basic dyes, these cell structures were termed as ‘chromosome.’ 

Discovery of Chromosomes: 

Chromosomes were first discovered by the German scientist, Walther Flemming in the year 1882. In 1888, Heinrich Wilhelm Gottfried Waldeyer-Hartz, a German scientist coined the term chromosome. Waldeyer is also remembered for summarizing the neuron theory. 

Location of Chromosomes: 

In eukaryotes, long chains of DNA are tightly packed into linear chromosomes. They are located in the nucleus of the cell. In prokaryotes (example: Bacteria), chromosomes are relatively small, single, and circular. Since prokaryotes do not have a well-defined nucleus, chromosomes are located in the cytoplasm of the cell. The region of cytoplasm where chromosome is located is referred to as the nucleoid. In addition to the main chromosome, bacterial cells have extra chromosomal genetic elements known as plasmids. 

Structure of Chromosomes: 

 

1. Centromere: 

Centromere is the constricted region of a chromosome. It is also known as kinetochore or primary constriction. It divides the chromosome into two arms. In other words, centromere links a pair of sister chromatids thereby dividing them into p and q arms. 

2. Telomeres: 

The two ends of chromosome are known as telomeres. These specialized ends are highly stable. Telomeres play an important role in stabilizing the ends of chromosomes. Each chromosome has 4 telomeres. 

3. Chromatid: 

One half of the duplicated chromosome is known as chromatid. Chromatids exist in two forms – sister chromatids (identical copies) and non-sister chromatids (non-identical copies). 

4. Chromomere: 

Chromomere are small bead like projections along the length of the chromosome. Genes are located within chromomere. Chromomere is also known as idiomere. They are also referred to as the smallest structural sub-unit of a chromosome.  

5. Chromonema: 

Chromonema are coiled thread like structures found in chromatids. Chromonema is also known as chromonemata. They carry genes and help in maintaining the proper structure of chromomeres. In other words, chromonema is arranged as coiled central filament in the chromatid to bear chromomeres upon them. The arrangement of chromomere on chromonema looks similar to the arrangement of beads on a string. 

6. Secondary Constriction: 

In addition to primary constriction, some chromosomes possess secondary constriction. These secondary constricts helps in the easy identification of a particular chromosome in a set. The chromosomal region that lies between the secondary constriction and the nearest telomere is known as satellite or Trabant. The chromosomes that contain these satellites are known as satellite or SAT chromosomes. Secondary constrictions are always associated with nucleolus. Hence these secondary constrictions are also known as nucleolus organiser region. SAT-chromosomes are also known as nucleolus organizer chromosomes. 

7. Pellicle:  

Chromosome is bound by a membrane. This outer envelope of chromosome is known as pellicle. Pellicle is very thin and formed of achromatic substances. 

8. Matrix: 

The jelly-like substance present inside the pellicle is known as matrix. Matrix is formed of non-genetic materials. Chromonemeta is present in the matrix. In other words, matrix surrounds chromonemata. 

Functional Significance of Chromosomes: 

1. Chromosomes provide the genetic information required for the cellular function of an organism. 

2. It protects the DNA from damage during cell division. 

3. For a particular species, the chromosome number remains constant. This helps in the determination of phylogeny and taxonomy of the species.  

4. Distribution of chromosome determines the sex of a child (Male – XY Female – XX). 

5. Chromosomes ensure the proper distribution of DNA to the daughter nuclei during cell division. 

Karyotyping: 

Karyotyping is the process of pairing and ordering all the chromosomes of an organism. It provides a genome-wide snapshot of an individual’s chromosome. A karyotype is the collection of the complete set of chromosomes in an individual. 

Karyogram / Idiogram: 

Karyogram/Idiogram refers to the diagram or photograph of the chromosomes of a cell arranged in homologous pairs in a numbered sequence. Karyotyping is used to detect chromosomal aberration. It is also used to detect a wide variety of genetic disorders. 

Types of Chromosomes: 

Humans have 23 pairs of chromosomes. In other words, a human cell has a total of 46 chromosomes. These chromosomes could be broadly classified into two types,  

1. Autosomes (Body Chromosomes) 

2. Allosomes (Sex Chromosomes) 

Human cells have 22 pairs of autosomes. These autosomes code for most of the genetic traits in human body. Autosomes are homologous chromosomes. These chromosomes have same genes in the same order along their chromosomal arms. Human cells have one pair of sex chromosome. Sex chromosomes are of two types – X and Y. Male have X and Y chromosomes while female possess two X chromosomes. 

Number of Centromeres: 

Based on the number of centromeres, chromosomes are classified into 4 different types:  

1. Monocentric 

2. Dicentric 

3. Polycentric 

4. Acentric 

1. Monocentric:  

Monocentric chromosomes have only one centromere. They form narrow constriction. They are commonly found in plants and animals. All the 46 chromosomes of human are monocentric. 

2.  Dicentric:  

Dicentric chromosomes have two centromeres. These chromosomes are considered to be abnormal chromosomes. Fusion of two chromosome segments result in the formation of dicentric chromosome. Chromosomal inversion is an important event in the formation of dicentric chromosome. 

3. Polycentric: 

Polycentric chromosomes have more than two centromeres. In other words, they have multiple centromeres. Chromosomal aberrations such as deletion, duplication, and translocation results in the formation of polycentric chromosomes. These types of chromosomes are found in algae and spirogyra. 

4. Acentric: 

Acentric chromosomes do not have centromere. They represent broken segments of chromosomes. They do not survive for a long time. Acentric chromosomes are commonly formed due to chromosome breaking. Exposure to radiations causes chromosome breaking which in turn results in the generation of acentric chromosomes. 

Location of Centromere: 

Based on the location/position of centromeres, chromosomes are classified into four different types:  

1. Telocentric 

2. Acrocentric 

3. Sub-metacentric 

4. Metacentric 

1. Telocentric:  

Telocentric chromosomes are rod-shaped. The centromere is located at one end of the chromosome. In other words, the centromere is at the terminal position. Telocentric chromosome has only one arm. Any mis-division in the centromere of normal chromosome may lead to the formation of telocentric chromosome. 

2. Acrocentric: 

Acrocentric chromosomes are also rod-shaped. In these chromosomes the centromere is located in the sub-terminal position. One of the arms is very long and the other arm is very short. 

3. Sub-metacentric: 

In sub-metacentric chromosomes, the centromere is located slightly away from the mid-point. Therefore, the arms of the chromosome are unequal. One arm of the chromosome is shorter than the other. Sub-metacentric chromosomes are often L-shaped. 

4. Metacentric: 

Metacentric chromosomes are V-shaped. In these chromosomes the centromere lies in the middle of the chromosome. Therefore, the two arms of the chromosome are almost equal. 

Chromosomal Disorders: 

A normal human cell, possess 22 pairs of autosomes and one pair of sex chromosome. At some circumstance, an individual may have an additional copy of the chromosome (trisomy), or an individual may lack any one pair of chromosomes (monosomy). These conditions may cause genetic / chromosomal disorders. The following are some of the examples of chromosomal disorders. 

Down’s syndrome:  

This disorder is caused due to the presence of an additional copy of chromosome number 21. An individual affected by Down’s syndrome have retardation in their physical, psychomotor, and mental developments. 

Turner’s Syndrome: 

This disorder is caused due to the absence of one of the X chromosomes (45 – X0). Females with Turner’s syndrome lack secondary sexual characteristics and are sterile. 

Klinefelter’s Syndrome: 

This disorder is caused due to the presence of an additional copy of chromosome X (47 – XXY). These individuals have overall masculine development but feminine development also gets expressed. These individuals are sterile. 

Summary

• Chromosomes are thread like structures that carry the genetic material

• In eukaryotic cell, chromosomes are located in the nucleus.

• In a prokaryotic cell, chromosome is circular and is located in the nucleoid region of the cytoplasm • Chromosomes are clearly visible during cell division.

• Chromosomes were first discovered by the German scientist, Walther Flemming in the year 1882.

• In 1888, Heinrich Wilhelm Gottfried Wa/deyer-Hartz, a German scientist coined the term chromosome.

• In eukaryotes, the chromosome structure is linear. It is composed of one centromere and one or two arms that project from the centromere.

• Each chromosome has a p arm (the shorter arm) and the q arm (the longer arm).

• The most important structural features of chromosomes include centromere, telo mere, chromatid, ch ro mo mere, chromonema, secondary constriction, pellicle and matrix.

• Chromosomes play significant role in protecting DNA during cell division and ensures their proper distribution to the daughter nuclei.

• Karyotyping is the process of pairing and ordering all the chromosomes of an organism.

• Human cell contains 22 pairs of autoso Ines and 1 pair of allosome (sex chromosome)

• Based on number of centromeres, chromosomes are classified into 4 types namely monocentric, dicentric, polycentric and acentric.

• Based on chromosome location, chromosomes are classified into 4 types namely telocentric, acrocentric, sub-metacentric and metacentric.

• Chromosomal disorders : Down’s syndrome, Turner’s Syndrome, Klinefelter’s syndrome.