Chromosomes , structure and functions

Chromosomes are the most essential cellular components that assist in determining the structural and functional integrity of all living organisms as they carry the genetic material necessary to regulate all human traits. They are the reason behind the existence of living organisms so is the reason, every living being has chromosomes, irrespective of genus, class, species etc. If we ask ourselves, why do individuals differ in their height, stature, the colour of eyes, and intelligence etc? The answer is that different parents passed on a variety of genetic recombination to their offerings, which makes individuals unique from each other.

Chromosomes

"Chromosome"  is derived from the Greek terms"khroma" meaning colour and "soma" refers to "body".  Eukaryotic cells have a distinct nucleus within which nucleolus is accommodated. Inside the nucleolus, we can find genetic material organized in a definite sequence but in a very compact manner. Compactness is one of the distinguished features chromosome . 23 pairs of chromosomes found in human cells when joined together, they make up of 200 nm. Similarly, if we unwind the entire genetic materials in a cell, they would measure roughly 2 meters.

DNA and chromosomes

Chromosomes are nothing but the tightly packed DNA molecules in a coiled fashion. Furthermore, a gene consists of many DNAs that codes for one protein. A genome is simply the sum total of  DNA of entire organisms. Genes come in pairs where a pair control both dominant and recessive version of a trait. For example, if the height of a person is a trait , one gene control dwarfness while the other controls tallness. 

Organization /packing of genetic material

DNA is the building block of the genetic material. A long chain of sequantially organized DNAs forms chromosome, in other words, a selected area of a chromosome contains DNAs. Genes code for some specific proteins and these proteins regulate us.

Discovery of chromosomes

Since chromosomes are highly sensitive to the biological stains, they can quickly absorb and retain various dyes which makes them stand out in the microscopic examination. Though, chromosomes were officially identified during the 1980s,  long before - in the year 1842,  Karl Wilhelm von Nageli has found some subcellular structures which were later confirmed as chromosomes by Walter Sutton and Theodor Boveri. Karl Wilhelm had observed the 'idioplasma', a network of string-like bodies. He had wrongly assumed that they form an interlinked network throughout the organism instead of being compactly packed inside the nucleolus.The complete meaning of the term ‘chromosome’ was first used by Waldeyer in 1888.

Karyotyping

Karyotyping is the pairing and ordering all the chromosomes of an organism to provide a blueprint of the individual chromosomes. It is prepared with the help of a set of standardized staining procedures that can reveal detailed analysis of organization of chromosomes. The human cell comprises 46 chromosomes distributed in 23 pairs, I,e 22+XX for a female and 22+XY for a male( see image below). The XX and XY unit is called allosomes or sex chrmosomes where a male chrmosome is distinguished by XY set while female`s has XX. An abnormal organization in the numbers or structure of chromosomes helps to diagnose many chrmosomal disorders such as polyploidy and aneuploidy etc. 

Characteristics of chromosomes

  1. Chromosomes are highly affinitive to colours due to which they form colored bodies.

  2. In many organisms including humans, chromosomes are diploid as they come in pairs. However, some organisms contain haploid, hexaploid, and tetraploid numbers.

  3. A pair of chromosomes vary in its  size and lengh from the other pair. For instance, if we take a sample of the human karyotyping, we can see that the first pair is 3 times bigger than the pair number 16( see image above).

  4.  Chromosomes are found in each and every cell of our body with some exceptions like RBCs as these are enucleated.

  5.  In almost all the eukaryotes,  DNA is found in the nucleus, however, mitochondrial DNA is found in the mitochondria.

  6. Chromosome numbers vary in different organisms, for instance, the common fruit fly has 8 diploid chromosomes, while humans have 46 chromosomes in 23 diploid sets. Similarly, aquatic creatures like god fishes have 100 chromosomes. Plants like maize have 20 diploid chromosomes, while wheat has 42 hexaploids ( 6 chromosomes per set)  chromosomes.

  7. Disorganization in the numbers and structure of chromosomes lead to chromosomal defects. Some examples are : Down syndrome has an extra chromosome at pair number 21 ( 21+xxx or 21+xxy),  Klinefelter’s syndrome lacks Y chromosomes (23+X0).

  8. In rare cases, we can find polytene chromosomes that are over-sized . They are developed from the normal chromosomes through repeated rounds of DNA replication without cell division (endomitosis).

Structure of human chromosome

Morphology

When stained, chromosomes resemble rod-shaped, dark-stained bodies during the metaphase. The average size of a chromosome is 5μm but some variation in the size is apparent depending upon their position in the karyotyping (between 1-22). Also, their shape varies according to the phases of cell division, for instance, they look-like thin, coiled, elastic and contractile threads during interphase they become much thicker and filamentous during metaphase and the anaphase.

Parts of the chromosome

1. Centromeres

A typical chromosome bears a centrally placed centromere or kinetochore which aid in dividing the chromosomes into arms. The position of the centromere determines the appearance of chromosomes. Depending upon the location of centromere, chromosomes are classified into telocentric, acrocentric, submetacentric, and metacentric( see image below).  Accordingly, telocentric chromosomes a have centromere placed on the proximal (nearest) end of the chromosome while acrocentric chromosomes are distinguished by a centromere pushed almost towards one end of the chrmosomes  resulting in one short and one long arm. On the other hand, sub-metacentric chromosomes look- like J or L shaped with their centromere located almost near the centre (but not exactly at the center) forming 2 unequal arms. The short arm of the chromosome is labeled the “p arm.” while the long arm  is labeled the “q arm.”. The most common and typical chromosome type is the metacentric type that look-like v-shaped as centromere is splaced exactly at the centre.The 2 chromatids held in place by the centromeres are very essential for the nuclear division where they assist in the exchange of genetic materials  from parents to the daughter cells. A chromatid is a non-stainable part of the chromosome and is also known as the primary constriction of a chromatid. Centrioles help in attaching the specific part of the chromosome with the spindle fibres during cell division.

 

 

2. Pellicle and Matrix

Pellicle is a membrane surrounding the 2 arms of chromosomes and is very thin formed by an achromatic substance. Pellicle contains a jelly-like substance called matrix which provides space for chromonemata. Pellicle and matrix are genetically nonsignificant and are not visible under the electron microscope during the resting phase.

3. Chromatids, Chromonema and Chromomeres

Chromonema or chromonemata( the plural form chromatins)  appears  spiral, coiled and filamentous during the early prophase. They are the arms of chromosomes, genetically significant parts and are clearly noticeable during the late prophase, metaphase and anaphase. The Chromonema is distinguished by knotting, granular and bead-like structures arranged in a single row along the length of arms called chromomeres. Chromomeres help in carrying the genes during inheritance. They appear thick, dark and slightly different in its shape and size during the late metaphase. Similarly, in the metaphase, they are clearly visible because of distinct chromatids. An individual chromonema consists of 8 mi­crofibrils, each of which is formed by a double helix of DNA.

 4. Satellite body

Satellite bodies are the terminal portion of the chromosome found behind the secondary constriction. They are round-shaped, elongated,  and sometimes invisible. Chromosomes with a clearly visible satellite body are called SAT-chromosomes. At least 2  SAT-chromosomes are found in a diploid nucleus.

5. Telomere

Telomeres are the terminal ends of chromosomes containing a 3, 0000 repetitive nucleotide sequence and may even reach up to 15,000 base pairs. They prevent sticking of the ends of the chromosomes with their adjoined chromosomes. Telomere comprises  TT-AG-GG sequence. They help in chromosome replication during cell division and are enclosed by a cap.

Function of Chromosomes

  1. Chromosomes asist in the storage of genetic code in the form of genes. Genes code for specific proteins required by the cell for its proper functioning.

  2. Chromosomes helps to determine gender based on whether the given cell  has  XX or XY allosomes.

  3. They regulate cell division by allowing parents to pass only the correct information to the daughter cells.

  4. They aid in the formation and storage of proteins .

Read More

  1. Disorders And Defects Of Chromosomes

  2. Chromosomal theory of inheritance

  3. Gene mutations

Questions

  1. What are chromosomes?

  2. Explain the discovery of chromosomes.

  3. Explain the structure of a chromosome.

  4. What is the average size of a chromosome?

  5. Why do chromosomes size vary in different phases of cell division?

  6. What are the functions of centrosomes?

  7. Explain the satellite bodies

  8. Classify chromosomes according to the centromere position.

  9. What are chromatids?

  10. Explain the role of satellite body.



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