Chromosomes

 Chromosomes are the structures within living cells that contain the genetic material. Its primary function is to store the information needed to produce the characteristics of an organism. Chromosomes are a condensed form of DNA. Each chromosome is composed of only one long DNA molecule. A eukaryotic chromosome is composed of DNA and proteins. A typical chromosome is composed of a few million base pairs (bp). For example, the chromosome of Escherichia coli has approximately 4.6 million bps.

Eukaryotes have chromosomes inside their nucleus and in a bacterial cell, a chromosome is highly compacted and found within a region of the cytoplasm known as a nucleoid. Chromosomes were discovered by Walther Flemming in 1876. The term chromosome was coined by Weldeyer in 1888.

Structure of chromosomes

In a eukaryotic cell, chromosomes are composed of a single molecule of DNA and five types of histone proteins. Histone molecules are rich in lysine and arginine residues. These proteins are positively charged and, therefore,  bind tightly to the negatively charged phosphates in the DNA molecule.  A small portion of non-histone proteins are also present. These proteins regulate transcription, thus called Transcription factors. 

During most of the cell’s life cycle, chromosomes are elongated fibers and cannot be observed under the microscope. In this state, it is active and able to duplicate and transcribe.  These fibers begin to condense at the beginning of cell division which can be stained and observed easily under the light microscope. These condensed chromosomes also have their duplicated copy and are known as dyads. A chromosome has the following components.

1. Chromatids

During the S phase of the cell cycle, the chromosomes are duplicated. These chromosomes when condensed during division form two arms called chromatids. The duplicated chromosomes are held together at the region of centromeres. The attached, duplicated chromosomes are commonly called sister chromatids. The shorter arm of the two arms of the chromosome extending from the centromere is called the p arm and the longer arm is known as the q arm.

2. Centromere

Centromeres are regions that attach the duplicated chromosomes. It plays a role in the proper segregation of chromosomes during mitosis and meiosis. Most of the eukaryotic chromosome contains a single centromere. The centromeres act as a site for the formation of kinetochores that link the centromere to the spindle apparatus during mitosis and meiosis. The length of the DNA in centromere can range in length from several thousand base pairs to over 1 million bp. Some chromosomes also have a satellite and a secondary constriction (nucleolar organizer). The satellites are knobs and have no known function it is considered junk DNA

            c.      Telomeres

Telomeres are the specialized regions found At the ends of linear chromosomes. These have important functions in the replication and structural stability of the chromosome. Telomeres prevent translocations and chromosome shortening. It also protects chromosomes from digestion by exonucleases.

Classification of chromosomes

Chromosomes are classified on the following basis.

A.     Autosomes and Sex Chromosomes

Human beings have 23 pairs of chromosomes in their cells. These chromosomes are of two types; autosomes and sex chromosomes. Sex chromosomes include the X and Y chromosomes. The other 22 pairs of chromosomes are autosomes. Characters that are linked to the sex of the person are passed on through the sex chromosomes. The rest of the genetic information is present in the autosomes.

B.      Based on the Number of Centromeres

Chromosomes are also classified based on the number of centromeres. Monocentric chromosomes have one centromere, Dicentric have two centromeres, and Polycentric have more than two centromeres. Chromosomes without centromere are called Acentric chromosome fragments.

C.      On the Basis of Location of Centromere

Chromosomes have the following types based on the position of their centromere.

i.       Metacentric chromosome; the chromosomes with the centromere at the center. These chromosomes have equal arms.

ii.     Sub-metacentric chromosome; the chromosomes with the centromere closer to one end. These chromosomes have unequal arms, the shorter arm is called as p arm, and the longer arm is called the q arm.

iii.   Acrocentric chromosome; the chromosomes with the centromere are too close to the one end. This chromosome also has a secondary constriction called the nucleolar organizer.

                   iv. Telocentric chromosome; the chromosomes with the centromere at one side.

Number of chromosomes

 

The number of chromosomes is variable in different species. Each species has its specific number of chromosomes. Organisms with only one set of chromosomes are called Monoploid, with two sets of chromosomes called Diploid, with three sets called Triploid, with four sets called Tetraploid, and with six sets of chromosomes are called Hexaploid. The term Haploid usually refers to half of the somatic chromosome number. A haploid number of chromosomes may be diploid or triploid.

The haploid chromosome numbers of a few representative species are given in the table below.

Chemical composition of chromosomes

A eukaryotic chromosome is chemically composed of 60% proteins and 40% DNA. Each chromosome is composed of a single molecule of DNA and five types of histone proteins. These are H₂A, H₂B, H_3, H₄ and H_1. Histone molecules are rich in basic amino acids lysine and arginine residues.

Fine structure of chromosomes

A chromosome has three levels of organization.

1. The first level of organization;

Each chromosome is chemically composed of DNA and basic proteins. The DNA and histone proteins when combined are referred to as nucleosomes. Two molecules of each histone protein H₂A, H₂B, H_3, and H₄ combine to form an octamer called histone Core Particle. Around the histone Core Particle DNA coils for two turns forming nucleosomes. The nucleosome is bead bead-like structure that is 10nm in thickness.

The DNA that coils around the histone core is called core DNA. Core DNA is 146 base pairs in length. A short sequence of nucleotides extends between two nucleosomes called Linker DNA. The length of linker DNA is variable and ranges from 8-114 base pairs.

The H₁ binds the linker DNA to the nucleosome where it enters the nucleosome and leaves the nucleosome.

1. Secondary level of organization:

The second level of organization is a 30nm thick fiber called solenoid. The H₁ binds the two nucleosomes forming loops with six nucleosomes in a turn called Solenoid.

2. The final level of organization:

The final level of organization is a 700nm thick structure called a chromosome. It is the result of condensation by coiling and supercoiling of solenoid. The densely packed chromatin is called heterochromatin and appears as dark regions. The loosely packed chromatin is called the Euchromatin. It appears as light regions.

23.1.4 Karyotype:

A karyotype is a chromosomal chart of an organism that displays the chromosomes of an organism arranged into pairs. A karyotype shows the number of chromosomes of an organism, their shapes, and sizes. Every species has a characteristic karyotype.

There are 46 chromosomes in a human somatic cell arranged in 23 pairs. 22 pairs of these are called autosomes and one pair is a sex chromosome that determines the sex of an organism.

A normal karyotype is shown in Fig below.