Chromosome Vs Chromatid and History of Chromosomes
Chromosomes are the rod-shaped or thread-like structures of cellular organisms which contain the DNA information. In eukaryotes, these are located in the cell nucleus while in prokaryotes, there is a single ring-shaped chromosome which is generally known as genophore. The chromosomes contain the genes which contain the heredity information of the individuals.
One chromosome consists of two identical structures which are held together at a constricted point by Centromere. Each of these identical structures is known as a chromatid. So, a chromosome is made up of two chromatids connected by the Centromere. Chromosomes are hence X shaped structures. The study of chromosomes and cell division is known as cytogenetics.
History of chromosomes
The word "chromosome" comes from the Greek word "Chroma" meaning color and "Soma" meaning body. The first scientists who recognized the chromosomes are the German scientists Schleiden, Virchow, and Butschli. Theodor Boveri gave the definitive demonstration that chromosomes are the conveyors of heredity after a series of experiments beginning in the middle of 1880s;
His two main principles were:
• the continuity of chromosomes
• the individuality of chromosomes
Wilhelm Roux propounded that each chromosome carries a different genetic load. Boveri tested and confirmed this hypothesis. Boveri pointed out that there is a connection between the rules of inheritance and the behavior of the chromosomes. Two generations of American cytologists were influenced by Boveri. Edmund Beecher Wilson, Nettie Stevens, Walter Sutton, and Theophilus Painter were all influenced by him.
Wilson linked together the independent work of Boveri and Sutton (both around 1902) and named the chromosome theory of inheritance - the Boveri–Sutton chromosome theory (the names are sometimes reversed) in his eminent textbook named “The Cell in Development and Heredity’. Ernst Mayr points out that the theory was debated by some famous geneticists: William Bateson, Wilhelm Johannsen, Richard Goldschmidt, and T.H. Morgan. However, the complete proof came from chromosome maps in Morgan's own lab.
In 1923, Theophilus Painter published that the number of human chromosomes is 24 pairs, which would mean 48 chromosomes. His error continued until 1956. The true number of chromosomes in the human body, i.e. 46, was determined by Indonesia-born cytogeneticist Joe HinTjio.
Structure of chromosomes
The above structure shows a simpler diagram of the chromosome. Male bodies contain XY chromosomes while females have XX chromosomes. The eukaryotic and the prokaryotic cells have slightly different chromosome structure. The similarities between eukaryotic and prokaryotic chromosomes are:
• Both types of chromosome contain genetic information DNA.
• The negatively charged DNA present in both types reacts with some positively charged protein to neutralize their changes.
• Both coding and non-coding sequences are contained in the genetic material.
• The methylation of DNA in the chromosome is responsible for its inactivation in both groups.
Extra chromosomal genetic materials are contained in both groups. (DNA of mitochondria and chloroplasts in eukaryotes and plasmids in case of prokaryotes.)
The differences between eukaryotic and prokaryotic chromosomes are:
• In eukaryotes, the genetic material is organized in a proper way as distinct structural bodies known as chromosomes while the typical chromosome formation is absent is a prokaryotic cell.
• A eukaryotic cell always contains two or more chromosomes while a prokaryotic cell contains only a single chromosome per cell.
• Eukaryotic chromosomes are comparatively larger than the prokaryotic ones.
• Each eukaryotic chromosome has a linear DNA with two ends while a prokaryotic chromosome contains a covalently closed circular DNA.
• The nuclear membrane separates the eukaryotic chromosomes from the cytoplasm while owing to the absence of a nucleus, the prokaryotic chromosomes stay in direct contact with the cytoplasm.
In short, the prokaryotic chromosome is of primitive type while the eukaryotic chromosome is of an advanced type.
Similarities between chromosomes and chromatids
• Both the chromosomes as well as the chromatids are formed by DNA molecules.
• Both are found within the nucleus of a eukaryotic cell.
• Genetic information is embedded in both chromosomes and chromatids and they both play an important role in heredity.
• Histone proteins are associated with both of them.
Differences between chromosomes and chromatids
In terms of their occurrence, chromosomes can be found throughout the life cycle of a cell while chromatids appear only when the cell undergoes mitosis (cell division resulting in formation of two identical daughter cells.) or meiosis (cell division when a single cell divides twice and form four daughter cells reducing the chromosome number to half.)
A chromosome is the most condensed form of DNA as DNA is condensed 10,000 times to form a chromosome. On the other hand, a chromatid is lesser condensed than chromosome as a DNA is condensed 50 times to form a chromatid.
Chromosomes contain tightly packed DNA molecules while in case of chromatids, the DNA molecules are unwound. A chromosome is made up of a single, double-stranded DNA molecule while a chromatid comprises of two DNA strands joining collectively by their centromere. The chromatids contain a substance called chromatin. It is a single, very long strand of DNA.
Structurally a chromosome is a thin, ribbon-like structure while a chromatid is a thick, fibrous structure. Chromosomes may or may not have an exact trait like that of a parent. Homologous chromosomes are exact to each other. However, they are non-identical. In the case of chromatids, each sister chromatid is an exact replication of the other. However, during mutation(s), they will have slight differences, in which case they may be heterozygous.
Chromosomes can be duplicated but chromatids cannot be duplicated. Chromosomal duplication is also known as gene duplication. It is a vital mechanism through which new genetic material can be generated during molecular evolution.
In the case of chromosomes, DNA is being used during macromolecule synthesis, while in case of chromatids, DNA is not being used during macromolecule synthesis.
Chromosomes contain the hereditary information for every organism. They are thus the gene carriers. A chromosome is also termed as the "Thread of Life". The main function of chromatid is that it helps cells to duplicate. It also helps to keep the proper DNA count during mitosis or meiosis.
The picture below shows the cell division process during mitosis.
One of the two identical copies of a duplicated chromosome is the chromatid. On the other hand, chromosomes are known as the vectors of heredity. Hence the difference between chromosomes and chromatid is very subtle and chromatid is a part of the chromosome.