As we know about how important is DNA i.e. DeoxyRiboNucleic Acid in passing characteristics from one generation to another. It acts as a blueprint for a living thing and codes for proteins. It is a spiraling chain-like molecule that is made up of 4 different types of nucleotides. Namely A, T, C and G.
We have also learnt that DNA is present in genes and how do we define a gene? A gene can be defined as a long stretch of DNA that codes for usually a protein or a group of proteins, also considering a few exceptions. In human beings or other multi-celled creatures, the proteins that are coded by genes interact with other proteins and molecules thereby making up living cells. These cells make up tissues that make up organs and organs entirely make up living creatures. Different kinds of genes lead to different traits in living organisms also due to differences in genetic coding or mutations. The mutation is any change in the nucleotide sequence of a chain of DNA or RNA (Ribonucleic Acid).
Knowing all of this information, we will move further with the chromosome definition and how it is related to the above.
Who Discovered Chromosomes?
The first person to discover Chromosomes was Strasburger in the year 1815 and Waldeyer used the term ‘chromosome in the year 1888.
A chromosome can be defined as an entire chain of DNA and it comes along with a group of stabilizing proteins. Chromosomes seem to be thread-like in appearance and are located inside the nucleus of an animal and plant cells. As discussed above, each chromosome is made up of protein and DNA (a single molecule). And as we already know the function of DNA in passing unique characteristics from parents to offspring, it contains specific instructions that make each type of living organism. And here, we can highlight the chromosome function which is to carry this basic genetic material known as DNA and it is essential for growth, survival, development and reproduction which is performed by various cells.
The thread-like structure in the chromosome is referred to as chromatin fibre and is arranged in the chromosome in such a manner that the chromosome looks like a spool of yarn. This chromatin fiber is a collection of protein molecules called histones that are wrapped with a string-like structure. If we zoom in the string we can find there is a chain of DNA in it which is extremely long with millions of nucleotides and contains hundreds of genes along its length.
The ultrastructure of chromosome can be briefly explained as- it consists of a single very long DNA that is compacted approx 10,000 fold by interactions with proteins. This structure fits within a eukaryotic nucleus of only 10 microns diameter.
During the reproductive stage, the chromosome is tightly packaged and helps with durability during the splitting of the cell into two. In most of the other cell’s life, chromosomes are present as a loose and noodle-like structure accompanied by other chromosomes and inside the nucleus of a cell. There are 23 pairs of chromosomes in the human genome, making it 46 altogether. There are a few exceptions where each cell in the body consists of its full copy of our chromosomes.
The eyeball cells will make use of only the eyeball cell genes and the rest are turned off. Liver cells use only live cell genes and the rest are turned off. Similarly, this is applicable for each cell type.
In humans, one member from each pair of chromosomes comes from the mother and the other comes from the father.
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Let’s understand all the parts of the above Chromosome Structure one by one.
Centromere: It is also known as Kinetochore and is the primary constriction at the center where the chromatids or spindle fibers are attached. It functions in the movement of the chromosome during a stage called anaphase during cell division.
Chromatid: When a chromosome is divided into two identical strands during cell division, a chromatid is formed as half of the chromosome. Each half strand is joined by a centromere, both are known as sister chromatids; and it contains DNA and separates at Anaphase to form a separate chromosome.
Chromatin: Chromatin is a complex of DNA consisting of DNA, RNA, and protein and it forms chromosomes within the nucleus of eukaryotic cells. It doesn’t exist freely as linear stands, rather it is highly condensed and wrapped around nuclear proteins.
Telomere: The terminal region of each side of the chromosome is called a telomere.
Chromosomes carry the basic genetic material DNA which is responsible to provide hereditary characteristics and genetic information to the various cells. The cellular functions are important for the growth and survival of living organisms.
Talking about the chemical composition of chromosomes, they are covered by histones and other proteins whose role is protection from chemical enzymes and physical forces. This, in turn, helps DNA to have a safeguard during the process of cell division.
Non-histone proteins present in the chromosomes help in the regulation of gene action.
Knowing about the chromosome structure and function, we can say that the chromosome shape appears like a butterfly with a centromere in between. Actually, its shape depends on the location of the centromere which is different at each stage of the cell division. Example- The X-shape structure of the chromosome is found at the metaphase stage.