DNA, Gene, Chromosome and Genome – What is the Difference?
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DNA:
It is a double helix structure. It has sugar phosphate backbone and it is made up of four nucleotides A, T, G and C.
In humans, the DNA is about 2 meters long and this much long DNA needs to fit in our tiny nucleus which is around 5-10 µm in size. So in order to fit inside the nucleus; DNA has a packaging strategy where it undergoes coiling by formation of nucleosome.
Figure 1. DNA
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Gene:
In our 2 meters long DNA, there are segments or chunks of DNA i.e. specific nucleotide sequences around 100s of bp long which codes for specific proteins and RNA. These segments of coding DNA is called gene and we have about 20,000 genes spread across all the chromosomes.
Every gene has a specific site in our DNA. For example, cystic fibrosis gene is present on chromosome 7. The traits that we get from our parents is because of the genes that we get from them so traits passes from parents to child in the form of genes.
Figure 2. Gene
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Chromosome:
As we just saw above that the DNA is too long to fit inside the nucleus. So it undergoes packing mechanism where the DNA starts wrapping itself around the histones proteins. This assembly where DNA is wrapped around histones is called nucleosome. Nucleosome formation condenses the DNA and therefore the space it requires to fit inside a nucleus would reduce. These nucleosomes would further undergo condensation by arranging them self in coiled form called chromosome.
So chromosome is nothing but super condensed form of DNA where it is wrapped around histone proteins. In humans, there are total of 46 chromosomes. We are diploid so that means 23 chromosomes comes from mother and 23 chromosomes comes from father; giving us total of 46 chromosomes or 23 pairs of chromosomes.
As saw above that DNA is about 2 meters long so when you take all the 46 chromosomes, unwind them and put them in one stretch that makes the 2 meters long DNA.
Figure 3. Chromosome
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Genome:
A genome is organism’s complete set of DNA. This means the entire DNA present in a cell and for humans other than nuclear DNA; there is also DNA in mitochondria so it is all the nuclear DNA distributed over 24 chromosomes (22 autosomes and 2 sex chromosomes) and DNA in mitochondria. For example, if we are talking about plants we will take chloroplast DNA in consideration.
Since we are diploid, we have two copies of genome in nucleus because we have all the chromosomes in pair.
So when we put together all the genetic information present inside a cell; it becomes the genome of an organism.
So think of it this way that genome is huge book carrying lot of information and for easy access to this information; nature has divided it into 24 chapters (22 autosomes + 2 sex chromosomes). So genome is divided into 24 chromosomes.
Figure 4. Summary
I hope this helps 🙂
Watch a video on this topic here.