What’s Bigger – DNA or Chromosome?

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What’s Bigger – DNA or Chromosome? What do you think?

If we talk about the uncoiled DNA, then DNA is larger than the chromosome. And if we talk about the coiled DNA, then DNA is smaller than the chromosome.

When DNA gets coiled, it becomes smaller in size just in order to fit the nucleus of the cell. This is known as DNA packaging.

So, we can say that the chromosome when present, is bigger than the DNA because millions of nucleotides of DNA add up to form a Chromosome.

DNA is made up of polynucleotide chains. This means that it is a polymer of nucleotides.

On the other hand, Chromosome is made up of a long chain of DNA. On average, a single human chromosome consists of a coiled DNA molecule that is about 2 inches long.

DNA simply stands for “deoxyribose nucleic acid.” It is the biomolecule that is the storehouse of genetic information. DNA is shaped like a double helix and is made up of nucleotides sequence arranged one after another using covalent and hydrogen bonds.

Chromosomes are made up of very long DNA strands that are wrapped around proteins. In other words, Chromosome is the supercoiled, tightly packaged, and compressed structure of long DNA strands.

If you talk about the order of packaging, then DNA → Genes → Chromosome. This means that DNA coils itself to make Genes, and Genes further supercoils itself to produce Chromosomes.

The diametre of DNA is just 2nm which when folded into 11nm fiber gets the shape of a nucleosome structure. This when folds further become 300 nm chromatin fiber and this folds further into a 700 nm chromatid, and when two chromatids join together it forms a chromosome. So, basically DNA is the unfolded form of a chromosome.

So, a coiled DNA molecule is smaller in size than a chromosome. It’s because a chromosome is made up of a long chain of DNA molecule that can have about 249 million DNA building blocks (base pairs).

And, if the same DNA gets uncoiled, then it can gain 109 times more length than that of a chromosome.

It’s incredible, right? So, that’s why DNA packaging is very much important to fit the whole length of DNA into a cell’s nucleus which is just 6 μm across.

DNA→Gene→Chromosome
DNA→Gene→Chromosome Structure
This diagram shows how a very long DNA strand is coiled and then supercoiled to form a very small size in order to fit in a portion inside the chromosome.
(Image: Thomas Splettstoesser (www.scistyle.com) / CC BY-SA)

Size Info: DNA & Chromosome

Size of DNA

There are a total of 3 billion base pairs in each human cell’s uncoiled DNA which is about 2 metres long.

All the DNA in all your cells put together would be about twice the diameter of the Solar System. That’s how long all of the cells uncoiled DNA is if added together.

DNA is so compressible that a DNA helix with a diameter of 2nm (2 x 10-9 m) can be supercoiled to become a chromatid of 700nm (700 x 10-9 m) diameter or so. A chromosome consists of two chromatids attached together.

Size of Chromosome

There are a total of 46 chromosomes in a diploid human cell. The length of each chromosome is between 20 to 85 mm.

On average, a single human chromosome consists of a coiled DNA molecule that is about 2 inches long.

And, if that same 2 inches long DNA molecule gets uncoiled then it can gain 109 times more length than that of a chromosome.

The smallest human chromosome is Chromosome Y which has a length of 20 mm with 57,227,415 bp of DNA.

And, the largest human chromosome is Chromosome 1 which has a length of 85 mm with 248,956,422 bp of DNA.


Reasons why DNA is smaller than Chromosome when coiled

1. DNA after replication, and transcription starts to coil itself around the histone octamer forming nucleosome. Each nucleosome has 1.7 turns of DNA and it holds the DNA strand firmly.

2. Nucleosomes help in controlling the replication and transcription of the DNA. As the whole length of DNA is packaged firmly by the nucleosomes, thus only allowing a small section of the DNA to replicate and transcript.

3. Histone proteins of the nucleosome are basic proteins, and their positive charges allow them to associate with DNA, which is negatively charged. This allows the DNA to decrease its length by winding itself around the histone octamer.

4. DNA Topoisomerases are universal enzymes found in all cell types. These enzymes act on DNA Helix to coil and supercoil it, turning it into a small compressed size.

5. DNA is the complete genetic information of an organism that needs to transferred to the next generation and so, it cannot be transferred so easily in its uncoiled shape. So, all of the DNA gets transformed into a compressed structure called chromosome so that it can carry the genetic information from one generation to the next with the less use of energy.

6. The long strands of DNA need to be coiled and supercoiled in order to preserve the whole genetic information in every cell. Or else, it can’t fit the cell size.

7. Moreover, the replication and transcription of the DNA need biochemical energy. So, DNA cannot stay uncoiled as it is not possible for the two strands of DNA to unwind in the entire length due to high energy requirements. This is very important in order to save the biochemical energy of the body to run other metabolic activities as well.


Where DNA and chromosomes are found?

Both DNA and Chromosomes are found inside the nucleus of the eukaryotic cell. The DNA mainly remain condensed as chromatin fibers in the nucleolus region of the cell’s nucleus.

The nucleolus is a concentrated round body located inside the nucleus of a eukaryotic cell. It is not surrounded by a membrane but sits in the nucleus.

The nucleolus occupies up to about 25% of the volume of the cell’s nucleus. It contains most of the cell’s genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, histones to form chromosomes.

During cell division- mitosis or meiosis, the DNA chromatin fibers further condenses itself forming chromatid fibers. Two such chromatid fibers bind together at the sight of the centromere forming a chromosome.

The binding of two chromatids to become one chromosome is necessary as the chromosomes are the carrier of genetic information from one generation to the next.

When the chromosomes or chromatids are formed, they start to get aligned inside the nucleus. Later nuclear membrane disappears and the following patterns as mentioned in the prophase, metaphase, anaphase, and telophase stage of mitotic or meiotic cell division takes place in the cytoplasm.

DNA replication and transcription happen only inside the nucleolus of the cell. That’s why that region is full of a large variety of proteins, histone octamers, polymerase enzymes, etc.

It is important to note that DNA, chromatin, genes, etc. all remain inside the nucleolus of the cell’s nucleus.

But when chromosomes or chromatids are formed, they don’t remain confined only inside the nucleolus but inside the whole nucleus of the cell, and later remain in the cytoplasm during cell division.

It is also important to note that, the human mitochondrial DNA (mtDNA) is not present inside the nucleus but in the mitochondria. It is a double-stranded, circular DNA molecule of 16,569 bp and contains 37 genes coding for two rRNAs, 22 tRNAs, and 13 polypeptides.

Unlike most of our DNA, mitochondrial DNA is not found in our chromosomes or even in the nucleus. Also note that, mtDNA do not form chromosmes.

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