|How many number of coils of DNA stand in a single chromosome?|
|Question Date: 2019-11-15|
Me and my fellow scientists had some fun discussing the different ways you could interpret this question! The simplest answer is that, before replication, a chromosome is made up of one very long strand of DNA very tightly coiled in on itself. After replication, this becomes two equivalent tightly coiled strands.
Within the chromosome these strands are coiled around proteins called histones, forming complexes called nucleosomes. There are about 30 million nucleosomes present in a human cell, and if we assume that each of the 23 chromosomes comprise 1/23rd of this number, there are about 1.3 million nucleosomes per chromosome.
If you are talking about coils within the DNA itself, you can also calculate the number of helical turns within a chromosome. There are about 3 billion base pairs in the human genome, and if we imagine a "perfect" chromosome made up of 1/23rd of these base pairs, the DNA in the chromosome would be 130 million base pairs long. According to Watson, Crick, and Franklin's DNA model, there are about 10.5 base pairs per turn in DNA. That means that within one chromosome there are about 12.4 million turns.
Of course, in real life you can't assume that each chromosome in the cell is an equal length. For example, chromosome 1 is quite a lot larger than chromosome 21, which is smaller than chromosome 22, and so on. This makes it trickier to look at the coils within each chromosome! If you're interested in a specific chromosome, you can look up the length of the DNA in that chromosome and work out the dimensions involved, like I did above. Thanks for asking!
DNA is organized and packed tightly in eukaryotic organisms, but not so in prokaryotes, so we will assume that eukaryotic DNA packing is what you're asking about.
In eukaryotes, DNA is organized on several levels. The most basic level is the wrapping of the double-helix around 8 proteins that make a thick disk (that looks like a very short cylinder). There are about 146 base-pairs wrapped around each of these "disks", and we call the combination of DNA and protein disk a nucleosome. Nucleosomes are connected by linker DNA that is not wrapped around proteins, and the average length of linker DNA is about 20 base-pairs, so altogether, a nucleosome with its linker DNA take about 170 base-pairs.
The nucleosomes are coiled into a structure that we call the 30nm fiber, because the fiber itself has a diameter of about 30nm. Each coil of the fiber takes about 6 nucleosomes plus their linker DNA. If we take human chromosomes, the average length of a chromosome is approximately 131.4 million base-pairs. If we want to find the number of coils in the 30nm fiber, then we would take the 131 million base-pairs and divide them accordingly, first by 170 to find the number of nucleosomes in the chromosome, and then by 6 to find the number of coils in a 30nm fiber.
However, if we look at higher-order (more complex) packaging of DNA, then we see that the 30nm fiber is further coiled into loops about 300nm in length, and these loops are folded into 250nm-wide fibers, so if we want to find the number of coils in these higher-level packing structures, we would have to find the number of base-pairs in the 300nm loop, and/or the 250nm-wide fibers, and so on. It depends on what you mean by the word "coil". For more DNA packing information, please feel free to visit this site as a starting point: DNA packing.
Depending on the stage of cell division, each chromosome has one to two double stranded DNA molecule. The DNA molecule winds on a type of protein called histone to form the nucleosome. Nucleosomes then are packed together to form coils. Those coils forms a 30 nanometer fiber called the chromatin fiber. However, the structure of chromatin fiber is not well known and we don't know the exact number of coils in a chromosome.
Our chromosomes are all different lengths. Chromosome #1 is more than 4 times as long as the Y chromosome. The Y chromosome is only 20 milimeters long = 20,000 nanometers long.
We have about 6 feet of DNA in each of our cells from each of our parents. It's amazing to imagine such a length of DNA in each of our tiny cells. We have 23 pairs of chromosomes. One chromosome in each pair is from our mother and the other chromosome is from our father.
DNA forms lots of coils. There's the DNA double helix [dsDNA], where 10 base pairs of DNA coil into 1 turn of the double helix. Each turn of the DNA double helix is 3.3 nanometers long. That gives over 6,000 turns of the DNA double helix in the Y chromosome.
The DNA double helix coils around histone proteins to form nucleosomes that have 167 base pairs of DNA wrapped around each one, forming something like beads on a string, except that the 'beads' of histone proteins have the 'string' of DNA wrapped all around them.
A nucleosome is a fundamental unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound in sequence around eight histone protein cores. This structure is often compared to thread wrapped around a spool.
The nucleosome beads-on-a-string are coiled up more, into a '30 nanometer fiber' that scientists are still discovering the structure of.
That depends on the size of the chromosome. From what I was able to find in a quick search, it's in the tens of millions for a mid-sized chromosome.
Each chromosome is composed of a single strand of DNA. When not being actively read each strand is wrapped around proteins called nucleosomes, which are further assembled into structures called histones. This hierarchical structure greatly compacts the DNA strands, which helps protect against damage and makes their transport (such as during cellular division) easier. For visual learners, the structure is shown in this video.
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