within a typical human cell which number is the greatest
Chromatin is the substance of a chromosome and consists of a complex of DNA and protein in eukaryotic cells. It can be made visible by staining (thus the name which literaly means coloured material). The nucleic acids are generally in the form of double-stranded DNA. The major proteins involved in chromatin are histone proteins. In a eukaryotic cell, nearly all DNA is found compacted in chromatin. DNA is packaged into chromatin both to constrain the size of the molecule and to allow the cell to control expression of the chromatin packaged genes. Changes in chromatin structure are effected mainly by methylation and acylation of the nucleosome proteins. Chromatin structure is also of importance with respect to the DNA replication. Genetic material (DNA) is also present in the nucleus, the DNA is present as a DNA-protein complex called chromatin. The DNA is present as a number of discrete units known as chromosomes. There are two types of chromatin: euchromatin and heterochromatin. Euchromatin is the least compact form of DNA, and the regions of DNA which constitute euchromatin contain genes which are frequently expressed by the cell.
A simple question deserves a simple answer. How many cells are in your body? Unfortunately, your cells can t fill out census forms, so they can t tell you themselves. And while it s easy enough to look through a microscope and count off certain types of cells, this method isn t practical either. Some types of cells are easy to spot, while others such as tangled neurons weave themselves up into obscurity.
Even if you could count ten cells each second, it would take you tens of thousands of years to finish counting. Plus, there would be certain logistical problems you d encounter along the way to counting all the cells in your body for example, chopping your own body up into tiny patches for microscopic viewing. For now, the best we can hope for is a study published recenty in Annals of Human Biology, entitled, with admirable clarity, The authors a team of scientists from Italy, Greece, and Spain admit that they re hardly the first people to tackle this question. They looked back over scientific journals and books from the past couple centuries and found many estimates. В But those estimates sprawled over a huge range, from 5 billion to 200 million trillion cells. В And practically none of scientists who offered those numbers provided В an explanation for how they came up with them. Clearly, this is a subject ripe for research. If scientists can t count all the cells in a human body, how can they estimate it? The mean weight of a cell is 1 nanogram. For an adult man weighing 70 kilograms, simple arithmetic would lead us to conclude that that man has 70 trillion cells. On the other hand, it s also possible to do this calculation based on the volume of cells. The mean volume of a mammal cell is estimated to be 4 billionths of a cubic centimeter. (To get a sense of that size, check out. ) Based on an adult man s typical volume, you might conclude that the human body contains 15 trillion cells.
So if you pick volume or weight, you get drastically different numbers. Making matters worse, our bodies are not packed with cells in a uniform way, like a jar full of jellybeans. Cells come in different sizes, and they grow in different densities. Look at a beaker of blood, for example, and you ll find that the red blood cells are packed tight. If you used their density to estimate the cells in a human body, you d come to a staggering 724 trillion cells. Skin cells, on the other hand, are so sparse that they d give you a paltry estimate of 35 billion cells. So the author of the new paper set out to estimate the number of cells in the body the hard way, breaking it down by organs and cell types. (They didn t try counting up, sticking only to human cells. ) They ve scoured the scientific literature for details on the volume and density of cells in gallbladders, knee joints, intestines, bone marrow, and many other tissues. They then came up with estimates for the total number of each kind of cell. They estimate, for example, that we have 50 billion fat cells and 2 billion heart muscle cells. Adding up all their numbers, the scientists came up withвdrumrollв37. 2 trillion cells. This is not a final number, but it s a very good start. While it s true that people may vary in size and thus vary in their number of cells adult humans don t vary by orders of magnitude.
The scientists declare with great confidence that the common estimate of a trillion cells in the human body is wrong. But they see their estimate as an opportunity for a collaboration perhaps through an online database assembled by many experts on many different body parts to zero in on a better estimate. Curiosity is justification enough to ponder how many cells the human body contains, but there can also be scientific benefits to pinning down the number too. Scientists are learning about the human body by building sophisticated computer models of lungs and hearts and other organs. If these models have ten times too many cells as real organs do, their results may veer wildly off the mark. The number of cells in an organ also has bearing on some medical conditions. The authors of the new study find that a healthy liver has 240 billion cells in it, for example, but some studies on cirrhosis have found the disease organ have as few as 172 billion. Perhaps most importantly, the very fact that some 34 trillion cells can cooperate for decades, giving rise to a single human body instead of a chaotic war of selfish microbes, is amazing. В But our ancestors went way beyond a simple sponge-like anatomy, evolving a vast collective made of many different types. To understand that collective on a deep level, we need to know how big it really is.
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