It’s hard convey the scale of very small or very large things by just quoting measurements as the measurements don’t relate well to our perception of everyday objects.
This interactive infographic presented by the Genetic Science Learning Centre at the University of Utah lets users zoom in from small everyday objects to the components of life. (To zoom in, drag the slider below the image.)
Below the interactive graphic are some explanations that are well worth reading.
One outlines why a sperm head is not much bigger than the (mitotic) X chromosome they show (see illustration to left):
How can an X chromosome be nearly as big as the head of the sperm cell?
No, this isn’t a mistake. First, there’s less DNA in a sperm cell than there is in a non-reproductive cell such as a skin cell. Second, the DNA in a sperm cell is super-condensed and compacted into a highly dense form. Third, the head of a sperm cell is almost all nucleus. Most of the cytoplasm has been squeezed out in order to make the sperm an efficient torpedo-like swimming machine.
The X chromosome is shown here in a condensed state, as it would appear in a cell that’s going through mitosis. It has also been duplicated, so there are actually two identical copies stuck together at their middles. A human sperm cell contains just one copy each of 23 chromosomes.
A chromosome is made up of genetic material (one long piece of DNA) wrapped around structural support proteins (histones). Histones organize the DNA and keep it from getting tangled, much like thread wrapped around a spool. But they also add a lot of bulk. In a sperm cell, a specialized set of tiny support proteins (protamines) pack the DNA down to about one-sixth the volume of a mitotic chromosome.
The X chromosome is not the biggest human chromosome either.
This video, originally created for the DNA Interactive learning site, illustrates the packaging of DNA into chromosomes. Near the middle of the video you’ll see mitotic chromomsomes like the one in the infographic, with their characteristic ‘X’ shape.
Like others who study molecular biology, one thing I like about this video is that the molecules are not rigid, but shown in constant motion. Molecules move!
I’d like to add two general thoughts that I hope to expand on in later posts.
Packing is not only for compaction, but to control what portions of a genome is used Historically, the ‘support proteins’ that they refer to have been thought of as mostly just that. In addition to just compacting the DNA into a small space, chromatin proteins are used to set the structure of local regions of the genome according what use is being made of them.
They are not ‘just’ passive packaging proteins, but are actively involved in how genes are regulated. Regions that not being used are packed away as heterochromatin. Regions that are actively being used are opened up, allowing them to be read. This changing of the local structure of a genome is one outcome of epigenetics.
At a more detailed level, nuclesomes near genes are placed and moved as part of the process of controlling genes. (Nucleosomes are a core made of eight chromatin packaging proteins called histones, shown in the video. The ’tails’ of the histones that extend away from the core are modified as part of controlling the packaging of genes. This has been called the ‘histone code’ that is part of epigenetic control of gene expression.)
One genome, many genome structures While all cells in your body have the same genome,* the chromosomes making up the genome in each kind of cell have different packing arrangements reflecting what genes are being used. Sperm cells are an extreme case of this, with tight packing of the whole genome. Other interesting examples of different chromosome structures are the polytene puff chromosomes best known in some insect salivary glands,** which you may remember from high school, and the DNA in the nuclei of retinal cells from the retina in your eyes.***
My personal research interests (that is, beyond what I am asked to work on by those that hire my services) are the role of chromatin in gene expression and genome structure, and new algorithms for computational biology.
* Leaving aside the difference in ploidy, or number of copies of the genome in a cell. Body cells (somatic cells) have two copies, they are diploid; germ cells (eggs, sperm) have half the number of copies as the body cells, i.e. they are haploid.
** They’re also found in a surprisingly wide range of other species, including plants and animals.
*** Actually, the ones I am thinking of are from mice, but they very likely to be similar to the ours.
Other posts at Code for life: