Which row shows three structures found in a healthy human male liver cell, in order of size from largest to smallest?
[Assume that there are no mutations in the cell.]
A. row 1
B. row 4
C. row 5
D. row 3
E. row 2
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The correct answer is A.
Histone
Histones are proteins that support DNA strands. Their purpose is to compact DNA (so it can fit inside the nucleus) and to regulate gene expression.
Nucleosome
A nucleosome is a portion of DNA wrapped around proteins.
A “bead” of nucleosome is made of 8 histones and about 150 base pairs of DNA. Their diameter is about 11 nm.
It is considered to be the basic structural unit of chromatin : its purpose is to condensate the DNA.
The chain of nucleosome is even further compacted, and make up the structure that we know as chromatin, which will then make up chromosomes (the most condensed structure, about 1 400 nm).
Y chromosome
It is the smallest human chromosome, with the height of about one third of the X chromosome ; it also contains way less genes (55 compared to ~900 in the X chromosome).
Understanding why the Y chromosome is shorter has been one of the Human Genome’s goals.
Nowadays, it is pretty safe to say that the Y chromosome has been shortened over time due to chromosomic mutations : X and Y couldn’t recombine genes significantly during meiosis and therefore, it has lost a lot of genes.
Here’s a recap (from smallest to biggest structure) :
DNA → Histones → Nucleosomes → Chromatin → Chromosome
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