Video Transcript
In the formation of chromosomes,
what is the primary function of the histone proteins? (A) To break hydrogen bonds between
double-stranded DNA to make it ready for replication. (B) To bind the two ends of the DNA
molecule together to form a circular chromosome. (C) To tightly coil and package
large amounts of DNA into a small space. (D) To digest the DNA at specific
sites to divide it into smaller sections. (E) To hold pairs of chromosomes
together at a central point.
You’ll recall that eukaryotes, like
humans, are characterized by the presence of a membrane-bound nucleus to contain
their DNA. In humans, if you were to unpack
all the DNA inside a single nucleus, you would get about two meters worth of
DNA. This isn’t one long continuous
piece of DNA. Instead, it’s organized into
structures called chromosomes.
In humans, there’s 23 pairs of
chromosomes or 46 in total. Here’s what one of these
chromosomes might look like. Each chromosome is a highly
packaged molecule of DNA that’s coiled up and wrapped around special proteins called
histones. Because of this extensive packing,
a lot of DNA can fit inside a single cell.
Histones are able to wrap DNA
tightly because they carry a positive charge, while DNA carries a negative
charge. Nearly 150 base pairs of DNA can
wrap around a histone complex of eight histone proteins. This forms a structure called a
nucleosome. Nucleosomes coil up tightly to form
chromatin, which wraps up further to form the chromosome.
Therefore, going back to our
question, the option that best describes the primary function of the histone protein
is given by answer choice (C): to tightly coil and package large amounts of DNA into
a small space.
