Video Transcript
Which of the following statements
correctly compares the DNA of eukaryotic cells with the DNA of prokaryotic
cells? Eukaryotic chromosomes contain
specialized proteins that help form chromatin, whereas most prokaryotic chromosomes
do not. Both eukaryotic and prokaryotic DNA
condense to form X-shaped chromosomes before the cell divides. Or eukaryotic DNA forms a single
chromosomal loop, whereas prokaryotic DNA forms many small circular plasmids.
To help us answer this question,
let’s remove the options for now and remind ourselves of the similarities and
differences between eukaryotic and prokaryotic DNA. You may recall that eukaryotic
cells, such as plant cells or animal cells, are those which contain a membrane-bound
nucleus, whereas prokaryotic cells, like bacteria, are much more primitive. They don’t have a nucleus or any
other membrane-bound organelles. Although DNA in both eukaryotic and
prokaryotic cells is incredibly thin, it can also be very long. For instance, if we stretched out
all the DNA in an average human cell end to end, although some estimates vary, it is
thought that it would be about two meters long. And yet it can fit inside the
nucleus, which is only around five micrometers in diameter. That’s nearly 500,000 times
smaller.
So how is this possible? Well, in order for the DNA to fit
inside the comparatively small space it occupies inside the cell, it has to be
carefully packaged to make it more compact. This is a bit like folding your
clothes so that you can fit them all inside your suitcase when you go on
holiday. The packaging of DNA in eukaryotic
cells involves specialized proteins called histones. DNA binds to the histones and is
wound around them, forming what’s known as the beads on a string structure because
that’s exactly what it looks like. The beads on a string are then
folded into tighter coils called chromatin. This is what all eukaryotic
chromosomes are made of.
During mitosis or miosis, before
the cell divides, eukaryotic chromosomes replicate themselves and eventually form
the X-shaped structures we’re familiar with. In this state, DNA is condensed
into even tighter coils to avoid tangles during the division. These structures are then pulled
apart, so we have two copies of every chromosome, one for each new cell. Rather than existing as many
chromosomes, prokaryotic DNA is usually found as a single chromosomal loop, which
lies freely in the cytoplasm. In addition to this, prokaryotic
cells often contain plasmids, which are small rings of extra DNA that is only useful
to the organism under certain conditions. For example, bacterial plasmids
often contain antibiotic resistance genes.
Unlike eukaryotic cells, most
prokaryotic cells do not contain histones and therefore also do not form
chromatin. Instead, their DNA is packaged by a
process called supercoiling, which is catalyzed by enzymes. During prokaryotic DNA replication,
because the chromosome is circular, instead of forming an X-shaped structure, it
forms a figure of eight shape, which is then separated into two identical
chromosomes before the cell divides.
Using the information we’ve
discussed, we can conclude that the correct statement is eukaryotic chromosomes
contain specialized proteins that help form chromatin, whereas most prokaryotic
chromosomes do not.
