This next series of podcasts is going to deal with cytogenetics. I'm going to show
you a list of learning objectives for the next several podcasts. Before I do
that let's just consider this diagram that represents two human cells. One, the
big one is an example of a neuron and little red dot will be an example of a
lymphocyte. These two human cells have identical genetic makeup, that is they
have the same genotype. So, question to keep in the back of your mind:
how can cells that have the same genotype exhibit such vastly different
phenotypes? Our learning objectives for the next few podcasts: You should be able to
describe the structural organization and packing of DNA into chromosomes, you
should be able to explain the differences between chromatin and
chromosomes, you should be able to explain the differences between
heterochromatin and euchromatin, be able to give examples of each of these,
you should be able to explain the function of the nucleolus and ribosome
biogenesis and be able to identify the nor-chromosomes in humans, the nor-
chromosomes would stand for the nuclear or organizer chromosomes, and you should
be able to explain how molecular genetic processes such as transcription,
replication, recombination would relate to chromatin and chromosome structure.
Here is an example of a set of images that look at levels of chromatin packing
from a native DNA molecule to a metaphase chromosome. And you should be
familiar with this from your studies of biochemistry. So, you can look at DNA
existing as a naked DNA double helix. You of course know that the DNA double helix
associates with nucleosomes, the nucleosomes back into the basic
chromatin fiber, the so-called 30 nanometer chromatin fiber, that fiber
continues to pack and condense until you get a fully condensed
metaphase chromosome. You might imagine that is this chromatin fiber
perhaps you might have DNA sequences that that may be many, many, many base
pairs apart from each other, but as the chromatin fiber packs together these DNA
sequences may be brought closer together in three-dimensional space. Just to
remind you that nucleosomes consists of a protein core of histones and DNA. The
histones would be a histone octamer that consists of two moles each of the
histones 2A 2B histones 3 and histone 4. On average about 200 base
pairs of DNA, there's perhaps 140 to 160 base pairs in the nucleosome core region
that has approximately 1.65 turns of the double helix in a left-handed coil.
The linker region about 40 to 60 base pairs of DNA. You should maybe recall
from your studies in biochemistry that the diploid human genome perhaps 6.4
x 10 to the 9th base pairs of DNA, that's about 30 million
nucleosomes. You might remember that histone H1 binds to the linker region
and to the core particles. The bottom image here is prepared under
non-physiologic conditions that shows the nucleosomes. As such, the so-called beads
on a string appearance, so the core region would be like that, like that, or
like that with the linker region in between. The top image is under
physiologic conditions. This would represent the 30 nanometer chromatin
fiber. This would be what the chromatin fiber would look like in an interphase
cell. As you might imagine, the 30 nanometer chromatin fiber can pack
together or unpack depending on the state of genetic activity or gene
expression. Modifications to histones play crucial roles in not only packing
of DNA into chromatin but an also making DNA sequences available for replication,
for repair, for recombination, and for
transcription. Modifications in DNA itself are also important in regulating
gene activity and you should have a reasonable appreciation of this from
your studies of biochemistry. As you might remember, histone modifications
occur at the end terminal end and these modifications may explain how DNA can
relax. The diagram shows the DNA packing into the nucleosomes and ultimately into
the 30 nanometer chromatin fiber, and eventually compacting into a metaphase
chromosome. The diagrams on the right show a space-filling model that is
actually based on x-ray defraction studies of nucleus ohm particles with
the typical ribbon diagram that's commonly generated by computer of
complex protein molecules. So, you can see, the red is the H-2A molecules that look
kind of orange the H2B, green showing the H3, and the blue showing the H4. The
gray areas in this diagram show where covalent modifications to histones might
occur and also note that the n-terminal ends of the histones stick out from the
main center of the molecule. As you understand, DNA can wrap around this
histone octamer and that wrapping of DNA may be influenced by how the histone
tails might be modified. And so, you might imagine that modification of the histone
tails can allow DNA binding proteins to remodel the nucleosomes within the 30
nanometer fiber and that may be very important as one considers molecular
genetic processes like transcription or like replication. So, the cartoon might
show here the histone molecules with a remodeling complex allowing DNA to spin
a little bit loose from the histones so that these sequences might be available
for molecular genetic events. You could have molecular genetic event
occurring then a remodeling complex where you dissociate DNA binding
proteins and the DNA might then re-associate with the nucleosome core
particles. We also talked about epigenetic regulation of transcription and
this table looks at some general principles. What you need to understand
that epigenetics relates to heritable changes in gene activity that are not
caused by changes in DNA sequences. So, for example, you may methylate GC rich
regions in DNA. If these GC rich regions are
unmethylated, the genes that may be active. If they're methylated, the genes
may be inactive. Also, you might acetylate histones. Acetylated histones
suggest that genes may be active, whereas deacetylated histones suggests that
genes would be inactive. As we said, epigenetic inheritance relates to
modifications to histones and to non-histone chromosomal proteins and
potentially to the DNA itself, but it's not changing the nucleotide sequence.
And this is a hypothetical example to demonstrate regulation of gene activity
in the development of the nervous system. So, for example, the chart looks at a
lifespan. Embryonic life through fetal life infancy childhood all the way to
adolescence and it might look at some significant types of genes that might be
expressed. You might, for example, expect that genes for cell proliferation are
going to be expressed very early on an embryonic development and they're going to
decline through feat of life and infancy and of course will drop to lower levels
in post fetal life. You might look at genes that are involved in the
development of the nervous system like, for example, dendrite development of
myelination and you may see lower expression of these genes earlier in the
life span but then the expression of these genes
certainly would get much higher as the lifespan continues. And this is just
an example to demonstrate regulation of gene activity in this case for the
nervous system, but it's just to indicate that gene activity is regulated in a
time-dependent manner during an individual's lifespan. This diagram shows
of simplified mitotic cycle, so it would be looking at somatic cells and if you
try to relate what's going on with the DNA in chromatin during a cell cycle. So,
interphase consists of G1 and G2, DNA replication, repair, and recombination, RNA
transcription, splicing, translation, and modification all of this would occur
during interphase. During the cell division cycle, of course, the chromosomes
segregate and somatic cells you have one cell division in mitosis. In gametes there
are two cell divisions the mitotic divisions. The cartoon indicates the
state of the chromatin at interphase at G1 as DNA is replicated then you see the
chromatin at G2 and of course the chromatin gets highly compacted during
the division cycle, and during the division cycle all of these molecular
genetic events would be halted. We know the DNA exists in the configuration of
chromosomes at the M phase, that is at the cell division phase, but the DNA
exists as chromatin at interphase. As we've suggested, replication,
transcription, and translation stop during the M phase and of course during
the M phase the chromosomes are captured by the spindle.