Defining the Genome
As most already know, at the very heart of any genome is
something called DNA, or deoxyribonucleic acid. DNA consists of two strands
twisted into a double helix, (the sugar-phosphate backbones) with “ladder
rungs” connecting in the middle. These “ladder rungs” are actually called
nitrogenous bases, and they are the smallest units of the structure.
There
are four nitrogenous bases; adenine, thymine, cytosine and guanine. Most
geneticists shorten these to A, T, C and G. To save things from getting too
complicated, DNA is read from only one strand at a time; this is what is meant
by a DNA sequence. DNA is double-stranded so that each base can form a chemical
pair with the base across from it. These unions are called “basepairs”, and
they are at the heart of DNA’s function. But, certain bases can’t pair with
each other; the pairs are limited to A/T or vice versa, and C/G or vice versa.
And it is the different combinations of these bases that makes your DNA, and by
extension you, unique. And together, they form deoxyribonucleic acid.
DNA
is an incredibly complicated thing. The amount of basepairs in a single human
genome alone amounts to roughly 3 billion.
Early estimates to the size of the human genome were done by measuring the
amount of genes in a cell. This gave us a pretty good idea of how things worked
on that level, but the idea to go about sequencing the entire genome from start
to finish didn’t come up until the mid-80’s, when capillary sequencing was
developed. When it first got started in the 1990’s, the first five years were
devoted to just mapping the chromosomes. Chromosomes display banding patterns
when treated with a chemical agent. These parents are very unique, and can be
used to identify individual chromosomes.
Within
each chromosome are certain regions. That way you can find out which genes are
located in each region. All of this “physical mapping” was done before the sequencing even started.
When
mapping with chromosomes, they wouldn’t pick a single chromosome and build up
from there. No, they would pick many and go out.
That way, they could easily see how the different chromosomes interconnected.
Of
course, the project eventually succeeded. And we now know much, much more about
the human genome, and even genomes in general, than ever before. For instance,
it’s been discovered that the amount of basepairs in a genome doesn't actually do
much to contribute to the complexity of a species. In fact, sheer size is
insignificant compared to the specific sequence. It’s how the bases pair, not
how many pairs there are, that makes the difference. Salamanders, for instance,
have many more basepairs than humans. And in fact, large genomes can sometimes
mean problems; genomes often get big because viruses sneak their way in and
replicate. This is one cause of repetitive regions, regions in a genome where
the same sequence repeats over and over. These are the remnants of dead viruses
which still function to some extent, and can jump around and insert themselves
elsewhere in the genome, causing problems.
Now,
here’s the fun part; every human being has two copies of the genome in each of
the trillions of cells in our body, each cell reading a different part of the
“instructions”. Each copy, if you were to stretch it out to its full length,
would measure out to almost two meters. That’s pretty impressive. The problem?
Each set of genomes has to fit neatly into a chromosome – where the DNA lives –
which is considerably smaller than DNA is long. So how does it all fit? The
answer is in histones. Histones are highly alkaline proteins found which help
package and order the DNA into structural units called nucleosomes. Think of it
as a strand of yard wrapping itself over and over around one post, then
stretching to the next. Then wrapping itself over and over around that post,
and proceeding to the next. And so on and so forth. This storage method works
so well that all 3 billion basepairs – a copy of each set, one from mom and one
from dad – fits into a single chromosome.
The
chromosomes, when ordered, occur in pairs. In humans, each cell contains 23
pairs of chromosomes, for a total of forty-six. Some are duplicated chromosomes
(the ones that look like X’s) and some are non-duplicated (the ones that look
like I’s) but this doesn’t make much of a difference. Except in the last pair,
which are a bit different; in each cell there are 22 pairs of autosomes, and
one pair of sex cells; these are the famous X and Y chromosomes that determine
whether an individual has male or female biology. Every female in the human
species has two X chromosomes, and every male has one X and one Y. The reason
males can’t have a matching pair is because without the X chromosome, a human
being would die. The X chromosome is ultimately more complex than the Y, and
vital to human survival.
And
so, all in all, there are 23 pairs of chromosomes in each cell, including 44
autosomes and two sex cells. But there’s also another set of DNA, that lives
outside the cytoplasm cell, inside another organelle (an organelle is basically
an organ for a cell.) This brings us to the mitochondria, a vital part of cell
biology, which has its very own set of DNA.
A
mitochondrion produces boatloads of energy in the form of the molecule ATP, or
Adenosine Triphosphate. A cell can actually have many mitochondria at a time,
ranging in number from a few to several thousand. They are passed down only
through the maternal line, and, (trivia time!) used to be their own free-living
bacteria.
Scientists
theorize that mitochondria were a separate entity long ago, but were devoured
by some ancestral cell and integrated into our biology. Now it works for us,
despite having its very own set of DNA. Mitochondrial DNA exists in the form of
a chromosome just like nuclear chromosomes. (Nuclear being chromosomes that
reside within the nucleus of a cell, rather than separately in a mitochondrion.)
But the mitochondrial chromosomes aren’t linear; they’re circular. And each
chromosome has only 17,000 basepairs. Does your head hurt yet? Mine did. And we’re
just getting started…
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