Hereditary
material is
found in the
nucleus of
each cell.
Chromosomes
are passed
down from
cell to cell
during cell
division.
These
chromosomes
consist of
protein and
DNA
Scientists knew that Chromosomes in cells were the Units of Heredity,
but they did not know if it was the Protein component (Histones) of the
Chromosomes or if it was the DNA component of a chromosome that was
the important.
Scientists also knew that Viruses consist of nothing more
than DNA and Protein, so they radioactively labeled the
DNA in some viruses. Then they let those viruses infect
bacteria to see whether the radioactively labeled DNA was
transferred into the bacteria.
This resulted in the Radioactively
labeled DNA being passed onto the
bacteria
During the second test, only the protein capsids of the
viruses were radioactively labeled. When they let those
viruses infect the bacteria, no radioactivity was found inside
the bacteria. So they concluded that it must be the DNA, not
the protein, that was the hereditary component that gets
passed on.
DNA STRUCTURE
Recall that to build a nucleic acid, whether it be DNA or RNA, cells
combine Nucleotides together to form long chains. As illustrated in the
“Dehydration Synthesis” A.K.A. “Condensation Synthesis” reaction
shown below.
Also recall that DNA is built from four different DNA
nucleotides. All four have the same pentose sugar
(deoxyribose), the same phosphate group, but they differ in
the type of Nitrogenous base that comes off of the
Deoxyribose.
This part will vary
from one base to
another
It is at this carbon that an
oxygen is missing, therefore it
is called “Deoxyribose” sugar
There are two categories of Nitrogenous bases. One category of
nitrogenous base will have a double ringed structure. This group is
called the PURINES, includes Adenine and Guanine. The other category
which have a single-ringed structure. This group, called PYRIMIDINES
include Cytosine and Thymine. Note the letter “Y” in Pyrimidine,
Cytosine and Thymine – good memory device.
To build a complete molecule of DNA, one strand of DNA will bond with
a second strand, then both will form a spiral twist. This unique structure
is known as a “DOUBLE HELIX”
The two strands are held together
by weak Hydrogen Bonds between
each adjacent base. But the base
pairing is very specific. A double
ringed Purine will always hydrogen
bond to a single ringed Pyrimidine.
But it is even more specific than
that. An Adenine will always pair
and bond with a Thymine, while
Cytosine will always pair and bond
with a Guanine. One should also
note that between each A and T
there will be two H-Bonds, while
between each C and G there will
always be three H-Bonds. Good
memory devices : “TA-Two” like
“tatoo” and “C –G –Three”
So you should be able to look at DNA and determine which base is a
double-ringed Purine and which are a single-ringed Pyrimidine. Then
you should observe how many H-bonds are found between the bases.
From the above information you should then be able to identify each
base. Try it.
Try this pairing first, take
note that there are only two
H-bonds therefore; think
“TA-Two”. It must be
Adenine to Thymine. Then I
need to recall that “Any Geek
Can Teach”, in this mnemonic
my first two words represent
the two Double-Ringed
Purines. So the double ringed
base in this pairing has to be
either adenine or guanine, but
it can only be Adenine,
because only adenine uses two
H-bonds
Try this pairing now, take
note that there are three Hbonds therefore; think “c-g3”. It must be Cytosine to
Guanine. Then I need to
recall again that “Any Geek
Can Teach”, in this
mnemonic my first two
words represent the DoubleRinged Purines. So the
double ringed base in this
pairing has to be either
adenine or guanine, but it
can only be GUANINE,
because only guanine uses
three H-bonds. The other
base must be Cytosine.
First noticeable
difference is in
the sugar found
in RNA
Nucleotides. The
5-carbon
(pentose) sugar
has all its
oxygens. It is
called “RIBOSE”
sugar
We will study RNA- Ribonucleic Acid in more detail later on
this chapter. But for now we just need to know the basic
differences between it and DNA – Deoxyribonucleic Acid
The second noticeable difference is that one of the four nitrogenous bases
is different than the DNA bases. Recall, “Any Geek Can Teach” – DNA.
“Any Geek Can U” – RNA. The DNA base Thymine is replaced with the
RNA base Uracil
A third difference is that RNA is never double
stranded
RNA is not Helical either, it is just straight (linear)