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Genetic Molecules | MCAT 2015
Nucleic Acid
• Nucleosides consist of a
nitrogenous base and a pentose
sugar
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Nucleoside
A nucleoside consists of a nitrogenous base (red)
and a pentose sugar (blue). The nitrogenous base
shown here is adenine, making this nucleoside
adenosine.
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Nucleotides are formed when a
phosphate group is bonded to the
nucleoside
o Adenine, guanine,
cytosine, thymine, and
uracil
We will see their
structures later,
but you need not
memorize them
Nucleotides polymerize to form
nucleic acids
o (more specifics soon)
Deoxyribonucleic Acid (DNA)
• DNA is a nucleic acid built from
nucleotides with 4 nitrogenous
bases:
o Adenine, guanine,
cytosine, & thymine
o Uracil is not found in
DNA; it is found in RNA,
which we will see later
• Adenine and guanine are:
o Double-ringed
o Known as “purines”
• Cytosine and thymine are:
o Single-ringed
o Known as “pyrimidines”
• As a pneumonic, remember that
“cytosine,” “thymine,” and
“pyrimidine” have the letter “y”
Nucleotide
A nucleotide is formed when a phosphate group
(green) is appended to a nucleoside. The
nucleotide formed here is adenosine phosphate.
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Nucleotides are frequently
referred to by the name of their
nitrogenous base
o “Adenosine phosphate”
above can simply be
called “adenine”
There are 5 nitrogenous bases we
will use:
The 4 Nucleotides of DNA
Cytosine and thymine are pyrimidines, while
adenine and guanine are purines.
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Genetic Molecules | MCAT 2015
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Though not found in DNA, uracil
is a purine
DNA forms when nucleotides
polymerize via phosphodiester
bonds
o These are bonds between
a pentose sugar’s 3rd
carbon and the phosphate
of another nucleotide
Ordering 5’ to 3’
Using the blue nucleotide as an example, an
arrow pointing from the pentose’s 5-carbon to its
3-carbon points down. This arrow indicates that
our nucleic acid should be denoted green blue
red and not in the reverse direction.
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One DNA Strand (aka A Nucleic Acid)
DNA consists of several nucleotides adjoined
between the 3rd carbon on one nucleotide’s
pentose sugar and the phosphate group of
another nucleotide. These bonds, indicated by
black arrows, are called phosphodiester bonds.
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We denote nucleic acids as a list
of all its nucleotides
o But in which direction?
5’ to 3’
o That is, we order
nucleotides in the
direction indicated by an
arrow from one pentose
sugar’s 5 carbon to its 3
carbon
In the above nucleic acid, an
arrow from a pentose’s 5-carbon
to its 3-carbon points downwards
o So, we’d order that
nucleotide from green
blue red
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Nucleic acids can be exceedingly
long
For simplicity, we will no longer
draw each nucleotide in its
entirety
o Rather, we will show the
“backbone” of phosphate
groups coupled with each
nucleotide represented by
a unique geometric shape
So, the nucleic acid we’ve been
working with will be shown as:
Simple Nucleic Acid Illustration
We will abbreviate the expansive nucleic acid
structure to a phosphate backbone (black)
coupled with each nucleotide represented by a
unique geometric shape and color. Ordered from
5’ to 3’, this strand is AGC, representative of
adenine guanine cytosine.
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Genetic Molecules | MCAT 2015
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The 5’ end of a nucleic acid is
capped with a phosphate group
The 3’ end is capped with a
hydroxyl
o (shown in previous
illustrations)
DNA consists of two nucleic
acids lying side-by-side
The two strands lie in opposite
directions
o One strand points 5’ 3’
in one direction, and the
other points 5’ 3’ in
the other direction
DNA Complementary Strands
Given the bottom strand of AGCGAT, the top
strand must be its complement: TCGCTA. Each
A is paired with T, and each G is paired with C.
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DNA Structure
DNA consists of two nucleic acids lying side-byside oriented in opposite directions. The two
strands are held together by hydrogen bonds
(black dotted lines). An adenine on one strand is
always coupled with thymine on the other strand.
The same is true of guanine and cytosine.
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A DNA nucleotide and it’s
“partner” on the complementary
strand is called a base pair
o We’d say the above DNA
is 3 base pairs long
Since A is always coupled with T
and G is always coupled with C,
we can predict the
complementary sequence
o i.e. suppose know that
from 5’ 3’, one strand
of DNA has the sequence
AGCGAT
o The complementary
strand must be TCGCTA
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But be careful! Since the bottom
strand was 5’ 3’ going
rightward, the top strand is 5’
3’ going leftward
o And since we report
nucleic acids 5’ 3’, the
complementary strand is
technically ATCGCT
We mentioned earlier that the
two strands are held together via
hydrogen bonds
o C and G share 3 H bonds
o A and T share 2 H bonds
Think, C & G
both rhyme with 3
In fact, a common exam question
will show a few DNA fragments
and ask which is most resistant to
decomposition
o The answer will be the
fragment containing the
most C-G base pairs
More C-G base
pairs means more
H bonds, making
stronger DNA
Identifying the Strongest DNA Sequence
The circled DNA fragment, which contains the
most C-G base pairs, will be the strongest.
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Genetic Molecules | MCAT 2015
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We’ve been drawing DNA as
linear side-by-side nucleic acids
But in reality, DNA actually
forms a 3D double helix
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(a)
(b)
True Structure of DNA
We’ve depicted DNA as (a) linear side-by-side
nucleic acids. In actuality, DNA is twisted into
(b) a double helix. This may be easier to
visualize in the lecture video, in which the
twisting DNA strands are animated.
Genes
• Several nucleotides within DNA
make up a gene
o Genes encode RNA,
which in turn encode the
proteins organisms rely
upon to survive
o We will learn more about
RNA and protein
production in the coming
sections
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In the previous illustration, we
show 7 nucleotides comprising a
gene
o But in actuality, several
hundred or thousand
nucleotides will comprise
a gene
Prokaryotes, which are simple
organisms lacking a nucleus and
membrane-bound organelles,
have just one copy of each gene
Eukaryotes, which are more
complex organisms that do have
a nucleus and membrane-bound
organelles, can have multiple
copies of each gene
o Eukaryotes even have
“spacer” DNA that
doesn’t code for RNA
In eukaryotes, DNA commonly
transcribed into RNA is stored as
euchromatin
o Which is lightly-packed
DNA, which is easy to
unfold and reference
DNA not commonly needed is
stored as heterochromatin
o Which is more tightly
packed away
The entirety of an organism’s
genes is called the organism’s
genome
o Though everyone’s
genome is unique, all
human genomes differ by
less than 0.1%
o And, the human genome
is only ~2% different
from that of a
chimpanzee
Genes
DNA nucleotides make up a gene, which
encodes RNA, which encodes proteins. More on
this in coming sections
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