Video Transcript
In this video, we will learn about
the structure and the function of the nucleic acids, DNA and RNA. We’ll learn about their
similarities and their differences, and we’ll learn about their molecular subunit,
the nucleotide.
Nucleic acids are one of four types
of biological macromolecules. Nucleic acids are specifically
adapted to store and transfer information. Nucleic acids got their name
because they were initially found in the nucleus of the cell. There are two types of nucleic
acids, DNA and RNA. Even though they were initially
discovered in the nucleus of eukaryotic cells, nucleic acids exist in all living
things, including prokaryotes, which don’t possess a nucleus at all. DNA stands for deoxyribonucleic
acid and RNA stands for ribonucleic acid. These two molecules are closely
related, but they have several important differences in their structure. This makes them adapted to related
but different functions.
Let’s begin by examining the
general structure of a nucleic acid. Nucleic acids are polymers, which
means that they are large molecules that are made up of several repeating molecular
subunits or monomers. The monomers of nucleic acids are
called nucleotides. The nucleotide itself is made up of
three parts: a phosphate group, a pentose sugar, and a nitrogen-containing base. The phosphate group is always the
same. There are two types of pentose
sugars found in nucleic acids, ribose sugar and deoxyribose sugar, which contains
one less oxygen atom than ribose sugar does. As you can likely tell by their
names, deoxyribose sugar is found in the nucleotides of DNA, and ribose sugar is
found in the nucleotides of RNA.
Finally, there are five types of
nitrogen-containing bases. They are adenine, guanine,
cytosine, thymine, and uracil. The nitrogen-containing bases are
often represented by their initials. The base thymine is only found in
DNA, and the base uracil is only found in RNA. Adenine, guanine, and cytosine are
found in both. The polarization of nucleotides is
what makes a nucleic acid. The pentose sugar of one nucleotide
bonds with the phosphate group of the adjacent nucleotide. The atoms of their two hydroxyl
groups rearrange so that both molecules are sharing one oxygen atom. The two hydrogen and one oxygen
atoms that are released in the process bond together to form one molecule of
water. This chemical process is called a
condensation reaction.
The resulting bonds between the
phosphate group and these two sugars is called a phosphodiester bond. The bonds between the pentose
sugars and the phosphate groups form a stable structural chain, which is referred to
as a sugar phosphate backbone. Well, we know that nucleic acids
are adapted to store and transfer information, and that information is contained in
the sequence of the nucleotide basis. And in order for that information
to make sense, that sequence must follow a specific order, which brings us to a
concept called directionality.
It’s common scientific convention
to number the carbon atoms in a sugar molecule. The carbon atoms in our pentose
sugar are numbered one to five like this. The fifth carbon is bonded to the
phosphate group of its own nucleotide. So this end of the molecule is
called the five prime end. The third carbon atom in the
pentose sugar is bonded to the phosphate group of the adjacent nucleotide. So this end of the molecule is
called the three prime end. We read nucleotide sequences from
the five prime to the three prime direction because that’s how the polymerization
occurs in living organisms.
Let’s practice quickly by writing
down the nucleotide sequence from our example. From five prime to three prime, it
reads GATCG. Now that we’ve learned about the
general structure of nucleotides and how they’re able to form a sequence of readable
nucleotide bases, let’s look at the specific structure of DNA and RNA. DNA is a nucleic acid that’s
specifically adapted to store information and to pass that information on to
offspring cells or organisms. DNA is made of two strands of
nucleotides bonded to each other at their nitrogen-containing bases. Because of the structure of the
molecular subunits, DNA forms a twisted ladder shape, also called a double
helix. The sugar phosphate backbone makes
up the sides of the ladder, and the bonded nitrogenous bases are the runs.
If we untwist this ladder, we can
see the nucleotide sequence more clearly. And I’ve also added the letters
that represent the nitrogen-containing bases. If we zoom in a little more closely
on the structure, we can see that the molecules in one strand are facing in one
direction, and the molecules in the opposite strand are pointing the opposite
way. For this reason, one strand reads
from five prime to three prime, while if we read in the same direction on the
opposite strand, we’re going from three prime to five prime. This arrangement is known as
antiparallel, since the two DNA strands are aligned but oriented in opposite
directions. The direction that we read a DNA
strand in is important, since the sequence of the nucleotide bases is what
determines the genetic information.
Another feature of the DNA molecule
is that each of the nitrogen-containing bases only bonds with a complementary
match. In DNA, adenine always pairs with
thymine, and cytosine always pairs with guanine. You’ll also notice that a larger
base always pairs with a smaller base. This allows the DNA molecule to
maintain the same width along its length. This is what we refer to as the
base pairing rules. I’ve left some blanks on our
example DNA strand so that we can practice base pairing together. If the nucleotide sequence on one
strand of DNA, reading from the five prime to the three prime direction, is CTA,
then what would be the sequence on the complementary strand reading from the three
prime to the five prime direction? Well, the complementary nucleotide
sequence would be GAT.
The complementary strands of DNA
are held together by hydrogen bonds. Unlike the phosphodiester bonds
found in the sugar phosphate backbone, these hydrogen bonds are relatively easy to
break. All of these features of DNA are
what allow it to be replicated quickly and precisely, which is essential in its
function as hereditary information. The function of RNA differs from
that in DNA because RNA is adapted to transfer information from place to place. We’re also already aware that the
RNA nucleotide includes ribose sugar instead of deoxyribose sugar. mRNA is
responsible for carrying information from the DNA where it’s stored to the ribosome,
which translates it into a protein. In order to accomplish this, mRNA
follows a similar set of base pairing rules to DNA.
Here we have a molecule of DNA
where the two complementary nucleotide strands have been separated. In order to transfer the
information, a complementary RNA strand must be built. Just like in DNA, small cytosine
always pairs with large guanine. And where we have thymine on the
DNA molecule, adenine will always be the complementary RNA base. But where DNA possesses adenine,
the complementary RNA base will be uracil instead of thymine. Why don’t you try to fill in the
remaining complementary RNA bases on your own?
Once the RNA has copied the
information from the strand of DNA, it detaches and travels to the ribosome, where
the protein synthesis process begins and the two DNA strands rejoin. Besides the differences that we’ve
already noted, we can also see here that DNA tends to exist as two bonded strands of
nucleotides, while RNA tends to exist as a single strand of nucleotides. Also in eukaryotic cells, DNA is
stored in the nucleus, while RNA can be found inside of the nucleus and in the
cytoplasm. Now that we’ve learned about how
the differences in the structure of DNA and RNA support their different functions,
let’s try a practice question.
DNA and RNA are both examples of
nucleic acids. Compare and contrast the structures
of DNA and RNA.
This question is asking us to
compare, which is to describe similarities, and to contrast, which is to describe
differences. And they want us to describe the
similarities and the differences between the structures of DNA and RNA. And we’re being asked to
specifically focus on the structure or the parts and not the function or what it is
these molecules do.
One way to organize your
information when you’re working on a compare-and-contrast question is to make a
table with a column for DNA, a column for RNA, and a column for both. Then you simply fill in whatever
you know which will help you to write out your answer. Another way to organize your
information is to make a Venn diagram with a circle for DNA, a circle for RNA, and
information about both where the two circles overlap. I like a more visual approach, so
I’m going to continue with the Venn diagram, but you can choose to use whichever
method is most comfortable for you. I’ve also added some visual clues
which we’ll annotate together before writing out our final answer.
Let’s start by noting that this
question already told us that DNA and RNA are both nucleic acids. And we know that nucleic acids are
polymers and that a polymer is a large molecule made up of repeating subunits, which
we call monomers. The monomers of nucleic acids are
called nucleotides, and each nucleotide is made up of a phosphate group, a pentose
sugar, and a nitrogen-containing base. However, DNA is made of two strands
of nucleic acids bonded together into a double helix, while RNA is typically a
single-stranded molecule.
There are also some differences
between the nucleotides of DNA and RNA. The pentose sugar in the
nucleotides of DNA is deoxyribose sugar, while in RNA it’s ribose sugar. Also the bases in DNA are adenine,
thymine, cytosine, and guanine. RNA molecules possess adenine,
guanine, and cytosine, but the base thymine is replaced by the base uracil. Now that we’ve summarized the
similarities and differences in the structure of DNA and RNA in our Venn diagram,
we’re ready to write out our answer. Your answer, of course, will be
written in your own words, but it should look something like this.
Both DNA and RNA are polymers
formed from multiple subunits called nucleotides. DNA and RNA nucleotides are both
formed from a pentose sugar, a phosphate group, and a nitrogenous base. However, the pentose sugar in DNA
is deoxyribose, and in RNA it is ribose. The nitrogenous bases in DNA are
adenine, thymine, cytosine, and guanine, but in RNA thymine is replaced by
uracil. DNA also forms a double-stranded
molecule, whereas RNA forms single-stranded molecules.
Let’s wrap up our lesson by taking
a moment to review what we’ve learned. In this video, we learned about
nucleic acids, which are polymers made up of nucleotide monomers. We learned about deoxyribonucleic
acid or DNA and ribonucleic acid or RNA. We learned about the structure of
both. We also learned how those
structures are related to their functions. The function of DNA is storage of
information, and the function of RNA is transfer of information.
