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Prophets or
Evolution - An LDS Perspective Chapter 21 DNA and RNA Introduction The rest of
this book will get increasingly involved with DNA. This chapter will introduce the reader to the
molecule called: DNA. Subsequent
chapters will get more and more involved with DNA, so it is important to have a
sold basis for understanding DNA. It would
also be wise to look at outside resources which discuss DNA. It is far beyond the scope of this book to
discuss many issues related to DNA. This
book will focus on very specific issues.
A broader understanding of DNA would be very helpful to the reader as
this book progresses. DNA (Deoxyribonucleic
Acid) DNA is a
very large, very complex molecule made up of several different kinds of small
molecules. "Nucleotides"
are a type of molecule found in DNA.
There are only 4 different kinds "nucleotides." Nucleotides are the key to what DNA is able
to do. The sequence (i.e. order) of
nucleotides is what provides the "information" that is needed for the
cells to function. To
understand DNA, consider a large encyclopedia of 100 volumes. Even though a large set of encyclopedia
volumes is a huge set of books, and would be very, very heavy to lift, the
encyclopedia only consists of 26 letters of the English alphabet. A person could say that an encyclopedia
consists of only 26 different letters; however, all of the letters are repeated
many, many times in an encyclopedia. Yes, an
encyclopedia will also have numbers (e.g. 0,1,2,3,...,9),
special characters (e.g. space, %, $, @, &, etc.), but still an
encyclopedia, no matter how many volumes, still has only 26 letters of the
alphabet, plus a few other characters. For
example, consider that a word in the English language, such as the word:
"tergiversation" (which is the act of being deliberately ambiguous)
is nothing but a permutation (i.e. a unique way of ordering the letters) of the
26 possible letters of the English alphabet which happens to have 14 consecutive
letters in it. An
encyclopedia has a lot of words and thus a lot of individual letters, if we
ignored the words and only looked at the letters. In fact, when we look at DNA we may look at
the letters (i.e. the nucleotides) and forget to look at DNA as
"words" or "segments" which have meaning as a group of letters. DNA only
has 4 letters in its alphabet but the "words" in DNA are far more
complex than the words in an Encyclopedia.
Human DNA consists of a chain of 3 billion nucleotides or 3 billion "letters"!! Thus, DNA is really nothing but a long string
of 4 nucleotides. Actually the
nucleotides are paired together, thus there are technically 3 billion
"pairs" of nucleotides in human DNA, meaning 6 billion individual
nucleotides. More About
DNA Visually,
DNA can be imagined to be like a very long ladder. A ladder has two sides (i.e. two long
side-rails). These two sides are joined
together by the steps, which, in a real ladder; people step on to climb the
ladder. When a person is climbing a
ladder, their hands usually grab onto the two side-rails of the ladder, and
their feet step on the rungs. When
comparing a ladder to DNA, the side-rails of the ladder can be compared to a
long sequence of alternating types of molecules. A sugar molecule and a phosphate molecule
alternate many times to create each side-rail of the ladder. In other
words, many "sugar-phosphate" molecule pairs make up each side-rail
of the DNA ladder to create two "sugar-phosphate backbones," one on
each side of the DNA molecule. For
example, human DNA has 3,000,000,000 alternating pairs of the sugar-phosphate
backbone in one long row. This is just
one side-rail of the ladder. The other
side-rail also has 3 billion alternating pairs of the sugar-phosphate backbone. The two
side-rails of DNA are designed such that the sugar molecules are across from
each other. Each of the sugar molecules
(dioxyribose) attaches to a "nucleotide"
molecule on the "step." Each
sugar molecule (on each side-rail) has a nucleotide attached to it. Because each of the side-rails attach to a
nucleotide, the "steps" of the ladder are a pair of nucleotides
because each of the two nucleotides attach to an opposite side-rail. Thus, when
there is a rung on the DNA ladder, there are four consecutive molecules. A sugar molecule on each side-rail and a
nucleotide attached to each side-rail.
The two nucleotides are bound together by hydrogen bonds. On each
side-rail, and between each sugar molecule (which is where the rungs are), is a
phosphate molecule holding the sugar molecules together on the side-rails. DNA can be
drawn like this (the hydrogen bond is not a physical object, but a type of
attraction between two molecules): (S=sugar,
P=phosphate, N=nulceotide, H=hydrogen bond) P P S-N-H-N-S P P S-N-H-N-S P P S-N-H-N-S P P S-N-H-N-S P P S-N-H-N-S P P ... The two
alternating P and S molecules are the two side-rails of the DNA ladder. The S-N-H-N-S combination
constitute a rung. There are
four different kinds of nucleotides (which are also referred to as
"bases"): 'A' is for
Adenine 'T' is for
Thymine 'C' is for
Cytosine 'G' is for
Guanine Substituting
one of these nucleotide bases for each 'N' in the above chart, we might see
this sequence: P P S-A-H-T-S P P S-T-H-A-S P P S-C-H-G-S P P S-G-H-C-S P P S-C-H-G-S P P ... Note in the
above graph that if an 'A' attaches to one sugar molecule, on one side-rail of
the DNA, a 'T' is supposed to attach to the opposite sugar molecule, on the
other side-rail of the ladder. These two
molecules are bound together by hydrogen bonds. Likewise,
if a 'T' attaches to one sugar molecule, on one side of the DNA, an 'A' attaches to the opposite sugar molecule, on the other
side of the ladder. In other
words, an 'A' and a 'T' should always be joined together on the same rung. Likewise, a
'C' and a 'G' should always be joined together to form a single rung of the
ladder. Adenine
actually forms two hydrogen bonds with thymine.
Guanine forms three hydrogen bonds with cytosine. These hydrogen bonds, along with the two
nucleotides, are referred to as "base-pairing." Now we can
take the above chart and improve on it ('2H' means two hydrogen bonds and '3H'
means three hydrogen bonds): P P S-A-2H-T-S P P S-T-2H-A-S P P S-C-3H-G-S P P S-G-3H-C-S P P S-C-3H-G-S P P ... Because if
we see an 'A' on one side of the rung, we know that there is a 'T' on the other
side of the rung, generally scientists only talk about one side of the
"ladder," and thus when the reader sees an 'A' in a chart, they are supposed
to mentally supply the 'T' on the other side.
Or if the reader sees a 'T" they are supposed to understand that an
'A' is on the other side. Ditto for 'G'
and 'C'. Thus, the
entire chart above would normally be designated as simply: ATCGC "Unzipping DNA" In the
process of protein synthesis, which will be discussed in the next chapter, DNA
is "unzipped." What does that
mean? What it means is that all of the
hydrogen bonds are broken and the DNA, instead of being one ladder with two
sides, becomes two ladders with one side.
For example, the above diagram would look like this: P
P S-A
T-S P
P S-T
A-S P
P S-C
G-S P
P S-G
C-S P
P S-C
G-S P
P ... Once
"zipped apart," the two half-sides of DNA can be built by new
nucleotides either to make two complete DNA strands (which is how one DNA
molecule is converted into two DNA molecules) or a side can be used to make
RNA, which will be discussed later. More Terminology As
mentioned above, when describing the sequence of nucleotides in a DNA strand,
scientists generally only talk about the nucleotides which are connected to one
side-rail of the ladder. Thus, when
you see something like this (the space is simply to make it easier to view): ATCGCATCTG
GGAAGCTACC these
"letters" represents the consecutive nucleotides on one side of the ladder.
The reader is expected to fill in the other side, if needed. The above DNA
section can be represented without showing the hydrogen bonds, and it can be
shown like this (a '-' represents a phosphate molecule and a '=' represents a
sugar molecule on the side of the ladder): -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- A T C G C A
T C T G G G A A G C T A C C T A G C G T
A G A C C C T T C G A T G G -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- A DNA
molecule is a double-helix (a 'helix' means a spiral shape), meaning it is
looks like a very, very long ladder that has been twisted like a spiral
staircase. When
"counting" how many nucleotides there are in a DNA strand, generally
only one side is counted, as mentioned above.
Thus, when it is said that human DNA has about 3,000,000,000
nucleotides, they are really talking about 3,000,000,000 nucleotide pairs, meaning they are really
talking about 6,000,000,000 nucleotides in a single human DNA. The human
body has about 100 trillion cells. Each
cell has a complete DNA strand inside its cell nucleus. Exceptions to this rule are eggs, sperm and
red blood cells. Thus, your body
contains about 100 trillion DNA molecules, each of which has 6,000,000,000
nucleotides (counting the nucleotides on both sides of the DNA). "If the chromosomes in one of
your cells were uncoiled and placed end to end, the DNA would be about 6 feet
long. If all the DNA in your body were connected in
this way, it would stretch approximately 67 billion miles! That's nearly
150,000 round trips to the Moon." The New Genetics Chapter 1 But the
most amazing thing about human DNA is not its shape or size, but what it does. The
sophistication of what human DNA accomplishes is literally beyond the ability
of the human mind to comprehend. As if that
weren't enough, every cell in our body also has multiple mitochondria. Mitochondria have their own DNA, though the
DNA in mitochondria is very short compared to the very, very long main DNA of
the cell. But
mitochondria are not the only organelle in the human cell that has its own
short DNA. This
discussion of DNA is very primitive. It
is like describing the space shuttle as a "big grey tube with
wings." People get PhDs and Nobel
Prizes for discovering things about DNA. RNA An RNA
molecule has only a single side-rail.
RNA is much like one of the unzipped DNA molecules above. Each sugar on this one side-rail has a
nucleotide attached to it. The sugar in
the side-rail of RNA is a different combination of sugars than that found in
DNA. In RNA, the
four nucleotides are: 'A' is for
Adenine 'U' is for
Uracil (replaces Thymine) 'C' is for
Cytosine 'G' is for
Guanine Thus, an
RNA strand might include this segment: -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- A A U G C A U C U G G G A A G C U A C C RNA can
also be looped. RNA has
many functions, and thus there are many different kinds of RNA. DNA Replication Due to the
way that DNA is designed, it is easy (from a visual perspective) to understand
how DNA is replicated (i.e. how one DNA strand is converted into two DNA
strands). Now instead
of looking at DNA as a ladder, let us think of it as a very long zipper. In DNA
replication, the DNA is "unzipped," as shown above, and the DNA is
broken into two half-DNA strands, which look like RNA, but have a different set
of nucleotides. Each
half-strand consists of one side-rail and the nucleotide that is associated
with the sugar molecule on that side-rail. A process
essentially takes each of the
two half-DNA strands, and builds a new, complete DNA strand by combining new
nucleotides with each half; by matching an A with a T, a T with an A, etc. So if we
follow a single rung, after unzipping the DNA, suppose there is an A on one
rung. Attached to the A will be a T
(which will be on the new rung). The other
side of the unzipped DNA will have a T on that rung. Thus, an A will added
to the new rung. This
results in a single DNA molecule becoming two identical copies of the original
DNA strand. Using the
above unzipped DNA section, this is what it would look like: P P P P S-A-T-S S-A-T-S P P P P S-T-A-S S-T-A-S P P P P S-C-G-S S-C-G-S P P P P S-G-C-S S-G-C-S P P P P S-C-G-S S-C-G-S P P P P ... Note that
the two DNA sequences are identical. If the
reader has any problems understanding this chapter they are encouraged to look
at a biology book to get more of a graphical
understanding of what is going on. |