Molecular Genetics ❮ 119RNA Structure and Function
Ribonucleic acid is known to the world as RNA. There are some similarities between DNA
and RNA. They both have a sugar-phosphate backbone. They both have four different
nucleotides that make up the structure of the molecule. They both have three letters in their
nickname—don’t worry if you don’t see that last similarity right away,... remember that
we have been studying these things for years. These two molecules also have their share of
differences. RNA’s nitrogenous bases are adenine, guanine, cytosine, and uracil.There is
no thymine in RNA; uracil beat out thymine for the job (probably had a better interview
during the hiring process). Another difference between DNA and RNA is that the sugar for
RNA is ribose instead of deoxyribose. While DNA exists as a double strand, RNA has a bit
more of an independent personality and tends to roam the cells as a single-stranded entity.
There are three main types of RNA that you should know about, all of which are
formed from DNA templates in the nucleus of eukaryotic cells: (1) messenger RNA
(mRNA), (2) transfer RNA (tRNA), and (3) ribosomal RNA (rRNA).Replication of DNA
Human cells do not have copy machines to do the dirty work for them. Instead, they use
a system called DNA replicationto copy DNA molecules from cell to cell. As we discussed
in Chapter 9, this process occurs during the S-phase of the cell cycle to ensure that every
cell produced during mitosis or meiosis receives the proper amount of DNA.
The mechanism for DNA replication was the source of much debate in the mid-1900s.
Some argued that it occurred in what was called a “conservative” (conservative DNA repli-
cation) fashion. In this model, the original double helix of DNA does not change at all; it
is as if the DNA is placed on a copy machine and an exact duplicate is made. DNA from
the parent appears in only one of the two daughter cells. A different model called the semi-
conservative DNA replicationmodel agrees that the original DNA molecule serves as the
template but proposes that before it is copied, the DNA unzips, with each single strand
serving as a template for the creation of a new double strand. One strand of DNA from the
parent goes to one daughter cell, and the second parent strand to the second daughter cell.
A third model, the dispersive DNA replication model,suggested that every daughter
strand contains someparental DNA, but it is dispersed among pieces of DNA not of parental
origin. Figure 11.3 is a simplistic sketch showing these three main theories. Watson and
Crick would not be pleased to see that we did not draw the DNA as a double helix... but
as long as you realize this is not how the DNA truly looks, the figure serves its purpose.
An experiment performed in the 1950s by Meselson and Stahl helped select a winner
in the debate about replication mechanisms. The experimenters grew bacteria in a medium
containing^15 N (a heavier-than-normal form of nitrogen) to create DNA that was denser
than normal. The DNA was denser because the bacteria picked up the^15 N and incorpo-
rated it into their DNA. The bacteria were then transferred to a medium containing normal(^14) N nitrogen. The DNA was allowed to replicate and produced DNA that was half (^15) N and
half^14 N. When the first generation of offspring replicated to form the second generation of
offspring, the new DNA produced was of two types—one type that had half^15 N and half
(^14) N, and another type that was completely (^14) N DNA. This gave a hands-down victory to
the semi-conservative theory of DNA replication. Let’s take a look at the mechanism of
semi-conservative DNA replication.
During the S-phase of the cell cycle, the double-stranded DNA unzips and prepares to
replicate. An enzyme called helicaseunzips the DNA just like a jacket, breaking the hydrogen
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