54 BIOCHEMISTRY FUNDAMENTALS
RNA (mRNA) codons needed to synthesize a complete protein. Genes may
be collected together in a chromosome, the DNA packaging unit in eukaryotes.
The human genome is an ordered sequence of almost 3 billion adenine,
guanine, cytosine, and thymine bases found on 46 chromosomes. Most cells
transfer genetic information DNA → RNA, but retroviruses (such as the
human immunodefi ciency virus, HIV) transmit their information RNA →
DNA. Cells have three major types of RNA. Messenger RNA (mRNA) is
produced directly from a DNA template (the sequence of nitrogenous bases
along the DNA chain) in a process called transcription. Transfer RNA (tRNA),
found in the cytosol, transfers an amino acid to the ribosome for protein syn-
thesis in the translation process. Ribosomal RNA (rRNA) is an essential
component of the ribosome, the organelle in which particles of RNA and
protein come together to synthesize proteins. The process of RNA interfer-
ence, RNAi, prevents genes from being translated into proteins. This signifi cant
“ gene - silencing ” discovery, described experimentally in 1998 by biologists
Andrew Fire and Craig Mello,^16 revolutionized genetics, transformed under-
standing of cellular behavior, and has spurred development of new medical
treatments. Fire, of Stanford University School of Medicine, and Mello, of the
University of Massachusetts Medical School, shared the 2006 Nobel Prize in
Physiology or Medicine for their work.^17
The processes of replication, transcription, and translation are illustrated
schematically in Figure 2.17 and further explained in the following
paragraphs.
When a cell divides, DNA duplicates itself in a multistep process called
replication. DNA replication takes place in a semiconservative manner,
meaning that the DNA double strand unwinds and each single strand acts as
a template for a new complementary strand. Thus, each daughter DNA strand
is half comprised of molecules from the old strand. DNA replication is neither
a passive nor spontaneous process. Many enzymes are required to unwind the
double helix and synthesize a new DNA strand. The enzyme topoisomerase,
for instance, is responsible for the initiation of DNA unwinding. Nicking a
single DNA strand releases the tension holding the helix in its coiled and
supercoiled structure. Another enzyme, helicase, requires energy in the form
of ATP to unwind the original double strand, held together by their A – T and
G – C hydrogen bonds, fi nally forming the single - stranded DNA (ssDNA) to
be replicated. DNA polymerase holoenzyme, a complex aggregate of several
different protein subunits, then proceeds along the single DNA strand, bring-
Figure 2.17 Schematic representation of replication, transcription, and translation.replicationtranscription
proteintranslation
DNA RNA