NEL Molecular Genetics 667
Gene Expression 20.220.2
As you learned in previous chapters, specific segments of DNA on a chromosome are called
genes. Genes determine the inherited characteristics, or traits, of an organism. Every
somatic (body) cell in an organism contains identical copies of DNA, and each of these
DNA copies is a genetic blueprint for the organism. Once scientists knew the structure
of DNA and how it replicated, they used this knowledge to further investigate another
question: How do the genes in DNA determine an inherited trait?
The way the information in a gene is converted into a specific characteristic or trait
through the production of a polypeptide is called gene expression. Recall that a polypep-
tide is a chain of amino acids and that proteins are made up of polypeptides. Proteins
form many structures in an organism, such as skin and muscle, and they also form all
of the enzymes in a cell.The products of all genes are polypeptides.
A second type of nucleic acid is involved in converting the instructions in a gene into
a polypeptide chain.Ribonucleic acid(RNA) is a polymer of nucleotides similar to
DNA. There are three main structural differences between RNA and DNA. First, the
sugar in RNA has an extra hydroxyl group and is called ribose rather than deoxyribose
(Figure 1). Second, instead of the base thymine found in DNA, RNA contains the base
uracil. Like thymine, uracil can form complementary base pairs with adenine (Figure 2).
Third, RNA is single-stranded and not double-stranded like DNA. There are three types
of RNA that are needed to convert genes into proteins: messenger RNA (mRNA), transfer
RNA (tRNA), and ribosomal RNA (rRNA).
gene expressionconversion of a
gene into a specific trait through
the production of a particular
polypeptideribonucleic acid (RNA)a nucleic
acid consisting of nucleotides
comprised of the sugar ribose and
nitrogenous basesThe Central Dogma
There are two main stages of gene expression, transcription and translation. In
transcription, the genetic information is converted from a DNA sequence into
messenger RNA(mRNA). In all cells, the mRNA carries the genetic information from
the chromosome to the site of protein synthesis. In eukaryotic cells, which contain a
nucleus, the mRNA carries the genetic information from the nucleus to the cytoplasm
as it passes through the pores in the nuclear envelope.
The second stage of gene expression is translation. During translation, the genetic
information carried by the mRNA is used to synthesize a polypeptide chain.
The two-step process of transferring genetic information from DNA to RNA and then
from RNA to protein is known as the central dogma of molecular genetics (Figure 3,next
page). We will explore transcription and translation in more detail in this section. You
transcriptionthe process of
converting DNA into messenger
RNAmessenger RNA (mRNA)the
product of transcription of a gene;
mRNA is translated by ribosomes
into proteintranslationthe process of
synthesizing a specific polypeptide
as coded for by messenger RNAHO CH 2 OH
H
OH
O
ribose sugarH
H
OH
H
5 3 4 1
2 HO CH 2 OH
H
OH
O
deoxyribose sugarH
HH
H
5 3 4 1
2 Figure 1
A ribose sugar possesses an OH group (hydroxyl) on the 2carbon. The
deoxyribose sugar is missing the OH group on the 2carbon. The deoxypart
of the name deoxyribose indicates a “loss of oxygen” at position 2.
DNA AAT G C ARNA UUA C G UFigure 2
Base pairing of RNA with DNA
during transcription. Notice that
thymine does not exist in RNA but is
substituted with uracil.