Chapter 4 • Lesson 21
The Structure and Role of RNA
Objectives: 3,1.1,3.1.2,4,1,2
Key terms
RNA • protein synthesis • transcription • messenger RNA (mRNA) • codon • translation transfer RNA (tRNA) •
anticodon • ribosomal RNA (rRNA) • gene expression
Getting the Idea
An organism's traits, or characteristics, are determined by its proteins. Recall that proteins
are large molecules made up of smaller units called amino acids. The combination and
arrangement of the amino acids determine a protein's form and purpose. The instructions
for protein production are carried in DMA. RNA (ribonucleic acid) copies this information
and transports it to the ribosomes, where proteins are made.
The Structure of RNA
RNA and DNA share some characteristics. Both molecules are nucleic acids that are
composed of many linked nucleotides. The nucleotides of RNA, like those of DNA, are
composed of a sugar, a nitrogenous base, and a phosphate group. The RNA nucleotides
are shown below.
RNA Nucleotides
The illustration shows that the nucleotides in RNA differ from those
of DNA in two important ways. First, the sugar in RNA is ribose.
Ribose is a five-carbon sugar that has one more oxygen atom than
deoxyribose (the sugar in DNA). The second difference between
RNA and DNA is in the nitrogenous bases. RNA molecules contain
a nucleotide called uracil instead of thymine. When an RNA
molecule pairs with another nucleic acid molecule, uracil pairs with
adenine.
The shapes of RNA and DNA also differ. Recall that a DNA
molecule is a double helix, or twisted ladder. RNA usually exists as
one strand twisted into a single helix, as shown.
Making Proteins
Recall that proteins are assembled on ribosomes, which are located in the cytoplasm of a
cell. However, the instructions for making proteins are in the nuclear DNA. RNA carries the
information needed to make proteins from the nucleus to the ribosomes. RNA molecules
then direct and carry out the processes that make proteins.
Protein synthesis is the complete process by which cells make proteins. Protein synthesis
occurs in two stages: transcription and translation. These stages use different types of RNA.
Cells contain three main kinds of RNA: messenger RNA (mRNA), transfer RNA (tRNA), and
ribosomal RNA (rRNA).
Transcription
The instructions in DNA are encoded in the nucleotide bases. The nucleotides are arranged
in triplets, or groups of three. Because there are four possible nucleotide bases, there are
64 possible triplets (4 x 4 x 4). The code for making a protein is passed from the DNA to an
RNA molecule during transcription. Transcription uses DNA as a template, or pattern, to
make a complementary strand of RNA. Transcription is similar to DNA replication, with two
major exceptions. First, transcription produces only one strand of nucleotides. Second,
uracil replaces thymine.
To produce a protein, a cell releases chemical signals that start transcription. Recall that
DNA consists of two linked strands of nucleotide bases. Unlike in DNA replication, in which
the entire double helix separates, during transcription only the portion of the DNA to be
used as a template is separated. Complementary nucleotides of RNA are added to this
portion of the DNA strand. The result is a piece of RNA called messenger RNA (mRNA).
Once the mRNA strand is completed, the DNA strands used as a template reconnect.
The mRNA strand then carries the instructions from the nucleus to the cytoplasm, where
they will be translated. The instructions in mRNA are arranged in codons. A codon is a
group of three consecutive nitrogenous bases that either codes for a specific amino acid or
marks the end of a protein. Recall that amino acids are the building blocks of proteins.
The chart above shows all 64 codons and the amino acids they code for. To use the chart,
find each letter in the codon along the designated side. Consider CGU. First find the C block
under "First Base" along the left side of the chart. That narrows your choices to the second
row. Next, find the G block across the top under "Second Base." This points you to the
fourth block of the second row. Finally, find the U in the "Third Base" column on the right
side of the chart. This example shows that CGU codes for the amino acid arginine.
Notice that there are 64 codons but only 20 different amino acids. These amino acids can
be combined in a multitude of ways to form many different proteins. The order of the amino
acids determines a protein's shape and function. Three codons—UAA, UAG, and UGA—
indicate the end of the instructions for a protein. These are called stop codons.
Translation
Translation is the conversion of the information in the mRNA into a sequence of amino
acids that make up the specified protein. Translation begins after the mRNA moves from the
nucleus into the cytoplasm. The mRNA joins with a ribosome, where the information is
translated from nucleotide codons to amino acids.
Transfer RNA (tRNA) carries amino acids to the mRNA, where they are joined together to
make proteins. In order to translate the code, mRNA codons must join with the correct
anticodon sites on the tRNA. An anticodon is a set of three nitrogenous bases on a tRNA
molecule with bases that are complementary to a codon on an mRNA molecule. The
sequence of bases in an anticodon is specific to the amino acid it carries.
The ribosome's job is to assemble amino acids into polypeptide chains, which are in turn
combined into proteins. Ribosomal RNA (rRNA) is RNA that makes up part of the structure
of a ribosome and assists in translation.
During translation, up to two tRNA molecules are connected to the mRNA at a time. Each
tRNA molecule carries an amino acid. Once the amino acid carried by one tRNA molecule
bonds to the growing amino acid chain, the anticodon of another tRNA molecule connects
to the next codon on the mRNA. Peptide bonds form between the successive amino acids
to form the polypeptide chains.
The first amino acid then disconnects from the tRNA molecule. Once it has released its
amino acid, the tRNA molecule separates from the mRNA. The depleted tRNA molecule
then bonds with a free-floating amino acid in the cytoplasm. It can now be used again in
translation. The ribosome continues to attach amino acids along the mRNA strand until it
reaches a stop codon.
Gene Expression
Recall that a gene is a section of a chromosome that codes for a specific trait. The main
function of genes is to control the production of proteins. The process by which the
information carried in genes is used to make proteins or direct other cell activities is called
gene expression.
Multicellular organisms can be made up of millions or even trillions of cells. In most cases,
all or most of the cells in an organism have the same DMA. However, the cells are not
identical because of differences in how their genes are expressed. As you learned in
Lesson 6, every gene in an organism is expressed, or makes proteins. In fact, only a
fraction of the genes in a cell are expressed at any time. For example, the genes for a
protein needed in skin cells are not expressed in liver cells. A complex mechanism known
as gene regulation determines whether a gene is turned on or off in a cell. If the gene is
turned off, the protein it codes for is not produced.
Cells do not always produce the same amounts and types of proteins. Instead, the proteins
a cell produces depend in part on its environment. For example, a cell may respond to an
injury (to itself or other parts of the organism) by producing proteins to repair the injury.
Another example is the production of proteins called antibodies when a cell detects a
foreign substance in its environment.
Which proteins a cell produces is important to both the individual cell and the organism of
which a cell is a part. Producing too little or too much of a protein, or producing a protein at
the wrong time, can cause problems for an organism. For example, overproduction of some
proteins can lead to cancer. The cancer occurs when proteins disrupt the cell cycle, causing
worn or damaged cells to reproduce rather than be replaced by newer, healthy cells.