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Chapter 13 RNA

Learning Objectives
 Contrast RNA and DNA.
 Explain the process of transcription.
Comparing RNA and DNA
The sugar in RNA is ribose instead of deoxyribose.
RNA is generally single-stranded, not double-stranded.
RNA contains uracil in place of thymine.
What can be found in both….
Phosphate groups, guanine, and cytosine
The Role of RNA
The roles played by DNA and RNA molecules in directing protein
production are like the two types of plans builders use:
 A master plan
 A blueprint
Types of RNA
The three main types of RNA are:
Transfer RNA
Ribosomal RNA
Messenger RNA
Messenger RNA
 An mRNA molecule is a copy of the portion of DNA that will be
used to make a protein.
 After being made in the nucleus, mRNA travels to the
cytoplasm, the site of protein synthesis.
 The complementary strand of RNA that is produced during
transcription is called MRNA
Ribosomal RNA
 Protein synthesis occurs on ribosomes, which are made up of
two subunits.
 These subunits consist of several molecules of ribosomal RNA
(rRNA) and as many as 80 different proteins.
Transfer RNA
During protein synthesis, transfer RNA molecules (tRNA)
carry amino acids from the cytoplasm to the mRNA.
RNA Synthesis: Transcription
• In transcription, segments of DNA serve as templates to
produce complementary mRNA molecules.
• Takes place in the nucleus
RNA Synthesis: Promoters
RNA polymerase binds only to regions of DNA that have specific
base sequences. These regions are called promoters.
RNA Synthesis: RNA Editing
New RNA molecules sometimes require a bit of editing
before they are ready to be read.
The pieces of
molecules that
are cut out, or
“edited out,”
and discarded
are called
pieces are
known as
Ribosomes and Protein Synthesis
Learning Objectives
 Identify the genetic code and explain how it is read.
 Summarize the process of translation.
 Describe the central dogma of molecular biology.
The Genetic Code
RNA has four bases: adenine, cytosine, guanine, and uracil.
These bases form a “language”: A, C, G, and U.
The Genetic Code: Codons
 The genetic code is read in three-letter groupings called
 A codon is a group of three nucleotide bases in
messenger RNA that specifies a particular amino acid.
Genetic Code Table
There are 64 possible three-base codons in the genetic
Several different codons can specify the same amino acid
Reading Codons
Start at the middle of the circle with the first letter of the codon and
move outward.
CAC = Histidine
Start and Stop Codons
The methionine codon AUG serves as the “start” codon for
protein synthesis. There are three “stop” codons.
and UGA
are “stop”
methionine =
“start” codon
 Transcribed mRNA directs the translation process.
 Translation is the process that produces proteins by
decoding the sequence of mRNA codons.
 During translation, the type of amino acid that is added to
the growing polypeptide depends on the codon on the
mRNA and the anticodon on the tRNA.
 Takes place in the cytoplasm
Translation: Transfer RNA
• Translation starts when a ribosome attaches to an mRNA
molecule. Then, tRNA molecules, carrying amino acids
with them, bind to mRNA codons.
• Transfer RNA is important in adding amino acids to
Translation: The Polypeptide Assembly
The ribosome helps form a peptide bond. It breaks the bond
holding the first tRNA molecule to its amino acid.
Translation: Completing the Polypeptide
Third step: The ribosome reaches a stop codon, releasing the newly
synthesized polypeptide and the mRNA molecule, completing the
process of translation.
Roles of RNA in Translation
All three major forms of RNA—mRNA, tRNA, and rRNA—are
involved in the process of translation.
The Molecular Basis of Heredity
• The central dogma of molecular biology is that information is
transferred from DNA to RNA to protein.
• An exception to the central dogma of molecular is that viruses
sometimes transfer information from RNA to DNA
Gene Expression
When a gene (segment) of DNA code is used to build a protein,
scientists say that gene has been expressed.
Learning Objectives
 Define mutations and describe the different types of
 Describe the effects mutations can have on genes.
Mutations are heritable changes in genetic information.
Types of Mutations
Mutations fall into two basic categories:
 Gene mutations
 Chromosomal mutations
Gene Mutations: Point Mutations
A point mutation is a change in a single nucleotide.
There are three types of point mutations:
Point Mutations: Substitutions
In a substitution, one base is changed to a different base.
Point Mutations: Insertions and Deletions
• Insertion mutation: when a
single extra base is added
into the code
• Deletion mutation: when a
single base is removed from
the code
• Frameshift mutations usually
produces a more drastic
change than a point mutation
Chromosomal Mutations
 Deletion
 Duplication
 Inversion
 Translocation
Effects of Mutations
Mutations can harm, help, or
have no effect on an organism.
Some mutations arise from
mutagens—chemical or
physical agents in the
Effects of Mutations: Harmful
Some of the most harmful mutations are those that dramatically
change protein structure or gene activity.
Example: Sickle cell
disease affects the
shape of red blood cells.
Sickle cell
Normal red blood cell
Effects of Mutations: Beneficial
• Mutations often produce proteins with new or altered functions
that can be useful to organisms in different or changing
• For example, when a complete set of chromosomes fails to separate
during meiosis, the gametes that result may produce triploid (3N) or
tetraploid (4N) organisms. The condition in which an organism has
extra sets of chromosomes is called polyploidy.
• Polyploid plants are often larger and stronger than diploid plants
Gene Regulation and Expression
Learning Objectives
 Describe gene regulation in prokaryotes.
 Explain how most eukaryotic genes are regulated.
 Relate gene regulation to development in multicellular
Prokaryotic Gene Regulation
DNA-binding proteins in prokaryotes regulate genes
by controlling transcription.
An operon is a group of
genes that are regulated
One of the keys to gene
transcription in bacteria is
the organization of genes
into operons.
The Lac Operon
When lactose is not
present, the lac genes are
turned off by regulatory
proteins that bind to DNA
and block transcription.
Promoters and Operators
Located in front of the operon’s three genes are two regulatory
 A promoter - an RNA polymerase binding site on a typical gene
 An operator- where a DNA-binding protein known as the lac repressor can
bind to DNA.
The Lac Repressor Blocks Transcription
When the lac repressor binds to the O region, RNA polymerase
cannot reach the lac genes to begin transcription.
Lactose Turns On the Operon
When lactose is added to the medium, it diffuses into the cell
and attaches to the lac repressor.
Repressor protein with
changed shape
Eukaryotic Gene Regulation
A typical eukaryotic gene has a TATA box binds a protein
that helps position RNA polymerase by marking a point just
before the beginning of a gene.
Transcription Factors
By binding DNA sequences in the regulatory regions of
eukaryotic genes, transcription factors control the expression
of those genes.
Cell Specialization
Complex gene regulation in eukaryotes is what makes
differentiation and specialization possible.
RNA Interference
Small RNA molecules that do not belong to any of the major
groups of RNA play a powerful role in regulating gene
They do so by interfering with mRNA.
RNA Interference
The small interfering RNA molecules fold into double-stranded
hairpin loops. The dicer enzyme cuts the double strands into
microRNA (miRNA).
RNA Interference
The two strands of the loops separate.
One of the miRNA pieces attaches to a cluster of proteins,
forming a silencing complex.
RNA Interference
Blocking gene expression by means of an miRNA silencing
complex is known as RNA interference.
The Promise of RNAi Technology
The discovery of RNAi has made
it possible for researchers to
switch genes on and off at will,
simply by inserting doublestranded RNA into cells.
It may provide new ways to treat
and perhaps even cure diseases.
Genetic Control of Development
• Regulating gene expression
is important in shaping how a
multicellular organism
• Transcription factors regulate
the expression of most
eukaryotic genes
• Each of the specialized cell
types found in the adult
originates from the same
fertilized egg cell.
Homeotic, Homeobox, and Hox Genes
 Homeotic genes
regulate organ
 Homeobox genes
code for transcription
 Hox genes determine
the identities of each
body segment.
Environmental Influences
Environmental factors can
affect gene regulation.
Metamorphosis is an
example of how organisms
can regulate gene
expression in response to
change in their environment.