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RNA
Chapter 13
Learning Objectives
 Contrast RNA and DNA.
 Explain the process of transcription.
 Text video
 Complete Study Guide
RNA carries DNA’s instructions.
•
The central dogma states that
information flows in one
direction from DNA to RNA to
proteins.
 DNA is the genetic material that carry a code
(genes)
 Genes carry instructions for making proteins
 The first step in making a protein is making a
copy of the instructions
 RNA is the copy of the instructions
Comparing RNA and DNA
The sugar in RNA is ribose instead of
deoxyribose.
 single-stranded, not double-stranded.
RNA contains uracil in place of thymine.
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 (ribosome).
Ribosomal RNA (rRNA)
• Protein synthesis occurs on ribosomes, which
are made up of two subunits.
• rRNA makes up part of the ribosome.
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.
Steps in Transcription
1. RNA polymerase binds to DNA and unzips
DNA beginning at the gene (one strand acts a
template)
2. Free nucleotides pair with their
complementary bases on the exposed DNA
template
3. RNA polymerase continues until it reached the
terminator sequence and stops
4. mRNA is released and goes to the ribosome
5. DNA helix reforms.
RNA Synthesis: RNA Editing
New RNA molecules sometimes require a bit of editing
before they are ready to be read.
Exons
Cap
Introns
Tail
Transcription video
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.
 Complete 13-2 Outline/Study Guide
 The first step in decoding the message
(instructions) is to transcribe the base sequence
form DNA to RNA
 DNA carries the instructions for making proteins
(polypeptides)
 Proteins have specific amino acid sequences
 RNA carries this code to the ribosome for
translation (the second step in protein
synthesis)
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 codons. (a group of three
nucleotide bases in mRNA that specifies a
particular amino acid)
AUG
AAC
UCU
Genetic Code Table
There are 64 possible three-base codons in the genetic
code.
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.
UAA, UAG,
and UGA
are “stop”
codons
AUG =
methionine =
“start” codon
Translation
 Transcribed mRNA directs the translation
process.
•
Translation is the process that produces proteins by
decoding the sequence of mRNA codons.
Translation
Steps of Translation
1. Ribosome attaches to an mRNA molecule.
2. tRNA
molecules,
carrying
amino
acids
with them,
bind
to mRNA
codons.
anticodon
Translation: The Polypeptide Assembly
3. The ribosome helps form a peptide bond. It breaks the
bond holding the first tRNA molecule to its amino acid.
Translation: Completing the Polypeptide
4. 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.
Gene Expression
When a gene (segment) of DNA code is used to build a protein,
scientists say that gene has been expressed.
Mutations
Learning Objectives
 Define mutations and describe the different types of
mutations.
 Describe the effects mutations can have on genes.
 Transcription/translation video
 Game
 What happened when the genetic message or instructions
change?
 Mistakes can happen in copying (wrong, missing, or extra
bases)
Mutations
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
-insertions and deletions are called frameshift mutations
because the shift the reading frame of the genetic message
• change in amino acid sequence
• Can cause change in the protein
Chromosomal Mutations
Involve changes in the
number of chromosomes.
Types
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
environment (mutagens)
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
Diseases caused by a point mutaton
Effects of Mutations: Beneficial
Mutations often produce proteins with new or altered functions
that can be useful to organisms in different or changing
environments.
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
organisms.
Prokaryotic Gene Regulation
DNA-binding proteins in prokaryotes regulate genes
by controlling transcription.
-can help them respond to
changing environments by
turning genes off and on
when needed
Eukaryotic Gene Regulation
- Gene expression is more complicated.
Genetic Control and Development
Cell Specialization
Complex gene regulation in eukaryotes is what makes
differentiation so the can specialize in structure and
function.
Genetic Control of Development
Regulating gene expression
is important in shaping how a
multicellular organism develops.
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
development.
 Homeobox genes
code for transcription
factors.
 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.