Gene Expression and Control
Chapter 7
Part 1
7.1 Impacts/Issues
Ricin and Your Ribosomes
 The ability to make proteins is critical to all life
processes – ricin kills because it inactivates
ribosomes that assemble proteins
7.2 The Nature of Genetic Information
 DNA carries all the genetic information needed
to build a new individual
• Genetic information consists of base sequences
• Genes are subunits of that sequence
 Gene
• Part of a DNA base sequence
• Specifies structure of an RNA or protein product
From Gene to RNA to Protein
 Gene expression involves transcription (DNA to
RNA), and translation (mRNA, or messenger
RNA, to protein)
 Gene expression
• Process by which the information in a gene
becomes converted to an RNA or protein product
Transcription
 A gene’s nucleotide base sequence encodes
instructions for building an RNA or protein
product
 A cell transcribes the base sequence of a gene
into mRNA
 mRNA carries a protein-building message
Transcription
 Transcription
• Process by which an RNA is assembled from
nucleotides using the base sequence of a gene
as a template
 Messenger RNA (mRNA)
• Type of RNA that has a protein-building message
Translation
 Translation requires the participation of tRNA
(transfer RNA) and rRNA (ribosomal RNA)
 Translation
• Process by which a polypeptide chain is
assembled from amino acids in the order
specified by an mRNA
RNA and DNA Nucleotides
base
(guanine)
3 phosphate groups
sugar
(ribose)
An RNA nucleotide: guanine (G),
or guanosine triphosphate
Fig. 7-2a, p. 117
base
(guanine)
3 phosphate groups
sugar
(deoxyribose)
A DNA nucleotide: guanine (G), or
deoxyguanosine triphosphate
Fig. 7-2b, p. 117
7.3 Transcription: DNA to RNA
 Base-pairing rules in DNA replication also apply
to RNA synthesis in transcription, but RNA uses
uracil in place of thymine
The Process of Transcription
 In transcription, RNA polymerase binds to a
promoter in the DNA near a gene
 RNA polymerase
• Enzyme that carries out transcription
 Promoter
• In DNA, a sequence to which RNA polymerase
binds
The Process of Transcription
 Polymerase moves along the DNA, unwinding
the DNA so it can read the base sequence
 RNA polymerase assembles a strand of RNA by
linking RNA nucleotides in the order determined
by the base sequence of the gene
 The new mRNA is a copy of the gene from which
it was transcribed
Transcription: DNA to RNA
RNA
polymerase
gene region
promoter sequence in DNA
1 RNA polymerase binds to a promoter in the DNA. The binding
positions the polymerase near a gene. In most cases, the base sequence
of the gene occurs on only one of the two DNA strands. Only the DNA
strand complementary to the gene sequence will be translated into RNA.
Fig. 7-3a, p. 118
RNA
DNA winding up
DNA unwinding
2 The polymerase begins to move along the DNA and unwind it. As it
does, it links RNA nucleotides into a strand of RNA in the order specified
by the base sequence of the DNA. The DNA winds up again after the
polymerase passes. The structure of the “opened” DNA at the
transcription site is called a transcription bubble, after its appearance.
Fig. 7-3b, p. 118
direction of
transcription
3 Zooming in on the gene region, we can see that RNA polymerase covalently
bonds successive nucleotides into an RNA strand. The base sequence of the new
RNA strand is complementary to the base sequence of its DNA template strand,
so it is an RNA copy of the gene. Figure It Out: After the guanine, what is the next
nucleotide that will be added to this growing strand of RNA?
Answer: Another guanine (G)
Fig. 7-3c, p. 119
Gene transcription details
Three Genes, Many RNA Polymerases
 Many polymerases can transcribe a gene region
at the same time
Pre-mRNA transcript processing
Transcription
Transcription
7.4 RNA Players in Translation
 Three types of RNA are involved in translation:
mRNA, rRNA, and tRNA
 mRNA produced by transcription carries proteinbuilding information from DNA to the other two
types of RNA for translation
mRNA and the Genetic Code
 The information in mRNA consists of sets of
three nucleotides (codons) that form “words”
spelled with the four bases A, C, G, and U
 Codon
• In mRNA, a nucleotide base triplet that codes for
an amino acid or stop signal during translation
mRNA and the Genetic Code
 Sixty-four codons, most of which specify amino
acids, constitute the genetic code
• 20 amino acids in proteins; most have more than
one codon
 Genetic code
• Sixty-four mRNA codons; each specifies an
amino acid or a signal to start or stop translation
The Genetic Code
Animation: Genetic code
Translating mRNA to Amino Acids
rRNA and tRNA – the Translators
 Ribosomes and transfer RNAs (tRNA) interact to
translate an mRNA into a polypeptide
 Ribosomes consist of two subunits of rRNA and
structural proteins
 Ribosomal RNA (rRNA)
• A type of RNA that becomes part of ribosomes
Ribosomes
 During translation, one large and one small
ribosomal subunit (rRNA) converge as a
ribosome on an mRNA
 rRNA reads the mRNA and acts as an enzyme
to form peptide bonds between amino acids,
assembling them into a polypeptide chain
A Ribosome
tRNA
 tRNAs deliver amino acids to ribosomes in the
order specified by mRNA
 Transfer RNA (tRNA)
• Type of RNA that delivers amino acids to a
ribosome during translation
tRNA
 Each tRNA has two attachment sites
• An anticodon that can base-pair with a codon
• A site that binds to the kind of amino acid
specified by the codon
 Anticodon
• Set of three nucleotides in a tRNA
• Base-pairs with mRNA codon
tRNA for Tryptophan
anticodon
amino acid
attachment site
Fig. 7-7a, p. 121
Structure of a tRNA
7.5 Translating the Code: RNA to Protein
 Translation, the second part of protein synthesis,
occurs in the cytoplasm of all cells
 Translation is an energy-requiring process that
converts the protein-building information carried
by an mRNA into a polypeptide
Three Stages of Translation
 Initiation
• mRNA joins with an initiator tRNA and two
ribosomal subunits
 Elongation
• Ribosome joins amino acids delivered by tRNAs
in the order specified by mRNA codons
 Termination
• Polymerase encounters a stop codon; mRNA and
polypeptide are released; ribosome disassembles
Elongation
start
codon
(AUG)
initiator
tRNA
first amino
acid of polypeptide
peptide bond
Stepped Art
p. 122-123
Polysomes
 In cells making a lot of protein, many ribosomes
may simultaneously translate the same mRNA
 Polysome
• Cluster of ribosomes that are simultaneously
translating an mRNA
Translation in Eukaryotes
1
Transcription
2
RNA
transport
ribosome
subunits
tRNA
5 Polysomes
3 Convergence of RNAs
mRNA
4 Translation
polypeptide
Fig. 7-8, p. 122
Animation: Translation
The major differences between prokaryotic and
eukaryotic protein synthesis
Overview of transcription and translation