Chapter 17
From Gene to Protein
17.1 – Genes specify proteins via
transcription & translation
 Gene Expression
 DNA directs the synthesis of proteins (or RNA)
 Includes translation & transcription
 Proteins are the links between genotype and phenotype
 One gene-one polypeptide hypothesis
 Each gene codes for a polypeptide
 Can be a protein or part of a protein
 Messenger RNA (mRNA)
 Produced during transcription
 Carries the genetic message of DNA to the protein
making machinery of the cell (ribosome)
 In Eukaryotes
 Transcription results in pre-mRNA, which undergoes
RNA processing to yield the final mRNA
 In Prokaryotes
 Transcription directly makes mRNA
 Transcription & Translation occur at the same time
Transcription
 Synthesis of RNA using DNA as a template
 Occurs in the nucleus
 Only one strand of DNA is transcribed (called
template strand)
 The mRNA produced is a complementary strand
 The mRNA base triplets are called codons
 Written in the 5’ to 3’ direction
 The genetic code is redundant
 More than one codon codes for the 20 Amino Acids
 Read based on a consistent reading frame
 Groups of 3 must be read in the correct groupings in
order for translation to be successful
 All 64 codons were deciphered by the mid-1960s
 Of the 64 triplets, 61 code for amino acids; 3 triplets
are “stop” signals to end translation
Translation
 Production of a polypeptide chain using mRNA
 Occurs at the ribosomes
 The instructions for the PP chain are written as a
triplet code
 The genetic code is nearly universal, shared by the
simplest bacteria to the most complex animals
 Genes can be transcribed and translated after being
transplanted from one species to another
17.2 – Transcription is the DNA-directed
synthesis of RNA
 RNA polymerase
 Enzyme that separates the two DNA strands
 Connects the RNA nucleotides as they base-pair
 Can add RNA nucleotides only to the 3’ end so it
elongates in the 5’ to 3’ direction
 Uracil replaces thymine
 Promoter
 DNA sequence that RNA polymerase attaches
 Terminator
 DNA sequence that signals the end of transcription
 Transcription unit
 Entire stretch of DNA that is transcribed into RNA
 May code for a polypeptide or an RNA such tRNA or
rRNA
3 stages of transcription
 1) Initiation
 2) Elongation
 3) Termination
1) Initiation
 In bacteria
 RNA polymerase recognizes & binds to the promoter
 In Eukaryotes
 RNA polymerase II cannot bind to the promoter
without supporting help from proteins known as
transcription factors
 Transcription Factors
 Assist the binding of RNA polymerase to the promoter,
& the initiation of transcription
 Transcription initiation complex
 The whole complex of RNA polymerase II &
transcription factors
 A promoter called a TATA box is crucial in forming
the initiation complex in eukaryotes
2) Elongation
 RNA polymerase moves along the DNA (untwists the
double helix)
 10 to 20 bases at a time
 RNA nucleotides are continually added to the 3’ end
of the growing chain
 40 nucleotides per second
 As the complex moves down the DNA strand, the
double helix re-forms with the new RNA molecule
straggling away from the DNA template
3) Termination
 RNA transcript is released & the polymerase detaches
upon transcribing a terminator sequence in the DNA
17.3 – Eukaryotic cells modify RNA after
transcription
 Modifications to RNA after transcription:
 Adding a 5’ cap & a poly-A tail
 Facilitate the export of mRNA from the nucleus
 Help protect mRNA from degradation by enzymes
 Facilitate the attachment of the mRNA to the ribosome
 RNA splicing (in Eukaryotic Cells)
 Large portions of the newly made RNA strand are
removed – called INTRONS
 The ones left behind are called EXONS & are spliced
together by a spliceosome
 Special RNA called small nuclear RNA (snRNA) aid
the spliceosomes
 Play a role by catalyzing the excision of the introns &
joining the exons
 When RNA is an enzyme it is called a RIBOZYME
17.4 – Translation is the RNA-directed
synthesis of a polypeptide
 Utilizes mRNA, tRNA, & rRNA
 tRNA
 Transfers AA from a pool of AA in the cytoplasm to a
ribosome
 The ribosome accepts the AA & adds it into a growing
PP chain
 Each tRNA is specific for an AA
 One one end, it binds to the AA & the other end has a
triplet called an anticodon which allows it to pair with a
codon on an mRNA
 Codon – mRNA triplet (there are 64)
 mRNa is read codon by codon & one AA is added to
the chain for each codon read
 The rules for base-pairing between the third base of a
codon & the corresponding tRNA anticodon are not
as strict as DNA & mRNA so it is called a wobble
 rRNA complexes with proteins
 Forms the 2 subunits that form ribosomes
 Ribosomes have 3 binding sites for tRNA
 P-site – holds the tRNA that carries the growing PP
chain
 A-site – holds the tRNA that carries the AA that will be
added next
 E-site – exit site for tRNA
3 stages of Translation
 1) Initiation
 2) Elongation
 3) Termination
Initiation
 A) a small ribosomal subunit binds to mRNA in a way
that the first codon of the mRNA strand (AUG) is
placed in the proper position
 B) tRNA with the anticodon UAC (carries the AA
methionine), hydrogen bonds to the first codon
(proteins called initiation factors aid)
 C) Large subunit of ribosome attaches
 Allows the tRNA with methionine to attach to the P-
site
 The A-site will now be available for the next tRNA with
the 2nd AA
2) Elongation
 A) Codon Recognition

The codon in the A-site is matched by the incoming tRNA
anticodon
 B) Peptide bond formation

The incoming AA in the A-site forms a peptide bond with
the existing chain of AA held in the P-site

Catalyzed by an rRNA (ribozyme)
 C) Translocation

Occurs when tRNA in the A-site is moved to the P-site & the
tRNA in the P-site is moved to the E-site

A-site is now clean and is ready for another AA
3) Termination
 A stop codon in the mRNA is reached & translation
stops
 A protein called release factor binds to the stop codon
& the PP is freed from the ribosome
 PP’s then will fold to assume their specific shape
 May be modified further to make them functional
 The destination of the protein is determined by the
sequence of about 20 AA’s at the leading end of the
PP chain (signal peptide)
17.5 – Point mutations can affect structure &
function
 Point mutation are alterations of just one base pair – 2
basic types:
 1) Base-pair substitutions
 The replacement of one nucleotide & its
complementary base pair in the DNA with another pair
of nucleotides
 Missense – enable the codon to still code for an AA
although it may not be the correct one
 Nonsense – change a regular AA codon into a stop
codon
 2) Insertions & deletions
 Additions & loses of nucleotide pairs in genes
 If they interfere with the codon groupings they can
cause a frameshift mutation
 Causes the mRNA to be read incorrectly
 Mutagens
 Substances or forces that interact with DNA in ways
that cause mutations
 X-rays & chemicals