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Ch. 17: From Gene to Protein
 Mendel’s work revealed that proteins are the
link between genotype and phenotype
 Tall vs. dwarf height in pea plants was due to a
growth hormone synthesized or not; due to a
presence of an enzyme!!
 DNA directs synthesis of proteins:
 Transcription
 Translation
Beadle and Tatum
 Worked with breadmold; x-rayed and examined
mutant growth
 Discovered the function of a gene is to dictate the
production of a specific enzyme
 Restated hypothesis as one gene – one polypeptide
 However, keep in mind…some genes code for RNA that
have important functions but are not translated into
protein
Genes
 The DNA provides the instructions to make the
protein
 RNA is the link between gene and protein
 DNA codes for RNA and RNA codes for the
protein….known as the “central dogma” of biology
Transcription/Translation
 The DNA and RNA molecules are composed of
nucleotide monomers.
 When converting from DNA to RNA you are simply
transcribing the code from the language of DNA
nucleotides to RNA nucleotides
 Proteins are “written” in the language of amino acids.
 When converting from RNA to protein we are
translating from the nucleotide language to amino
acid language
RNA
 In what ways are RNA molecules different from DNA?
 RNA is single stranded
 In RNA Uracil replaces Thymine
 Nucleotides have ribose instead of deoxyribose.
 In eukaryotes RNA leaves the nucleus
What are the functions of these 4 different types of RNA?
4 types of RNA
 mRNA  takes DNA’s message out to the ribosome for
protein synthesis
 tRNA  brings amino acids to the ribosome for
protein synthesis
 rRNA  structural component of ribosomes
 snRNA  involved in RNA splicing
The Genetic Code
 mRNA strand is complementary and antiparallel
to DNA template
 RNA consists of four “letters”  A, U, G, and C
 Proteins consist of 20 “letters”  the amino acids
 If 1 RNA base codes for 1 amino acid, then only 4
amino acids can be coded for.
 How many different amino acids can be coded for if
2 RNA’s code for 1 amino acid?
 42 = 16 : Not enough!
 How many different amino acids can be coded for if
3 RNA’s code for 1 amino acid?
 43 = 64: More than enough for the 20 different
amino acids….
Codons
 mRNA base triplets are called codons
 Codons are read in the 5’3’ direction
 # of nucleotides making up the genetic message is 3x
the # of amino acids
 64 codons deciphered by mid 1960’s
 Stop codons: UAA, UGA, UAG
 Start signal and methionine: AUG
There is
redundancy
…2 codons
for one a.a.,
but not
ambiguity
…1 codon
doesn’t
code for
2+a.a.!
The Code is Universal
 The code is shared by almost all organisms
 CCG codes for what amino acid?
 Proline. This holds true for all species of living
organisms.
 Bacteria, therefore can be programmed to synthesize
human proteins by inserting human DNA
Eukaryotic Transcription
 3 steps:
 Initiation
 Elongation
 Termination
 RNA polymerases are used
 RNA pol. II used for mRNA synthesis
 RNA pol. I and III used for all other RNA (not coded into
protein)
 Direction of transcription  downstream (5’3”)
Initiation
 Signaled by a promoter
 DNA sequence is TATAAAA, called a “TATA” box
Elongation
 RNA pol. moves along DNA and untwists it 10-20 bases
at a time
 RNA nucleotides are added to 3’ end (about 60/sec in
eukaryotes)
 DNA double helix reforms as new RNA peels away
Termination
 Prokaryotes: terminator sequence on DNA causing
RNA pol. to detach and mRNA to be released
 Eukaryotes: premRNA is cleaved due to a particular
DNA sequence but needs to be processed into mRNA!
 1. 5’end cap is added
 2. 3’ tail called a poly-A tail is added
In prokaryotes, RNA is directly translated into the
polypeptide
RNA in eukaryotes is processed before
translation
 The function of the cap is:
 prevent mRNA degradation by hydrolytic enzymes
 helps attach to the ribosome
 Function of the 3’ tail:
 same functions as the 5’cap
 also helps facilitate export of mRNA from nucleus
RNA splicing
 Removes non-coding regions (introns)
 snRNP (short nuclear ribonucleoproteins) recognize
the splicing signals that are at the ends of introns
 The RNA in the snRNP is called snRNA (small
nuclear RNA)
 snRNP + protein = spliceosome
 The spliceosome cuts and releases the introns, and
then joins exons together
Evolutionary role of introns
 Introns may play regulatory role
 Different intron removal may lead to different proteins
 Introns may enhance crossing over between
homologous regions by increasing the distance
between exons
Translation
 mRNA delivers the message in the “nucleotide
language”
 tRNA translates the message into the “amino acid
language”
 End of tRNA molecule is an anticodon…triplet,
complementary to mRNA
 Ex. mRNA  UUU; tRNA  AAA + phenylalanine
Structure and function of tRNA
 Transcribed from template DNA strand in nucleus
 Used repeatedly
 About 80 nucleotides long, single stranded with H-
bonds causing a 3D structure
Recognition steps to translation
 1. Amino acid joined to correct tRNA by
aminoacyl-tRNA synthetase…20 of those (each
specific to an individual amino acid)


This step is catalyzed ATP
The tRNA with the amino acid is known as aminoacyl tRNA
 2. Correct match between tRNA anticodon and
mRNA codon

Wobble  relaxation in the base pairing rules with 3rd base at the
3’ end of mRNA
tRNA Assembly
Messenger RNA consists of leader,
reading frame, and trailer sequences.
Ribosomes
 2 subunits (large and small)
 Constructed of protein and rRNA
 Only functional when attached to mRNA
 2/3 of ribosomal mass is rRNA (most abundant type of
RNA)
Ribosomal binding sites
 P site  peptidyl tRNA site; holds the tRNA carrying
the growing polypeptide chain
 A site  aminoacyl tRNA site; holds the tRNA
carrying the next amino acid
 E site  exit site; site where tRNAs leave the ribosome
Ribosomes, consist of two subunits, each of which contains
rRNA and ribosomal proteins…rRNA serves as the catalyst
(called a ribozyme)of peptide bond formation!
Building a polypeptide
 3 stages of translation
 Initiation
 Elongation
 Termination
Initiation
 Small ribosomal subunit binds to mRNA and initiator




tRNA carrying methionine
Small subunit scans downstream along mRNA until it
reaches start codon … AUG, establishing the “reading
frame”.
Initiator tRNA H-bonds to start codon
mRNA + initiator tRNA + small ribosomal subunit + large
subunit = translation initiation complex … requires
proteins called initiation factors and energy in the form of
GTP
Proteins synthesized from N-terminus  C-terminus
Elongation

Proteins called elongation factors are required to
add new amino acids to preceding ones
GTP required
Ribosomes moves along mRNA in the 5’  3’
direction



3 steps to elongation
1.
2.
3.
Codon recognition
Peptide bond formation
Translocation (moving along A, P, E sites)
Termination
 Protein called release factor binds to stop codon in the
A site bringing in a water molecule instead of an
amino acid
 Polypeptide is released through the exit tunnel of the
ribosome’s large subunit
 Translation assembly comes apart
Initiation of
Translation
Elongation
Termination
Polyribosomes  a string of ribosomes trailing along one
mRNA to make many copies of a polypeptide very quickly