Chapter 17
From Gene to Protein
Main Idea!
• The DNA inherited by an organism leads to
specific traits by dictating the synthesis of
proteins
• Proteins are the links between genotype and
phenotype
Study of Metabolic Defects Provided
Evidence that Genes Specify Proteins
• 1909: Garrod suggested that genes dictate
phenotypes through enzymes that catalyze
specific chemical reactions in the cells
• Symptoms of an inherited disease reflect the
person’s (in)ability to make a particular
enzyme
– Inborn errors of metabolism
• Studied alkaptnuria- body lacks enzyme to
break down alkapton
One Gene – One Enzyme
• Beadle and Tatum: worked with bread mold
• Function of a gene is to dictate the production
of a specific enzyme
One Gene – One Polypeptide
• Problem: not all proteins are enzymes
• Because proteins that are not enzymes are
gene products, we think of one gene- one
polypeptide
– Polypeptide = Proteins
Transcription and Translation Overview
• Genes provide instructions for making specific
proteins, but does not build a protein directly
• Bridge between DNA and proteins is RNA
– What are the differences between DNA and RNA?
• Transcription: synthesis of RNA under the
direction of DNA
• Translation: synthesis of proteins under the
direction of mRNA Quick Video
Transcription
• Both DNA and RNA use the same language
and the information is copied from one
molecule to another
• DNA provides the template for assembling a
sequence of RNA nucleotides
– Messenger RNA
Translation
• Change in the language, the cell must
translate the base sequence of an mRNA
molecule into the amino acid sequence of a
polypeptide
• Site is in the ribosome
Pro versus Euk
• Prokaryotes: Because bacteria lack nuclei,
their DNA is not segregated from ribosomes
and the other protein equipment
– Transcription and translation are coupled
• Eukaryotes: transcription occurs in the nucleus
and mRNA travels to the cytoplasm to join a
ribosome
– Before they leave the sequence undergoes RNA
processing and becomes the primary transcript
Triplet Codes
• Triplets of nucleotide bases are the smallest
units of uniform length that can code for all
the amino acids
– There are 20 different amino acids
• mRNA is complementary to DNA
– In RNA, A binds with U instead of T
• mRNA has a triplet code called codons that
are transcribed from DNA, always in a 5’ to 3’
direction
Cracking the Code
• 1961: Nirenberg made synthetic RNA in
repeating subunits of UUU, AAA, GGG, and
CCC to determine what amino acid it made
• Learned that the third letter of the codon can
change, but the amino acid generally stays the
same
– Wobble Theory
Evolution of Genetic Code
• Since the genetic code is the same for all living
organisms, it is safe to say that the earliest of
the ancestors shared the same vocabulary
Transcription
• RNA polymerase pries the two strands of DNA
apart and hooks together the RNA nucleotides as
they base pair along the DNA template
– Adds only in the 5’ to 3’ direction
• The promoter is the DNA sequence where RNA
polymerase attaches and initiates transcription
– TATA Box
• It stops at the terminator
• The stretch of DNA that is transcribed into RNA is
called the transcription unit
Stages of Transcription
• Initiation
– Transcription factors, a group of proteins, mediate
the binding of RNA polymerase and the initiation
of transcription, put together it is called a
transcription initiation complex
• Elongation
– The growing of the RNA strand
• Termination
– Continues until it hits a stop codon
Modify RNA After Transcription
• Enzymes modify RNA before the message is
dispatched to the cytoplasm
• Both ends of the primary transcript are
altered, and certain interior sections are cut
out and the remainders are spliced together
Alteration of mRNA Ends
• Each end is modified in a particular way
– 5’ end is immediately capped off with a modified
version of Guanine
• Cap helps protect mRNA from degradation by enzymes
and functions as an attachment point for the ribosomes
– 3’ end makes a poly(A) tail consisting of repeating
A units that facilitate the transport from the
nucleus
Split Genes and RNA Splicing
• RNA splicing
– Most genes and their RNA transcripts have long
noncoding stretches of nucleotides that are not
translated
– Most of these segments are interspersed between
coding segments
• Noncoding segments are called introns
• Coding segments are called exons (expressed)
To Cut or Not to Cut?
• Signals for RNA splicing are short nucleotide
sequences at the end of introns
• Particles called small nuclear ribonucleoproteins
(snRPS) recognize the splice sites
• Several snRPS put together with other proteins is
a splicesome which interacts with the splice sites
at the end of an intron
– Cuts the introns out and splices the remaining exons
together
Evolution of Introns
• Introns play regulatory roles in the cell
– Control gene activity in some way
• Alternative RNA splicing is where different
polypeptides are made from the same strand
of mRNA depending on what are introns and
exons
Translation
• A cell interprets a genetic message and builds
a protein accordingly, the message is a series
of codons on an mRNA molecule, and the
interpreter is tRNA (transfer RNA)
– Transfers amino acids from the cytoplasm to a
ribosome
– Has a specific amino acid at one end, and an
anticodon on the other that is complementary to
mRNA
tRNA Structure and Function
• Looks like a clover leaf with one leaf
containing the anticodon and the other end
houses the amino acid
Aminoacyl-tRNA synthetase
• Enzyme that bonds the correct amino acid to
the correct tRNA molecule
• Twenty different enzymes, one for each amino
acids
Ribosomes
• Facilitate the specific coupling of tRNA
anticodons with mRNA codons during protein
synthesis
• Made up of two subunits, called the large and
small units
– Constructed of proteins and rRNA
rRNA Binding Sites
• In addition to a binding site for mRNA, each
ribosome has three binding spots for tRNA
– P site holds the tRNA carrying the growing
polypeptide chain
– A site holds the tRNA carrying the next amino acid
to be added to the chain
– E site is where the discharged tRNA leaves the
ribosome
Building a Polypeptide
• Initiation: brings together mRNA, tRNA, and the
ribosomal subunits, looks for AUG starting at the
5’ end
• Elongation: amino acids are added one by one
– Requires elongation factors
• Codon recognition
• Peptide bond formation
• Translocation: ribosome moves down
• Termination: continues until a stop codon
reaches the A site of the ribosome
– Release factor: protein binds directly to stop codon,
adds a water molecule, and releases the polypeptide
from the ribosome
Did You Learn Anything?
• Quick Video
Polyribsomes
• A single mRNA is used to make many copies of
a polypeptide simultaneously
• Once a ribosome moves past the initiation
codon, a second ribosome can attach
Signal Peptides
• All protein synthesis begins in the free
ribosomes found in the cytoplasm
• Sometimes the growing polypeptide will have
a signal peptide which targets the protein to
endoplasmic reticulum
• It is then recognized as it emerges by a protein
called signal-recognition particle (SRP)
– Functions as an adapter that brings the ribosome
to the ER membrane
Type of RNA
Function
Messenger RNA (mRNA)
Carries information specifying amino acid
sequences of proteins from DNA to ribosomes
Transfer RNA (tRNA)
Serves as an adapter molecule in protein
synthesis; translates mRNA codons into amino
acids
Ribosomal RNA (rRNA)
Plays catalytic roles and structural roles in
ribosomes
Primary Transcript
Serves as a precursor to mRNA and may be
processed by splicing
Small nuclear RNA (snRNA)
Plays structural and catalytic roles in
splicesomes
SRP RNA
Component of the signal recognition particle
Mutations
• Mutations are changes in the genetic material
of a cell
• Point mutations are a chemical change in just
one base pair
– Sickle cell anemia: single base pain that codes for
one of the parts of hemoglobin
Types of Point Mutations
• Base pair substitution: replacement of one
nucleotide and its partner with another pair
– Could be silent mutations because of the nature
of the wobble effect
• Substitution mutations are usually missense
mutations; altered codon still codes for an
amino acid, and thus makes sense
• Alterations that change an amino acid codon
to a stop signal are called nonsense mutations
• Insertion and deletion are the worst because
they lead to a change in the reading frame
called a frameshift mutation
– Since amino acids are coded for in triplets, you
shift the reading frame over from where it should
be and it alters the rest of the sequence
Mutagens
• There are many places that cause errors- DNA
replication, repair, or recombination can lead
to mutations
– Spontaneous mutations
• Mutagens are physical or chemical agents that
interact with DNA to cause mutations
– X-Rays, UV light
– Chemical mutagens
• Some are chemicals similar to normal DNA bases that
insert themselves, others cause changes in the base
pairs