12.1 What Is the Evidence that Genes Code for Proteins?
The gene-enzyme relationship is one-gene,
one-polypeptide relationship.
Example: In hemoglobin, each polypeptide
chain is specified by a separate gene.
12.2 How Does Information Flow from Genes to Proteins?
Expression of a gene to form a
polypeptide takes 3 main processes:
• Transcription—copies information from
gene to a sequence of pre-mRNA.
• RNA Processing-converts pre-mRNA to
mRNA
• Translation—converts mRNA sequence
to amino acid sequence.
12.2 How Does Information Flow from Genes to Proteins?
RNA, ribonucleic acid differs from DNA:
• Single strand-so what’s that mean?
• The sugar is ribose
• Contains uracil (U) instead of thymine (T)
12.2 How Does Information Flow from Genes to Proteins?
RNA can pair with a single strand of
DNA, except that adenine pairs with
uracil instead of thymine.
Single-strand RNA can fold into much
more unique and differing shapes by
internal base pairing. (This flexibility is
not seen in DNA)
Figure 12.2 The Central Dogma
The central dogma of molecular biology for
eukaryotes: information flows in one direction
when genes are expressed (Francis Crick).
12.2 How Does Information Flow from Genes to Proteins?
ONE Exception to the central dogma:
Viruses: acellular particles that reproduce
inside cells; many have RNA instead of DNA
so reverse the process. Synthesis of DNA
from RNA is called reverse transcription.
Viruses that do this are called retroviruses
12.2 How Does Information Flow from Genes to Proteins?
Messenger RNA (mRNA) forms as a
complementary copy of DNA and
carries information to the cytoplasm.
(WHY use a copy of DNA?)
This process is called transcription and
occurs in the nucleus.
RNA polymerase is the enzyme that runs
the same direction as it’s “cousin”. Will
we have a leading or lagging strand
now? Why or why not?
12.3 How Is the Information Content in DNA Transcribed to
Produce RNA?
Transcription occurs in three phases:
• Initiation
• Elongation
• Termination
12.3 How Is the Information Content in DNA Transcribed to
Produce RNA?
Initiation requires a promoter—a special
sequence of DNA.
RNA polymerase binds to the promoter.
Promoter tells RNA polymerase where to
start, which direction to go in, and which
strand of DNA to transcribe. In
eukaryotes it is the “TATA” region called
the initiation site.
12.3 How Is the Information Content in DNA Transcribed to
Produce RNA?
Elongation: RNA polymerase copies
base pairs of DNA into pre-mRNA.
RNA polymerase also runs in a 5-3
direction. (So what DNA template will
we use? Why? What about the other
one?)
12.3 How Is the Information Content in DNA Transcribed to
Produce RNA?
Termination: specified by a specific DNA
base sequence.
Mechanisms of termination are complex
and varied.
Eukaryotes—first product is a pre-
mRNA that is longer than the final
mRNA and must undergo processing.
The Pre mRNA must be readied for
travel so 5’ caps and poly A tails (3’)
are added to the strand. Non coding
regions called introns are also
removed leaving only exons. Once
RNA processing is complete, we
have mRNA
Before we begin today, we need to understand that RNA is extremely flexible!
• There are 4 types of RNA, each encoded by its own type of gene:
• mRNA - Messenger RNA: Encodes amino acid sequence of a
polypeptide. Linear and made as the first product of transcription (pre-
MRNA)
• tRNA - Transfer RNA: Brings amino acids to ribosomes during
translation. (Folded mRNA bonded together into a “t” shape using H
bonds)
• rRNA - Ribosomal RNA: With ribosomal proteins, makes up the
ribosomes, the organelles that translate the mRNA. (Quaternary
protein structure, many more complex H bonds holding many linear
mRNA’s together)
• snRNA - Small nuclear RNA: Work with regulatory enzymes
(spliceosomes) to form complexes that are used in RNA processing in
eukaryotes. (Not found in prokaryotes.) (This is what splices out
introns!)
12.4 How Is RNA Translated into Proteins?
Let’s look at each type of RNA now….
Functions of tRNA:
• Carries an inactive amino acid
• Carries an active amino acid
• Interacts with ribosomes by providing
the anticodon
12.4 How Is RNA Translated into Proteins?
The conformation (three-dimensional shape) of tRNA results from base pairing (H bonds) within the molecule.
Anticodon: site of base pairing with mRNA. Unique for each species of tRNA.
Formula for building a protein is
Codon + anticodon + inactive aa= specific aa in polypeptide chain
12.4 How Is RNA Translated into Proteins?
Example:
DNA codon for alanine: GCC
Complementary mRNA: CGG
Anticodon on the tRNA: GCC
Active amino acid would be: alanine
12.4 How Is RNA Translated into Proteins?
Wobble: specificity for the base on tRNA
so one tRNA can decode up to 3
different codons.
Example: codons for alanine—GCA,
GCC, and GCU—are recognized by the
same tRNA.
Allows cells to produce fewer tRNA.
12.4 How Is RNA Translated into Proteins?
Ribosome: the workbench—holds
mRNA and tRNA in the correct positions
to allow assembly of polypeptide chain.
Ribosomes are not specific, they can
make any type of protein.
12.4 How Is RNA Translated into Proteins?
rRNA:
AKA the Ribosomes have two subunits,
large and small.
The subunits are made of rRNA or
ribosomal RNA.
12.4 How Is RNA Translated into Proteins?
Large subunit has three tRNA binding sites:
• A site binds with anticodon of charged tRNA. Activation
• P site is where tRNA adds its amino acid to the growing chain. Polypeptide chain is held and built
• E site is where tRNA sits before being released. Exit
12.4 How Is RNA Translated into Proteins?
Translation also occurs in three steps:
• *Initiation-start codon (AUG) first amino
acid is always methionine
• Elongation of the polypeptide chain
• Termination- stop codon enters the A
site.
Methionine (AUG) hits the P site of the small
ribosomal sub-unit
that action initiates the process.
One of the first things that happens is the large
ribosomal sub-unit
joins with the small unit and makes an rRNA
• http://highered.mheducation.com/sites/
0072507470/student_view0/chapter3/a
nimation__how_translation_works.html
• http://www.stolaf.edu/people/giannini/fla
shanimat/molgenetics/translation.swf
• http://www.phschool.com/science/biolog
y_place/biocoach/transcription/difgns.ht
ml
12.6 What Are Mutations?
Somatic mutations occur in somatic
(body) cells. Mutation is passed to
daughter cells, but not to sexually
produced offspring.
Germ line mutations occur in cells that
produce gametes. Can be passed to
next generation. This is the key to
evolution and are available to occur in
transcription.
12.6 What Are Mutations?
All mutations are alterations of the
nucleotide sequence. 2 levels of
mutation….
Point mutations: change in a single
base pair—loss, gain, or substitution
of a base.
Chromosomal mutations: change in
segments of DNA—loss, duplication, or
rearrangement.
12.6 What Are Mutations?
Point mutations can result from replication and proofreading errors, or from environmental mutagens.
Silent mutations have no effect on the protein because of the redundancy of the genetic code.
Silent mutations result in genetic diversity not expressed as phenotype differences.
12.6 What Are Mutations?
KEY! These CAN be beneficial!
Missense mutations: base substitution
results in amino acid substitution.
12.6 What Are Mutations?
Sickle allele for human β-globin is a
missense mutation.
Sickle allele differs from normal by only
one base—the polypeptide differs by
only one amino acid.
Individuals that are homozygous have
sickle-cell disease.
12.6 What Are Mutations?
Frame-shift mutations: single bases
inserted or deleted—usually leads to
nonfunctional proteins.
12.6 What Are Mutations?
Chromosomal mutations:
Deletions—severe consequences unless
it affects unnecessary genes or is
masked by normal alleles.
Duplications—if homologous
chromosomes break in different places
and recombine with the wrong partners.
12.6 What Are Mutations?
Chromosomal mutations:
Inversions—breaking and rejoining, but segment is “flipped.”
Translocations—segment of DNA breaks off and is inserted into another chromosome. Can cause duplications and deletions. Meiosis can be prevented if chromosome pairing is impossible.
12.6 What Are Mutations?
• Replication errors—some escape
detection and repair.
• Nondisjunction in meiosis.
12.6 What Are Mutations?
Mutation provides the raw material for
evolution in the form of genetic diversity.
Mutations can harm the organism, or be
neutral.
Occasionally, a mutation can improve an
organism’s adaptation to its
environment, or become favorable as
conditions change.
12.6 What Are Mutations?
Induced mutation—due to an outside
agent, a mutagen.
Chemicals can alter bases
Prokaryotic gene regulation much simpler!
Operons are repeating regions that make up the prokaryote’s genome
They include; regulatory genes, promoter, structural genes
2 main regulatory options for ALL genes, inducible (lac)
or repressible (trp)