Simultaneous transcription and
translation in prokaryotes
Green arrow = E. coli DNA
Red arrow = mRNA combined with ribosomes
Eukaryotic RNA
Differences
RNA processing
– Primary transcript produced in the
nucleus
– Processed before transported to the
cytoplasm
A cap consisting of 7-methylguanosine is
added to the 5’ end of the transcript
3’ poly (A) tail
Eukaryotic RNA
RNA processing
– 5’ cap
• Protects RNA from degradation
• Required for binding to the ribosome during
initiation of protein synthesis (translation)
– 3’ poly (A) tail
• Protects RNA from degradation by
nucleases
Eukaryotic RNA
RNA processing
– Splicing
• Removes intervening sequences in RNA
Many eukaryotic genes contain internal
sequences that do not encode amino acids –
introns (light colored areas)
Sequences that encode amino acids –
exons (darker colored areas)
Splicing
removes the
introns and
brings together
the coding
regions
Gene Splicing
• Consensus sequence at intron-exon
junction
• snRNAs pair complementarily with
the splice site
• Splicing enzymes can then cut-out
introns
Gene Splicing
• Sometimes, different introns are
spliced-out determining the function
(type) of protein that is made
The Central Dogma
(Francis Crick, 1958)
(Transcription)
DNA
(Gene)

(Translation)
RNA

Protein
(Phenotype)
An informational process between the genetic
material (genotype) and the protein (phenotype
Proteins
• Proteins are just long polymers of
amino acids
– So, the basic unit of a protein is an
amino acid
– 20 different amino acids
Proteins
• Amino acids in a protein are held
together by peptide bonds
– Facilitated by peptidyltransferase
Proteins
• A long string of amino acids is called
a polypeptide
• A protein has an amino (the first
amino acid in the chain) and a
carboxyl (the last amino acid in a
chain) ends
Translation (protein synthesis)
Peptidyl site:
peptidyltransferase
attaches amino acid
to chain
Aminoacyl site:
new amino acid
brought in
Ribosome
moves in this
direction
Animation of
protein synthesis
• http://highered.mcgrawhill.com/sites/0072556781/student_view0/c
hapter12/animation_quiz_2.html
Cells have adapter molecules called tRNA with a three nucleotide
sequence on one end (anticodon) that is complementary to a codon
of the genetic code.
• There are different transfer RNAs
(tRNAs) with anticodons that are
complementary to the codons for each of
the twenty amino acids.
• Each tRNA interacts with an enzyme
(aminoacyl-tRNA synthetase) that
specifically attaches the amino acid that
corresponds to its anticodon.
• For example, the tRNA to the right with
the anticodon AAG is complementary to
the UUC codon in the genetic code
(mRNA). That tRNA would carry the
amino acid phenylalanine (see genetic
code table) and only phenylalanine to the
site of protein synthesis.
• When a tRNA has its specific amino acid
attached it is said to be “charged.”
Proteins
Protein can have a
• Primary structure
• Secondary structure
• Tertiary structure
• Quaternary structure
Primary structure
•The order of the amino acids
•The order is the primary determinant of protein
function
•The primary structure is determined by the code on
the DNA/RNA
synthesized
Primary structure
Amino end
Carboxyl end
Tryptophane Synthase A Protein
268 amino acids long
Secondary structure
•Interaction of side groups, giving polypeptides a
periodic structure
•Stabilized by hydrogen bonds
Alpha Helix
Alpha Helix
Beta Pleated Sheet
Tertiary structure
•The folding or
bending of the
polypeptide
Tertiary structure can be affected by environmental
factors such as temperature
Enzymes are proteins: if the tertiary
structure is changed (mutation or
temperature) the enzyme cannot carry out
its function
Quaternary structure
•Two or more polypeptides combine to form
a functional protein
Bovine Insulin Protein
Proteins
• The order of the amino acids (the
primary structure) can affect the
secondary, tertiary and quaternary
structures
– Possibly affecting the function of the
protein
Beta chains each
have 146 amino
acids
Alpha chains each
have 141 amino
acids
Hemoglobin
Change in beta chain at amino acid 6 out of the 146
amino acids (change in codon from GAG to GUG)
Proteins
• The order of the amino acids in a
polypeptide is like the order of words
in a sentence
Proteins
• If you change one word you can
change the meaning significantly
– John only punched Jim in his eye.
Proteins
• If you change one word you can
change the meaning significantly
– John only punched Jim in his eye.
– John only punched Jim in his dreams.
Proteins
• This is what happens in mutations
– If the code changes (DNA), new amino
acids can be put in the polypeptide,
changing “the meaning” of the
polypeptide
Genetic Code
• One fundamental question: How can
DNA and RNA, each consisting of
only four different nucleotides
(bases), encode proteins consisting
of 20 amino acids?
– Solving the genetic code became the
most important biological question of
the late 1950s and early 1960s