Download The Universal Dogma of Genetics

The Universal Dogma of
Genetics
Introduction
• The question is how does it get from a fragment of DNA
(gene) to……………PTC tasting?
• How is the information interpreted and then put into action
as a real physical thing?
• The answer to these questions is: Everything is written in the
instructions.
• Contrary to most students taking Bio101 at SCCC (who do not
seem to read the instructions to anything very often) cells
have developed mechanisms to be able to read the
instructions written in code using the nitrogen basis (A,C,G
and T) of the DNA and translate them into…proteins
(enzymes)
How does it work?
• For example. We have studied that the inability to digest
lactose is due to the lack of the enzyme lactase. The equation
is as follows
Glucose + Galactose
Enzyme
Energy
Lactose
H2O
• What is the key to this equation???
Yes! The lack of the enzyme.
• What causes that an individual does not have an enzyme? The
lack of information (instructions or the recipe) to make the
enzyme or we may have switched of the reading mechanism.
From Genotype to Phenotype
• The DNA (sequence of nitrogen bases) makes
the genotype (genetic make up). The DNA is
expressed as proteins (enzymes), which provide
the molecular basis for phenotypic traits
•
•
The information constituting an organism's
genotype is carried in its sequence of DNA bases
A particular gene—a linear sequence of many
nucleotides—specifies a particular polypeptide.
DNA is life…the rest is just translation
• In eukaryotic organisms, DNA is stored in the
nucleus where it is transcribed into mRNA; a
process called transcription (In the nucleus)
• mRNA translates the information that was
copied or transcribed from DNA into the
language of proteins (amino acids). It takes
place in the cytoplasm (or to be more precise,
in the ribosomes… we’ll come back to this); a
process called translation
The Central Dogma
• Basically, a gene dictates production of a specific
polypeptide. This is known as the one gene-one
polypeptide hypothesis (Beadle and Tatum, 1953).
• Based on this concept the Universal Dogma of
Genetics has been stated. It reads as follows
DNA
Nucleotides
transcription
mRNA
Nucleotides
translation
Protein
Amino Acids
DNA
Transcription of DNA into RNA
RNA
Nucleus
Cytoplasm
Translation of RNA into Protein
Protein
How does it happen? Transcription
• A typical gene consists of hundreds or
thousands of nucleotides in a specific
sequence.
• The sequence (and number) of these
nucleotides determines the protein produced
by this gene.
• The Genetic information written in the DNA is
transcribed (or copied) to mDNA ( from
nucleotide to nucleotide replacing T’s with
U’s)
• DNA must first be re-written (transcribed) as a
LE 10-7a
DNA molecule
Gene 1
Gene 2
Gene 3
DNA strand
A
A
A
C
C
G
G
C
A
A
A
A
U
U
U
G
G
C
C
G
U
U
U
U
Transcription
RNA
Codon
Translation
Polypeptide
Amino acid
RNA nucleotides
RNA
polymerase
Direction of
transcription
Newly made RNA
Template
strand of DNA
Messenger RNA
• The type of RNA that encodes amino acid
sequences is called messenger RNA (mRNA)
• In eukaryotic cells, mRNA leaves the nucleus
where it had been transcribed and enters the
cytoplasm
• Before mRNA can leave the nucleus, it is
modified
– A ‘tail’ and ‘cap’ are added
– Introns are removed
A cap and
tail are
added to
protect the
mRNA
strand,
facilitate its
transport
out of the
nucleus and
to help
ribosomes
bind to it
Exon Intron
Exon
Intron Exon
DNA
Cap
RNA
transcript
with cap
and tail
Transcription
Addition of cap and tail
Introns removed
Tail
Introns are
intervening
sequences
of DNA
which do
not code for
amino
acids; must
be removed
Exons spliced together
mRNA
Coding sequence
Nucleus
Cytoplasm
Exons are
the coding
regions,
parts of the
gene which
remain and
are
translated
into amino
acids
How does it happen? Translation
• Translation: The Genetic information in the
DNA “coding strand” is copied into mRNA
(replacing T with U).
• The mRNA is then translated using the code into
amino acid sequences.
• Each 3 mRNA bases make a CODON.
• Each codon codes for an AA.
DNA strand
Transcription
RNA
Codon
Translation
Polypeptide
Amino acid
Genetic information written in codons is
translated into amino acid sequences
• Translation then converts the nucleic acid
‘language’ into the polypeptide (protein)
‘language’
• The sequence of RNA nucleotides dictates the
sequence of amino acids of the polypeptide
being produced
• Thus, the RNA molecule acts as a messanger
carrying genetic information from DNA
Second base
C
U
A
UAU
UCU
UUU
Phe
U
UGU
Cys
Tyr
UAC
UCC
UUC
G
C
UGC
Ser
U
UUA
UCA
UAA
Stop
UGA
Stop
A
UUG
UCG
UAG
Stop
UGG
Trp
G
CUU
CCU
CAU
CUC
CCC
CAC
Leu
U
CGU
His
Leu
C
CUA
CGU
Pro
CCA
C
Arg
CAA
CGA
A
CGG
G
Gln
CUG
CCG
CAG
AUU
ACU
AAU
AUC
lle
ACC
U
AGU
Ser
Asn
AAC
AGC
C
ACA
AAA
AGA
A
ACG
AAG
AGG
G
GCU
GAU
GGU
U
Thr
A
AUA
AUG
Met or
start
GUU
Arg
Lys
Asp
GUC
G
GCC
Val
GUA
GAC
GCA
C
GGC
Gly
Ala
GAA
GGA
A
GGG
G
Glu
GUG
GCG
GAG
Genetic information written in codons is
translated into amino acid sequences
• In order for translation to proceed, the
sequence of the 4 nucleotides in RNA (A,U,
C,G) must somehow specify the 20 amino
acids used to make up proteins
• The flow of information from gene to protein
is based on a triplet code; genetic instructions
for the amino acid sequences of a polypeptide
chain are written in DNA and RNA as a series
of 3-base ‘words’, called codons
The Genetic Code
• The genetic code is a set of instructions
indicating which codons are translated into
which amino acid
• The genetic code does not only specify which
codons code for which amino acids, but also
specify ‘start’ and ‘stop’ signals, which begin
and end protein synthesis, respectively
• For each of the 20 amino acids, there are 2-4
codons which code exclusively for them
Third base
First base
Second base
The Genetic Code
• The genetic code is nearly universal; humans
cells can translate bacterial RNA and vice versa
Transfer RNA
• In order to convert the 3-letter codons of
nucleic acids into a single amino acid, a cell
must employ a molecular interpreter, transfer
RNA (tRNA)
• tRNA recognizes the codons in the mRNA
molecule and picks out the appropriate amino
acids for incorporation into the growing
polypeptide
Transfer RNA
• tRNA recognizes codons from mRNA via a
special triplet of bases called an anticodon,
which is complimentary to the codon on the
mRNA
• When the codon of mRNA complements the
anticodon of tRNA, the appropriate amino
acid is laid down at the other end of the tRNA
molecule
Amino acid attachment site
Hydrogen bond
RNA polynucleotide chain
Anticodon
Translation
• Translation begins with a ‘start’ codon, and
ends with a ‘stop’ codon
• The amino acid methionine (Met) is always
translated by the start codon (AUG)
• What would the anticodon look like?
• Stop codons (UAA, UAG, and UGA) do not
code for amino acids but instead act only as
signals to end translation
DNA strand
Transcription
RNA
Codon
Translation
Polypeptide
Amino acid
Review
• Describe the differences between mRNA,
tRNA and rRNA
• What bases are found in DNA? In RNA?
• Which molecule has codons? Anticodons?
• What is transcription? Translation? Which
happens first and where does each occur in
the cell?