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The blueprint of life; from DNA
to Protein
Biology 261
Medgar Evers College
Prof. Santos
• DNA, the substance of inheritance
– Is the most celebrated molecule of our time
• Hereditary information
– Is encoded in the chemical language of DNA
and reproduced in all the cells of your body
• It is the DNA program
– That directs the development of many
different types of traits
G
C
A
T
T
A
1 nm
C
G
C
3.4 nm
G
A
T
G
C
T
A
T
A
A
T
T
A
G
C
0.34 nm
A
Figure 16.7a, c
T
(a) Key features of DNA structure
(c) Space-filling model
5 end
O
OH
Hydrogen bond
P
–O
3 end
OH
O
O
A
T
O
O
O
CH2
P
–O
O
H2C
O
–O
P
O
O
G
O
C
O
O
CH2
P
O
O–
O
P
H2C
O
O
C
O
G
O
O
O
CH2
P
–O
O–
O
O
O–
O
P
H2C
O
O
A
O
T
O
CH2
OH
3 end
O
O–
P
O
Figure 16.7b
(b) Partial chemical structure
O
5 end
H
N
N
N
N
Sugar
O
H
H
CH3
N
N
N
O
Sugar
Thymine (T)
Adenine (A)
H
O
N
N
Sugar
N
H
N
N
N
N
N
Figure 16.8
H
H
Guanine (G)
H
O
Sugar
Cytosine (C)
DNA
The Components and Structure of DNA
DNA is made up of nucleotides.
A nucleotide is a monomer of nucleic acids made up
of:
• Deoxyribose – 5-carbon Sugar
• Phosphate Group
• Nitrogenous Base
There are four kinds of bases in in DNA:
adenine
guanine
cytosine
thymine
Chargaff’s rule
Chargaff's Rules
Erwin Chargaff discovered that:
• The percentages of guanine [G] and cytosine [C] bases are almost
equal in any sample of DNA.
– The percentages of adenine [A] and thymine [T] bases
are almost equal in any sample of DNA.
DNA Double Helix
• There are 2 hydrogen bonds between
adenine and thymine and three hydrogen
bonds between cytosine and guanine.
DNA to RNA to Protein connection
Protein synthesis
3 steps
1- transcription
2- RNA processing
3- translation
transcription
• 3 steps
1- initiation
2- elongation
3- termination
Transcription
• Copying the genetic code directly from DNA.
• We make a single strand of messenger RNA.
• We begin initiation by unwinding the double
stranded DNA and copying only one of the
strands. The strand that is copied is called the
nonsense strand. It serves as a template for the
production of messenger RNA.
• Transcription begins when an enzyme
called RNA polymerase binds to a special
region of the DNA called promoter
sequence. Unlike DNA polymerase, RNA
polymerase doesn’t need a primer.
• RNA polymerase brings free floating RNA
nucleotides to the nonsense strand.
• The immediate product of this transcription
is a resultant initial RNA transcript, which
contains a sequence of nucleotides that is
identical to the that of the sense strand.
The exception to this is that uracil is used
for nucleotide sequencing of RNA
molecules rather than thymine.
Role of transcription factors
• Transcription factors are proteins that aid
in the process of transcription. They can
be involved in promoting or blocking
(repressing) the action of RNA
Polymerase.
• Guanine and cytosine pair up.
• But, there is no thymine in RNA. Another
base called Uracil pairs up with adenine.
• The messenger RNA strand will continue
to elongate until it reaches a termination
point.
RNA processing
• The pre messenger RNA made must be
modified before it can leave the nucleus
and direct the production of proteins.
• 3 steps
1- removal of introns
2- addition of cap at 5 end
3- poly A tail at 3 end
• The coding regions are called exons and
the non-coding regions are called introns.
• The introns are removed by an enzymeRNA complex known as the spliceosome.
• A tail of adenine bases is added to the 3
prime end and a modified guanine
nucleotide is added to the 5 prime end.
• Once the messenger RNA has been
processed, it is ready to leave the nucleus
and bind to a ribosome.
• The mature messenger RNA carries the
message from DNA in the forms of
codons.
• A codon is a group of 3 bases that
correspond to one of the 20 amino acids.
• There are 64 possible codons and only 20
amino acids. There is redundancy with
some of the amino acids!
• The initial codon is AUG or methionine
and there are three stop codons, UAA,
UGA and UAG.
CUU
CUC
C
CUA
CUG
CCU
CCC
Leu CCA
CCG
Pro
AUU
AUC
A
AUA
AUG
ACU
ACC
ACA
ACG
Thr
GUU
G GUC
GUA
GUG
lle
Met or
start
GCU
GCC
Val
GCA
GCG
Ala
G
U
UGU
Cys
UGC
C
UGA Stop A
UGG Trp G
U
CAU
CGU
His
CAC
CGC
C
Arg
CAA
CGA
A
Gln
CAG
CGG
G
U
AAU
AGU
Asn
AAC
AGC Ser C
A
AAA
AGA
Lys
Arg
G
AAG
AGG
U
GAU
GGU
C
GAC Asp GGC
Gly
GAA
GGA
A
Glu
GAG
GGG
G
Third mRNA base (3 end)
First mRNA base (5 end)
Figure 17.5
Second mRNA base
U
C
A
UAU
UUU
UCU
Tyr
Phe
UAC
UUC
UCC
U
UUA
UCA Ser UAA Stop
UAG Stop
UUG Leu UCG
Translation
• The messenger RNA attaches to the ribosome
and the message in the form of codons is
“translated” and the appropriate amino acid is
put in place.
• A molecule of RNA called transfer RNA brings
along the amino acid. It resembles a four leaf
clover.
• On the top is the amino acid and on the bottom
is a sequence known as the anti codon.
3
A
C
C
A 5
C G
G C
C G
U G
U A
A U
A U
U C
UA
C A C AG
*
G
*
G U G U *
C
C
* *
U C
*
* G AG C
G C
U A
* G
A
*
A
C
U
*
A
G
A
Amino acid
attachment site
Anticodon
C U C
G A G
A G *
*
G
A G G
Hydrogen
bonds
• The anticodon pairs up with the codon.
This allows the amino acids to put placed
in the correct sequence or order.