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DNA
&
THE GENETIC CODE
THIS SECTION OF SLIDES IS
AVAILABLE AT CLASS WEBSITE
Recommended reading:
“The Double Helix: A Personal Account of the Discovery of the Structure of DNA,
by James D. Watson
Deoxy
Nucleic
Deoxy
l 
Complex long molecule
l 
“Double helix” with 2 identical
strands running in opposite
directions (“anti-parallel”)
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“Steps” composed of four
“bases”
•  Adenine (A)
•  Thymine (T)
•  Cytosine (C)
•  Guanine (G)
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The sequence of bases
corresponds to the “code”
l 
Genetic Information Carrier
l 
Used by all life to store and
transmit all hereditary
information (some use RNA)
Nucleic
A
C
A
C
T
G
G
T
“All That Can Grow”
DNA: DeoxyriboNucleic Acid
The bases
A single nucleotide
(“monomer”)
A polynucleotide
(“polymer)
A
T
C
G
T
A
G
C
A
T
C
G
All the hereditary information is
contained in the order of the bases
along either strand of double helix
è the hereditary library!
•  Each base = “letter”
•  Each 3 base set = “word” è codon
T
A
•  Large segments of many codons
= “sentence” è gene è physical trait
G
C
l 
Contains sets of instructions
–  Each complete set of instructions =
–  Each gene is a segment of the DNA chain
–  Each gene defines the structure of a protein
l 
Contains sets of instructions
–  Each complete set of instructions =
–  Each gene is a segment of the DNA chain
–  Each gene defines the structure of a protein
Each pair of chromosomes contains one chromosome
from the mother & one from the father
l 
Contains sets of instructions
–  Each set of instructions =
–  Each gene is a segment of the DNA chain.
–  Each gene defines the structure of a protein.
l 
Entire set of genes =
–  Humans have 3 billion base pairs per cell
–  Spread across all 46 chromosomes
(2 “copies” of 23 types; 1 copy is from the mother
and the other is from the father, so the 2 copies are
not exactly the same –why not all siblings look the
same!)
–  About 21,000 genes in total
l 
Every cell has complete genome (spread
across the 23 chromosome pairs)
l 
Rules for how to read it:
l 
98% of human DNA is non-coding!
(“Non-functional” for protein synthesis—left over
from evolutionary dead-ends? Not all!)
l 
DNA molecules are very large
–  Human chromosome 1 is a single DNA molecule
containing 243 million base pairs!
Note: DNA is only located in chromosomes for part of life cycle of a cell, in particular, just before cell division.
During the “interphase,” DNA is more distributed around the nucleus.
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Accurate replications
l 
A ó T and C ó G
contains the information needed to make “stuff” from the amino acids. A
part of one strand of the DNA becomes the template for the RNA (a process
called “transcription”)
feeds selected instructions to ribosomes (molecule factories) about what
amino acids must be located and carried to the ribosome to make proteins.
“worker”- build structure, function as enzymes to catalyze chemical
reaction in the cell important for maintaining life function, such as metabolism.
most complex & versatile “macromolecules”
à  Cell structure (fingernails, hair,..)
à  Enzymes (make reactions faster à digestion)
Composed of
l  70 known, only ~20 used in life
l  “Left-handed” only
Long chains of amino acids are called polypeptides or proteins.
Short chains (<30 or so) are usually just called peptides.
Amine
group
(NH2)
Carbolic acid
group (COOH)
l 
l 
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4 letters (U,C,G,A)
3 bases = codon = “word”
Each word = one amino acid or an instruction
to “start reading” or “stop reading”
l 
l 
l 
4 letters (U,C,G,A)
3 bases = codon = “word”
Each word = one amino acid or an instruction
to “start reading” or “stop reading”
DNA à RNA à Amino Acids à Proteins
(one strand)
Using the genetic code, RNA strands are translated to specific sequences of amino acids which are the building
blocks of proteins. These RNA strands are created using DNA strands as the template (a process called
“transcription”)
DNA à RNA à Amino Acids à Proteins
(one strand)
Using the genetic code, RNA strands are translated to specific sequences of amino acids which are the building
blocks of proteins. These RNA strands are created using DNA strands as the template (a process called
“transcription”)
DNA tells mRNA (messenger RNA) what must be made.
At the factory (ribosome), mRNA sends tRNA (transfer RNA) out for the parts (amino acids).
tRNA brings parts back to factory, where they are assembled into proteins.
Using the genetic code, RNA strands are translated to specific sequences of amino acids which are the building
blocks of proteins. These RNA strands are created using DNA strands as the template (a process called
“transcription”)
DNA tells mRNA what must be made.
At the factory (ribosome), mRNA sends tRNA out for the parts (various amino acids).
tRNA brings parts back to factory, where they are assembled into proteins.
(transfer RNA)
(messenger RNA)
Using the genetic code, RNA strands are translated to specific sequences of amino acids which are the building
blocks of proteins. These RNA strands are created using DNA strands as the template (a process called
“transcription”)
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DNA replication very accurate
–  One error per billion bases copied
l 
Errors form the basis of mutations.
l 
Most mutations are neutral, some are bad, some are
beneficial, some are both.
! different shaped red blood cells
stiff and sticky, can block blood flow
resistance to malaria
Scientist can determine, or sequence, the genomes of
organisms on Earth and look for similarities.
The Tree of Life
Modern version based on detailed genetic analysis (DNA)
Modern version based on detailed genetic analysis (DNA):
microbes
microbes
“Last Universal Common Ancestor” LUCA
Tree of life points toward common ancestor
(“Handedness”)
In the lab, usually make equal numbers of L and R handed
Molecules, but for life on earth:
•  Most amino acids are left handed
•  Most sugars are right handed
l 
All life uses similar cells
l 
All life uses same 20 amino acids, while over 70 are known
l 
All life uses left-handed amino acids
l 
All life uses DNA
l 
Tree of life points toward common ancestor
All life uses similar cells
http://www.eva.mpg.de/neandertal/index.html