THIE
pLANNIER^
b. Structure and replication of DNA
1
DNA is made of two strands twisted in a
spiral staircase-like structure called a
double helix.
1
Each strand consists of nucleotides
bound together.
1
Each nucleotide consists of a
deoxyribose sugar bound to a
phosphate group and one of 4
nitrogenous bases [adenine (A),
thymine (T), guanine (G), cytosine (C)].
1
The nitrogenous bases pair together
through hydrogen bonding to form the
“steps” of the double helix.
1
Adenine pairs with thymine and guanine
pairs with cytosine.
Components of nucleotides
Phosphate
group
Nitrogenous bases
О ......
H —N.
OH
N H
OH
Deoxyribose
sugar
О
Thymine (T)
H H
Adenine (A)
О-
H2C— О=Р-
I
О-
OH
О-
H2C— О=Р-О
I
О-
Nitrogenous bases
New strand
Old strand
Portion Of
a
DNA molecule
^
___1. The double helix unwinds into 2 strands. The
Hvdr0aen b0nd
hydrogen bonds between the existing nitrogenous
i t
/
V a
bases break. Each strand serves as a template for
_
_a new DNA molecule.
^
7
71
^
2. New nucleotides pair with those in
each old strand (A to T, G to C) through
Key to bases.
^
,-----------------hvdrogen bonds.
Key to bases:
< A P = Adenine
,
Phosphate group
3. The new nucleotides bond
^
A
S
'L
.........
” " I
" " ' .
. . . /
Deoxyribose sugar
to each other through covalent
= Guanine
bonds to form a new strand of
= Thymine
------------------------------- ------------------------------------------------------ DNA.
{
'C
= Cytosine
4. Each new strand winds around
each old strand to make a double
helix.
5. As the process becomes
complete, there are 2 DNA
molecules; each molecule contains
one old strand from the original DNA
and a new strand.
Old
strand
H
H
One of four different nitrogenous bases, ring-shaped
molecules that contain C, H, O, and N
A five-carbon monosaccharide—deoxyribose or ribose
A phosphate group (PO43-)
The two kinds of nucleic acids are deoxyribonucleic acid
(DNA)
(de-ok'-se-n'-bo-noo-KLE-ik), which forms the ge-
netic material inside each human cell, and ribonucleic acid
(RNA), which relays instructions from genes to guide each
Life Uses Important Chemicals 41
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