Transcript Slide 1

DNA
•Deoxyribonucleic acid
•A polymer
•Monomers are nucleotides
Nucleotides
• Building blocks of DNA
• 4 types
• Each type has three parts
–Ring shaped sugar called
deoxyribose
–Phosphate group
–Nitrogenous base
•Single or double ring of carbon
and nitrogen atoms with
functional groups
Nitrogenous bases
• The four nucleotides found in DNA differ
only in their bases.
–Thymine (T)
–Cytosine (C)
•These are single ring structures
called pyrimidines
-Adenine (A)
-Guaninie (G)
- are larger, double ring structures
called purines
DNA Strands
•Nucleotides are joined to
one another by covalent
bonds that connect the
sugar of one nucleotide to
the phosphate group of the
next.
This repeating pattern of sugarphosphate-sugar-phosphate is
called the “BACKBONE”
The nitrogenous bases are lined
up along this backbone.
DNA Structure
• Double Helix:
–Watson and Crick model DNA this
way
–They created a new model in
which two strands of nucleotides
wound about each other.
–Forming a twisting shape called
the DOUBLE HELIX
–Their model placed the sugarphosphate backbones on the
outside of the double helix and the
nitrogenous bases on the inside.
–They hypostasized that the
nitrogenous bases that aligned
across the two strands formed
hydrogen bonds.
Complementary Base Pairs
• Individual structures of the
nitrogenous bases determine
very specific pairing between the
nucleotides of the two strands of
the double helix.
• These pairing are due to the
sizes of the bases and their
abilities to form hydrogen bonds
with each other
Adenine pairs with Thymine
Cytosine pairs with Guanine
While the sequence of nucleotides along
the length of one of the two DNA strands
can vary in a number of ways, the bases
on the second strand of the double helix
are determined by the sequence of the
bases on the first strand.
Each base must pair up with its
complementary base.
DNA replication
• is the process of copying a doublestranded deoxyribonucleic acid
(DNA) molecule,
• a process essential in all known life
forms.
• The general mechanisms of DNA
replication are different in
prokaryotic and eukaryotic
organisms.
DNA replication
• In a cell, DNA replication must
happen before cell division.
• Prokaryotes replicate their DNA
throughout the interval between
cell divisions.
• Eukaryotes, timings are highly
regulated and this occurs during
the S phase of the cell cycle,
preceding mitosis or meiosis I.
DNA replication
• Each DNA strand holds the
same genetic information.
• Both strands can serve as
templates for the reproduction
of the opposite strand.
• The template strand is
preserved in its entirety and
the new strand is assembled
from nucleotides
Enzymes link the nucleotides
together and form the two new
DNA strands, called daughter
strands.
This process of copying the DNA
molecule is called
DNA replication
Replication of the Double Helix
• More than a dozen enzymes
are involved in DNA
replication.
• Each "incoming" nucleotide
pairs with its complementary
nucleotide on the parent
strand.
•Enzymes called DNA polymerases
(PAHL ih mur ayz ez) make the
covalent bonds between the
nucleotides of the new DNA strand.
•The process is fast and accurate
•an error occurs in only about one of
a billion nucleotides.
DNA replication begins at specific
sites called origins of replication.
The copying proceeds outward in
both directions, creating replication
"bubbles".
The parent DNA strands open up as
daughter strands grow on both
sides of each bubble.
• A eukaryotic DNA molecule has
many origins where replication
can start at the same time.
• This shortens the total time to
copy all the DNA. Eventually, all
the bubbles merge
• End product: two doublestranded DNA molecules, each
with one new and one old
strand.
DNA replication occurs before a cell
divides, ensuring that the cells in a
multicellular organism all carry the
same genetic information.
It is also the mechanism for
producing the DNA copies that
offspring inherit from parents during
reproduction.
DNA replication
•The resulting doublestranded DNA molecules
are identical; proofreading
and error-checking
mechanisms exist