DNA
A Look at Its Structure & Function
DNA
• Is often called a “double helix”
–Twisted ladder
• That means that it has two strands that twist
around each other. (double helix)
• The strands are made up of four nitrogen
bases:
–
–
–
–
Adenine
Thymine
Cytosine
Guanine
• The two sides of the ladder are made up of
molecules of sugar. (deoxyribose) C5H10O4
alternating with molecules known as
phosphate.
PO4
• The two strands of DNA complement each
other.
• The nitrogen bases pair up in a certain way.
• Adenine always pairs with Thymine
• Guanine always pairs with Cytosine
Complementary Strands
• There are only four letters that make up the
language of DNA.
– A, T, C & G (Adenine, Thymine, Cytosine &
Guanine)
• Wherever there is an A you will have a T, Where
there is C you will have a G and so on…. Called
CHARGOFF’s RULE (or the complementary base
pairing rule
Hydrogen Bonds
-between a pair of bases
-a WEAK bond
3 hydrogen bonds
between guanine and
cytosine
2 hydrogen bonds
between adenine and
thymine
Figure out what the
complementary strand (other
side) would be to these bases:
• ACG TAC GAA TCG ACG
• You should have:
– TGC ATG CTT AGC TGC as a
complementary copy of the strand on the
previous slide.
• Each group of three bases listed above, for
example TGC is considered a codon.
• A codon is a group of three nitrogenous
bases that codes for a particular amino acid.
Code Letters: Most of the
time Scientists do not want
to write out the names of
the bases so they refer to
them by their first letters.
•Adenine: A
•Thymine: T
•Cytosine: C
•Guanine: G
Nucleotides
• DNA is made up of puzzle
pieces called nucleotides.
• Each nucleotide is composed
of:
– One deoxyribose sugar
– One phosphate group
– One nitrogenous base (A,
T, C or G)
A Nucleotide
How do we know all this???
•
James Watson & Francis Crick
• Crick and Watson, together with
Maurice Wilkins, won the 1962 Nobel
Prize in Medicine for their discovery of
the structure of DNA. This was one of
the most significant scientific
discoveries of the 20th century.
Where is DNA Found?
• DNA is found in the Nucleus of the cell.
• DNA makes up the chromosomes in the
nucleus of the cell.
• DNA duplicates during the S phase of
interphase so that each new cell will have a
complete copy of DNA.
DNA Replication
1. Replication means: making a copy of
DNA.
2. It occurs during the S phase of interphase.
3. There are four important components for
DNA replication to take place:
1.
2.
3.
4.
Helicase
DNA polymerase
Ligase
Free nucleotides
1. Helicase: An enzyme that attaches to
DNA and unwinds the strand by breaking the
hydrogen bonds between the bases .
• Ligase: is an enzyme that can connect two strands of
DNA together. It works like glue to connect the
parent strand to the daughter strand.
• DNA polymerase is an enzyme that assists in DNA
replication binding to a single strand of DNA and
initiating the attachment of nucleotides to the DNA
strand.
Free nucleotides
start to fill in the
open sides of the
nitrogen bases.
Complementary
base pairing must
happen. (Adenine
with Thymine,
Cytosine with
Guanine)
• When all bases are
matched up you will
have two identical
strands of DNA.
• The cell will now
enter the G2 phase
of cell division
preparing to begin
Mitosis
FYI
A complete strand of
human DNA is approx. 2
meters in length, and is
composed of a chain of
about 2.8 billion
nucleotides .
The End