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DNA
The Molecule of Heredity
Chapter 11.1
DNA - Deoxyribonucleic
Acid
•Strands of repeating molecules that
make up our chromatin and
chromosomes
•Contains genetic
information (genes)
DNA is called the “Blueprint of
Life”
Why?
• DNA tells cells which proteins to make
• The proteins then cause our individual
characteristics
The Structure of DNA
- DNA is in a Double-helix shape
- It is double-stranded and
wound up like a twisted ladder
- 2 strands attached to each other
Discovery credited to
James Watson and
Francis Crick.
Watson and Crick with
DNA Model, 1953.
DNA Structure continued
 It is a very long molecule made up of repeating
subunits called nucleotides
3 Parts of a Nucleotide
1) Phosphate group
2) Simple sugar
-DNA has a Deoxyribose sugar
-RNA has a Ribose sugar
3) Nitrogenous base
-A, T, C, or G
4 Nitrogenous Bases
in DNA
1.
2.
3.
4.
Adenine
Thymine
Guanine
Cytosine
Pyrimidines
Purines
Special Base
Pairing Rules
*Adenine pairs with
Thymine:
A-T
T-A
*Cytosine pairs with
Guanine:
C-G
G-C
The DNA
Strand
* Bases held
together by
hydrogen bonds.
* Sugar and
phosphate are
considered
backbone
What are the proper pairings
for the base strands shown:
1. A T C G T T T A C G C G T
2. T G C T A T A C C G T A A
3. C T A G C G A T G T C G T
Answers
1. A T C G T T T A C G C G T
TA G CAAAT G C G C A
2. T G C T A T A C C G T A A
AC GATATG G CATT
3. C T A G C G A T G T C G T
GATC G CTACAG CA
New Strand = Complementary strand
Old Strand = Template Strand
RNA - ribonucleic acid
- Molecules that “read”
the DNA and make
proteins using the
information in DNA
-RNA are the worker
molecules.
3 ways RNA is different
from DNA
• 1. RNA is single stranded
• 2. RNA has the sugar ribose
(not deoxyribose)
• 3. RNA has the nitrogen base
uracil instead of thymine.
RNA nitrogenous bases
• RNA bases
– Adenine
– Uracil (in DNA this is thymine)
– Cytosine
– Guanine
– Rules:
• Same pairing as DNA except a U goes where a T
was in DNA
– A pairs with U (A-U) and C pairs with G (C-G)
RNA base pairing examples
IF the DNA strand is
ATC G G CAT
IF the DNA is
ATAC GTAC G
Then the RNA
complement strand
is
UAGCCGUA
Then the RNA
complement strand
is
UAUGCAUGC
Practice Time!
Three types of RNA
• mRNA = Messenger RNA
– Carries information from nucleus to
cytoplasm (ribosomes in cytoplasm)
• rRNA = Ribosomal RNA
– Make ribosomes which are where amino
acids are assembled into proteins
• tRNA = Transfer RNA
– Carries amino acids to the ribosome to be
assembled into proteins
DNA Replication - 2 from 1
1.
Why do cells
need to
replicate their
DNA before
dividing?
2.
During what
phase of the cell
cycle does
replication
occur?
4 Steps to DNA Replication
Step 1:
Enzyme (helicase) initiates (starts)
replication by unzipping the DNA
Step 2:
Another enzyme (DNA polymerase)
attaches new nucleotides following
base pairing rules
Step 3: Error Checking
Mistake causes a bulge
As it moves down the
strand, the enzyme DNA
polymerase:
DNA poly. finds & piece is cut out
1. Checks for errors
2. Then fixes them
This process
is called
proofreading
DNA poly. repairs with correct base
DNA poly. continues down strand
Step 4:
Original
DNA
2 Identical copies
-1 old strand
(template)
-1 new strand
(complementary)
Original
DNA
Enzymes twist the new
strands (creating the
double helix)
New DNA
Steps of DNA Replication
1. Enzyme (helicase) unzips the DNA
2. Enzyme (DNA polymerase) attaches
new nucleotides according to base
pairing rules
3. DNA polymerase checks for errors
and fixes them.
4. End up with two identical daughter
DNA strands that enzymes retwist.
STOPPING
POINT
DNA Sequencing
Each organisms has a unique sequence
of DNA
The closer two individuals are related, the
more alike their sequences are.
What are some ways that we use this
knowledge?
The Central Dogma
DNA Structure and
Base Pairs
(Adenine) A = T (Thymine)
( Cystosine) C = G (Guanine)