Vsauce Science Friday

http://www.youtube.com/watch?v=2lR7s1Y6Zig
&list=UU6nSFpj9HTCZ5t-N3Rm3-HA
Bell-Ringer

What do we call the monomer that makes up
DNA?

What two substances make up the DNA
Backbone?

What are the names of the four bases that
make up DNA?
DNA’s Discovery and Structure
Important Scientists in the “Race
for the Double Helix”

James Watson & Francis Crick (1953)
–
–
Created the first accurate model of DNA
Won the Nobel Prize
DNA’s Structure
is a double helix – this makes
DNA look like a “twisted ladder”.
 Each molecule of DNA is composed
of a “backbone” on the outside and
bases that point towards the middle
of the molecule.
 The monomer for DNA is the
nucleotide
 DNA
Nitrogenous Bases
 DNA
is made of nitrogenous bases.
 These four bases are:
adenine (A)
– cytosine (C)
– guanine (G)
– thymine (T)
–
DNA Bases

Purines – double-ringed molecule
–

Ex. Adenine and Guanine
Pyrimidines – single-ringed molecule
–
Ex. Cytosine and Thymine
• Complementary Base-Pairing
Rules


DNA is built following the complementary basepairing rules.
These state that:
–
–
–
–

A always pairs with T
T always pairs with A
C always pairs with G
G always pairs with C
In RNA, adenine (A) pairs with uracil (U). We
will look at this in a later lesson.
Sugar-Phosphate Backbone
Crashcourse DNA Video

http://www.youtube.com/watch?v=8kK2zwjRV0
M
DNA Replication

Replication = Duplication
–
–
Happens in the S phase of Interphase
Must replicate before mitosis or meiosis I
Part of the double helix is unwound
 Replication in long and small pieces
(Okazaki fragments)
 Enzyme stitches pieces together later

Semi-Conservative Replication


One-half of
each new
molecule of
DNA is old
(template
strand)
One-half of new
molecule of
DNA is new
(complementary
strand)
DNA Replication
•
•
13
Enzyme Helicase unwinds
and separates the 2 DNA
strands by breaking the
weak hydrogen bonds
DNA polymerase can then
add the new nucleotides
copyright cmassengale
14
copyright cmassengale
DNA Replication Animations
15

http://www.youtube.com/watch?v=5_vzY7H9G
Yk – Simple Animation

http://www.youtube.com/watch?v=teV62zrm2P
0 – Detailed Animation
copyright cmassengale
DNA Replication
•
•
•
Begins at Origins of Replication
Two strands open forming Replication
Forks (Y-shaped region)
New strands grow at the forks
5’ Parental DNA Molecule
3’
16
copyright cmassengale
3’
Replication
Fork
5’
DNA Replication
•
•
DNA polymerase can only add
nucleotides to the 3’ end of the DNA
This causes the NEW strand to be built in
a 5’ to 3’ direction
5’
3’
Nucleotide
17
DNA Polymerase
copyright cmassengale
RNA
Primer
5’
Synthesis of the New DNA
Strands
•
5’
The Leading Strand is
synthesized as a single
strand from the point of
origin toward the opening
replication fork
Nucleotides
18
DNA Polymerase
copyright cmassengale
RNA
Primer
3’
5’
Synthesis of the New DNA
Strands
•
•
19
The Lagging Strand is synthesized
discontinuously against overall
direction of replication
This strand is made in MANY short
segments It is replicated from the
replication fork toward the origin
copyright cmassengale
Lagging Strand Segments
•
•
Okazaki Fragments - series of short
segments on the lagging strand
Must be joined together by an
enzyme
DNA
Polymerase
Okazaki Fragment
RNA
Primer
5’
3’
20
Lagging Strand
3’
5’
copyright cmassengale
What is a Gene?
A length of DNA on a chromosome
 May be in several parts

–
–

Exon – the expressed parts of the DNA
sequence
Intron – the intervening, “junk DNA”, not
expressed
Parts are cut, put together and then used
to make RNA and proteins
Odd Types of Genes
 Multigene
families
 Transposons (Barbara McClintock)
 Protective Genes
 Viruses
may give us new DNA that
can be kept over time to cause new
evolutionary changes!