DNA Replication

Before a
, DNA is replicated so that both
receive exactly the same
cells
as the parent cell.

Replication is the process of making an

Replication is possible due to the double-strands of DNA and the
of DNA.
of
.

Each strand becomes a
, then
creates identical strands.

This is known as
old (
replication as each new helix is composed of an
) strand & a new (
) strand
The diagram shows the overall process:
Steps in DNA Replication
1. Unzipping - the molecule of DNA is
. The weak
bonds between the complementary base pairs are broken
by an
called
. The DNA bases on both
strands are now exposed and can be used as a template to create 2 identical strands
due to the process of
2. Complementary base pairing –
attached to the exposed strands by an
(Free nucleotides are always present in the
(matching) bases are
(DNA polymerase).
)
3. Proofreading--DNA polymerase makes very few
, and most of
those that are made are quickly
by enzymes that "proofread"
the nucleotides added into the new DNA strand. If a newly added nucleotide is not
to the one on the template strand, these enzymes remove the nucleotide and
replace it with the correct one. With this system, a cell's DNA is copied with less than
mistake in a
nucleotides.
* This is equal to a person copying 100 large (1000 page) dictionaries word for word, and symbol for
symbol, with only one error for the whole process!
T-A-C-T-G-A-A-C (parent strand)
T-A-C-T-G-A-A-C
A-T-G-A-C-T-T-G
(new strand)
(new strand)
A-T-G-A-C-T-T-G (parent strand)
There is ONE small problem with this method of DNA replication…..
REPLICATING THE ANTIPARALLEL STRANDS OF DNA
(a) What are Antiparallel Strands?
 When nucleotides combine, the phosphate of the
nucleotide bonds to the sugar of the previous
nucleotide (I.E. the sugar-phosphate backbones of
each strand run in opposite directions)
 since the nucleotides’ own phosphate attaches to
the 5th carbon of the deoxyribose sugar, the
beginning of the strand is called 5’ (5 prime) and
the opposite end is called 3’ (3 prime)
 DNA synthesis must always occur in the 5’ to 3’
direction to allow removal of phosphates, release of energy & proper bonding to carbon
(b) What’s the Big Problem Here?
If you examine the replication to the right, you will notice:
(1) _____________________________
 nucleotides are continuously added in the 5’ to 3’
direction
 polymerase can continue to add nucleotides as the fork
moves forward
(2) ______________________________
 cannot add nucleotides in the same direction as the
replication fork (must be 5’ to 3’)
 nucleotides must be added in the 5’ to 3’ direction, which
is __________________ to the movement of the fork
(c) Solution to the Problem?
 lagging strand is put together in short segments called _________________________________
 Okazaki Fragments can be about 100 – 200 nucleotides in length
 __________________ enzymes produce an RNA primer that is about 10 nucleotides long
 Primer attaches just behind the replication fork
 DNA polymerase starts adding nucleotides to the primer (Polymerase cannot start DNA synthesis alone)
 Polymerase adds 100 – 200 nucleotides away from the fork, to complete the Okazaki Fragment
 _______________ enzyme then joins the Okazaki fragments together to complete the lagging strand of
DNA
So, in summary, DNA polymerase only works in
direction, so nucleotides are added
on the
strand and in
strand. These pieces are called
on the
and
are joined by
Animations:
http://www.youtube.com/watch?v=teV62zrm2P0&NR=1
http://www.mcb.harvard.edu/Losick/images/TromboneFINALd.swf
http://www.nobel.se/medicine/educational/dna/a/replication/replication_ani.html.
http://www.nobel.se/medicine/educational/dna/a/replication/lagging_ani.html
*Because human DNA is so very long (with up to 80 million base pairs in a chromosome) it unzips at
multiple places along its length so that the replication process is going on simultaneously at hundreds of
places along the length of the chain. Eventually these areas run together to form a complete chain. In
humans, DNA is copied at about 50 base pairs per second. The process would take a month (rather than
the hour it actually does) without these multiple places on the chromosome where replication can begin.
** DNA has approximately 3 billion base pairs. It was completely mapped in 2003.
Practice Questions:
1. Which of the following is a difference between RNA and DNA?
a) the presence of adenine
b) the presence of a five-carbon sugar
2.
c) the presence of uracil
d) the presence of a nitrogenous base
X and Y could represent which of the following?
a) A and U
b) G and C
c) C and A
d) T and T
X
3. In a strand of DNA, the percentage of thymine is 30 %.
What is the percentage of cytosine in the same DNA strand?
a) 20%
b) 30%
c) 50%
d) 70%
4. In DNA replication, the term “unzipping” refers to
a)
b)
c)
d)
denaturing the DNA helix
unwinding the mRNA from the DNA after transcription
breaking the bonds between complementary DNA strands
breaking the bonds between the sugar and phosphate molecules
5. Which of the following is a valid contrast between RNA and DNA?
RNA
a)
b)
c)
d)
helical
sugar is ribose
found only in the nucleus
thymine is one of its bases
6.
Name:
Description—write at the side (point form)
DNA
not helical
sugar is deoxyribose
found in the nucleus and cytoplasm
uracil is one of its bases
Y