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DNA & RNA
Before We Knew about DNA
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Gregor Mendel –
the “father of
genetics” was a
monk who, in the
1800’s, was the
first person to
hypothesize
“factors” which
were passed
down, or inherited,
from generation to
generation
Before We Knew about DNA
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But what were these “factors”?
Many scientists were on the case.
In the first half of the 1900’s,
scientists Griffith, Avery, Hershey
and Chase all conducted
experiments contributing to the
isolation of the DNA molecule.
Before We Knew about DNA
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In 1950, Erwin Chargaff
conducted chemical
experiments and concluded
there was the same amount
of adenine as of thymine in
DNA.
In addition, there is the same
amount of cytosine as
guanine in DNA.
This became known as
Chargaff’s rule, and from this
we know that:
adenine always pairs with
thymine, and that cytosine
always pairs with guanine
but wait
I’m jumping ahead
We’ll get back to this
Before We Knew about DNA
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In the early
1950’s, Rosalind
Franklin worked
with X-ray
photography to
take the famous
photo, showing
the structure of
DNA for the first
time.
Sadly, she died of
radiation poisoning
Before We Knew about DNA
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Then in 1953,
James Watson
and Francis
Crick took the
work of all these
scientists before
them and built a
model of the DNA
molecule.
Think “Whats in
the Crick?”
Discovering DNA
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So how long have
we known about
DNA?
If you count from
Watson and Crick,
only about a half
century….
But Mendel had a
pretty good idea
long before that.
The Structure of DNA
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DNA: deoxyribonucleic acid
Genetic material that contains
the instructions for the functions
of the cell
Found in the nucleus of
eukaryotic cells
In prokaryotes, shaped as a ring
DNA is a very long, thin molecule
Shape is double helix – like a
twisted ladder
Structure of DNA
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“railing” or “backbone” of
ladder consists of 2
alternating parts :
deoxyribose (a sugar),
and phosphate molecules
Steps, or “rungs” of ladder
consist of pairs of nitrogen
bases:
Adenine & Thymine
Guanine & Cytosine
Structure of DNA
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Each nitrogen base “step” is connected to the
railing at the sugar molecule
Each sugar molecule is connected to its own
phosphate molecule
One nitrogen base, together with its sugar and
phosphate backbone make one unit called a
nucleotide
Nucleotides are made up of three components:
1. deoxyribose (sugar)
2. phosphate
3. nitrogenous base
DNA structure
Nitrogen bases:
1. Purines: molecules
whose shape is 2 ringed
-Adenine and Guanine
2. Pyrimidines: 1 ring
-Thymine and Cytosine
o Adenine only bonds with
thymine and guanine
only bonds with cytosine
o Remember Chargaff’s
rule?
o A=T and C=G
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DNA structure
o
o
o
This bonding is called base pairing
The nitrogen bases are held
together with loose hydrogen
bonds
These are usually shown as dotted
lines
DNA Structure
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There are millions of “steps”
in each of our 46 DNA
molecules,
in each of our cells’ nuclei.
If you unraveled one single
DNA molecule all the way
and stretched it out,
It would be about 5 cm long
– that’s about 2 inches!
How does all that DNA fit
inside a nucleus?
DNA Structure
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Histones are
proteins that the
DNA molecule
wraps itself
around to make a
smaller compact
unit to fit in the
nucleus.
DNA Replication p 334
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DNA must
replicate before
cell can
reproduce.
Each new copy
must be exactly
the same as the
original
DNA Replication – Big Picture
Two complete
strands will result,
these will be
exactly alike
 Each resulting
strand has one
“old” half and one
“new” half
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DNA Replication p 334
 First
step: DNA
unwinds and
enzymes “unzip”
DNA into two
separate strands
by breaking the
hydrogen bonds,
exposing
unpaired
nucleotides
DNA Replication – p 334
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Because
nitrogen bases
can only match
in specific pairs,
this allows exact
copies to be
made from each
open half
DNA Replication
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Second step: free
nucleotides join onto
open strands to form
two new strands of
DNA, aided by
enzymes called DNA
polymerese.
DNA polymerese: an
enzyme that helps
new nucleotides bond
onto open strand of
DNA during
replication.
DNA Replication
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When DNA
replicates, each
strand works in
opposite
directions, from
5 prime to 3
prime ends.
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The leading
edge replicates
in one smooth
motion.
The lagging
strand adds
nucleotides in
pieces, working
toward open
end.
DNA Replication
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The segments
produced on the
lagging end are
called Okazaki
segments
DNA ligase
helps connect
the Okazaki
segments
RNA
o
o
o
RNA: ribonucleic acid
Nucleic acid which
carries out protein
synthesis
Differences from
DNA: different sugar
(ribose), single
strand, different
base (no thymine,
uracil instead)
Types of RNA:
three types
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Messenger RNA:
carries nucleotide
sequence from
nucleus to ribosome
Transfer RNA: picks
up amino acid in
cytoplasm and
carries them to
ribosome
Ribosomal RNA:
found in ribosome,
joins mRNA and
tRNA; forms protein
Unit 4 -- Quiz #1
1. In DNA, adenine always bonds with ?
a. cytosine
b. guanine
c. thymine
2. In DNA, guanine always bonds with ?
a. adenine
b. cytosine
c. thymine
3. The shape of DNA is a double ?
a. coil
b. helix
c. ladder
4. What sugar is found in RNA?
a. deoxyribose b. glucose
c. ribose
5. In RNA, what takes the place of thymine?
a. adenine
b. cytosine
c. uracil