DNA
 Molecular basis of herdity

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What is DNA?
Importance of DNA
 DNA stands for deoxyribonucleic acid.
 It is one of two nucleic acids found in the cell.

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What is DNA?
Importance of DNA
 DNA is the blueprint for life.
 Every living thing uses
DNA as a code for making proteins , which determines traits .

What is DNA?
 Importance of DNA
 For example, DNA contains the instructions for making the proteins (called pigments) which give your eyes color.

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What is DNA?
DNA is packaged into chromosomes .
Each chromosome is composed of one continuous DNA molecule.
The DNA molecule is wrapped around proteins and coiled tightly for protection

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What is DNA?
Remember, chromosomes are found in the nucleus of eukaryotic cells.
Prokaryotic cells have a single chromosome free-floating in the cytoplasm .

Fill It In …

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Discovery of DNA structure
Many scientists worked to determine the source of heredity .
 Heredity is the passing of traits from parent to offspring.
 But how are those traits passed?
Video

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Discovery of DNA structure
First, scientists determined that chromosomes controlled heredity and are made of DNA and proteins.

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Discovery of DNA structure
Then, scientists determined DNA was the chemical that controlled characteristics (traits of the organism)

Discovery of DNA structure
 Then, the race was on to reveal the structure of the DNA molecule

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Discovery of DNA structure
Rosalind Franklin was the first to take a clear
“picture” of DNA using a technique called X-ray crystallography
The “picture” offered a clue of the shape of DNA

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Discovery of DNA structure
Watson and Crick received credit for finalizing the model of DNA by using the picture taken by
Franklin (given to them by
Franklin’s research assistant - Maurice
Wilkins), and by syntehsizing work completed by other scientists.

Structure of the DNA molecule
 DNA is a double helix .
 The double helix looks like a twisted ladder .

Structure of the DNA molecule
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The building blocks of
DNA are called nucleotides .
A nucleotide consists of three parts:
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A sugar (named deoxyribose )
A phosphate group
 One of four nitrogen bases

Structure of the DNA molecule
 The four possible nitrogen bases in a
DNA molecule are named:
 Adenine (A)
 Thymine (T)
 Guanine (G)
 Cytosine (C)

Fill It In …
(From small to large)
N _____ base + sugar + phosphate
N __________
Two strands held together by _______ ______
D_______ H_________

Structure of the DNA molecule
 There are two strands of nucleotides in every
DNA molecule held together by weak hydrogen bonds between the nitrogen bases.

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Structure of the DNA molecule
The nitrogen bases bond in a specific way.
Adenine bonds with thymine (A-T).
Guanine bonds with cytosine (G-C ).
This pattern is called complementary base pairing.

Anti-parallel strands
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 DNA molecule has
“direction”
 complementary strand runs in opposite direction
5
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3
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3
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5
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Copying DNA
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 base pairing allows each strand to serve as a template for a new strand
 new strand is 1/2 parent template &
1/2 new DNA

Fill It In …
On this diagram:
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Highlight a nucleotide
Then write out the
NAME of the sugar beside one of the sugar molecules
Finally, draw a box around the
“backbone” and label.

Check Yourself!
1.
How is DNA connected to your traits?
2.
What larger structure is composed of DNA?
3.
What two parts of the nucleotide make up the sides
(backbone) of a DNA molecule?
4.
What makes up one rung/”step” of the DNA “ladder”?
5.
What type of bond holds the rungs together?

Check Yourself!
1.
2.
How is DNA connected to your traits? DNA CONTAINS
THE INSTRUCTIONS FOR MAKING PROTEINS
What larger structure is composed of DNA?
3.
What two parts of the nucleotide make up the sides
(backbone) of a DNA molecule?
4.
What makes up one rung/”step” of the DNA “ladder”?
5.
What type of bond holds the rungs together?

Check Yourself!
1.
2.
3.
How is DNA connected to your traits? DNA CONTAINS
THE INSTRUCTIONS FOR MAKING PROTEINS
What larger structure is composed of DNA?
CHROMOSOMES
What two parts of the nucleotide make up the sides
(backbone) of a DNA molecule?
4.
What makes up one rung/”step” of the DNA “ladder”?
5.
What type of bond holds the rungs together?

Check Yourself!
1.
2.
3.
4.
How is DNA connected to your traits? DNA CONTAINS
THE INSTRUCTIONS FOR MAKING PROTEINS
What larger structure is composed of DNA?
CHROMOSOMES
What two parts of the nucleotide make up the sides
(backbone) of a DNA molecule? SUGAR
(DEOXYRIBOSE) AND PHOSPHATE GROUP
What makes up one rung/”step” of the DNA “ladder”?
5.
What type of bond holds the rungs together?

Check Yourself!
1.
2.
3.
4.
5.
How is DNA connected to your traits? DNA CONTAINS
THE INSTRUCTIONS FOR MAKING PROTEINS
What larger structure is composed of DNA?
CHROMOSOMES
What two parts of the nucleotide make up the sides
(backbone) of a DNA molecule? SUGAR
(DEOXYRIBOSE) AND PHOSPHATE GROUP
What makes up one rung/”step” of the DNA “ladder”?
(COMPLEMENTARY) NITROGEN BASE PAIRS
What type of bond holds the rungs together?

Check Yourself!
1.
2.
3.
4.
5.
How is DNA connected to your traits? DNA CONTAINS
THE INSTRUCTIONS FOR MAKING PROTEINS
What larger structure is composed of DNA?
CHROMOSOMES
What two parts of the nucleotide make up the sides
(backbone) of a DNA molecule? SUGAR
(DEOXYRIBOSE) AND PHOSPHATE GROUP
What makes up one rung/”step” of the DNA “ladder”?
(COMPLEMENTARY) NITROGEN BASE PAIRS
What type of bond holds the rungs together?
HYDROGEN BOND

Structure of the DNA molecule

Do all my cells have the same
DNA?
 DNA replication copies DNA for new cells
What is DNA?

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DNA replication
DNA is needed in each cell to make necessary proteins .
 Because DNA is so important, when a cell divides, it must pass on an exact copy of the DNA to function correctly.

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DNA replication
Therefore, DNA is copied (replicated) during the S phase of the cell cycle (part of interphase, before mitosis/meiosis)

Fill It In …
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Let ’ s meet the team …
DNA Replication
Large team of enzymes coordinates replication

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Process of DNA replication
An enzyme breaks the weak hydrogen bonds between the paired nitrogen bases.
 This allows DNA to
“unzip” as the two strands move apart.

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Replication: 1st step
 helicase enzyme
 unwinds part of DNA helix
 stabilized by single-stranded binding proteins helicase single-stranded binding proteins replication fork

Process of DNA replication
 The newly unpaired nucleotides are paired
(A-T and G-C) with extra nucleotides present in the nucleus.
 This process is catalyzed by another enzyme .

Process of DNA replication
 Enzymes then link the nucleotides along the newly constructed side of the DNA ladder by bonding sugar to phosphate .

Replication: 2nd step
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 add new complementary bases
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DNA polymerase III
DNA
Polymerase III
But
We ’
… re missing for the bonding!

5
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Replication
DNA
Polymerase III
 can only add nucleotides to
3
 end of a growing
DNA strand
 need a “starter” nucleotide to bond to
 strand only grows
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3
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B.Y.O. ENERGY!
The energy rules the process energy
DNA
Polymerase III energy
DNA
Polymerase III
Polymerase III
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3
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5
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3
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5
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3
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Replication fork / Replication bubble
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3
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DNA polymerase III
3
 leading strand
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5
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3
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5
 lagging strand
5
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3
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5
 3 lagging strand
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5
 growing replication fork
3
 leading strand
5
 leading strand lagging strand
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5
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5
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3
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5
 growing replication fork
5
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3
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Starting DNA synthesis: RNA
 can only build onto 3
 end of an existing DNA strand
3
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5
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3
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5
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5
 growing replication fork
3
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3
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5
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DNA polymerase III
5
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3
 primase
RNA 5
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3
 RNA primer
 built by primase
 serves as starter sequence for DNA polymerase III

Replacing RNA primers with DNA
DNA polymerase I
 removes sections of RNA primer and replaces with
DNA nucleotides
DNA polymerase I
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3
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3
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5
 growing replication fork
3
 ligase
5
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RNA 5
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3
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But DNA polymerase I still can only build onto 3
 end of an existing DNA strand

Process of DNA replication
 The DNA is proofread by enzymes for any errors.

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Editing & proofreading DNA
1000 bases/second = lots of typos!
 DNA polymerase I
 proofreads & corrects typos
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 repairs mismatched bases removes abnormal bases
 repairs damage throughout life reduces error rate from
1 in 10,000 to
1 in 100 million bases

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Result of DNA replication
Two identical DNA molecules have been produced .
Each “ daughter ” DNA molecule is composed of one “old” strand and one “new” strand
(Here a “strand” refers to one chain of nucleotides)

Result of DNA replication
 Each copy of DNA is packaged as a chromatid on a doubled chromosome.

Result of DNA replication
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After mitosis , each daughter cell will receive one of the two identical copies of DNA.
This happens when the doubled chromosome is split, each new chromosome going to a new daughter cell.

Replication
 What?
 Where?
 Used for?
 When?
Fill It In …

1.
Check Yourself!
Why does each cell need DNA?
2.
3.
What is the name of the process which makes a copy of DNA?
When does DNA replication occur?
4.
What catalyzes each step of DNA replication?
5.
At the end of DNA replication, each molecule is composed of one _____ strand and one _____ strand.

1.
2.
3.
Check Yourself!
Why does each cell need DNA?
TO MAKE NECESSARY PROTEINS
What is the name of the process which makes a copy of DNA?
When does DNA replication occur?
4.
What catalyzes each step of DNA replication?
5.
At the end of DNA replication, each molecule is composed of one _____ strand and one _____ strand.

1.
2.
3.
Check Yourself!
Why does each cell need DNA?
TO MAKE NECESSARY PROTEINS
What is the name of the process which makes a copy of DNA? REPLICATION
When does DNA replication occur?
4.
What catalyzes each step of DNA replication?
5.
At the end of DNA replication, each molecule is composed of one _____ strand and one _____ strand.

1.
2.
3.
4.
Check Yourself!
Why does each cell need DNA?
TO MAKE NECESSARY PROTEINS
What is the name of the process which makes a copy of DNA? REPLICATION
When does DNA replication occur?
INTERPHASE/S-PHASE OF INTERPHASE
What catalyzes each step of DNA replication?
5.
At the end of DNA replication, each molecule is composed of one _____ strand and one _____ strand.

1.
2.
3.
4.
5.
Check Yourself!
Why does each cell need DNA?
TO MAKE NECESSARY PROTEINS
What is the name of the process which makes a copy of DNA? REPLICATION
When does DNA replication occur?
INTERPHASE/S-PHASE OF INTERPHASE
What catalyzes each step of DNA replication?
ENZYMES
At the end of DNA replication, each molecule is composed of one _____ strand and one _____ strand.

1.
2.
3.
4.
5.
Check Yourself!
Why does each cell need DNA?
TO MAKE NECESSARY PROTEINS
What is the name of the process which makes a copy of DNA? REPLICATION
When does DNA replication occur?
INTERPHASE/S-PHASE OF INTERPHASE
What catalyzes each step of DNA replication?
ENZYMES
At the end of DNA replication, each molecule is composed of one OLD strand and one NEW strand.