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A HISTORY OF DNAA HISTORY OF DNA
• Discovery of the DNA double helixDNA double helix
A. Frederick Griffith – Discovers that a factor in diseased bacteria can transform harmless bacteria into deadly bacteria (1928)
B. Rosalind Franklin - X-ray photo of DNA.(1952)
C. Watson and Crick - described the DNA molecule from Franklin’s X-ray.(1953)
SEE p. 230 - 233
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Discovering the structure of DNA
Rosalind Franklin’s DNA image
“Chargoff’s rule”
A = T & C = G
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Chargaff’s RuleChargaff’s Rule
• AdenineAdenine must pair with ThymineThymine
• GuanineGuanine must pair with CytosineCytosine
• Their amounts in a given DNA molecule will be about the sameabout the same.
G CT A
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DNA stands for deoxyribose nucleic acid
This chemical substance is present in the nucleusof all cells in all living organisms
DNA controls all the chemical changes which take place in cells
The kind of cell which is formed, (muscle, blood,nerve etc) is controlled by DNA
The kind of organism which is produced (buttercup,giraffe, herring, human etc) is controlled by DNA
DNA 2
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DNA is a very large molecule made up of a long chain of sub-units
The sub-units are called nucleotides
Each nucleotide is made up of
a sugar called deoxyribose
a phosphate group -PO4 and
an organic base
DNA molecule 3
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Ribose is a sugar, like glucose, but with only fivecarbon atoms in its molecule
Deoxyribose is almost the same but lacks one oxygen atom
Both molecules may be represented by the symbol
Ribose & deoxyribose 4
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The most common organic bases are
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
The bases 5
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Nitrogenous BasesNitrogenous Bases
• PURINESPURINES
1. Adenine (A)Adenine (A)
2. Guanine (G)Guanine (G)
• PYRIMIDINESPYRIMIDINES
3. Thymine (T)Thymine (T)
4. Cytosine (C)Cytosine (C)T or C
A or G
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The deoxyribose, the phosphate and one of the bases
adenine
deoxyribose
PO4
Combine to form a nucleotide
Nucleotides 6
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A molecule of DNA is formed by millions of nucleotides joined together in a long chain
PO4
PO4
PO4
PO4
sugar-phosphate backbone
+ bases
Joined nucleotides 7
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In fact, the DNA usually consists of a double strand of nucleotides
The sugar-phosphate chains are on the outsideand the strands are held together by chemical bonds between the bases
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PO4
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2-stranded DNA 9
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The bases always pair up in the same way
Adenine forms a bond with Thymine
and Cytosine bonds with Guanine
Bonding 1 10
Adenine Thymine
Cytosine Guanine
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PO4
PO4
PO4
thymine
PO4
PO4
PO4
PO4
adenine
cytosine
PO4
guanine
Bonding 2 11
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PO4
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Pairing up 12
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The paired strands are coiled into a spiral called
A DOUBLE HELIX
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sugar-phosphatechain
bases
THE DOUBLE HELIX
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A DIY model ofpart of a DNA molecule
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Before a cell divides, the DNA strands unwind and separate
Each strand makes a new partner by addingthe appropriate nucleotides
The result is that there are now two double-stranded DNA molecules in the nucleus
So that when the cell divides, each nucleus contains identical DNA
This process is called replication
replication 16
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DNA DNA ReplicationReplication• DNA must be copiedDNA must be copied
• The DNA molecule produces The DNA molecule produces 2 2 IDENTICALIDENTICAL new complementary new complementary strands following the rules of base strands following the rules of base pairing: pairing:
A-T, G-CA-T, G-C
•Each strand of the Each strand of the original DNA serves as original DNA serves as a template for the new a template for the new strand strand See p. 298See p. 298
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Proteins that are used1. Helicase: Unwinds a portion of the DNA
Double Helix
2. DNA Polymerase – brings in complementary bases. (one strand continuous; the other fragmented)
3. DNA Ligase: Adds phosphate in the remaining gaps of the phosphate - sugar backbone
4. Nucleases: Remove wrong nucleotides from the daughter strand
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DNA DNA ReplicationReplication
• Semiconservative Semiconservative Model:Model:
1. Watson and Crick Watson and Crick showed:showed: the two strands of the parental molecule separate, and each functions as a template for synthesis of a new complementary strand.
. Parental DNA
DNA Template
New DNA
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PO4
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The strands separate
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PO4
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Each strand builds up its partner by adding the appropriate nucleotides
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The sequence of bases in DNA forms the Genetic Code
A group of three bases (a triplet) controlsthe production of a particular amino acid in the cytoplasm of the cell
The different amino acids and the order in which they are joined up determines the sort of protein being produced
Genetic code 1 19
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Ser-Cyst-Val-Gly-Ser-Cyst Ala Val Val-Cyst-Ser-Ala-Ser-Cyst-Gly
Val- Cyst-Ala-Ala-Ser-Gly
This is a small, imaginary protein molecule showing how a sequence of 5 different amino acids could determine the shape and identity of the molecule
Each amino acid (Serine, Cysteine, Valine, Glycine and Alanine) is coded for by a particular triplet of bases
Genetic code 2 20
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For example
Cytosine
Adenine Codes for Valine
Cytosine (C)
Guanine (G)
Adenine (A)
Codes for Alanine
Thymine
Coding 21
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This is known as the triplet code
Each triplet codes for a specific amino acid
CGA - CAA - CCA - CCA - GCT - GGG - GAG - CCA -
Ala Val Gly Gly Arg Pro Leu Gly
Ala Val Gly Gly Arg Pro Leu Gly
The amino acids are joined together in the correct sequence to make part of a protein
Triplet code 22
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The proteins build the cell structures
They also make enzymes
The DNA controls which enzymes are made andthe enzymes determine what reactions take place
The structures and reactions in the cell determinewhat sort of a cell it is and what its function is
So DNA exerts its control through the enzymes
DNA and enzymes 23
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A sequence of triplets in the DNA molecule may code for a complete protein
Such a sequence forms a gene
There may be a thousand or more bases in one gene
Genes 24
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Question 1
Which of the following are components of nucleotides?
(a) deoxyribose
(b) amino acids
(c) phosphate
(d) enzymes
(e) organic bases
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Question 2
Which of the following represent a correct pairing of bases?
(a) adenine with thymine
(b) adenine with guanine
(c) thymine with adenine
(d) guanine with cytosine
(e) thymine with thymine
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Question 3
DNA molecules are formed from
(a) organic bases
(b) amino acids
(c) deoxyribose
(d) nucleotides
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Question 4
Which of the following are organic bases?
(a) Valine
(b) Guanine
(c) Thymine
(d) Serine
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Question 5
Replication of DNA occurs
(a) During cell division
(b) before cell division
(c) at any time
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Question 6
A nucleotide triplet codes for
(a) a protein
(b) an amino acid
(c) an enzyme
(d) an organic base
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Answer
CORRECT
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Answer
INCORRECT