dna structure
TRANSCRIPT
DNAStructure and Replication
DNA STRUCTURE
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History of DNAEarly on, protein thought as the cell’s hereditary material because it was more complex than DNA
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Fred Griffith worked with virulent S and nonvirulent R strain Pneumoccocus bacteria.He found that R strain could become virulent when it took in DNA from heat-killed S strainStudy suggested that DNA was probably the genetic material
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History of DNAChromosomes are made of both DNA and proteinExperiments on bacteriophage viruses by Hershey & Chase proved that DNA was the cell’s genetic material
Radioactive 32P was injected into bacteria!
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Discovery of DNA StructureErwin Chargaff showed the amounts of the four bases on DNA ( A,T,C,G)In a body or somatic cell: A = 30.3% T = 30.3% G = 19.5% C = 19.9%
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Chargaff’s Rule
Adenine must pair with ThymineGuanine must pair with CytosineThe bases form weak hydrogen
bonds
G CT A
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James Watson (L) and Francis Crick (R), and the model they built of the structure of DNA
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HelixMost DNA has a right-hand twist with 10 base pairs in a complete turnLeft twisted DNA is called Z-DNA or southpaw DNAHot spots occur where right and left twisted DNA meet producing mutations
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The structure of DNA and RNA
Genetic material of living organisms is either DNA or RNA.
DNA – Deoxyribonucleic acidRNA – Ribonucleic acid
Genes are lengths of DNA that code for particular proteins.
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DNA and RNA are polynucleotidesBoth DNA and RNA are polynucleotides.They are made up of smaller molecules called nucleotides.
DNA is made of two polynucleotide strands:
RNA is made of a single polynucleotide strand:
Nucleotide NucleotideNucleotide
Nucleotide
Nucleotide
Nucleotide
Nucleotide
Nucleotide
Nucleotide Nucleotide Nucleotide Nucleotide
NucleotideNucleotide Nucleotide
Nucleotide
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Structure of a nucleotide
2.A Phosphate group
Phosphate groups are important because they link the sugar on one nucleotide onto the phosphate of the next nucleotide to make a polynucleotide.
S
P
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Structure of a nucleotide
3. A Nitrogenous base
In DNA the four bases are:
ThymineAdenineCytosineGuanine
In RNA the four bases are:
UracilAdenineCytosineGuanine
S
PN-base
Sugar phosphate bonds (backbone of DNA)
Nucleotides are connected to each other via the phosphate on one nucleotide and the sugar on the next nucleotideA Polynucleotide
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Nitrogenous bases – Two types
PyrimidinesHave single ring
Thymine - TCytosine -
CUracil - U
PurinesHave double
rings of Carbon and Nitrogen
atom
Adenine - AGuanine -
GBase-Pairings: Purines only pair with Pyrimidines
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Adenine
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Guanine
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Cytosine
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Base pairingSides made of a pentose sugar deoxyribose bonded to phosphate (PO4) groups by phosphodiester bondsCenter made of nitrogen bases bonded together by weak hydrogen bonds
Adenine links with Thymine or Uracil by 2 hydrogen bonds
Cytosine links with Guanine by 3 hydrogen bonds
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AS Biology. Gnetic control of protein structure and function
Again:
Adenine always base pairs with Thymine (or Uracil if RNA)
Cytosine always base pairs with Guanine.
This is because there is exactly enough room for one purine and one pyrimidine base between the two polynucleotide strands of DNA.
Complementary Base Pairing
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Nature of the Genetic Material
1. It must contain, in a stable form, information encoding the organism’s structure, function, development and reproduction2. It must replicate accurately so progeny cells have the same genetic makeup3. It must be capable of some variation (mutation) to permit evolution
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DNA
P
P
P
O
O
O
1
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4
5
5
3
3
5
P
P
PO
O
O
1
2 3
4
5
5
3
5
3
G C
T A
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Antiparallel StrandsOne strand of DNA goes from 5’ to 3’ (sugars)The other strand is opposite in direction going 3’ to 5’ (sugars)
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Question:
What would be the complementary DNA strand for the following DNA sequence?
DNA 5’-CGTATG-3’
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Answer:
DNA 5’-CGTATG-3’DNA 3’-GCATAC-5’
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