Chapter 10 Flashcards
Even before nucleic acids were identified as the genetic material, it was understood that:
1.Genetic material must contain complex information.
2.Genetic material must replicate faithfully.
3.Genetic material must
encode the phenotype.
4.Genetic material must
have the capacity to vary
The Hershey-Chase Experiment
Used T2 bacteriophage (virus that infects bacteria) to observe phage replication.
Watson and Crick’s discovery
Watson and Crick applied laws of structural chemistry to Franklin’s images to identify 3D structure of DNA.
Watson realized A could bind with T and G with C!
Accounted for Chargaff’s base ratios!
Tobacco mosaic virus
Single molecule of RNA surrounded by a helically arranged cylinder of protein molecules.
Singer and Fraenkel-Conrat created hybrid viruses by mixing RNA and protein from different strains of TMV and produced new viral particles!
Three levels of DNA structure
Primary structure
Secondary structure – stable three-dimensional configuration (helical structure worked out by Watson/Crick)
Tertiary structures – the complex packing arrangements of double-stranded DNA In chromosomes
Primary structure –
nucleotide structure and how the nucleotides are joined together
consists of a string of nucleotides joined together by phosphodiester linkages
DNA is fairly simple – a chain (polymer) of many repeating nucleotide units.
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Nucleotides consist of three main parts:
Sugar
Phosphate group
Nitrogen-containing base
Sugars
The sugars in DNA and RNA are slightly different in structure.
Ribosome and Deoxyribose
Nitrogen Bases
Purine: Bigger, Adenine (A) and Guanine (G)
Pyrimidine: Smaller Cytosine (C), Thymine(T) (DNA), Uracil (U) (RNA)
Phosphate group
Found in every nucleotide
Frequently carry a negative charge,
making DNA acidic.
Always bound to the 5’– carbon atom
of the sugar.
Polynucleotide Strands
Nucleotides connected by covalent bonds.
5’-phosphate group to the 3’-hydroxyl group
phosphodiester linkages – strong covalent bonds.
A series of these nucleotides = polynucleotide strand.
Double helix
Two polynucleotide strands
Sugar-phosphate linkages are on the outside
Bases are stacked in the interior of the molecule
The two strands are antiparallel - run in opposite directions
5’ end of one strand is opposite 3’ end of the other
Hydrogen bond imposes a limitation on bases that can pair.
3 H bonds between C -G
2 H bonds between A - T
This gives the the characteristic of complementary DNA strands – a critical feature for efficient DNA replication
Stacking pattern helps to stabilize the DNA molecule.
C-G pairing is_______than A–T pairing
stronger
DNA methylation:
Methyl groups (CH3)added to nucleotide bases
Related to gene expression in eukaryotes
Affects the three-dimensional structure of DNA
H-DNA:
three-stranded (triplex); formed when DNA unwinds and one strand pairs with double-stranded DNA from another part of the molecule
Long sequences of only purines or only pyrimidines
H-DNA breaks more readily than double-stranded DNA
RNA molecules may contain numerous hairpins, allowing them to
fold up into complex structures.
DNA normally consists of two polynucleotide strands that are
antiparallel and complementary.
B-DNA
Three-dimensional structure identified by Watson and Crick refers to B-DNA
Plenty of water
No unusual base sequences
Most stable configuration
Predominant
A-DNA
Exists if less water is present
Right-handed helix
Shorter and wider
Base pairs are tilted away from the main axis
Detected in some DNA-protein complexes and in spores of some bacteria.
Z-DNA
- Radically different in structure
Left-handed helix
Sugar-phosphate backbone zigzags back and forth
Can result if the molecule contains particular base sequences (i.e. stretches of alternating C and G nucleotides).
May play a role in gene expression
Hairpin structure
In single strands of nucleotides, when sequences of nucleotides on the same strand are inverted complements, a hairpin structure will be formed.
If complementary sequences are contiguous (close together/touching), the hairpin has a _____ but no _____
stem,loop
