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A Closer Look at DNA: DNA Structure and Replication
A Closer Look at DNA: DNA Structure and Replication
History
James Watson and Francis Crick (1953)
:
Determined the structure of DNA.
Rosalind Franklin contributed through X-ray crystallography.
Structure of DNA
Nucleotides
: Building blocks of DNA (and RNA).
Components
:
Phosphate group
Pentose sugar
Nitrogenous base
Double Helix
:
Two strands form a ladder-like structure.
Backbone
: Sugar-phosphate.
Rungs
: Nitrogenous bases.
Purines
: Guanine (G), Adenine (A).
Pyrimidines
: Cytosine (C), Thymine (T).
Key Features
:
Antiparallel strands.
Hydrogen bonds connect bases.
Covalent bonds link the backbone.
Conventional Numbering System
:
Sugar carbons numbered 1' to 5'.
Base attached to 1', phosphate attached to 5'.
Facts about DNA
Human Genome
:
Each cell contains 46 DNA molecules (one double helix per chromosome).
Total of 6 billion base pairs in humans.
Stretched DNA: 5 feet long, 50 trillionths of an inch wide.
Human genome contains 3,164.7 million chemical nucleotide bases.
Genes & Estimates
:
Average gene: 3000 bases; largest gene (dystrophin): 2.4 million bases.
Estimated number of genes: 30,000 (much lower than previous estimates of 80,000 - 140,000).
99.9% of nucleotide bases are identical in all humans.
Functions of over 50% of genes are unknown.
Less than 2% of genome codes for proteins.
"Junk DNA" (non-coding sequences) constitutes at least 50% of genome.
The order of bases determines species identity.
Genomics
: Study of genomes.
DNA replication errors: 1 error per 10 billion nucleotides.
DNA Replication
Models of Replication
(1950s):
Semiconservative
Conservative
Dispersive
Key Terms
:
Daughter strands
: Newly made strands.
Parental strands
: Original strands.
Process
:
Begins at origin points (replication bubbles and forks).
Parental strands separate, serve as templates.
Follow A-T / G-C base pairing rules.
Result: 2 new double helices with identical base sequences.
Directionality
:
Synthesis proceeds 5' to 3'.
Leading strand synthesized continuously in direction of fork.
Lagging strand synthesized in Okazaki fragments.
Enzymatic Proteins in DNA Replication
DNA Polymerase
:
Catalyzes elongation of new DNA.
Cannot initiate synthesis on an empty template strand.
Requires RNA primer from DNA primase.
Primase
:
Synthesizes short RNA primers (5-10 nucleotides).
DNA Ligase
:
Joins Okazaki fragments.
DNA Helicase
:
Binds to DNA, unwinds strands using ATP.
DNA Topoisomerase
:
Relieves strain ahead of replication fork.
Single-Strand Binding Proteins
:
Keep parental strands open during replication.
Telomeres and DNA Repair
DNA Repair
:
Enzymes like DNA polymerase help repair damage.
Telomeres
:
Non-repairable regions of DNA.
Composed of repeating sequences (TTAGGG).
Function of Telomeres
:
Protect genes from erosion during cell division.
Telomerase
:
Enzyme that counters telomere shortening.
Active in gamete-producing cells and cancer cells.
Discovered by Carol Greider and Elizabeth Blackburn in 1984.
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Chapter 2: Beginnings of English America (1607-1660)
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Studied by 58 people
5.0
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y9 health
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Studied by 6 people
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Ultimate AP Pre Calc Notes (original)
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Studied by 1444 people
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APUSH midyear
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Studied by 23 people
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Chapter 26: Prosperity and Anxiety: The 1950s
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Studied by 18 people
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Cell/Molecular Biology
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Studied by 31 people
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