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Questions and Answers
What is the primary characteristic of negative supercoiling in DNA?
What is the primary characteristic of negative supercoiling in DNA?
Which enzyme is responsible for introducing negative supercoils in prokaryotic DNA?
Which enzyme is responsible for introducing negative supercoils in prokaryotic DNA?
How does positive supercoiling affect the DNA structure compared to negative supercoiling?
How does positive supercoiling affect the DNA structure compared to negative supercoiling?
Which of the following statements about eukaryotic DNA is true?
Which of the following statements about eukaryotic DNA is true?
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What is the role of topoisomerases in DNA supercoiling?
What is the role of topoisomerases in DNA supercoiling?
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What is the primary factor that contributes to the formation of Z-DNA?
What is the primary factor that contributes to the formation of Z-DNA?
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In which situation is the A-helix form of DNA predominantly favored?
In which situation is the A-helix form of DNA predominantly favored?
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Which structural feature distinguishes single-stranded DNA from double-stranded DNA?
Which structural feature distinguishes single-stranded DNA from double-stranded DNA?
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What results from the presence of regions of Z-DNA near genes?
What results from the presence of regions of Z-DNA near genes?
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How does the G+C content in complex organisms typically vary?
How does the G+C content in complex organisms typically vary?
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What is a distinct characteristic of the A-helix compared to the B-helix?
What is a distinct characteristic of the A-helix compared to the B-helix?
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Which characteristic defines the structural composition of the DNA backbone?
Which characteristic defines the structural composition of the DNA backbone?
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What is the base pairing arrangement between strands in DNA?
What is the base pairing arrangement between strands in DNA?
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Study Notes
DNA Structure and Supercoiling
- DNA is composed of two helical strands—a double-stranded helix.
- Sugar-phosphate molecules form the DNA backbone.
- Nitrogenous bases are stacked in a helical arrangement within the central core.
- Bases from one strand are hydrogen-bonded to bases on the opposite strand (A-T; G-C).
- Major and minor grooves allow proteins to interact with the bases.
- Strands run antiparallel.
G+C Content
- G+C content is approximately 50% in most complex organisms.
- Extremes in most plants and animals are 48% and 52%.
- The percentage of G+C varies widely across single-cell organisms.
- Examples include 27% for Clostridium and 76% for Sarcina; E. coli is 50%.
Alternate DNA Structures
- B-DNA: Right-handed helix, the most common form.
- A-DNA: Wider and shorter than B-DNA, with a 30° tilt of the base pairs. Favored under dehydration conditions, and common in RNA-DNA hybrids, and some DNA-protein complexes.
- Z-DNA: Left-handed helix with a zigzag backbone. Has 12 base pairs per turn, and is longer and thinner than B-DNA. Primarily associated with alternating G-C sequences. Regions of Z DNA can affect gene expression.
Single-Stranded DNA
- Occurs in some viruses.
- Structure is irregular.
- Can fold back on itself.
- Short double-stranded regions can form between complementary regions.
- Single-stranded DNA (ssDNA) is denser than double-stranded DNA (dsDNA).
Triple and Four-Stranded DNA
- Found in DNA with repeating purine-pyrimidine sequences.
- Important in telomeres (chromosome ends) potentially influencing meiosis.
DNA Structure: Supercoiling
- Linearized E. coli chromosome would be over 1mm in length, considerably bigger than the cell itself.
- Supercoiling is the process of further twisting double-stranded DNA to compact it into the cell.
- Supercoiling can be negative (opposite twist to the right-hand helix) or positive (same twist).
- Negatively supercoiled DNA is most common in nature.
DNA Gyrase and Topoisomerases
- DNA gyrase: A key enzyme in prokaryotes that introduces negative supercoils in DNA and in turn, compacts it.
- Reverse gyrase introduces positive supercoiling.
- Topoisomerases: Enzymes in prokaryotes that control DNA supercoiling. Topoisomerase II (DNA gyrase) enhances supercoiling and Topoisomerase I releases supercoiling.
- In eukaryotes, DNA is wound around histone proteins to form nucleosomes.
- Little proteins are also associated with prokaryotic chromosomes.
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Description
Explore the fascinating world of DNA structure, including its double helical formation and the crucial roles of sugar-phosphate backbones and nitrogenous bases. This quiz also delves into G+C content variations in complex organisms and alternative DNA structures like B-DNA, A-DNA, and Z-DNA.
