DNA Structure and Genome Organization

What's the repeating unit of DNA/RNA, and what 3 parts make it up?

A nucleotide: a 5-carbon sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base.

Structural differences between DNA and RNA?

DNA: deoxyribose, thymine, usually double-stranded, B-form helix. RNA: ribose, uracil instead of thymine, usually single-stranded but folds into complex secondary/tertiary structures via internal base pairing.

What holds the two DNA strands together, and in what orientation do they run?

Hydrogen bonds between complementary bases (Watson-Crick-Franklin pairing: A-T, G-C). The two strands are antiparallel — one runs 5'→3', the other 3'→5'.

What did Chargaff's rules establish about base composition?

-A=T and G=C always; base ratios (like %AT vs %GC) differ between species but stay constant across cell types, age, and environment within one organism.

What non-standard base pair is allowed only in RNA?

G-U wobble pairing, when RNA base-pairs with itself or another RNA molecule (not with DNA).

Describe B-DNA, the dominant cellular conformation.

Right-handed helix, ~10.5 base pairs per turn, bases stacked flat and perpendicular to the backbone, hydrophobic bases buried inside, major and minor grooves exposed for protein binding.

How do A-DNA and Z-DNA differ from B-DNA?

-DNA: forms when DNA is dehydrated; models dsRNA and RNA-DNA hybrids.

Z-DNA: left-handed, <1% of cellular DNA, found in high G-C regions.

How is a prokaryotic genome organized, using E. coli as the example?

One circular chromosome (~4.6 million bp, ~4,435 genes), no bound histone proteins, gene positions fixed within the species.

How is a eukaryotic genome organized, using humans as the example?

46 linear chromosomes; DNA is wrapped around histone proteins into nucleosomes, packaged into chromatin; chromosomes have telomeres (ends) and a centromere; specific genes sit at fixed chromosomal positions (e.g., EDN1 at 6p24).

What is the central dogma of molecular biology?

DNA is transcribed into RNA, and RNA is translated into protein (DNA → RNA → protein). DNA also replicates itself, and this flow can be refined further with reverse transcription and multiple layers of regulatory control.

Why do ~200 different cell types exist if every cell has identical DNA?

Because gene expression — which genes get turned on/off — differs by cell type, even though the underlying DNA sequence is the same everywhere in the body.

How can a tiny DNA change cause a major phenotype change? (CF example)

The ΔF508 mutation deletes just 3 nucleotides from the CFTR gene. This misfolds the CFTR protein, blocking chloride transport, causing thick mucus buildup and elevated sweat chloride — a small sequence change with large downstream effects.

What question was Avery's experiment designed to answer, and using what organism?

Which molecule carries genetic information in a cell? Tested using Streptococcus pneumoniae: smooth (S, capsule-forming, pathogenic) vs rough (R, non-capsule, nonpathogenic) strains.

What is bacterial transformation, and what did it show in Avery's setup?

External DNA taken up by a bacterial cell that changes its traits. A cell-free extract from heat-killed S cells could transform R cells into S cells — proving genetic material could pass between cells outside a living organism.

How did Avery pinpoint DNA (not protein or RNA) as the genetic material?

He treated the transforming extract with enzymes that each degrade one macromolecule: proteinase and RNase still allowed R→S transformation, but DNase blocked it — so DNA had to be the genetic material.

What did Rosalind Franklin's Photo 51 reveal, and how?

Using X-ray diffraction (X-rays bending based on atomic positions), it showed DNA is a helix with a consistent diameter — each turn 34 Å long with 10 base pairs spaced 3.4 Å apart.

How did Chargaff's data serve as evidence for base pairing rules later confirmed by the double helix model?

His finding that A always equals T and G always equals C (regardless of species) hinted at a fixed pairing scheme, which Watson, Crick, and Franklin's model explained structurally via hydrogen bonding.

Whose combined evidence led to the final DNA double helix model?

Franklin's X-ray diffraction data (helical, dimensions) + Chargaff's base ratios (A=T, G=C) were integrated by Watson and Crick into the antiparallel, base-paired double helix model.