DNA Structure Overview

DNA Structure

• DNA is a double-stranded molecule (dsDNA).
• The two strands are held together by hydrogen bonds through complementary base pairing.
• Guanine (G) pairs with cytosine (C) forming three hydrogen bonds.
• Adenine (A) pairs with thymine (T) forming two hydrogen bonds.
• The DNA sequence is a primary structure, and the DNA helix is a secondary structure.
• DNA strands run in opposite directions (antiparallel).
• The 3' end of DNA has a hydroxyl group at the 3' position on the deoxyribose sugar.
• The 5' end of DNA has a phosphate group at the 5' position on the deoxyribose sugar.
• DNA exists in different forms: B-form, A-form, and Z-form.
• The B-form is a right-handed helix with 10 base pairs (bps) per 360° turn.
• The A-form has 11 bps per 360° turn and the bases tilted 20° from perpendicular.
• The Z-form is a left-handed helix with 12 bps per turn.
• The transition between B and Z forms may affect gene expression regulation.
• The hydrophilic deoxyribose-phosphate backbones are on the outside of the helix.
• The hydrophobic bases are stacked in the interior.
• The spatial relationship between the strands creates major and minor grooves.
• Each helical turn is separated by 0.34 nanometers.

Nucleotides

• Nucleotides are the building blocks of DNA and RNA.
• Each nucleotide has three components: a nitrogenous base, a pentose sugar, and a phosphate group.
• DNA nucleotides contain deoxyribose sugar, while RNA nucleotides contain ribose sugar.
• DNA has thymine (T), while RNA has uracil (U).
• DNA is a polymer of deoxynucleoside monophosphates (dNMPs).
• RNA is a polymer of nucleoside monophosphates (NMPs).
• Purines are adenine (A) and guanine (G) (two rings).
• Pyrimidines are cytosine (C), thymine (T), and uracil (U) (one ring).

Chargaff's Rule

• In double-stranded DNA:
  • The amount of adenine (A) equals the amount of thymine (T).
  • The amount of guanine (G) equals the amount of cytosine (C).
  • The total amount of purines (A + G) equals the total amount of pyrimidines (T + C).

Denaturation

• Denaturation occurs when hydrogen bonds between complementary bases are disrupted.
• This process can happen under specific lab conditions (pH change, high temperatures).
• Tm (Melting Temperature) is the temperature at which half of the helical structure in DNA becomes single stranded.
• DNA with a higher proportion of AT base pairs has a lower Tm compared to GC-rich DNA.

Quiz Question

• If adenine = 30%, the percentage of guanine will be 20% (since G always pairs with C, and A with T).

This quiz explores the fundamental structure of DNA, focusing on its double-stranded nature, base pairing, and the various forms such as B, A, and Z. Understand the significance of antiparallel strands and how structural differences can influence gene expression. Perfect for students studying molecular biology.