Nucleotides and the structure of DNA
Nucleotides and DNA
- DNA and RNA are nucleic acids — polymers of small units called nucleotides.
- The same building block also powers the cell as ATP.
- DNA's famous shape, the double helix, comes from how its bases pair up.
Nucleotides and bases
- Each nucleotide has three parts: a phosphate, a 5-carbon sugar, and a base.
- ATP is a nucleotide with the base adenine, the sugar ribose, and three phosphates — breaking off the last one releases energy.
- Five bases, in two groups:
- purines (double ring): adenine (A), guanine (G).
- pyrimidines (single ring): cytosine (C), thymine (T), uracil (U).
Practice
A single nucleotide is made of:
Every nucleotide has three parts: a phosphate group, a pentose sugar, and a nitrogen-containing base.
Practice
ATP releases energy for the cell when:
ATP has three phosphates; breaking off the terminal phosphate releases energy.
Practice
Which pair are both purines (double-ring bases)?
Purines (double ring) are adenine and guanine; the pyrimidines (single ring) are cytosine, thymine and uracil.
The DNA backbone
- DNA is two strands twisted into a double helix.
- Each strand has a backbone of alternating deoxyribose sugar and phosphate, joined by phosphodiester bonds.
- The two strands run in opposite directions — they are antiparallel (one 5′→3′, the other 3′→5′).
Complementary base pairing
- The strands are held together by their bases in the middle, paired exactly:
- A pairs with T — 2 hydrogen bonds.
- C pairs with G — 3 hydrogen bonds (so C–G is harder to separate).
Practice
In DNA, the base pairs are:
Complementary base pairing: A–T held by 2 hydrogen bonds, C–G by 3 (C–G is harder to separate).
Practice
The two DNA strands are described as antiparallel because they:
Antiparallel means the strands point in opposite directions: 5′→3′ on one, 3′→5′ on the other.
You've got it
Key idea
- nucleotide = phosphate + sugar + base; ATP = adenine + ribose + 3 phosphates (energy)
- purines (A, G, double ring) vs pyrimidines (C, T, U, single ring)
- DNA = double helix; backbone = deoxyribose + phosphate (phosphodiester bonds); strands antiparallel
- A–T (2 H-bonds), C–G (3 H-bonds) — complementary base pairing