6.1 DNA and RNA Structure
- Syllabus
- 2025
- Topic
- 6.1
- Level
- —
Learning objectives
6.1A—Describe the structures involved in passing hereditary information from one generation to the nextDescribe the structures involved in passing hereditary information from one generation to the next.• Genetic information is stored in and passed to subsequent generations through DNA molecules and, in some cases, RNA molecules.- i. Prokaryotic organisms typically have cir cular chromosomes.- ii. Eukaryotic organisms typically have multiple linear chr omosomes that are comprised of DNA. These chromosomes are condensed using histones and associated proteins.• Prokaryotes and eukaryotes can contain plasmids, which are extra-chromosomal circular molecules of DNA.6.1B—Describe the characteristics of DNA that allow it to be used as hereditary materialDescribe the characteristics of DNA that allow it to be used as hereditary material.• Nucleic acids exhibit specific nucleotide base pairing that is conserved through evolution.- i. Purines (guanine and adenine) have a double ring structur e.- ii. Pyrimidines (cytosine, thymine, and uracil) ha ve a single ring structure.- iii. Purines pair with pyrimidines: adenine with thymine (or uracil in RNA) and guanine with cytosine.
How Cells Organize Hereditary Information
Hereditary information is stored in the nucleotide sequence of DNA and, in some biological systems, RNA. Cells organize DNA into chromosomes and may also carry smaller extra-chromosomal DNA molecules called plasmids.
| Structure | Typical organization | Hereditary role |
|---|---|---|
| Prokaryotic chromosome | Usually one circular DNA molecule | Carries the organism's main genetic information |
| Eukaryotic chromosomes | Multiple linear DNA molecules condensed with histones and associated proteins | Package and transmit the nuclear genome |
| Plasmid | Small, circular, extra-chromosomal DNA found in prokaryotes and some eukaryotes | Carries additional genes outside the main chromosome |
The shape and packaging differ, but each DNA structure preserves information in its base sequence. Before cell division or reproduction, the information must be copied and then distributed to descendant cells or organisms.
A plasmid is DNA but is not part of the main chromosome. Circular does not mean unorganized, and linear does not mean unprotected; both are structured hereditary molecules.
Why Complementary Base Pairing Supports Heredity
DNA is suitable hereditary material because its base sequence stores information and each strand has a predictable complementary partner. Specific base pairing allows one strand to guide the construction of another with the corresponding sequence.
| Base | Ring class | Complementary partner |
|---|---|---|
| Adenine (A) | Purine: double ring | Thymine (T) in DNA or uracil (U) in RNA |
| Guanine (G) | Purine: double ring | Cytosine (C) |
| Cytosine (C) | Pyrimidine: single ring | Guanine (G) |
| Thymine (T) | Pyrimidine: single ring | Adenine (A) in DNA |
| Uracil (U) | Pyrimidine: single ring | Adenine (A) in RNA |
Pairing one purine with one pyrimidine keeps the paired structure consistent. Hydrogen bonds hold complementary bases together, yet the strands can separate so each can serve as a template. The conserved A-T and G-C rules therefore connect molecular structure to accurate hereditary copying.
DNA does not use uracil, and RNA does not normally use thymine. Complementarity means a specific partner, not that any purine can pair with any pyrimidine.