6.1 DNA and RNA Structure
Keywords
| English Term | 中文翻译 | Definition & Explanation |
|---|---|---|
| Chromosome | 染色体 | A cellular structure consisting of one DNA molecule and associated protein molecules, carrying genetic information. |
| Histone | 组蛋白 | Small proteins that bind to DNA and play a key role in its chromatin structure and packaging in eukaryotes. |
| Plasmid | 质粒 | A small, circular, double-stranded DNA molecule that carries accessory genes separate from those of a bacterial chromosome. |
| Purine | 嘌呤 | A type of nitrogenous base with a double-ring structure (Adenine and Guanine). |
| Pyrimidine | 嘧啶 | A type of nitrogenous base with a single-ring structure (Cytosine, Thymine, and Uracil). |
| Base Pairing | 碱基配对 | The specific hydrogen bonding between purines and pyrimidines (A-T/U, G-C). |
1. Organizing the Genetic Blueprint
Genetic information is the blueprint of life. It is stored in and passed to subsequent generations primarily through DNA molecules (and, in the case of some viruses, RNA molecules).
Because DNA molecules are incredibly long, cells must organize and package them efficiently so they can fit inside the cell and be accurately divided during cell division. However, prokaryotes and eukaryotes handle this packaging differently:
- Prokaryotic Organisms: Typically have a single, circular chromosome. It is relatively naked (lacking complex packaging proteins) and floats freely in a region of the cytoplasm called the nucleoid.
- Eukaryotic Organisms: Typically have multiple, linear chromosomes located inside a membrane-bound nucleus. Because eukaryotic genomes are vastly larger, the DNA must be highly condensed. The linear DNA wraps around specialized proteins called histones (like thread wrapping around a spool) to form neatly packed chromosomes.
2. Plasmids: The Extra Baggage
In addition to the main chromosomal DNA, both prokaryotes and eukaryotes can contain plasmids.
- Plasmids are extra-chromosomal, circular molecules of DNA.
- They replicate independently of the main chromosome.
- While they are found in some eukaryotes (like yeast), they are most famous for their role in bacteria, where they often carry "bonus" genes, such as those that provide antibiotic resistance. Because they are small and independent, bacteria can easily trade plasmids with one another, spreading advantageous traits rapidly.
3. The Rings of Nucleic Acids: Purines and Pyrimidines
As we learned in Unit 1, nucleic acids are polymers of nucleotides. The actual genetic information is encoded in the sequence of the nitrogenous bases. These bases exhibit specific pairing rules that are highly conserved through evolution, demonstrating the common ancestry of all living things.
The nitrogenous bases are divided into two chemical families based on their ring structures:
- Purines: Have a double-ring structure.
- Adenine (A)
- Guanine (G)
- Pyrimidines: Have a single-ring structure.
- Cytosine (C)
- Thymine (T) - found only in DNA
- Uracil (U) - found only in RNA
Memory Trick: The Ring Families
- Purines: Think "Pure As Gold" \(\rightarrow\) Purines are Adenine and Guanine. (Gold is valuable, so it gets two rings!).
- Pyrimidines: Think "CUT the Py" \(\rightarrow\) Cytosine, Uracil, and Thymine are Pyrimidines. (A pie is a single ring).
The Base Pairing Rules
To maintain a perfect, uniform width down the entire DNA double helix, a double-ring molecule must always pair with a single-ring molecule. Purines always pair with pyrimidines.
- Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds. (In RNA, Adenine pairs with Uracil (U)).
- Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds.
Quiz
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