DNA, RNA, Replication, Central Dogma & Genetic Engineering — Study Materials Summary & Study Notes

🧬 Structure of nucleic acids

Both DNA and RNA are nucleic acids built from three repeating parts: a sugar, a phosphate group, and a nitrogenous base. The backbone is a repeating sugar–phosphate chain; bases project inward and encode information.

🔬 Key chemical differences

In RNA the sugar is ribose; in DNA the sugar is deoxyribose (one less oxygen). RNA is usually single-stranded; DNA is a double helix composed of two complementary strands.

🧩 Nitrogenous bases

RNA bases: adenine (A), guanine (G), cytosine (C), uracil (U).
DNA bases: adenine (A), guanine (G), cytosine (C), thymine (T).
Uracil in RNA and thymine in DNA are chemically similar and serve the same base-pairing roles.

🔁 DNA replication (overview)

DNA replication occurs whenever a cell copies itself (cell division). The double helix is separated (unzipped) and each original strand serves as a template for a new complementary strand. After replication there are two DNA molecules, each containing one original strand and one newly synthesized strand.

⚙️ Replication mechanics

DNA synthesis enzymes (DNA polymerases) can only add nucleotides to the $3'$ end of a growing strand. This creates a smoothly synthesized leading strand and a discontinuously synthesized lagging strand (Okazaki fragments) on the opposite template. The end result is two identical DNA duplexes distributed to daughter cells.

🧾 Central dogma: flow of information

Francis Crick coined the term central dogma, summarizing information flow: DNA → RNA → Protein. The first step is transcription (DNA to RNA). The second step is translation (mRNA to protein).

✍️ Transcription

An enzyme RNA polymerase reads a gene (a DNA segment) and synthesizes a complementary messenger RNA (mRNA) strand. In eukaryotes the DNA remains in the nucleus and mRNA exits to the cytoplasm to be translated.

🧫 Translation

In the cytoplasm ribosomes read mRNA codons (three-base groups). Transfer RNA (tRNA) molecules carry specific amino acids and match codons via anticodons. Ribosomes catalyze peptide-bond formation, producing a polypeptide (protein) chain that folds and carries out cellular functions.

🎭 From genes to phenotype

Proteins produce observable traits (the phenotype). Small genetic changes (mutations) can alter mRNA and amino acid sequence and therefore protein function, sometimes producing noticeable phenotype changes subject to natural selection.

🐾 Extended phenotype

Richard Dawkins' concept of the extended phenotype expands selection targets beyond an organism's body to include structures or behaviors produced by genes (e.g., beaver dams). These external effects can also influence survival and reproduction.

🧪 Genetic engineering and plasmids

Many bacteria naturally exchange DNA via plasmids (small, auxiliary DNA). Scientists harness this by inserting genes (including human genes) into bacterial plasmids so bacteria express human proteins. Examples: recombinant insulin production in bacteria and experimental expression of human proteins in plants (e.g., safflower) for cost-effective production.

✅ Summary points to remember

• Backbone = sugar + phosphate; bases encode information.
• RNA has U, DNA has T; sugars differ by one oxygen.
• Replication is semi-conservative: each new duplex contains one old strand.
• Transcription (DNA → mRNA) occurs in the nucleus; translation (mRNA → protein) occurs at ribosomes in the cytoplasm.
• DNA polymerase adds only to the $3'$ end.
• Genetic engineering uses plasmids and recombinant DNA to produce proteins like insulin.