Concept 1 Unit 4 Summary & Study Notes

🧬 Nucleic Acids

Nucleic acids are the macromolecules that store and transmit genetic information. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Genes are specific sections of DNA that provide the instructions to make proteins, which carry out cellular functions.

🧩 Nucleotides

Nucleotides are the monomers of nucleic acids and consist of three parts: a sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base. The bases are adenine (A), guanine (G), cytosine (C), thymine (T, DNA only) and uracil (U, RNA only).

🧪 DNA Structure

DNA is a double helix that resembles a twisted ladder. The sugar-phosphate backbone forms the outside rails of the ladder, linked by strong covalent bonds. The nitrogenous bases pair in the middle via weaker hydrogen bonds, forming the rungs.

🧫 Base Pairing Rules

Base pairing is complementary: A pairs with T in DNA, and C pairs with G. These pairings are held together by hydrogen bonds and ensure accurate information storage and transfer.

🧬 RNA Structure

RNA is usually a single strand of nucleotides with exposed bases. In RNA-DNA interactions, A pairs with U (in RNA) and C pairs with G. RNA plays roles in coding, decoding, regulation, and expression of genes.

🔬 DNA vs RNA (Key Differences)

DNA contains A, T, C, G, uses deoxyribose, and is typically a double helix. RNA contains A, U, C, G, uses ribose, and is usually single-stranded. Their structural differences relate directly to their cellular roles.

📚 Genes and Chromosomes

A gene is a section of DNA that codes for a protein. Chromosomes are long, tightly coiled strands of DNA; organisms differ in chromosome number (humans: 23 pairs, 46 total). A single chromosome can contain thousands of genes linked along its length.

🔁 DNA Replication Overview

Before cell division, the cell must copy its DNA in a process called DNA replication (DNA → DNA). Replication occurs in the nucleus during the S phase of interphase and ensures each daughter cell receives an identical genome.

⚙️ Steps of Replication (Simplified)

1. The DNA double helix is unwound/unzipped by enzymes.
2. Each original (parent) strand serves as a template. Complementary nucleotides are matched to each template strand following base-pairing rules (A-T, C-G).
3. Enzymes (e.g., DNA polymerases) synthesize new strands and connect nucleotides with covalent bonds, producing two identical DNA molecules, each with one old and one new strand.

🧾 Semi-Conservative Replication

Replication is described as semi-conservative because each newly formed double helix conserves one original strand paired with one newly synthesized strand. This mechanism helps preserve genetic fidelity.

⚠️ Replication Errors and Consequences

Errors (mutations) can occur if the wrong base is incorporated or if structural problems arise. A mistake in a somatic (body) cell can affect tissue function or potentially lead to cancer, but it is not heritable. A mistake in a germ cell (sperm or egg) can be passed to offspring, affecting the entire organism.

A gene-level error alters a sequence that may change a single protein, while a chromosomal error (deletion, duplication, translocation) can affect many genes and have larger-scale consequences.

❓ Considerations and Big Picture

Accurate replication is essential for organismal survival and heredity. Cellular systems include proofreading and repair mechanisms to correct many mistakes, but unrepaired errors can lead to variation or disease. Understanding the distinctions between DNA/RNA, genes/chromosomes, and somatic/germline mutations is key to interpreting genetic outcomes.