Concept 3 Unit 4 Summary & Study Notes

🧬 DNA Structure & Replication

Nucleic acids are the macromolecules that store genetic information. DNA (deoxyribonucleic acid) contains genes, which are sections of DNA that code for proteins. DNA is made of nucleotides, each with a sugar (deoxyribose), a phosphate group, and a nitrogenous base (A, T, C, G).

Double helix describes DNA's shape: the sugar-phosphate backbones form the sides and nitrogen bases pair in the middle via hydrogen bonds. Base-pairing follows complementary rules: A—T and C—G.

DNA replication happens during S phase of interphase. The double helix is unzipped and each original strand serves as a template for a new complementary strand. Replication is semi-conservative: each new double helix contains one old strand and one newly synthesized strand. Enzymes ensure complementary base matching but mistakes (mutations) can occur with differing consequences depending on whether they occur in somatic or germ cells.

🔍 Key Concepts & Questions

• Chromosomes are condensed DNA; organisms have characteristic chromosome numbers (e.g., humans: 23 pairs). A gene is a specific DNA segment on a chromosome.
• Consider the difference between errors in a body cell vs errors in a germ cell: germ-line errors can be inherited, while somatic errors affect only the individual.

🧫 Protein Synthesis (Transcription & Translation)

Protein synthesis is the process of reading DNA instructions to build polypeptides (proteins). Because DNA stays in the nucleus, information is transferred via RNA in two steps: transcription (DNA → mRNA) and translation (mRNA → polypeptide).

There are three essential types of RNA: mRNA (messenger RNA) carries the code from nucleus to ribosomes; tRNA (transfer RNA) brings specific amino acids to the ribosome; rRNA (ribosomal RNA) combines with proteins to form the ribosome and helps catalyze peptide bond formation.

During transcription, a gene is unzipped and a complementary mRNA strand is synthesized using base-pairing rules (A pairs with U in RNA). Translation occurs at the ribosome: mRNA codons (triplets) are read, tRNA anticodons bring matching amino acids, and the ribosome links amino acids with peptide bonds until a stop codon terminates the polypeptide.

⚙️ Regulation & Expression

Gene expression is tightly regulated; cells selectively turn genes on or off to differentiate (e.g., skin vs. muscle cells). Transcription factors (activators/repressors) control this regulation. Epigenetics refers to heritable changes in gene expression without altering the DNA sequence (e.g., histone modification).

🔬 Meiosis & Sexual Reproduction

Organisms have two main cell types: somatic cells (body cells, diploid = 2n) and gametes (sex cells, haploid = n). In humans, somatic cells are 2n = 46 and gametes are n = 23. Autosomes are chromosome pairs 1–22; the sex chromosomes are pair 23 (X or Y).

Meiosis is the specialized cell division that produces haploid gametes in gonads (ovaries, testes). It involves two rounds of division: Meiosis I (separates homologous chromosome pairs and reduces chromosome number from 2n to n) and Meiosis II (separates sister chromatids).

Key stages of Meiosis I: Prophase I (homologous chromosomes pair to form tetrads and may undergo crossing over, exchanging DNA and increasing genetic variation), Metaphase I (paired homologs align at the metaphase plate), Anaphase I (homologs separate), and Telophase I/Cytokinesis (two haploid cells form, each with duplicated chromosomes).

Meiosis II resembles mitosis: Prophase II, Metaphase II (sister chromatids line up single file), Anaphase II (sister chromatids separate), and Telophase II/Cytokinesis result in four genetically unique haploid gametes.

🔁 Mitosis vs Meiosis (Summary)

• Mitosis produces two identical diploid somatic cells used for growth and repair (PMAT once).
• Meiosis produces four unique haploid gametes for sexual reproduction (PMAT twice). Crossing over and independent assortment generate genetic diversity.