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Biology Quick-Review: Carbohydrates, Nucleic Acids, Proteins, Cells, and the Cell Cycle

Carbohydrates

  • Building blocks: monosaccharides; polymers formed by dehydration synthesis; hydrolysis breaks polymers into monosaccharides.

  • Examples: glycogen (animal polysaccharide; highly branched) vs starch and cellulose (plants).

  • Glycogen: highly branched for rapid glucose release in animals.

  • Sugar classification by carbon number: hexose = C6H{12}O6; pentose = C5H{10}O5.

  • Digestible vs indigestible: cellulose is fiber (not digestible in humans).

Nucleic Acids

  • Types: DNA and RNA.

  • Building blocks: nucleotides (nitrogenous base + sugar + phosphate).

  • Nitrogenous bases:

    • Purines: A\,G (two-ring structures).

    • Pyrimidines: C\,T\,U (one-ring structures).

  • Base pairing:

    • DNA: A\text{ pairs with }T; C\text{ pairs with }G.

    • RNA: A\text{ pairs with }U; C\text{ pairs with }G.

  • DNA structure: double helix; backbone made of sugars and phosphates; covalent phosphodiester bonds hold the backbone; hydrogen bonds between bases hold the two strands.

  • RNA: single-stranded.

  • ATP and energy: nucleotides with three phosphates can act as energy packets; common example in class is ATP\rightarrow ADP + P_i.

  • Nucleotides components: nitrogenous base + sugar + phosphate.

Proteins

  • Building blocks: amino acids (20 types).

  • Structure levels:

    • Primary: amino acid sequence.

    • Secondary: alpha-helix or beta-pleated sheet.

    • Tertiary: three-dimensional folding.

    • Quaternary: multiple polypeptide chains.

  • Peptide bonds form via dehydration synthesis (water removed).

  • Protein folding depends on interactions; denaturation: loss of structure due to pH, temperature, salinity; renaturation is sometimes possible.

  • Function is tied to shape.

Lipids

  • Not formed by polymers like carbohydrates or proteins.

  • Major roles: membranes (phospholipids form a bilayer), energy storage (triglycerides), steroid hormones.

  • Building blocks vary by type (glycerol + fatty acids for triglycerides; rings for steroids).

Cells and Cell Structure

  • Cell theory: living organisms are made of cells; cells arise from preexisting cells; cells are the basic units of life.

  • Plasma membrane: mainly lipids (phospholipids) forming a boundary.

  • Cytoplasm: cytosol + organelles.

  • Nucleus: contains DNA; nucleus enclosed by a nuclear envelope.

  • DNA typically does not leave the nucleus; nucleoplasm is inside the nucleus.

  • Chromatin vs chromosome: relaxed form is chromatin; condensed form is chromosome.

  • Packaging: DNA + histone proteins form nucleosomes; DNA wraps around histones to form chromatin.

  • Histones analogy: DNA wraps around histones like toilet paper around a core roll.

  • Chromosome number in humans: 46 chromosomes total; 23 pairs; diploid (two sets).

  • Haploid vs diploid: haploid = one set (sex cells); diploid = two sets (somatic cells).

  • Homologous chromosomes: pairs of chromosomes (one from each parent) that are similar in shape and gene content.

  • Sister chromatids: identical copies held together at the centromere; during meiosis/mitosis they separate.

  • Centromere: region where sisters chromatids are held together; centrioles organize spindle apparatus in mitosis.

The Cell Cycle and Mitosis/Meiosis

  • Cell cycle phases:

    • Interphase: G1 (growth and metabolism), S (DNA synthesis/replication), G2 (preparation for division).

    • G0: resting state (quiescent).

  • Checkpoints:

    • Between G1 and S (G1/S checkpoint).

    • Between G2 and M (G2/M checkpoint).

    • Additional checkpoints exist during M phase.

  • M phase includes mitosis (somatic cells) and meiosis (sex cells).

  • Mitosis stages (somatic cells): Prophase, Metaphase, Anaphase, Telophase; sometimes cytokinesis counted separately.

    • Prophase: chromosomes condense (shorten/thicken); nuclear envelope dissolves; spindle forms; centrosomes migrate to poles.

    • Metaphase: chromosomes align along the metaphase/equatorial plate in single file.

    • Anaphase: sister chromatids separate and move to opposite poles.

    • Telophase: nuclear envelopes reform around two sets of chromosomes; chromosomes de-condense; two nuclei form.

    • Cytokinesis: cytoplasm divides, yielding two separate daughter cells.

  • In humans, daughter cells after mitosis are genetically identical to each other and to the parent cell (assuming complete S phase replication and proper division).

  • Chromosome numbers in daughter cells after mitosis: 46; number of chromosome sets remains diploid.

  • Meiosis creates haploid gametes: 23 chromosomes per gamete, ensuring genetic diversity when fertilization occurs.

  • Cancer concept: failure of cell cycle checkpoints can lead to uncontrolled division; cancer involves cells dividing when they should not.

  • Important note: DNA generally does not leave the nucleus; during certain rare circumstances, it may. The nucleus is enclosed by the nuclear envelope, made of lipids similar to the plasma membrane.

Quick Reference Facts (Recap)

  • Glycogen: ext{highly branched polysaccharide in animals}.

  • ATP: energy currency; commonly referenced; reaction: ATP\rightarrow ADP + P_i.

  • DNA base pairing: A!\leftrightarrow!T,\ C!\leftrightarrow!G; RNA uses A!\leftrightarrow!U,\ C!\leftrightarrow!G.

  • Nucleotides: sugar + phosphate + nitrogenous base; two classes of bases: purines (A,G) and pyrimidines (C,T,U).

  • Proteins are built from amino acids; peptide bonds formed by dehydration synthesis; denaturation disrupts function.

  • Chromosomes vs chromatids: sister chromatids are held by centromere; homologous chromosomes are paired by ancestry.

  • Haploid vs diploid: 23 vs 46 chromosomes; gametes are haploid; somatic cells are diploid.

  • Cell cycle checkpoints ensure proper division; failure leads to conditions like cancer.