Cell Function Notes (Membrane-bound Organelles & Genetics, Concise)

Membrane-bound organelles: overview

Have a lipid bilayer membrane; proteins and sometimes carbohydrates—composition varies by organelle and cell type.

Structure: network of sacs; rough ER has ribosomes on the surface; smooth ER lacks ribosomes.
Rough ER: site of protein synthesis for proteins that enter membranes or the lumen; proteins packaged into vesicles and sent to the Golgi.
Smooth ER: lipid metabolism and membrane lipid synthesis; also processes carbohydrates.

Protein processing center: adds/clips carbohydrates, inserts phosphates, and other modifications.
Vesicles bud off to destinations (lysosomes, plasma membrane, or outside the cell).

Contain digestive enzymes; acidic lumen; digest materials taken in by endocytosis.
Formed from the Golgi; fuse with endocytic vesicles to degrade contents.
Endocytosis/exocytosis involved in material intake and waste expulsion (future topics).

Double membrane (outer and inner); energy powerhouse of the cell.
Convert nutrients into ATP; ATP used for cellular processes and muscle contraction.
Glycolysis occurs in the cytoplasm; pyruvate enters mitochondria for ATP production.

Nuclear envelope with two membranes; pores allow large complexes (e.g., ribosomal subunits) to pass.
Houses DNA organized with histones into chromosomes.
DNA is the genetic material; keeps a long double-stranded code; stored as chromatin when not condensed.

DNA: double-stranded; base pairing: A\leftrightarrow T,\quad G\leftrightarrow C.
RNA: single-stranded; uses GAUC (U instead of T).
Central dogma: ext{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}.
DNA is replicated to produce two genome copies; transcription produces RNA copies used to synthesize proteins.

Most cells are diploid: 2n copies of the genome; RBCs lack nuclei and hence genomic copies.
Genes: regions coding for proteins; noncoding regions exist between genes.
Housekeeping genes: expressed in all cells for basic function (e.g., ion pumps, basic metabolism).
Cell-type specific genes: expressed depending on cell identity (e.g., neuron genes in neurons).
All cells carry all genes; expression determines phenotype.

DNA is transcribed into RNA; RNA moves to cytoplasm; ribosomes translate RNA into protein.
Codons: triplets of nucleotides; there are 4^3 = 64 codons; encode 20 amino acids and 3 stop signals.
Start codon: \text{ATG} \rightarrow \text{Methionine}.
Ribosome reads codons three nucleotides at a time to add corresponding amino acids, forming a polypeptide.
Wobble effect: third codon position often flexible; allows some codon variations to code for the same amino acid.

Not all DNA regions code for proteins; some are noncoding.
Mutations: changes in DNA sequence; can alter RNA and protein sequence; effects depend on location and codon position; some are silent, others deleterious.
Mutations can affect development and disease; some gene expression changes occur over time during development.

Endomembrane system: Rough ER (protein synthesis) → Golgi (protein processing) → vesicles to lysosomes, plasma membrane, or secretion.
Mitochondria: energy production via ATP; double membrane.
Nucleus: houses genome; transcription and RNA processing occur here; translation in cytoplasm.
Central dogma: DNA -> RNA -> Protein; codons translate to amino acids; genetic code is degenerate due to wobble.
Gene expression: housekeeping vs cell-type specific genes; development can alter expression patterns.