R

Nucleus: The Cell's Headquarters

  • Central Role: Stores genetic information (DNA), provides instructions for protein synthesis, and coordinates protein production with ribosomes and ER.
  • Acts as the "blueprint storage" for protein recipes.

The Nuclear Envelope and Nuclear Pores

  • Nuclear Envelope: Double-layered outer membrane continuous with the Rough Endoplasmic Reticulum (RER).
  • Nuclear Pores: Openings in the envelope that regulate molecular traffic (ions, molecules) between the nucleus and cytoplasm, essential for RNA export.
    • Messenger RNA (mRNA) exits through pores to the cytoplasm for protein synthesis.

RNA, DNA, and RNA Export

  • RNA Export: mRNA leaves the nucleus via nuclear pores to ribosomes in the cytoplasm or on the RER for translation.
  • Protein Synthesis: Nucleus provides instructions; cytoplasm/ER/ribosomes execute synthesis.

Chromatin and Chromosomes

  • Chromatin: Loosely structured DNA-protein complex inside the nucleus when the cell is not dividing. Contains genetic information but is uncoiled.
  • Chromosomes: Tightly packed, condensed DNA structures formed from chromatin during cell division.
    • Formed by DNA wrapping around histones (protein spools).

DNA vs RNA: Structure and Base Pairing

  • DNA: Double helix with sugar-phosphate backbones and paired nitrogenous bases.
  • Base Pairing Rules:
    • DNA: A-T\quad\text{and}\quad C-G
    • RNA: A-U\quad\text{and}\quad C-G (Uracil (U) replaces Thymine (T) in RNA).

Nucleotides: Building Blocks of Nucleic Acids

  • Structure: Each nucleotide has three components: a phosphate group, a five-carbon sugar, and a nitrogenous base.
    • Conceptual Representation: ext{Nucleotide} = {\text{phosphate group}, \text{5-carbon sugar}, \text{nitrogenous base}}
  • Bases: Adenine (A), Cytosine (C), Guanine (G), Thymine (T - DNA), Uracil (U - RNA).
  • Function: Nucleotides link to form DNA and RNA, and their specific base pairing encodes genetic information.

Connection to Protein Synthesis and Cellular Organization

  • Process: DNA (blueprint) \to RNA (instructions) in nucleus \to mRNA exits via nuclear pores \to Ribosomes (cytoplasmic or RER) translate mRNA into proteins.
  • RER Connection: Nuclear envelope is continuous with RER, which synthesizes proteins for secretion or membranes.

Non-Membranous Organelles

  • Definition: Organelles lacking a surrounding membrane.
  • Example: Ribosomes.