Cell Processes

• Cells are the smallest unit of life
• Cells are the basic functional units of the body
• Cells come in a variety of shapes and sizes, reflecting their diverse functions
• Principal parts of cells:
• Plasma (cell) membrane:

  • Selectively permeable
  • Gives form and separates from the external environment
• Cytoplasm and organelles:

  • Fluid part of the cell
  • Little organs that perform functions
• Nucleus:

  • Contains DNA, one or more nucleoli
  • Directs cell activities
• Ribosomes:
• Nonmembranous organelles that are the site of protein synthesis
• Made up of protein and ribosomal RNA (rRNA)
• Two switchable forms found in the cell:

  • Free ribosomes: free floating; site of synthesis of soluble proteins that function in cytosol or other organelles
  • Membrane-bound ribosomes: attached to the membrane of endoplasmic reticulum (ER); site of synthesis of proteins to be incorporated into membranes or lysosomes, or exported from the cell
• Nucleus:
• Largest organelle
• Contains the genetic material for the synthesis of nearly all cellular proteins
• Most cells are uninucleate (one nucleus), but some cells are multinucleate (many nuclei) or anucleate (no nucleus)
• Nuclear envelope:
• Double-membrane barrier that encloses the jelly-like fluid nucleoplasm
• Outer layer is continuous with rough ER and studded with ribosomes
• Nuclear pores allow substances to pass into and out of the nucleus
• Nucleolus:
• Involved in ribosomal RNA (rRNA) synthesis and ribosome subunit assembly
• Chromatin and chromosome structure:
• Chromatin consists of threadlike strands of DNA, histone proteins, and RNA
• Chromosomes are condensed chromatin
• Chemical alterations of histones can help regulate gene expression
• DNA replication:
• Uncoiling: enzyme helicase unwinds DNA
• Separation: two DNA strands separate so hydrogen bonds between base pairs are broken
• Assembly:

  • Each strand acts as a template for a new complementary strand
  • DNA polymerase synthesizes both new strands at one time
  • Process is called semiconservative replication
• Restoration: DNA ligase splices short segments of discontinuous lagging strand together
• Protein synthesis:
• DNA directs the order of amino acids in a polypeptide
• A segment of DNA that holds the code for one polypeptide is referred to as a gene
• The code consists of three sequential bases (triplet) that specify the code for a particular amino acid
• The role of RNA in protein synthesis:
• RNA copies the DNA code in the nucleus and carries it into the cytoplasm to ribosomes
• Three types of RNA: Messenger RNA (mRNA), Ribosomal RNA (rRNA), Transfer RNA (tRNA)
• Protein synthesis occurs in two steps: Transcription and Translation
• Transcription:
• Process of transferring the code held in DNA gene base sequence to the complementary base sequence of mRNA