Chapter 3 – Cells: The Living Units (Human Anatomy & Physiology I) – Study Notes

Chapter 3 – Cells: The Living Units

Key Definitions

This section introduces essential terminology for understanding cell structure, function, and molecular biology processes relevant to anatomy and physiology.

• Aminoacyl-tRNA: A tRNA molecule linked to its specific amino acid, used during translation.

• Anticodon: A sequence of three nucleotides in tRNA that pairs with the complementary codon in mRNA.

• Cell: The basic structural and functional unit of all living organisms.

• Cell membrane (Plasma membrane): The selectively permeable boundary of the cell, composed of a phospholipid bilayer with embedded proteins.

• Channel protein: Membrane protein that forms a passageway for specific molecules or ions.

• Cholesterol: A lipid molecule that stabilizes cell membrane fluidity.

• Coding strand: The DNA strand whose sequence matches the mRNA (except T/U substitution).

• DNA polymerase: Enzyme that synthesizes new DNA strands during replication.

• Elongation (of transcription/translation): The stage where the RNA or polypeptide chain is extended.

• G1, G2, S phase: Phases of the cell cycle; G1 and G2 are growth phases, S is DNA synthesis.

• Gene: A segment of DNA that codes for a specific protein.

• Genetic code: The set of rules by which nucleotide sequences are translated into amino acid sequences.

• Glycolipid: Lipid with a carbohydrate attached, found in cell membranes.

• Hydrophilic/Hydrophobic: Water-loving/water-fearing properties of molecules.

• Initiation (of transcription/translation): The beginning stage of RNA or protein synthesis.

• Integral protein: Protein embedded within the cell membrane.

• Interphase: The cell cycle phase between divisions, including G1, S, and G2.

• Messenger RNA (mRNA): RNA that carries genetic information from DNA to ribosomes.

• Peripheral protein: Protein attached to the surface of the cell membrane.

• Phospholipid: Major component of cell membranes, with hydrophilic heads and hydrophobic tails.

• Replication: The process of copying DNA before cell division.

• Replication bubble: The region where DNA is unwound for replication.

• Ribosomal RNA (rRNA): RNA component of ribosomes, essential for protein synthesis.

• RNA polymerase: Enzyme that synthesizes RNA from a DNA template.

• Semiconservative replication: Each new DNA molecule consists of one old and one new strand.

• Start codon/Stop codon: Codons that signal the start and end of translation ($AUG$ for start; $UAA$, $UAG$, $UGA$ for stop).

• Template strand: The DNA strand used as a template for RNA synthesis.

• Termination (of transcription/translation): The end stage of RNA or protein synthesis.

• Transcription: The process of synthesizing RNA from DNA.

• Transfer RNA (tRNA): RNA that brings amino acids to the ribosome during translation.

• Transmembrane protein: Protein that spans the entire cell membrane.

• Triplet: Three-nucleotide sequence in DNA or mRNA coding for an amino acid.

Learning Objectives

Cells

Cells are the fundamental units of life, performing all necessary functions for organismal survival.

• Function of Cells: Cells carry out metabolism, growth, response to stimuli, and reproduction.

• Cell Theory: All living things are composed of cells; cells are the basic unit of life; all cells arise from pre-existing cells.

• Basic Parts of a Cell: Major components include the plasma membrane, cytoplasm, and nucleus.

• Example: In a typical animal cell, the nucleus contains genetic material, the cytoplasm houses organelles, and the plasma membrane controls entry and exit of substances.

Plasma Membrane

The plasma membrane is a dynamic structure that separates the cell from its environment and regulates transport.

• Intracellular vs. Extracellular Materials: The plasma membrane maintains distinct internal and external environments.

• Principal Molecules: Phospholipids, cholesterol, glycolipids, and proteins.

• Phospholipid Arrangement: Phospholipids form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

• Structure of Cell Membranes: The fluid mosaic model describes the arrangement of lipids and proteins.

• Integral vs. Peripheral Proteins: Integral proteins span the membrane; peripheral proteins attach to the surface.

• Functions of Membrane Proteins: Include transport, signaling, cell recognition, and structural support.

• Example: Channel proteins facilitate ion movement; glycoproteins are involved in cell recognition.

DNA Replication

DNA replication ensures genetic continuity during cell division.

• Semiconservative Replication: Each daughter DNA molecule contains one parental and one new strand.

• Enzymes Involved: DNA polymerase synthesizes new DNA; DNA ligase joins fragments.

• Replication Bubble: The region where DNA strands are separated for replication.

• Flow of Genetic Information: DNA is replicated, transcribed into RNA, and translated into protein.

• Example: During S phase, DNA replication occurs before cell division.

• Equation: $\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

Protein Synthesis

Protein synthesis involves transcription and translation, converting genetic information into functional proteins.

• Role of DNA: DNA provides the template for mRNA synthesis.

• Role of mRNA: mRNA carries genetic instructions from DNA to ribosomes.

• Role of tRNA: tRNA brings amino acids to the ribosome, matching mRNA codons with anticodons.

• Genetic Code: The sequence of nucleotides in mRNA determines the amino acid sequence of proteins.

• Example: The codon $AUG$ codes for methionine and signals the start of translation.

• Equation: $\text{Codon} \rightarrow \text{Amino Acid}$

Transcription

Transcription is the process by which RNA is synthesized from a DNA template.

• Location: Transcription occurs in the nucleus of eukaryotic cells.

• Initiation: RNA polymerase binds to the promoter region of DNA.

• Elongation: RNA polymerase adds nucleotides to the growing RNA strand.

• Termination: Transcription ends when RNA polymerase reaches a termination signal.

• Promoter: DNA sequence where transcription begins.

• Example: The gene for hemoglobin is transcribed into mRNA in red blood cell precursors.

Translation

Translation is the process by which ribosomes synthesize proteins using mRNA as a template.

• Initiation: Ribosome assembles at the start codon of mRNA.

• Elongation: tRNA brings amino acids, and the polypeptide chain grows.

• Termination: Translation ends at a stop codon; the completed protein is released.

• Role of rRNA: rRNA forms the core of ribosome structure and catalyzes peptide bond formation.

• Predicting Amino Acid Sequence: The sequence of codons in mRNA determines the order of amino acids in the protein.

• Equation: $\text{mRNA sequence} \rightarrow \text{Amino Acid sequence}$

• Example: The mRNA sequence $AUG-GGC-UAA$ translates to Methionine-Glycine (stop).

Summary of Gene Expression

Gene expression encompasses the processes by which genetic information is used to synthesize functional gene products (proteins).

• Overall Process: DNA is transcribed into mRNA, which is then translated into protein.

• Predicting Protein Sequence: Given a DNA template strand, the corresponding mRNA and amino acid sequence can be determined using the genetic code.

• Equation: $\text{DNA} \rightarrow \text{mRNA} \rightarrow \text{Protein}$

• Example: If the DNA template is $TAC$, the mRNA is $AUG$, which codes for Methionine.

Table: Types of Membrane Proteins

This table summarizes the main types of proteins found in the plasma membrane and their characteristics.

Type	Location	Function
Integral Protein	Embedded within the membrane	Transport, signaling, structural support
Peripheral Protein	Attached to membrane surface	Cell signaling, cytoskeletal attachment
Transmembrane Protein	Spans the entire membrane	Transport of molecules, cell communication