Chapter 3: Cellular Level (Test Review)

Transcription vs. Translation

Transcription:

• Definition: The process of synthesizing messenger RNA (mRNA) from a specific segment of DNA, which serves as a template.

• Location: Takes place in the nucleus of eukaryotic cells.

• Stages:

  1. Initiation: A region called a promoter triggers the start of transcription.

  2. Elongation: RNA polymerase unwinds the DNA and synthesizes mRNA by aligning complementary nucleotides (A, C, G, U) with the DNA template.

  3. Termination: A terminator sequence signals the end of transcription, causing mRNA to fold and detach from the gene.

• Post-Transcriptional Modifications: The pre-mRNA undergoes splicing where introns (non-coding regions) are removed and exons (coding regions) are joined together before exiting the nucleus.

Translation:

• Definition: The process of interpreting the mRNA sequence into a specific amino acid chain (polypeptide), ultimately forming a protein.

• Location: Takes place in the cytoplasm, primarily at ribosomes.

• Stages:

  1. Initiation: The ribosome subunits bind to the mRNA at the start codon, preparing for translation.

  2. Elongation: Transfer RNA (tRNA) brings the appropriate amino acids based on the codons of mRNA, and ribosomes catalyze the formation of peptide bonds between amino acids.

  3. Termination: A stop codon on the mRNA signals the end of translation, resulting in the release of the newly synthesized polypeptide chain.

• Components Involved: Ribosomal RNA (rRNA) forms the structure of ribosomes, while tRNA delivers the respective amino acids matched to the codons on the mRNA.

Endocytosis vs. Exocytosis

• Endocytosis: Process of a cell ingesting material by enveloping it in a portion of its cell membrane and pinching off to form an intracellular vesicle.

  • Vesicle: Membranous sac (spherical organelle) bounded by a lipid bilayer membrane.

  • Types of Endocytosis:

    • Phagocytosis (“cell eating”): Ingests large particles (e.g., immune cells phagocytizing pathogens).

    • Pinocytosis (“cell drinking”): Brings fluid with dissolved substances into the cell.

    • Receptor-mediated Endocytosis: Selective process where surface receptors bind specific ligands and endocytose them.

      • Example: Iron is endocytosed by red blood cells via transferrin receptors.

• Exocytosis: Process of exporting material out of the cell using vesicular transport.

  • Involves packaging substances into membrane-bound vesicles within the cell.

  • When the vesicle membrane fuses with the cell membrane, it releases contents into interstitial fluid, becoming part of the cell membrane.

  • Examples: Secretion of digestive enzymes by stomach and pancreas cells; endocrine cells secrete hormones; certain immune cells secrete histamine.

Mitosis

Importance of Cell Division

• Cell division is prevalent in body cells, except for some (e.g., gametes, red blood cells, neurons, muscle cells).

• Somatic cells (body cells) divide regularly and have 2 copies of each chromosome (diploid).

Overview of the Cell Cycle

• Sequence of events from cell creation to division, generating two new cells.

• Composed of two main phases:

  • Interphase: Non-dividing phase, most cells spend their time here.

  • Mitosis and Cytokinesis: Division of genetic material and cytoplasm.

Interphase Phases

1. G1 Phase (Gap 1):

   • Cell growth and normal metabolic functions.

2. S Phase (Synthesis):

   • DNA replication, doubling the amount of DNA.

3. G2 Phase (Gap 2):

   • Final preparations for mitosis; growth continues.

4. G0 Phase:

   • Resting phase; cells may stop dividing temporarily or permanently.

Structure of Chromosomes

• During S phase, DNA doubles; resulting chromosomes consist of sister chromatids.

• Each chromosome has a centromere that bounds sister chromatids.

Mitosis and Cytokinesis

• Mitosis lasts 1-2 hours and includes four stages:

  1. Prophase:

     • Chromatin condenses into visible chromosomes.

     • Nucleolus disappears; nuclear envelope disassembles.

  2. Metaphase:

     • Sister chromatids align along the metaphase plate.

  3. Anaphase:

     • Sister chromatids separate and are pulled to opposite ends.

  4. Telophase:

     • Two new nuclei form at each end; chromosomes de-condense.

• Cytokinesis:

  • Division of cytoplasm and formation of two new cells; cleavage furrow forms.

Cell Cycle Control

• Regulation of the cell cycle is crucial for health; deficiencies can lead to cancer.

• Control systems use checkpoints to signal readiness for progression:

  • Cyclins and Cyclin-Dependent Kinases (CDKs) determine progression through checkpoints.

  • Specific checkpoints at G1, G2, and during mitosis ensure proper sequence of events and readiness.

Implications of Cell Cycle Control Failures

• Uncontrolled cell division can lead to tumors/cancer.

• Tumors are classified as benign (non-threatening) or malignant (capable of damaging tissues).

• Cancerous cells may go undetected but can proliferate and spread if regulation mechanisms fail.