A&P chapter 3 lecture 02/19/2025

Overview of Gene Expression and Protein Synthesis

Messenger RNA (mRNA)

• mRNA acts as a messenger that copies and transports genetic information from DNA to ribosomes for protein synthesis.

• The process of using mRNA to synthesize proteins is known as translation.

Stages of mRNA Synthesis

Initiation

• DNA contains genes that code for proteins. Only relevant genes are transcribed into mRNA as needed by the cell.

• RNA Polymerase is an enzyme that binds the DNA at the gene's start point to begin transcription.

• mRNA is synthesized based on the template strand of DNA, which is complementary to the coding strand.

  • E.g., if the coding strand has the sequence ACGTA, the mRNA will have the complementary sequence UGCAU (with 'U' replacing 'T').

Elongation

• RNA Polymerase continues to add nucleotides to the growing mRNA strand until it reaches a terminator sequence on the DNA.

• The terminator signals the end of transcription, and RNA Polymerase detaches from the DNA.

• After synthesis, mRNA exits the DNA template. The DNA returns to its helical shape for further usage.

Post-Transcriptional Modification

• mRNA undergoes several modifications before it exits the nucleus:

  • Introns (non-coding sequences) are spliced out, and exons (coding sequences) are joined together.

  • A 5' cap and a poly-A tail are added to protect the mRNA from degradation during transportation to the ribosome.

Genetic Code

• The genetic code is universal across all organisms and consists of codons (triplet sequences of bases) which code for specific amino acids.

• Out of approximately 64 codons, 20 correspond to amino acids with redundancy; some amino acids are specified by multiple codons.

• AUG is the start codon, while UAA, UAG, and UGA are stop codons.

Translation Process

tRNA and Ribosomes

• Transfer RNA (tRNA) carries amino acids to the ribosome where they match with the mRNA codons through complementary anticodons.

  • The interaction is crucial, as the tRNA must fit precisely to deliver the right amino acid.

• In the ribosome, mRNA is read in sets of three codons, and tRNAs deliver corresponding amino acids, leading to the formation of a polypeptide chain through peptide bonds.

Elongation of the Polypeptide Chain

• As each tRNA brings its amino acid, they are linked together by peptide bonds to form a growing protein chain.

• This process continues until a stop codon is read, signaling the end of protein synthesis.

Protein Folding

• The newly synthesized polypeptide chain will fold into a specific three-dimensional structure critical for its function.

Mutations and Their Impact

• Mutations are changes in the DNA sequence that can occur during replication or as a result of external factors (e.g., radiation, chemicals).

• Types of Mutations:

  • Silent Mutations: No change in the amino acid sequence; often occur due to redundancy in the genetic code.

  • Missense Mutations: Replace one amino acid in a polypeptide with another, potentially altering protein function.

  • Nonsense Mutations: Introduce a premature stop codon, resulting in truncated proteins, which usually lead to loss of function.

  • Frameshift Mutations: Caused by insertions or deletions of nucleotides; can drastically change the resulting protein sequence and function.

Plasma Membrane and Transport Mechanisms

Structure of the Plasma Membrane

• Composed of a phospholipid bilayer facilitating selective permeability.

• Integral (transmembrane) proteins and peripheral proteins assist in transport and communication across the membrane.

Transport Mechanisms

• Passive Transport: No energy required; substances move along a concentration gradient (high to low concentration).

  • Diffusion: Movement of molecules until equilibrium is reached. Factors influencing diffusion:

    • Distance (further distances slow down diffusion).

    • Molecule Size (smaller molecules diffuse faster).

    • Temperature (increased temperature speeds up diffusion).

    • Concentration Gradient (larger differences speed up diffusion).

• Active Transport: Requires energy (usually ATP) to move substances against their concentration gradients (low to high concentration).

  • e.g., Sodium-Potassium pump, which actively transports sodium out and potassium into the cell.

• Vesicular Transport (Active Transport): Includes endocytosis (bulk transport into the cell) and exocytosis (bulk transport out of the cell).

  • Phagocytosis: Cell 'eating', bringing in solid material.

  • Pinocytosis: Cell 'drinking', bringing in liquid.

Tonicity and Osmosis

• Tonicity: Refers to the ability of a solution to cause a cell to gain or lose water.

  • Isotonic Solutions: Concentrations of solute equal inside and outside the cell; no net gain/loss of water.

  • Hypotonic Solutions: Lower solute concentration outside, leading to water influx and potential cell lysis (rupture).

  • Hypertonic Solutions: Higher solute concentration outside, causing water efflux and cell shrinkage (crenation).

Summary

• The central dogma of molecular biology involves the flow of genetic information from DNA to RNA to protein.

• Understanding the processes of transcription and translation, as well as mutation impact, is crucial for comprehension of genetic expression and cell function.