Week 11

Protein Synthesis Overview

• Proteins are essential for various cellular functions.

• Two key processes:

  • Transcription: Information transfer from DNA to mRNA.

  • Translation: Conversion of mRNA information into a polypeptide chain.

• Occurs throughout the cell cycle.

Transcription in Detail

• RNA Synthesis: Begins with the formation of mRNA from a DNA template.

• Key components:

  • Gene expression: The process by which the information from a gene is utilized.

  • Regulatory elements: Control the transcription rate, including promoters, enhancers, and silencers.

  • RNA polymerases: Enzymes that synthesize RNA from the DNA template.

  • Direction: DNA is transcribed from the 3' to 5' direction.

Classes of RNA

• mRNA: Encodes polypeptides.

  • Translated into proteins.

• tRNA: Delivers amino acids to ribosomes for protein synthesis.

• rRNA: Forms the structural components of ribosomes.

• snRNA: Plays a role in mRNA processing.

Regulatory Elements in Transcription

• Promoters: Adjacent to initiation sites, essential for starting transcription.

• Enhancers: Distant regulatory elements that increase transcription rates.

• Silencers: Element that represses transcription.

• Transcription Factors: Proteins needed for RNA polymerase to initiate transcription.

mRNA Structure

• Composed of three segments:

  1. Leader sequence: Helps guide the mRNA during translation.

  2. Coding sequence: The actual sequence that is translated into a protein.

  3. Trailer sequence: Follows the coding sequence and aids in mRNA stability.

• Codons: Start (AUG) and stop (UAA, UAG, UGA) codons control translation.

Gene Architecture in Prokaryotes vs. Eukaryotes

• Prokaryotic genes: Continuous sequences.

• Eukaryotic genes: Contain introns (non-coding regions) and exons (coding regions).

  • Introns are removed during mRNA processing.

  • Exons are spliced together during RNA splicing to form the mature mRNA.

Ending Transcription

• 5' capping: Addition of a modified guanine nucleotide to the 5' end.

• Poly(A) tail: A sequence of adenine nucleotides (150-200 bases) added to the 3' end.

  • Functions to:

    • Protect mRNA from degradation.

    • Facilitate transport to the cytoplasm.

    • Stimulate translation.

DNA Replication Overview

• Process: DNA copies itself for cell division.

• Cell cycle: Different phases, including interphase and cell division.

DNA Double Helix Structure

• Composed of two strands running in opposite directions (anti-parallel).

• Nucleotides: Units made up of sugar (deoxyribose), a phosphate, and nitrogenous bases (A, T, C, G).

DNA Replication Mechanics

• Leading strand: Synthesized continuously toward the replication fork.

• Lagging strand: Synthesized in short segments (Okazaki fragments) away from the fork.

Primase and DNA Polymerase Roles

• Primase: Begins DNA synthesis by creating an RNA primer.

• DNA polymerase: Adds nucleotides based on the DNA template from 3' to 5'.

Filling in the Gaps

• Exonuclease: Removes RNA primers from the lagging strand.

• Ligase: Joins Okazaki fragments to create a continuous DNA strand.

• Semi-conservative replication: One old strand pairs with one new strand.

Proofreading and Termination

• Exonuclease: Helps remove errors in DNA.

• Telomeres: Added by telomerase at chromosome ends to prevent shortening after replication.

Genetic Material Composition

• DNA: Nitrogen bases (A, C, G, T) and deoxyribose.

• RNA: Nitrogen bases (A, C, G, U) and ribose.

• Nucleotides: Joined by phosphodiester bonds.

Gene Expression Process

• Step 1: Transcription - mRNA synthesis from DNA.

• Step 2: Translation - synthesis of protein from mRNA.

• Codons (three bases) represent specific amino acids.

Protein Functions

• Structural role in cells.

• Serving as hormones, carriers, and enzymes for reactions.

Protein Structure Levels

• Primary: Amino acid sequence.

• Secondary: Alpha-helix and beta-sheet formations.

• Tertiary: Overall 3D shape.

• Quaternary: Assembly of multiple peptide chains.

Mechanism of Translation

• Initiation: mRNA binds to ribosome, begins at AUG codon.

• Elongation: Sequential addition of amino acids via peptide bonds.

• Termination: Occurs when stop codons (UAA, UAG, UGA) are reached.

Protein Sorting

• Destination specified by sequences after translation.

• Common locations: cytoplasm, ER, mitochondria.

Genetic Mutations

• Types: Chromosomal and point mutations.

• Point mutations: Can be spontaneous or induced.

  • Substitutions: Transition (purine for purine) or transversion (purine for pyrimidine).

Understanding the Genetic Code

• Each codon corresponds to one amino acid; the code is universal and non-overlapping.

• Includes start and stop signals essential for translation.

tRNA and Ribosome Functionality

• tRNA: Delivers amino acids to the ribosome, each with specific anticodons.

• Ribosomes: Sites of protein synthesis, composed of rRNA and proteins.