(L8) DNA Transcription to Translation

Learning Outcomes

  • Acquire a fundamental knowledge of cellular organelles involved in protein synthesis. This includes understanding the structure and function of each organelle and their roles in the overall process of gene expression and regulation.

  • Develop a comprehensive understanding of genes and genetic coding. This entails exploring the different types of genes, their functions, and how they relate to an organism's traits and characteristics.

  • Gain a thorough understanding of the processes of transcription and translation. This involves detailing each stage of these processes and the molecular machinery involved.

  • Develop a basic understanding of genetic regulatory mechanisms. Understanding how genes are turned on and off is crucial for grasping how cells differentiate and respond to their environments.

  • Acquire a foundational knowledge of genetic mutations. Understanding the various types of mutations and their implications can help in comprehending genetic disorders and evolutionary biology.

Associated Organelles - Overview

  • Nucleus:

    • Contains DNA, responsible for genetic information storage and gene expression regulation. It serves as the control center of cell metabolism and reproduction.

  • Ribosomes:

    • The site of protein synthesis, composed of rRNA and proteins. Ribosomes can be found floating in the cytoplasm or attached to the endoplasmic reticulum, which plays a role in synthesizing proteins destined for secretion or for use within the cell.

  • Golgi Apparatus:

    • Processes and sorts proteins and lipids received from the endoplasmic reticulum. It modifies glycoproteins and lipids, adds sugars to them, and packages them into vesicles for delivery to their final destinations.

Associated Organelles - Nucleus

Constituents:

  • Nuclear Envelope:

    • A lipid bilayer that protects genetic material and regulates transport. It contains nuclear pores that control the entry and exit of molecules.

  • Nucleolus:

    • The site of ribosomal RNA synthesis. It is a dense structure found within the nucleus and is crucial for ribosome assembly.

  • Chromatin:

    • DNA packaging and gene control (histones assist in coiling), which controls access to genes for replication and transcription.

  • Chromosomes:

    • Discrete units of DNA that store genetic information, only visible during cell division, allowing for proper segregation.

Associated Organelles - Ribosomes

Consists of:

  • Small subunit (40S):

    • Contains 18S rRNA and 33 proteins, responsible for reading mRNA.

  • Large subunit (60S):

    • Contains three rRNA molecules and 49 proteins, responsible for forming peptide bonds between amino acids.

Sites:

  • A site:

    • Binds incoming tRNA carrying an amino acid.

  • P site:

    • Holds tRNA with a growing polypeptide chain where peptide bond formation occurs.

  • E site:

    • Exit site for tRNA, which has donated its amino acid.

Ribosome size:

  • 80S (Svedberg unit):

    • The sedimentation coefficient due to the combination of the small and large subunits, indicating its density and size.

Associated Organelles - Golgi Apparatus

  • Composed of cisternae:

    • Flattened membrane-bound sacs that facilitate the processing and sorting of proteins and lipids.

  • Polarity in structure and function:

    • Cis face: Entry point for newly synthesized proteins and lipids.

    • Trans face: Exit point for modified proteins that are packaged into vesicles for transport to different cellular or extracellular locations.

Genetic Code - Constituents

  • Genes encode protein types; DNA is transcribed to RNA: Understanding the central dogma of molecular biology provides insight into how genetic information leads to protein synthesis.

  • Three main types of RNA:

    • tRNA:

      • Brings amino acids to ribosomes, functioning as an adaptor molecule in translation.

    • rRNA:

      • Forms ribosomes, constituting the core structural and functional components of the ribosome.

    • mRNA:

      • Carries genetic code from nucleus to ribosomes for protein synthesis, directing the sequence of amino acids in proteins.

Genetic Code - Composition

Bases:

  • Purines: Adenine (A), Guanine (G)

  • Pyrimidines: Thymine (T), Cytosine (C)

Base pairing rules:

  • A pairs with T (or U in RNA).

  • C pairs with G.

Genetic Code - Codons

  • Use of triplet codons allows for 64 combinations encoding 20 amino acids: This redundancy in the genetic code provides a level of protection against mutations.

  • Degeneracy: Multiple codons can specify the same amino acid, allowing for variation without altering protein function.

  • Universality: Genetic code is nearly identical across all organisms, showcasing the shared ancestry of life.

Genetic Code - Reading Frames

  • Importance of proper grouping: Codons do not overlap; changing the reading frame can result in entirely different proteins.

  • Incorrect grouping alters meaning of sequences, potentially leading to dysfunctional proteins.

Genetic Code - Genes

  • A sequence coding for a functional RNA or protein: Genes serve as templates for building proteins that carry out organismal functions.

  • Gene expression can be regulated, including the role of introns (non-coding regions) and exons (coding regions) in eukaryotes, allowing for alternative splicing and regulation of protein diversity.

Transcription

  • Involves RNA polymerase that synthesizes RNA from a DNA template: RNA polymerase unwinds the DNA, reads the template strand, and builds the complementary RNA strand.

Stages of transcription:

  1. Initiation: RNA polymerase binds to the promoter region of DNA, signaling the start of transcription.

  2. Elongation: The RNA strand elongates by adding complementary RNA nucleotides, moving along the DNA template.

  3. Termination: RNA polymerase encounters a stop signal, prompting the release of the RNA strand and detaching from the DNA.

RNA Processing

  • In eukaryotic cells, mRNA undergoes several modifications:

    • Introns removal (splicing): Non-coding sequences are excised, and exons are joined together.

    • Addition of 5' cap and 3' poly-A tail: These modifications enhance mRNA stability and promote translation.

  • Processed mRNA exits to ribosomes, where it will be translated into a protein.

Translation - Overview

  • Initiation begins with AUG (start codon): The small ribosomal subunit binds to mRNA, and the large subunit assembles, marking the start of protein synthesis.

  • Elongation involves tRNA binding and peptide bond formation between amino acids: tRNA molecules deliver specific amino acids in accordance with the mRNA codons, creating a growing polypeptide chain.

  • Termination occurs at stop codons: Upon reaching a stop codon, the completed polypeptide is released from the ribosome, completing translation.

Genetic Control

  • Regulatory mechanisms control transcription: This includes the roles of promoter regions, repressor proteins, and activator proteins in regulating gene expression.

  • Proximity of gene to regulatory segments facilitates or hampers transcription, ensuring that genes are expressed at the right time and place.

Genetic Control - Epigenetics

  • Changes in gene function without altering DNA sequence: These modifications affect how genes are expressed and can be influenced by environmental factors.

  • Methylation can prevent transcription, influencing gene expression during development and throughout an organism's life cycle.

Mutations

  • Permanent DNA sequence alterations: Mutations can arise spontaneously or be induced by environmental factors, leading to genetic diversity.

  • Three main mutation types:

    • Point mutations (substitutions): Changes in a single base pair of DNA, potentially altering a single amino acid in a protein.

    • Frameshift mutations (insertions or deletions): Shifts the reading frame of the genetic code, often resulting in dramatically altered proteins.

    • Chromosome mutations (changes in chromosome structure): These can include duplications, deletions, inversions, and translocations affecting large segments of DNA.

Further Reading

  • Essential Cell Biology by Alberts et al, Chapter 7

  • Biochemistry by Berg et al, Chapters 29 and 30

  • Principles of Biochemistry by Voet et al, Chapters 26 and 27.