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Transcription and Translation end of chapter 3

Overview of Transcription and Translation Process

  • Definition of Transcription: The process that involves creating RNA from a DNA template.

DNA and RNA: Basic Structure

  • DNA Overview:

    • Composed of a sugar-phosphate backbone and nitrogenous bases.

    • DNA is represented metaphorically as a piece of paper with printed words, the words are the genetic information.

  • Metaphor of DNA Structure:

    • Sugar Phosphate Backbone: Represents the paper.

    • Nucleotide Bases: Represent the words printed on the paper.

RNA Polymerase and Promoter Region

  • RNA Polymerase Function:

    • Binds to the promoter region of DNA.

    • Unzips DNA by breaking hydrogen bonds between the nitrogenous bases.

    • Reads the DNA template to synthesize RNA.

  • Complementary Base Pairing:

    • The RNA polymerase reads a nitrogenous base on the DNA and selects a complementary RNA nucleotide from the nucleotide pool.

    • Base pairing rules:

    • Adenine (A) pairs with Uracil (U) in RNA (instead of Thymine (T) as in DNA).

    • Cytosine (C) pairs with Guanine (G).

Differences between DNA and RNA

  • DNA Characteristics:

    • Structure: Double-stranded

    • Sugar: Deoxyribose

    • Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)

  • RNA Characteristics:

    • Structure: Typically single-stranded (with exceptions like some viruses that possess double-stranded RNA)

    • Sugar: Ribose (contains an extra oxygen compared to deoxyribose)

    • Bases: Adenine (A), Uracil (U), Cytosine (C), Guanine (G)

Directionality in DNA and RNA

  • Antiparallel Nature of DNA:

    • One DNA strand runs in a 5' to 3' direction while the other runs 3' to 5'.

  • Directionality of RNA Synthesis:

    • RNA polymerase reads DNA from 3' to 5' but synthesizes RNA from 5' to 3'.

Transcription Process Steps

  1. Initiation: RNA polymerase binds to the promoter region and unwinds the DNA.

  2. Elongation: RNA polymerase reads the DNA template strand.

  3. Complementary Base Pairing Rules:

    • A pairs with U, C pairs with G.

  4. Termination: RNA polymerase continues until it reaches a terminator sequence.

RNA Processing

  • Pre-mRNA Modification:

    • Exons: coding regions that will be expressed.

    • Introns: non-coding regions that need to be removed.

    • Process involves splicing where exons are joined together, and introns are removed by spliceosome enzymes.

  • 5' Cap and Poly-A Tail Addition:

    • A 5' cap (modified guanine) is added to one end for protection and recognition.

    • A poly-A tail (adenosine residues) is added to the 3' end to protect the RNA from degradation.

Translation Overview

  • mRNA Function: Carries the information from DNA out of the nucleus to the ribosomes, where protein synthesis occurs.

  • Ribosome Structure: Consists of a small ribosomal subunit and a large subunit, where mRNA binds, and proteins are synthesized.

Steps of Translation:

  1. Initiation:

    • Ribosome binds to the mRNA molecule starting at the start codon (AUG).

  2. tRNA Structure:

    • tRNA molecules transfer specific amino acids to the ribosome. Each tRNA carries an anticodon that is complementary to the mRNA codon.

  3. Codon-Anticodon Interaction:

    • Ribosome reads the mRNA in three-base segments called codons.

    • Corresponding tRNA with the correct anticodon binds to each codon.

  4. Peptide Bond Formation:

    • The ribosome catalyzes the formation of peptide bonds between adjacent amino acids, creating a polypeptide chain.

  5. Termination of Translation:

    • Translation stops at a stop codon (UAA, UAG, UGA).

    • The polypeptide is released, and the ribosome dissociates.

Amino Acids and Codons

  • Genetic Code:

    • There are 64 different codons for 20 different amino acids, indicating redundancy in the genetic code.

    • AUG is always the start codon for translation, corresponding to methionine, while UAA, UAG, and UGA function as stop codons.

  • Polypeptide Sequence Formation:

    • The amino acids are added sequentially, forming the primary structure of proteins; further folding leads to secondary and tertiary structures.

Ribosomal Sites During Translation

  • P Site: Location where peptide bonds form between amino acids.

  • A Site: Accepts the incoming tRNA molecule.

  • E Site: The exit site where the tRNA, now without amino acid, leaves the ribosome.

Conclusion

  • Poly-ribosomal arrays: Multiple ribosomes can translate the same mRNA simultaneously, increasing efficiency in protein production.

  • DNA as a Blueprint: The flow of information from DNA to mRNA to protein is foundational to cellular function. Each gene expresses a specific protein necessary for cellular processes, exemplifying the significant role of transcription and translation in biology.