Unit 6: Lesson 56 - RNA Transcription and Codons

Unit 6: Lesson 56 - RNA Transcription and Codons

Overview of RNA and the Human Genome

  • The human genome consists of 20,000 to 25,000 genes.
  • These genes encode for various traits.
  • DNA contains all the genes necessary for an organism.

RNA: Structure and Function

  • RNA (Ribonucleic Acid) is produced in the nucleus where DNA is located.
  • Eukaryotic organisms: RNA can exit the nucleus after synthesis.
  • Polymer Composition of RNA:
    • RNA is a polymer of nucleotides.
    • Each nucleotide consists of three parts:
    1. Phosphate group (PO₄³⁻)
    2. Pentose sugar (ribose)
    3. Nitrogenous base
    • Two classes of nitrogenous bases:
    1. Purines: Adenine (A) and Guanine (G)
    2. Pyrimidines: Cytosine (C), Uracil (U)
  • In RNA, Uracil replaces Thymine, which is found in DNA.
  • RNA occurs as a single-stranded molecule rather than double-stranded and has a backbone of phosphate and sugar (p-s-p-g).

Types of RNA

  • There are three main types of RNA essential for protein synthesis:
    1. mRNA (messenger RNA):
    • Function: Carries instructions from DNA to synthesize proteins.
    • Analogy: Acts like a cookbook providing the recipe.
    1. tRNA (transfer RNA):
    • Function: Provides amino acids for protein construction, using codons as letters to form protein names.
    • There are 20 different amino acids found in nature.
    1. rRNA (ribosomal RNA):
    • Function: Forms the structural and functional core of the ribosome, the site of protein synthesis.
    • Analogy: Acts like a kitchen where the cooking occurs.

Definition and Process of Transcription

  • Transcription (noun): A written or printed representation of something.
  • Transcription (verb): The act of creating a transcription in a biological context, referring to the process of using a DNA template to synthesize a complementary mRNA strand.
  • Instructions for Transcription:
    • Identify the template strand of DNA that will be transcribed.
    • The non-template strand is not transcribed.

Steps of Transcription

  1. Helicase: Unzips the DNA double helix to expose the gene sequence by breaking hydrogen bonds between base pairs. Example:
    • Unzipping sequence: A-T-C-G
  2. RNA Polymerase:
    • Synthesizes a complementary mRNA sequence according to the DNA template.
    • Transcription progress: Goes down the DNA strand, creating an mRNA transcript complementary to the template DNA strand.
    • Example sequence: If DNA template is A-C-C-O-G, then mRNA transcribed would be U-G-G-C.
  3. Final Steps:
    • Once synthesized, mRNA can exit the nucleus and enter the cytoplasm, where it will participate in protein synthesis at ribosomes using rRNA as part of the structure.