|MODULE 5|

INTRODUCTION

  • DNA information is contained in nucleotide sequences

  • These sequences direct the production of proteins

  • Information flows in one direction

    • DNA → RNA → Protein

  • Known as the central dogma

RIBONUCLEIC ACID (RNA)

  • Bridge between DNA & protein synthesis

  • Characteristics:

    1. Sugar - Ribose

    2. Four Bases - A,G, C, Uracil

    3. Single Stranded

  • Three types

    1. Messenger RNA (mRNA) - transmits info from DNA & serves as a template for protein synthesis

    2. Ribosomal RNA ( rRNA) - combines w/ proteins to make ribosome

    3. Transfer RNA (tRNA) - brings amino acids to ribosome for protein synthesis

GENE EXPRESSION

  • The process by which DNA directs protein synthesis

  • Two stages - locations

    1. Transcription - inside the nucleus

      • Synthesis of RNA using information in DNA

      • A DNA strand is a template for building RNA strand

      • DNA info is written in the form of triplets

        • 3 nucleotide bases (Ex. AGC)

      • RNA nucleotide bases form a triplet called a condon

    2. Translation - Outside nucleus

      • The cell reads mRNA codons; and builds protein accordingly

      • Transfer RNA (tRNA) brings amino acids to ribosome (rRNA)

        • has its nucleotide triplet called an anticodon, pairs with the complementary mRNA codon

      • Ribosome adds amino acids to growing polypeptide (polymer) chain

mRNA SYNTHESIS

  • DNA information is “rewritten” into RNA

  • Has 3 stages:

    1. Initiation

      • Promoter - DNA sequence (TATA box) where RNA polymerase attaches

      • Transcription Factors - assists binding of RNA polymerase

      • Transcription Initiation Complex - RNA polymerase + factors

      • Starting Point - specific nucleotide downstream from TATA box where RNA synthesis begins

    2. Elongation

      • RNA polymerase moves along DNA, untwisting the helix

      • RNA nucleotides add to the 3’ end in a 5’ → 3 direction

      • RNA strand peels away

      • DNA strand reform

    3. Termination

      • RNA polymerase transcribes a DNA sequence called polyadenylation signal sequence (AAUAAA)

      • Signals RNA transcript to be released from RNA polymerase

      • RNA polymerase detaches

    4. BUT… pre-mRNA must undergo further processing

    RNA PROCESSING

  • Produces an mRNA molecule ready for translation

  • Processing

    1. Alternation to mRNA ends

      • Alternations include

        1. 5’ cap - modified guanine nucleotide

        2. poly A tail - made of 50-250 adenine nucleotides

      • Aides in

        1. Export

        2. Protection

        3. Ribosome attachment

    2. RNA splicing

      • Removes non-coding regions called introns

      • joined coding regions called exons

RIBOSOME BINDING SITES

  • Ribosoe has 3 binding sites for tRNA

  • Binding sites

    1. P site - holds tRNA with the growing polypeptide chain

    2. A site - holds tRNA carrying the next amino acid

    3. E site - exit site for tRNA

POLYPEPTIDE FORMATION

  • 3 stages

    1. Initiation

    2. Elongation

    3. Termination

  • All stages required various protein factors & energy (GTP)