BISC 130 - Chapter 15: Genes and Proteins

• The Central Dogma of Molecular Biology

• Describes the flow of information in cells.

• DNA --> RNA --> Protein

  ^Transcription      ^Translation

• Prokaryotic Transcription

  • Carried out by RNA Polymerase.

  • 3 Phases:

  1. Initiation

     1. RNA Pol. binds to Promoter

        1. Promoter - a region of DNA with a specific sequence

           1. This sequence allows the RNA Pol to know where it is and how to bind to it.

           2. If there isn't a correct sequence, RNA wouldn't bind.

        2. Upstream/right before where transcription (of genes) will begin.

1. Forms transcription bubble.

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1. Elongation

   • RNA Pol. Builds RNA 5' - 3'

   • RNA sequence is complementary to "template strand" of DNA.

     • Base pairing

       • DNA - C G T A

       • RNA - G C A U 

     • RNA uses U instead of T**

   • RNA sequence is the same as the "coding strand" of DNA

   • DNA re-winds as the transcription bubble passes by.

1. Termination

   • Rho protein follows behind transcription bubble.

     • At a sequence of many Gs, RNA Pol. Stalls or loses some speed/momentum

       • Rho catches up to it and ends the transcription OR

       • A hairpin structure forms in the RNA, which causes the transcription to end

• Eukaryotic Transcription

  • Fundamentally the same as Prokaryotic Transcription- Same 3 stages

  • BUT:

    • There are 3 diff. RNA Polymerases for different types of RNA.

    • Introns are spliced out, exons are kept in

      • Exons - An expressed sequence that is kept in the RNA after processing

        • Introns removed.

• RNA Is processed, to extend the stability and to facilitate export from the nucleus.

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• Structural components of ribosomes

• tRNA = transfer RNA

  • Plays a structural role in translation

• mRNA = messenger RNA

  • Its information directs protein synthesis

The Genetic Code

• # of DNA nucleotides: 4

• # of RNA nucleotides: 4

• # of amino acids: 20

• If 1 nucleotide encodes 1 amino acid

  • Not enough nucleotides

• If 2 nucleotides encode 1 amino acid:

  • 4^2 = 16 possible combinations

  • Not enough for all 20 amino acids

• If 3 nucleotides encode 1 amino acids

  • 4^3 = 64 possible combinations

  • More than enough for 20 amino acids.

• Genetic code is based on sets of 3 nucleotides called codons.

• The Genetic code is "degenerate"

  • Redundancy

  • Most amino acids are encoded by multiple codons

• There are two types of special codons

• 1 start codon

  • Signals start of the protein - 1st amino acid

  • Encodes the a.a. Methionine

• 3 different top codons

  • Signals end of translation

  • Do not encode an a.a.

• The genetic code is almost completely universal.

Translation:

• Carried out by by ribosomes

• 3 phases

  1. Initiation

     • Small ribosomal subunit binds to mRNA.

     • Initiator tRNA (carries the Methionine a.a.) binds to start codon

     • Large ribosomal subunit binds

1. Elongation

   1. tRNAs have an a.a. attached at one end, and an anticodon at the other end.

      1. Anticodons are complementary to codons in mRNA

• tRNAs enter ribosome one at a time, according to the mRNA sequence

• tRNA's a.a. is passed to the next tRNA's a.a.

  • Peptide bonds are created

    • Costs energy

• "empty" tRNA exits ribosome

• mRNA slides along

• Next tRNA enters, etc……

1. Termination

   • A stop codon in the mRNA causes a release factor protein to enter the ribosome.

   • 2 ribosomal subunits dissociate?

   • Protein released

   • mRNA released

     • Can be translated again

• Prokaryotes: Simultaneous transcription and translation

• Eukaryotes: Transcription in nucleus, translation in cyptoplasm

Point Mutations

• Changes a single base pair in DNA

• 3 types

  1. Silent

     • No effect on the a.a. sequence

     • New codon encodes same a.a.

1. Missense

   • Amino acids changed

1. Nonsense

   1. An a.a. encoding codon is changed to a stop codon.

Frameshift Mutation

• A few nucleotides are inserted or removed

  • If not a multiple of 3, completely changes further a.a. sequence