Protein Synthesis

  1. How is mRNA different from than DNA?
    Both DNA and mRNA are polynucletides however, they have three distinct difference to one another.
    – Unlike DNA, which is double stranded, long and is shaped in a helix. Whereas mRNA differs as it is single stranded, short and is not in a shape of a double helix
    – DNA’s nucleotides contain contains the sugar deoxyribose but mRNA nucleotides contain the sugar; ribose.
    – While base pairing occurs in DNA, Adenine will always pair with Thymine. In mRNA, there is no Thymine but instead, Adenine pairs with Uracil in terms of all types of RNA including mRNA.

    DNA Backbone (Thymine is being represented by blue beads)

    RNA Backbone (Uracil is being represented by the brown beads)

    2. Describe the process of transcription
    Transcription is the process in which one gene’s DNA sequence is copied (transcribed) onto a strand of mRNA. It occurs in three phases: unwinding and unzipping of DNA, complimentary base pairing with DNA, and separation from DNA.
    All phases are in occurrence because of the enzyme RNA Polymerase.

    DNA being unzipped. A process that is facilitated by RNA Polymerase

    PHASE 1: Unwinding and Unzipping
    During the process of transcription, a portion of the DNA helix unwinds and unzips, exposing one gene.

    RNA Polymerase joining adjacent nucleotides

    mRNA Molecule

    PHASE 2: Complimentary Base Pairing
    Only one strand will be used as a template to produce an mRNA strand, this is know as the ‘sense strand’. Along the sense strand the complimentary RNA bases will bond together. Since RNA does not contain Thymine, therefore Adenine will pair with RNA’s equivalent base pair; Uracil.
    The sense strand partners with what is known as the nonsense strand, containing information that is not useful; not yielding a protein, if transcribed.
    RNA Polymerase will join the adjacent nucleotides to one another using H-Bonds. The nucleotides will bond, forming covalent bonds and build an mRNA backbone.

    mRNA is detached

    PHASE 3: Separation
    Until the entire gene has been transcribed the mRNA strand will detach from the DNA strand. The DNA molecule will rewind itself, reforming its double helix shape. This results in a strand of mRNA that gets modified before it moves out towards the nucleus.

    DNA reforms its double helix

    3. How did today’s activity do a good job of modelling the process of RNA transcription? In what ways was out model inaccurate?
    Some ways that this activity accurately modeled the process of RNA transcription. It accurately displayed how instead of pairing with Thymine, Adenine will pair with RNA’s Uracil by the use of a different coloured bead, as well as the difference in their backbones, modeled using a red backbone. The activity also gave a good representation of how the DNA and RNA bases paired with one another. The only inaccuracy in this activity I believe was that it did not demonstrate how the mRNA strand is much smaller to the DNA strand and it did not show what follows after transcription occurs and how the mRNA strand is processed which was shown as an important step.
    4. Describe the process of Translation: initiation, elongation, and termination.
    Translation is the process in which the code carried by mRNA transforms into a polypeptide. This process of translation occurs in three steps: Initiation, elongation and termination

    Ribosome reaches the start codon and is instructed to start translating and to code for an Amino Acid

    STEP 1: Initiation
    – In an mRNA, the instructions to build a polypeptide come in groups called codons (3 letter words located on mRNA). There are 3 “stop” codons that mark the polypeptide as finished; and one codon, AUG, is a “start” signal that starts of translation.
    – During this stage of translation, mRNA binds to the small ribosomal subunit in the area of the start codon (AUG). The first tRNA pairs with this codon. A large ribosomal subunit and small subunit will then join together. There are two sites in the ribosome in which bonding can occur, the “A-site” and “P-site”. The ribosome will then start to move along the mRNA until it reaches the start codon, AUG, in the “P-site”. This instructs it to start translating and code for an amino acid. A matching tRNA will bring in an amino acid that is represented by the mRNA codon.

    Amino Acids in both the “A” and “P” sites

    The tRNA in the “P-site” is released and leaves the ribosome;

    STEP 2: Elongation
    A ribosome is large enough where it is able to contain two tRNA molecules; the incoming and outgoing. The tRNA also contains three letter codes called anti-codons which are complimentary to mRNA codons. During this process, the ribosome will hold the mRNA and allow for the complimentary tRNA to attach to the binding sites. As the tRNA binds to the “P-site”, another will bond with the “A-site”. A peptide bond will form between the two amino acids. This bond causes the amino acids to let go of the tRNA in the “P-site” and start to bind to the neighbouring amino acids. The ’empty’ tRNA will then leave the ribosome and as a result, the ribosome will begin to move along the mRNA. This is done in order to make the tRNA is at the “P-site” and the new tRNA will start to bind with the mRNA codon at the “A-site”.

    tRNA in the “P-site” gets the stop codon and the chain stops growing

    STEP 3: Termination
    The cycle for elongation will continue until the mRNA reads a stop codon, terminating the process. This would result in a three letter word (codon) with no matching tRNA amino acid. This means that there are no new amino acids being added to the chain, causing the ribosome to separate into its two subunits and release the polypeptide.
    How did today’s activity do a good job of modelling the process of translation? In what ways was our model accurate?
    It was accurate that the model showed that it went through each of the stages carefully which allowed a better grasp on the concept. We got to see how the ribosome moves along the chain. It was also very beneficial how this activity showed how the codons and the anticodons paired with one another. However, there were some inaccuracies with the activity where we were not able to model the release factor that bonds to the stop codon and how the polypeptide is detached. Furthermore, the changing of the tRNA shape was not displayed not was the accurate shape of the amino acids displayed very well.

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