Protein Synthesis › Transcription

Transcription, Part I

Notes

Transcription, Part I

Sections

RNA TRANSCRIPTION PART 1

CONVERTS INFORMATION IN DNA TO RNA

  • Protects original information
  • Multiple RNA copies allow for more protein to be produced

DEFINITIONS

Transcription factors

Proteins which bind the promoter and recruit the RNA polymerase

Template strand

Strand of DNA in which the gene to be transcribed is located

THREE STEP PROCESS:

Initiation

  • Promoter recognized by transcription factors
  • RNA Polymerase II recruited by the transcription factors
  • Transcription begins and continues 3' to 5' along template strand

Elongation

  • Complementary RNA nucleotides added to the growing RNA strand (uracil instead of thymine in the RNA strand)

Termination

  • Polyadenylation signal in the transcribed RNA signals proteins to cut it from the polymerase
  • Newly formed RNA is called Pre-mRNA because it still needs to be processed
  • RNA Polymerase eventually falls off the template strand

Full-Length Text

  • Here, we'll learn about RNA transcription, the first step of a two-step process that converts the information held in DNA to usable protein.
    • This tutorial is split into two parts, the first part focuses on transcription and the second part focuses on post-transcriptional modification.
  • First, start a table to summarize some important points.
  • Denote that transcription converts the information held in DNA to RNA, which not only protects the original information, itself, but also allows for multiple copies of RNA so more protein can be produced.
    • This RNA is called messenger RNA (aka mRNA) because it carries the message from the DNA to the ribosome for protein synthesis.
  • Note that prokaryotic transcription differs slightly from eukaryotic transcription. We will focus on eukaryotic transcription, but we will denote these differences at the end of Part 2.
  • Indicate that transcription follows a three-step process: initiation, elongation, and termination.

Let's illustrate the three-step process of transcription now.

  • Draw the two strands of DNA.
  • Label the 5' and 3' ends of each strand. This will be referenced later in the tutorial.
  • Label the lower strand as the template strand and highlight a section as the gene.
    • Any particular gene will only ever have one template strand, but templates can be found on both strands of DNA.
  • Now label a section of DNA before the gene as the promoter.
    • The promoter contains sequences that are recognized by a group of proteins called transcription factors.
  • Draw transcription factors binding to the promoter.
    • After binding to the promoter, these proteins recruit the RNA polymerase to the site.
  • Draw the RNA polymerase, and label it RNA polymerase II.
    • Eukaryotes have multiple forms of RNA polymerase, each transcribing a different kind of RNA.
    • For mRNA, eukaryotes use RNA polymerase II.
  • Label the start point right at the beginning of the gene.
    • This is where transcription initiates.
    • Some start sites are located within the promoter region, though still close to the gene.
  • Draw an arrow to indicate that transcription proceeds from 3' to 5' along the template strand because (like DNA synthesis) the polymerase can only add nucleotides to a free 3' hydroxyl group.
  • Indicate "downstream" as beyond the start point.
  • And "upstream" as before it.
  • Show that after transcription begins, the transcription factors fall off.

Next, we will look at the elongation step.

  • Again, draw the two strands of DNA, but this time, in the middle, have a space between the two to show that the double helix has been unwound.
  • Draw the RNA molecule that is being transcribed
  • Label the 5' end of the RNA.
  • Draw the RNA polymerase around this space to illustrate it is unwinding the strands as it progresses along.

Next, draw a zoomed-in view of the elongation process.

  • Draw the DNA template strand.
  • Attached to the template, draw 5 nitrogenous bases.
    • Label these bases guanine, adenine, thymine, cytosine and adenine.
  • Next, draw the RNA strand, labeling the 3' end, and add nitrogenous bases so that all but one of the DNA bases are paired.
  • Label these RNA nitrogenous bases appropriately:
    • Guanine bound to cytosine.
    • Adenine is bound to thymine, but where thymine would be present on the RNA, uracil is found instead. Recall that this is because RNA does not contain thymine.
  • For the unbound adenine, now draw its partner, uracil, free-floating.
  • Label the 5' and 3' end of this nucleotide.
  • Now draw an arrow to indicate that the 5' end of uracil will be added to the 3' end of the growing RNA strand.
  • Going back to the zoomed out view, finish by drawing an arrow so we don't forget the direction of transcription.

Finally we will look at the termination step.

  • Draw the two DNA strands.
  • Now draw the transcribed RNA, though it is shorter than the DNA strands.
  • Label a section at the 3' end of the RNA as the polyadenylation signal sequence.
    • This sequence in the newly transcribed RNA (which contains AAUAAA) is recognized by proteins which cleave the RNA.
  • Illustrate that the RNA polymerase continues on for a short while until it falls off, though the mechanism by which this occurs is currently unknown.
  • Label this newly formed RNA as Pre-mRNA because it still needs to undergo some processing before it is considered mRNA and can leave the nucleus.

This processing will be discussed in Part 2 of our tutorial.

UNIT CITATIONS:

  1. Campbell, N. A. & Reece, J. B. Biology, 7th ed. (Pearson Benjamin Cummings, 2005).
  1. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. Molecular Biology of the Cell, 5th ed. (Garland Science, 2008).
  1. Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. Essential Cell Biology, 3rd ed. (Garland Science, 2010).
  1. Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., Ploegh, H. & Matsudaira, P. Molecular Cell Biology, 6th ed. (W. H. Freeman and Company, 2008).
  1. Marieb, E. N. & Hoehn, K. Human Anatomy & Physiology, 10th ed. (Pearson, 2016).
  1. Jeanteur, P. (edited by). Alternative Splicing and Disease. (Springer, 2006).
  1. Elliott, D. & Ladomery, M. Molecular Biology of RNA. (Oxford University Press, 2015).