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Transcription - DNA to RNA

& mRNA Processing

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DNA Recap

The parts of a nucleotide are:

  • 5 carbon sugar (part of the backbone)
  • Phosphate group (part of the backbone)
  • Nitrogenous base (A,T,C,G)

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Backbone is linked by strong covalent bonds

Nitrogenous bases linked by weaker hydrogen bonds

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DNA Practice:

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What is the complementary DNA strand to the following?

3’TAC/TCT/ATG/ATA/CCG/TCG/ATG/ATG/TCT/CCG/GAC 5’

(Part of the gene for human skin keratin)

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Remember: T pairs with A

G pairs with C

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Remember: strands are antiparallel

Include the 3’ and 5’ labels in your complementary strand!

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Original: 3’TAC/TCT/ATG/ATA/CCG/TCG/ATG/ATG/TCT/CCG/GAC 5’

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New strand:

5’ATG/AGA/TAC/TAT/GGC/AGC/TAC/TAC/AGA/GGC/CTG 3’

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Amatoxin

Why am I showing a picture of a mushroom?

(Amanita muscaria and Amanita virosa (Destroying Angel))

Amanita muscaria toxin- amatoxin stops RNA polymerase (enzyme that helps make RNA) from working, what effect do you think this has?

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RNA Polymerase

RNA polymerase is an enzyme that is responsible for copying a DNA sequence into an RNA sequence. This is one of the first steps in making protein. Hence, your body stops making proteins and liver failure occurs when poisoned by specific mushrooms.

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Gene to mRNA to Protein = The Central Dogma

The central dogma of molecular biology describes the two-step process, transcription and translation, by which the information in genes is used to create proteins: DNA → RNA → protein. Transcription is the synthesis of a mRNA copy of a segment of DNA. mRNA is synthesized by the enzyme RNA polymerase.

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Gene to Protein

DNA cannot travel outside the nucleus...

However, protein synthesis occurs in the cytoplasm. How can the cell transfer that information?

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Transcription!

  • DNA is transcribed into a messenger RNA molecule
  • This step occurs in the nucleus

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RNA vs DNA

  • Quick review of differences between DNA and RNA….

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mRNA = one type of RNA

  • Messenger RNA
  • Transcribed from DNA
  • Carries genetic material outside the nucleus

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Transcription: The process of making messenger RNA using a DNA template. RNA polymerase creates mRNA.

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Transcription Step 1: Initiation

  • RNA polymerase binds to the promoter region of DNA

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Transcription Step 2: Elongation

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  • RNA polymerase opens the DNA and moves along the gene from 3’ to 5’, which means it’s building the RNA strand from 5’ to 3’.

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Transcription Step 3: Termination

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RNA polymerase reaches the termination sequence

The mRNA breaks away

RNA polymerase disengages from the DNA strand

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Quick recap: Transcription

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Differences between DNA and RNA

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mRNA processing

A 5’ tail is added (modified G), and a poly-A cap (string of A’s) to the 3’ end.

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mRNA processing: splicing

The mRNA has sections that must be edited out by a spliceosome

Exon: coding section of RNA

Intron: section of RNA that is edited out

What might happen if introns were not edited out?

What could be a benefit to having exons?

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Capping and Tailing = 5 prime guanosine cap and 3 prime poly adenosine tail is added to the mRNA.

This helps with getting through the nuclear pore, attaching to the ribosome, and keeping the mRNA in tact.

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Because of alternative splicing, one gene can code for many proteins.

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End Result of Transcription and mRNA processing: �a mature mRNA molecule

  • Single stranded
  • A,U,C,G
  • Ribose sugar

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mRNA then exits the nucleus for translation in the ribosome.