02/20: mRNA Processing & Intro To Translation

mRNA Processing

• mRNA processing only occurs in eukaryotes

• In prokaryotes, transcription and translation occur simultaneously

• Translation begins before mRNA transcription is even completed 

In Eukaryotic Cells

• In eukaryotic cells, mRNAs must first be processed and transported to the cytoplasm before they are translated 

• RNA processing includes 3 main steps 

1. A nucleotide “cap” is covalently attached to the 5’ end

2. A poly(A) “tail” is added to the 3’ end 

3. Introns are spliced out 

• Once steps are completed, RNA becomes a mature messenger RNA (mRNA)

• 5’ Cap Roles: 

  • A Guanine nucleotide that has been modified and flipped upside down

  • Gives mRNA stability

  • Provides a recognition site for ribosomes to bind (for translation)  

• 3’ PolyA tail Roles:

  • Gives mRNA stability 

  • The longer the tail/the more A’s it has the more stable it is

  • Provides a recognition site for “exporter” proteins to bind, to export mRNA from nucleus to cytoplasm

Exons & Introns

• A gene includes coding regions called exons and non-coding regions called introns 

• The whole gene will be transcribed during transcription 

• Thus the resulting (pre-processed) RNA transcript contains both exonic and intronic regions as well

• During splicing introns are removed from the RNA transcript and exons are joined together 

• Helpful Hint 

  • A way to remember the difference between exons and introns: 

    • EXons contain the code that’s EXpressed in protein 

    • INtrons are IN-between (or IN the trash) 

    • BUT! It’s not always just the introns that get spliced out…

Alternative Splicing 

• Often there are multiple ways an mRNA transcript can be spliced (i.e., it can contain different combinations of exons)

• This process is called alternative splicing

• Alternative splicing allows a single gene in DNA to code for MANY different proteins simply by editing the mRNA copy

• An extreme example is the gene Dscam can make almost 40,000 different proteins simply by the alternative splicing of its (many) exons

Translation

• Our mRNA carries the information (“blueprint”) for a protein in its nucleotide sequence (“genetic code”) 

• To make our protein, we need to translate the nucleotide sequence to an amino acid sequence 

Genetic Code

• Soon after discovery of DNA structure, scientists cracked the genetic code

• A series of experiments indicated that: 

  • The genetic code is read on mRNA 3 nucleotides at a time, from 5’ to 3’

  • Each triplet of nucleotides, termed “codons”, can be translated to a specific amino acid 

  • Translations for all the possible codon combinations using 4 nucleotides (A, C, U, & G) are summarized in this “codon table”

• Subsequent studies revealed the same nucleotide-to-amino acid translation is used by most prokaryotes, eukaryotes, and viruses 

• We say the genetic code is redundant or “degenerate” - often amino acids are encoded by multiple codons 

  • There are 61 codons that code for only 20 amino acids 

  • Buffer room for mutations 

• This includes a universal start codon AUG (which translates to methionine, or Met), signals that start of mRNA’s coding region 

• There are also 3 codons that don’t encode any amino acid, and therefore end or STOP the protein’s synthesis (“stop codons”) 

Codon Table 

• The table will be given to you on exams but you should be familiar with how it works:

  • Example: What amino acid does the codon 5’-CAU-3’ code for?

1. On left side of table, find the row corresponding to the first nucleotide at 5’ end of codon 

2. At the top, select the column corresponding to the second nucleotide to find the right “box”

3. Using the right side of the table, locate the exact amino acid by finding the proper 3’ nucleotide row within the box

General Rules of Translation 

• Read the mRNA 5’ to 3’ (as with the codons displayed in the table)

• Start at the start codon (AUG) - this assures the correct ‘reading frame”

  • AUG codes for Met thus, Met is always at the beginning of every new polypeptide chain 

• Read nucleotides in groups of 3 (i.e., one codon at a time)

• Once you reach a stop codon (UAA, UAG, or UGA), no more amino acids are added and the polypeptide ends