Points of possible confusion: termination signal vs. stop codon
- The termination signal is for terminating the copying of the mRNA from DNA in transcription.
- The stop codon is for stopping the adding of amino acids in translation.
Crazy Facts
- On average, 8000 nucleotides are transcribed to make an mRNA molecule (in a eukaryote).
- Proteins average only about 400 amino acids (1200 nucleotides) in length.
- Only 15 % of the mRNA is actually used to code for a protein!
Modifying the Middle
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The coding segment of the mRNA has two different regions:
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Introns, regions that do not code for proteins.
- These must therefore be removed before translation.
- Also known as “intervening sequences.”
- Exons, regions that code for proteins.
- Way to remember: exons are expressed.
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Molecules that remove introns are known as spliceosomes, and are composed of two main parts:
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Small nuclear ribonucleoproteins
- Abbreviated snRNPs (pronounced “snurps”).
- Made of RNA and proteins.
- Other proteins
The Final Product
- Once the introns have been removed, RNA processing is over: pre-mRNA has been converted into mRNA.
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The mRNA is ready to be translated.
- Before this can happen, the mRNA must be transported from the nucleus into the cytoplasm, where translation occurs.
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The final product is composed of, in 5' → 3' order:
- 5' cap
- Leader
- Start codon
- Exons
- Stop codon
- Trailer
- Poly(A) tail
A Nagging Question
- What is the function of introns?
- No one knows for sure, here are a few ideas. . .
- Perhaps they have some regulatory role in gene activity (turning genes on and off)
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They allow for a greater diversity of proteins since exons can be mixed and matched in different ways- this is called "alternative splicing."
- 92% to 94% of humans genes undergo alternative splicing (Nature, November 2008).
- Therefore, one gene can code for more than one protein.
- This may explain why we have 20,0000- 25,0000 genes and 100,000 proteins (perhaps upwards of 1 million).
- In other words, this may explain why the proteome (the collection of proteins in an organism) is much larger than the genome (the collection of genes in an organism).