Lecture 4 mRNA splicing and protein synthesis

	Another day in the life of a gene.

Adding a 5’ cap

Pre-mRNA has introns

The splicing complex recognizes semiconserved sequences

Introns are removed by a process called splicing

Splicing includes multiple proteins and small nuclear RNAs called snRNAs

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Complexity of genes

	Splicing in some genes seems straightforward such as globin

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For other genes splicing is much more complex

	Fibrillin is a protein that is part of connective tissue.  Mutations in it are associated with Marfan Syndrome (long limbs, crowned teeth elastic joints, heart problems and spinal column deformities.  The protein is 3500 aa, and the gene is 110 kb long made up of 65 introns.
	Titin has 175 introns.
	With these large complex genes it is difficult to identify all of the exons and introns.

Alternative RNA splicing

	Shortly after the discovery of splicing came the realization that the exons in some genes were not utilized in the same way in every cell or stage of development.  In other words exons could be skipped or added.  This means that variations of a protein (called isoforms) can be produced from the same gene.

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Gene Expression II

	Translation of the mRNA into protein

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How does DNA function as a code for protein synthesis?

	The experiments of Charles Yanofsky and Syndey Brenner demonstrated that the sequential arrangement of nucleotides along a gene code for a sequential arrangement of amino acids in its encoded protein.
	The code in DNA (and ultimately mRNA is read in triplets).
	The code is degenerate.

Yanofsky precisely mapped the positions of a series of mutations in the TrpA gene

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Discovery of the genetic code

The table of codons

Central Dogma

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Assembly of ribosomes

	Ribosomal RNA is transcribed as a 45S precursor RNA, synthesized in the nucleolus by polI from thousands of copies of the gene.
	The 45S precursor (13,000 nt)is processed into 3 smaller RNAs 28S (5000 nt), 18S (2000 nt) and 5.8S (160 nt)
	The 5S subunit is synthesized by polIII from a cluster of 2000 genes located separately from the other ribosomal genes
	Some 80 proteins associate with the rRNAs to make up complete ribosome.
	Small ribosomal subunit (40S) contains 18S rRNA while the large 60S subunit contains the remaining rRNAs

ribosomes

Transfer RNAs (tRNAs)

	tRNAs are small 70-90 nt
	there are about 32 different tRNAs in most organisms
	the tRNAs contain unusual modified nucleotides
	aminoacyl-tRNA synthetases charge tRNAs with amino acids
	tRNAs function to deliver the amino acids to the ribosomes for protein synthesis

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Wobble Hypothesis

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Protein synthesis

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Initiation of translation

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Termination

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Regulation of protein synthesis

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