MCDB 1041 Class 21 Splicing and Gene Expression

Transcription

1 MCDB 1041 Class 21 Splicing and Gene Expression Learning Goals Describe the role of introns and exons Interpret the possible outcomes of alternative splicing Relate the generation of protein from DNA to what we've previously discussed about the relationship between genotype and phenotype Define gene expression and identify parts of a DNA sequence that are inolved

2 There are two more levels of information control between transcription and translation Not all DNA within a gene is actually read into protein -exons are kept in the mrna -introns are removed from the mrna We don t really know all the reasons for introns, but they are not junk sometimes other gene coding sequences are within large introns sometimes an intron is used for another purpose (before it is removed) such as a binding site for a protein

3 Handout, part 1

4 Alternative splicing: 1 gene does not necessarily = 1 protein The human genome is 3.2 billion base pairs There are only about 25,000 genes, comprising only a small fraction of the actual genome But, it is estimated that humans can make over 100,000 proteins. How? One way is by having a single gene code for several variations on a protein: the process by which these different transcripts are created is called alternative splicing

5 Alternative Splicing when the same sequence of mrna is processed differently in different tissues to make slightly different versions of the same protein Your sequence of DNA has introns and exons; find these to make a sentence Which words can you leave out and still make a sentence that makes sense? Write these alternatives

6 Alternative Splicing general example

7 Stop codon tells ribosome to stop reading mrna Start codon tells ribosome to start reading mrna For transcrip:on, the start is indicated by the promoter region (NOT the ATG) For transcrip:on, the stop is indicated by a transcrip:onal stop (NOT the stop codon)

8 If a DNA gene sequence were altered so that it had and early stop codon, what will happen in transcrip:on and transla:on? a. Normal transcrip:on, early stop for transla:on b. Early stop for transcrip:on, early stop for transla:on c. Early stop for transcrip:on, normal transla:on

9 How many copies of the same protein (comprised of amino acids) are made from one mrna? a. 1 b. 2 c. Many

10 Many mrnas made from a single gene Many proteins made from a single mrna This process is called amplifica:on

11 Protein structure depends on the sequence of amino acids

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13 Each set of interactions between amino acids helps to shape the protein Any disruption of these could result in a protein without function, or with a different function

14 Changes in protein structure result in the protein functioning differently or not at all. If a person has many copies of a particular protein whose structure is changed from normal, where did this change most likely originate? a. In the DNA sequence of the gene b. In the mrna sequence c. In the amino acid sequence d. In the protein More on muta:ons next :me!

15 Thinking about the kinds of proteins that are made If you were comparing two somatic cells (like a skin cell and a neuronal cell), you would find that they contain: a.the same chromosomes and the same proteins b.different chromosomes and different proteins c. the same chromosomes but different proteins d.different chromosomes but the same proteins

16 This leads us to: Gene expression If all genes were made into RNA, processed, and translated into protein in all cells, all cells would contain the same proteins. This is not the case. Different cells have different proteins. The identity of a cell = what is transcribed and translated= gene expression

17 What controls whether a gene is transcribed or not? Transcription factors (proteins that turn on transcription by assembling at the promoter) Chemical modifications of DNA sequences that can prevent the DNA from being unwound (thus keeping it inactive ) Chemical modifications of the Histone proteins (the proteins that DNA is wound around)

18 Transcription factors -proteins that bind to the DNA (usually near or at the promoter region) and allow RNA polymerase to bind Transcription RNA factor promoter polyerase 3 5 template region of DNA 5 3 There are all-purpose TFs that bind to all sequences There are unique transcription factors that are produced in some cells and not others These unique transcription factors bind to regions near the promoter and allow transcription: this determine which genes will get expressed in which cells

19 Neuron Muscle Which of these cells has the neurogenin gene? a. Neurons b. Muscles c. Both d. Neither Which of these cells has neurogenin mrna? a. Neurons b. Muscles c. Both d. Neither Neuronal neuron cells contain the protein neurogenin Muscle cells contain the protein myosin

20 Alterations to the chromosomes determine how easily transcription factors (TFs) can bind to promoter regions: chemical modifications Addition of a methyl group (CH3) make DNA inaccessible; TFs can t bind Addition of an acetyl group (COCH3) to the histone proteins opens the structure of the chromosome, allowing TFs to bind