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Lecture 3

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Western University
Biology 1001A
Tom Haffie

Photochemistry occurring in photo receptor and reaction centre Why there is no antenna around a photo receptor but there is an antenna around photo system. Photosystem, want to harvest as much light energy as you can, In a photo receptor, the arrangement of rods and codes is attempting to provide to mimic to harvest light as information, where those photons come from conveys information. Photosynthesis don’t care. Photo system is probably made up of 50 proteins come together. Humans have 20 thousand express genes, so 20 thousand different proteins. Biochemistry is built upon the isolation of proteins from cells, purifying the protein, putting it into test tubes, and studying its structure…protein turns very abundant, and how do you isolate a specific one… Genetics looking at phenotype, try to link phenotype and genotype. Molecular biology link protein and gene What controls protein abundance? One type proteins, many present in one kind of cells, and none present in another 1) Control transcription Genes code for proteins The conversion, the copying, deoxyribonucleic acid (DNA) into messenger ribonucleic acid (RNA) 2) Control translation Control the conversion of mRNA into protein Transcript abundance Transcript: the product of the transcription. How much specific mRNA? Depends on transcription Transcription rate is high, should have lots of transcript. But not necessarily the case. mRNA decay controls transcript abundance. When mRNA is made, it doesn’t stick around forever. 20 minutes, or a few hours, or… before they break down. There is a competing process with transcription and that’s mRNA decay. Transcript abundance is the balance of those two processes. You can modulate transcript abundance by changing only one of these, or changing both. How do we measure protein abundance and transcript abundance? Measuring transcript abundance? How do we measure the abundance a specific mRNA within a cell? Isolate RNA from cell or tissue samples and run the RNA on a gel. Gel electrophoresis Very similar from we run a DNA on a gel So here we have tissues from human brain, heart, lung, spleen, and bacterium. So I can quantify the gel has equal amounts of RNA. Why don’t see mRNA on the gel? 97% of total RNA is ribosomal RNA. And these bands are ribosomal RNA. 2/3 of ribosome is RNA. Ribosomal RNA is much much more abundant than messenger RNA. Prokaryotic ribosome is different in size than the eukaryotic ribosome. This is reflected in these bands do not migrate to the same size on the gel. So these are ribosomal bands. But when you take a picture of this, I got equal amounts of RNA. But I can’t see mRNA. About 3% of the total RNA is mRNA. Hard to find one specific species of mRNA. Transfer the RNA to the membrane. Detect a specific RNA. Make a single stranded DNA probe. It would hybridized to the mRNA corresponding to that gene. That single stranded mRNA is mobilized on the membrane. I had a single stranded copy of DNA that I label radioactively and hybridization would take place. The sequences are identical. Hybridization will take place. That radioactive probe will stick to the membrane exactly where the complementary sequence is. So I have a gene specific probe, it is single stranded DNA, it will hybridized to the single stranded mRNA, and it will do so very specifically. It is radioactive, so I ca
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