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Lecture 3: "Gene & Genome Structure"

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Western University
Biology 2581B
Jim Karagiannis

Genetics Lecture No. 3: Gene & Genome Structure th Wednesday January 16 , 2013 Flow Of Information In Biological Systems: -The flow of information in biological systems is illustrative of the Central Dogma of molecular biology, which states that DNA is transcribed into mRNA that is subsequently translated into protein. DNA acts as double-stranded storage molecule that is based on complementarity of 5’-3’ and 3’-5’ strands. During transcription, a single-stranded mRNA (complementary to a DNA template) is later translated into protein (chain of amino acids) where the N-terminus and C-terminus correspond to the 5’ end and 3’ end of mRNA respectively. Genes: -A gene is the basic unit of biological information and represents a specific segment of DNA at a specific location in the genome (on a region of a chromosome) that serves as a unit of function in organisms. Genes encode RNA or protein products. Anatomy Of A Eukaryotic Gene: -Eukaryotic genes contain a regulatory flanking region (comprised of promoter sequence GC box and/or promoter enhancing CAAT box and/or TATA box) that are usually situated on the 5’ end, but can be found anywhere. mRNA is transcribed beginning at the 5’ UTR (Untranslatable Region) that contains the 5’ cap and the 3’UTR has a signal for the poly-A tail at the end of the gene. Newly-transcribed mRNA are subject to splicing (introns are removed according to GT splice donor sites and AG splice acceptor sites between exons). mRNA can degrade quickly after transcription due to the instability of single-stranded molecules because they resemble viruses to the body. During translation, ribosomes commence at the 5’ end with the start codon (AUG) and complete the process at the 3’ end (from N to C-terminus). In all eukaryotic genes, there is a coding strand (sense strand) and a non-coding strand (template strand). The sense strand is a similar 5’–3’ sequence as RNA (also called a Crick strand). The template strand is used as a template to transcribe RNA (also called a Watson strand). Genes Between Species: -Genes differ quite a bit between species. In simpler eukaryotic organisms like C. elegans, very few exons and introns are present in the pre-mRNA. In contrast, humans have in some of their larger transcripts many more exons and 4 or 5 different tissue-specific promoters that give rise to a large combination of mature mRNA transcripts. Sequence To Function: -An Open Reading Frame (ORF) is an in-frame sequence of DNA that starts with the start codon (ATG) and ends with any of the three termination (stop) codons (TAA, TAG, TGA). ORF’s can potentially code for proteins. A Coding Sequence (CDS) is a region of DNA that is translated to form proteins. Amino acids have different shapes, sizes and charges, which is why the amino acids sequence determines its 3- dimensional shape. The protein’s shape or conformation is what determines its biological function. Anatomy Of A Genome: -A genome is the sum total of genetic information in a particular cell or organism. Two copies of each chromosome in the genome are derived (one from each parent). As organisms increase in complexity, so too does the complexity of their gene structure. This is observed in the human genome as: decreasing gene density (coding sequences), increasing complexity of gene structure (more large introns and complex gene regulation), and increasing repetitive elements (LINES, SINES, and microsatellite loci which arose due to errors in
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