Lecture 3.docx

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University of Auckland
Biological Sciences
Craig Miller

BIOSCI202 Lecture 3: Eukaryote sequence organisation Eukaryote genome size  Eukaryote genomes tend to be larger than prokaryotes  Eukaryote genomes vary about 5000 folds from the largest to the smallest NO relationship between genome size and phenotypic complexity Number of genes  The number of genes varies considerably between prokaryotes and eukaryotes e.g. Bacteria have 1,500 – 7,500 while eukaryotes range from 5,000 – 4,0000 genes  BUT, in eukaryotes, the number of genes in a species is often lower than expected when compared to the size of its genome e.g. Human haploid genome is about 3,200 Mbp, 10x larger than size of C. elegans, with approximately the same number of genes (~21,000)  once you get to about 20,000 – 30,000 genes, the complexity of a species does not increase with the number of genes, BUT with how genes are regulated (turning on and off, splicing of intron and exons) e.g. In humans, 93% of the genes with multiple exons are spliced in at least 2 different ways Gene density and non-coding DNA  Eukaryotes genomes generally have a lower density of genes compared to prokaryotes e.g. Human and other mammals have the lowest gene density  Most bacteria genomes consist of genes for proteins, tRNA or rRNA with the remaining consisting of non- transcribed regulatory regions  Whereas, in eukaryotes the vast majority of DNA neither encodes protein nor is transcribed to RNA of known function  Human have 10,000x as much non-coding DNA as bacteria So what role does there non-coding sequences do? A classification of the different class of DNA sequences Types of DNA in the human genome 1 BIOSCI202 Unique sequence DNA Includes:  Exons – sequences that encode proteins (1.5% in human)  Introns – intervening sequences that are not translated (5% in human) Number and size of introns in gene is very variable  Gene-related regulatory sequences (accounts for 20% of the human genome)  Unique non-coding DNA (15%) Repeated sequence DNA Dispersed gene families – repeated at different regions  Exact DNA sequence of the genes with the family may diverge and different genes may come to have slightly different function  Mostly arise via gene duplication  Several type of proteins are encoded by families of homologous genes spread throughout the genome  Some of the genes within the family may become non-functional  pseudogenes e.g. Human globin genes Non-identical human α and β globin gene families are organized into clusters that contain functional and non-functional copies (pseudogenes ψ). Tandem gene families – repeated in a series  Cells need large amounts of the products of some gene and therefore the genes have evolved into tandem arrays  Often the multigene families consist of identical DNA sequences e.g. the genes for histones are arranged in tandem arrays in some species (sea urchin and Drosophila) e.g. ribosomal RNA genes found in the nucleolar organiser region of the chromosome are repeated tandemly (human has 250 copies of Nucleolar organiser) 2 BIOSCI202 Non-coding functional sequences Telomere  The telomere act as a cap stabilising the end of the chromosome and protects it from being degraded  Comprised of tandem arrays of simple DNA sequences repeated hundreds to thousands of times E.g. In the ciliate Tetrahymena (where telomere is first discovered) the telomere sequence is TTGGGG and in humans the repeat sequence is TTAGGG  The G-rich strand often protrudes beyond the complementary C-rich strand at the end of the chromosome special proteins bind to this single-strand sequence protecting the telomere from degradation In mammalian cells the G-rich strand folds over and pairs with a short stretch of DNA to form a t-loop.  The telomeric sequence also solved the problem of replicating the ends of a linear DNA molecule  The single stranded protruding end of the telomere
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