Chapter 19 - Chapter 19 Gene Regulation in Eukaryotes I The...

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Chapter 19: Gene Regulation in Eukaryotes I. The Structure and Organization of Chromatin A. Structural Levels in DNA Packing Eukaryotic DNA is composed of the DNA double helix in association with proteins called histones. The basic functional unit of DNA packing is the nucleosome, which consists of the DNA double helix wrapped twice around 4 pairs of histones and an H1 protein attached to a stretch of “linker” DNA. At the time of cell division (mitosis and meiosis), the DNA-histone complex (called chromatin) condenses into tightly coiled strands called chromosomes . The process of condensing chromatin into chromosomes is divided into four levels of organization or packing: Gene transcription does not occur during cell division while the chromosomes are condensed. However, when the DNA- histone complex uncoils, gene transcription is observed in the loosely organized regions. Loosely organized (uncoiled, de-condensed) DNA is called euchromatin. Tightly coiled regions of DNA (called heterochromatin) have also been observed during interphase but it is found that in these regions gene transcription does not occur. This suggests that the tight coiling (condensing) of DNA prevents gene transcription and the de-condensing (uncoiling) of DNA allows transcription to proceed. 1 of 17
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B. Non-coding DNA sequences Only 1.5% of the human genome codes for proteins. The bulk of the human genome (98.5 %) consists of non-coding DNA sequences. 24% of the human genome consists of regulatory sequences and introns. 15% consists of unique non-coding (and non-functional) DNA. The remaining 59% consists of repetitive DNA sequences, most of which are derived from transposable elements. 1. Repetitive DNA derived from transposable elements: long sequences made up of hundreds or thousands of N-base pairs scattered randomly throughout the genome, and whose N-base sequences are similar but not identical to each other. a. Transposons: DNA sequences that can move from one location to another within the same chromosome (a.k.a.: “jumping genes”). Transposons can move by a “cut-and-paste” mechanism or by a “copy-and-paste” mechanism. b. Retrotransposons move by means of a mRNA intermediate which must be converted back to DNA by means of reverse transcriptase. A portion of the retrotransposon codes for the reverse transcriptase enzyme. Reverse transcriptase uses a mRNA template to synthesize double-stranded DNA. The original retrotransposon always remains in its original locus. The copies are inserted at a different location within the genome. 2 of 17
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e.g., Alu elements are retrotransposons that lack the gene for reverse transcriptase. Alu elements “borrow” reverse transcriptase from other retrotransposons that are active in the cell. Alu elements are about 300 N-base pairs in length. Many Alu elements are transcribed and translated, but they appear to be “pseudogenes”. (The function of Alu elements is unknown at this time. However, they are suspected of being involved in the alternative splicing of exons.)
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This note was uploaded on 04/07/2008 for the course BIO 101 taught by Professor Stein during the Spring '07 term at South Carolina.

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Chapter 19 - Chapter 19 Gene Regulation in Eukaryotes I The...

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