bio325-lec12,13 - Chromosome Organization and Molecular...

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Chromosome Organization and Molecular Structure Chromosomes are complexes of DNA and proteins containing genetic material. The genome is all the genes of an organism. Bacteria is a single circular chromosome, eukaryotes refer to one complete set of nuclear chromosomes. Genetic material stores information needed for organisms; a DNA molecule has genes in base sequence. DNA sequences are necessary for: Synthesis of RNA and cellular proteins; Replication of chromosomes; Proper segregation of chromosomes; and Compaction of chromosomes so they can fit within living cells. Viral Genomes Viruses - small infectious particles containing nucleic acid surrounded by a capsid of proteins ~They rely on their host (the cells they infect) for replication. Most exhibit a limited host range; they infect only specific cells of one host species. ~Bacteriophages may also contain a sheath, base plate and tail fibers. (Bacteria has no nucleus) ~A viral genome (chromosome), can be DNA or RNA, single-stranded or double-stranded, circular or linear. They vary in size from a few thousand to more than a hundred thousand nucleotides. They are packaged into the capsid by an assembly process: During infection, need to be ssembled. Viruses with a simple structure may self-assemble ; genetic material and capsid proteins spontaneously bind to each other (Tobacco mosaic virus) Complex viruses, (like T2 bacteriophages), undergo directed assembly . This requires proteins that are not part of the mature virus; noncapsid proteins have 2 main functions: ~Carry out the assembly process (scaffolding proteins that are not part of the mature virus). ~Act as proteases that cleave viral capsid proteins; this yields smaller proteins that assemble correctly. Bacterial Chromosomes Found in the nucleoid region; the nucleoid is not membrane-bounded, so DNA is in direct contact with cytoplasm. They contain a few thousand different gene sequences that are interspersed with other functionally important sequences. This circular molecule is a few million nucleotides in length; (Escherichia coli has 4.6 million base pairs and Haemophilus influenzae has 1.8 million base pairs) Most contain a single type of chromosome, but it may be present in multiple copies. Structural gene sequences (encoding proteins) are the majority of bacterial DNA; nontranscribed DNA sequences between adjacent genes are called intergenic regions. One replication origin needed to initiate DNA replication. Repetitive sequences (which deal with genetic processes including DNA folding, DNA replication, gene regulation, and genetic recombination) may be interspersed throughout the chromosome. Loops help make the bacterial chromosome more compact. To fit in the cell, the DNA must compact about a 1000-fold; this involves forming loop domains. The number of loops varies according to the size and the species; E. coli has 50-100 with 40,000 to 80,000 binding proteins of DNA in each. The formation of additional coils due to twisting forces is DNA supercoiling
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bio325-lec12,13 - Chromosome Organization and Molecular...

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