1 Chromosome Organization and Molecular Structure Introduction Chromosomes are the structures that contain the genetic material. They are complexes of DNA and proteins. The genome comprises all the genetic material that an organism possesses. In bacteria, it is typically a single circular chromosome. In eukaryotes, it refers to one complete set of nuclear chromosomes. The main function of the genetic material is to store information required to produce an organism; the DNA molecule does that through its base sequence. DNA sequences are necessary for: Synthesis of RNA and cellular proteins. Replication of chromosomes. Proper segregation of chromosomes. Compaction of chromosomes so they can fit within living cells. Viral Genomes Viruses are small infectious particles containing nucleic acid surrounded by a capsid of proteins (see fig. 10.1). For replication, viruses rely on their host cells; i.e., the cells they infect. Most viruses exhibit a limited host range; they typically infect only specific types of cells of one host species. Bacteriophages may also contain a sheath, base plate and tail fibers (see fig. 9.4). Viral genomes are relatively small and are composed of DNA or RNA (see table 10.1). A viral genome, also termed the viral chromosome, is the genetic material of the virus; the genome can be DNA or RNA, single-stranded or double-stranded, circular or linear. Viral genomes vary in size from a few thousand to more than a hundred thousand nucleotides.
2 Viral genomes are packaged into the capsid in an assembly process. During an infection process, mature viral particles need to be assembled. Viruses with a simple structure may self-assemble; genetic material and capsid proteins spontaneously bind to each other (e.g., Tobacco mosaic virus – see fig. 10.2). Complex viruses, such as T2 bacteriophages, undergo a process called directed assembly. Complex virus assembly requires proteins that are not part of the mature virus itself; noncapsid proteins usually have two main functions: Carry out the assembly process (e.g., scaffolding proteins that are not part of the mature virus). Act as proteases that cleave viral capsid proteins; this yields smaller capsid proteins that assemble correctly. Bacterial Chromosomes The bacterial chromosome is found in a region called the nucleoid (see fig. 10.3); the nucleoid is not membrane-bounded, so the DNA is in direct contact with the cytoplasm. Bacterial chromosomes contain a few thousand gene sequences that are interspersed with other functionally important sequences (see fig. 10.4). Bacterial chromosomal DNA is usually a circular molecule. A typical chromosome is a few million nucleotides in length; e.g., Escherichia coli has about 4.6 million base pairs and Haemophilus influenzae has about 1.8 million base pairs. Most bacterial species contain a single type of chromosome, but it may be present in
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