Genome organization in various organisms

Organisms whole genetic DNA is referred as genome. Complete genetic informations involve nucleus and cytoplasm. It is obvious as phenotypes of organism of different population is not unique each genome differs in its size, shape, number of genes, number of chromosomes and organization inside the genome. Genome size increases with increase in gene number and vice versa. Exceptions are evident in case of viral genomes which are relatively smaller in size. Size of viral genome is only 5 kb to 25o kb in length. Comparatively bacterial genomes are larger in size and are nearly 600 kb to 7000 kb in length. However bacterial genome size is smaller than eukaryotic genomes. Depending on the eukaryotic organisms, genome size and their organization varies.

Viral genomes

A virus is referred as simplest non living particles and has a diversified evolutionary history. Virus can be of single stranded DNA, double stranded DNA, single stranded RNA or double stranded RNA. Viral genome can be either linear or circular in nature. They are very small containing very few genes and intergenic space. Morphology helps in easy classification of genomes. In spite virus particles contain only protein coat and genetic material; lacking other membranes and enzymes. Viral entry is the first stage of genome organization. After which viral nucleic acid replicates and viral proteins are produced. Viral genome is assembled finally and comes out from host cells. Virus is obligate parasites. Bacteriophage is a type of virus that infects bacteria. Eukaryotic viral genome is more similar to bacteriophage and has non coding gene regions referred as introns. It is unique characteristic of eukaryotic viral genome.

Prokaryotic genomes

Prokaryotic genomes are smaller in size and are bigger compared to viral genome. Prokaryotes consist of double stranded DNA that can replicate known as plasmids. Generally plasmids are 1 kb - 300 kb length. Bacterial genome has less intergenic regions and more density genes. Associated protein of bacterial genome is referred as nucleoid. DNA gets organized in chromatin. Prokaryotic nuclear genomes consist of functional DNA which codes different types of proteins and RNA’s.

Eukaryotic genomes

Eukaryotic genome organization is complex as it is larger in size. As of now two types of eukaryotic genomes are classified as nuclear genome and cytoplasmic genomes. Mitochondrial genomes are of cytoplasmic type. Chloroplast genomes are evident in plants.  For instance human genomes are of two type’s namely nuclear genome and mitochondrial genome. Coding regions are very less and eukaryotic genome is non functional nature. Genomic DNA is prevalently found in non coding region and intergenic regions. It comprises repeated sequences and introns. DNA condensed is expressed in terms of packing ratio. DNA is interwound in to smaller structure as superstructures. Nucleosome is one such superstructure. Nucleosomes are surrounded by histone proteins thereby forming histone octamer. Histone H1 separates nucleosomes by DNA of shorter length. Nucleosomes are condensed as chromatin and are seen as lamp brush chromosomes under electron microscope. Chromosomes are condensed further during late G2 phase of mitotic cell cycle. Numbers of genes are less in organelles like mitochondria and chloroplast.

Questions:

• Describe genome organization and gene expression in eukaryotes.
• What are different patterns of genome organization?
• Give genome organization evolution and its function.
• How organization genome occurs in nucleus?
• What are general steps in chromatin assembly?