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DIFFERENCE BETWEEN PROKARYOTIC GENOME AND EUKARYOTIC GENOME�

Submitted by:

Purnima , Department of Bioinformatics

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INTRODUCTION�Prokaryotes have highly organized genomes because their DNA interacts directly with molecular machines in the cytoplasm. But their genomes are also very plastic, with high rates of horizontal gene transfer and gene loss. Prokaryotic cells typically lack a clear physical separation between DNA and the cytoplasm.�However, all eukaryotes that have been studied have at least two chromosomes and the DNA molecules are always linear. The only variability at this level of eukaryotic genome structure lies with chromosome number, which appears to be unrelated to the biological features of the organism. For example, yeast has 16 chromosomes, four times as many as the fruit fly.�

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DIFFERENCE BETWEEN PROKARYOTIC AND EUKARYOTIC GENOME

EUKARYOTIC GENOME

Eukaryotic genome is the multiple linear chromosome that contain the genetic information of eukaryotes

PROKARYOTIC GENOME

DEFINITION
Prokaryotic genome is single circular chromosome that contains the genetic information of the prokaryotes.

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STRUCTURE

The Eukaryotic DNA is linear. All eukaryotes also possess smaller, usually circular, mitochondrial genomes.

Prokaryotic DNA is double-stranded and circular in structure which occupies nucleoid region of the cell. Prokaryotes may also have additional genes on independent smaller, circular or linear DNA molecules called plasmids.

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COMPOSITION

Nucleoid is composed of 60% DNA and small amount of RNA and proteins.

Chromatin is made up of 35% - DNA, 60% - Proteins, 5% - RNA

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LOCATION

It is present freely in the cytoplasm without any membrane bound organelle.

It is enclosed by membrane in nucleus, mitochondria, chloroplast etc depending upon the type of cell.

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�EUCHROMATIN

In Prokaryotes, euchromatin is the only form of chromatin present. [Euchromatin is the loosely packed form of chromatin (DNA, RNA and proteins) that is enriched in genes under active transcription

In eukaryotes, euchromatin comprises the most active portion of the genome within the cell nucleus. Euchromatin: poorly stained, less compact, contains transcribed genes.

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SUPERCOILING

In prokaryotes, plectonemic supercoils are predominant, because of the circular chromosome and relatively small amount of genetic material. It is facilitated by Topoisomerases.

In eukaryotes, DNA supercoiling exists on many levels of both plectonemic and solenoidal supercoils, with the solenoidal supercoiling proving most effective in compacting the DNA. Solenoidal supercoiling (negative supercoiling) is achieved with histones to form a 10 nm fiber.

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SIZE

The size is small(1-10Mb). It is because of the increased rates of mutation for evolution of the cells which is the driving force for genome reduction.

Example - The Escherichia coli genome varies in size from 4.5 to 5.5 Mb.

It is large in size (10-1,00,000 Mb) (where Mb is megabases) because of presence of many repeats.

Example - The human genome is roughly 3 billion base pairs long, or about 3,100 Mbp.

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HETROCHROMATIN

It is absent in prokaryotes.

Heterochromatin is a major component of the eukaryotic nucleus and is essential for the maintenance of genome stability. Heterochromatin is riddled with repetitive sequences and has evolved specific ways to compartmentalize, silence, and repair repeats

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REGULATION MACHINERY

Regulation of transcription in prokaryotes typically involves operons. An operon is a region of DNA that consists of one or more genes that encode the proteins needed for a specific function. The operon also includes a promoter and an operator.

Gene expression in eukaryotic cells is regulated by repressors as well as by transcriptional activators but no operons involved.

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PSEUDOGENES AND INTRONS

Few or rare.

Abundant in eukaryotes

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NUCLEOSOME

Absent in prokaryotic genome.

Nucleosome has octamer of Histones H2A, H2B, H3, H4 and a strand of DNA having 146 base pairs wrapped tightly around this.

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REPEATED DNA

Tandemly repeated DNA is found much less frequently in prokaryotes

Tandemly repeated DNA is a common feature of eukaryotic genomes. This type of repeat is also called satellite DNA because DNA fragments containing tandemly repeated sequences form ‘satellite’ bands when genomic DNA is fractionated by density gradient centrifugation.

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DNA TRANSPOSONS

DNA transposons are a much more important component of prokaryotic genome anatomies.

In eukaryotes, DNA transposons are less common than retrotransposons

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TELOMERE

Prokaryotes lack telomeres as it is circular with no endings.

Telomeres are the physical ends of eukaryotic chromosomes. They protect chromosome ends from DNA degradation, recombination, and DNA end fusions, and they are important for nuclear architecture.