How is the organisation of genes different between prokaryotes and eukaryotes?

The organisation of genes in prokaryotes and eukaryotes is quite different due to their structural, functional and evolutionary differences. These differences are seen in the way genes are arranged on the DNA, how they are regulated and how they are transcribed and translated. Below is a detailed explanation of their gene organisation based on major points:

1. Arrangement of Genes

Prokaryotes:
Genes are often arranged in clusters called operons. An operon is a group of genes under the control of a single promoter and transcribed together as one mRNA. These genes usually have related functions. For example, the lac operon in E. coli includes genes required for lactose metabolism.

Eukaryotes:
Genes are usually arranged individually. Each gene has its own promoter, enhancer and regulatory elements. Eukaryotic genes are not usually grouped by function. They are transcribed separately into different mRNAs.

2. Coding and Non-coding Regions

Prokaryotes:
Their genes are mostly made of exons (coding sequences). There are very few introns (non-coding regions) in prokaryotic genes. So the transcription and translation processes are direct and fast.

Eukaryotes:
Most genes contain both exons (coding sequences) and introns (non-coding sequences). After transcription, the primary mRNA undergoes splicing to remove introns before translation. This adds complexity and regulation to gene expression.

3. Regulation of Genes

Prokaryotes:
Gene regulation is simpler and often at the level of transcription initiation. For example, the presence or absence of a substrate (like lactose) can switch genes on or off.

Eukaryotes:
Gene regulation is more complex. It happens at multiple levels, transcription, post-transcription, translation and post-translation. Regulatory sequences like enhancers and silencers may be located far from the gene they regulate. Proteins called transcription factors and epigenetic modifications (like methylation) also play important roles.

4. Transcription and Translation Location

Prokaryotes:
Transcription and translation occur in the same compartment (cytoplasm), often at the same time. This is called coupled transcription and translation.

Eukaryotes:
Transcription takes place in the nucleus and translation occurs in the cytoplasm. The mRNA must first be processed and exported from the nucleus.

5. Gene Copy Number and Complexity

Prokaryotes:
Generally have a single circular chromosome with fewer genes. The genome is small and compact with minimal non-coding DNA.

Eukaryotes:
Have multiple linear chromosomes. Their genome is large and contains a high amount of non-coding DNA, including repetitive sequences, pseudogenes and regulatory elements.

6. Histones and Chromatin Structure

Prokaryotes:
Do not have histones (with few exceptions in Archaea). Their DNA is not wrapped in chromatin but exists as a simple nucleoid.

Eukaryotes:
DNA is tightly packed with histone proteins to form chromatin. The organisation into euchromatin and heterochromatin controls gene accessibility and expression.




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