Lecture 25. characteristics of eukaryotic genes
Eukaryotic organisms have essential differences in cell structure compared with prokaryotic ones. Eukaryotes have typical cell structure, mitosis and meiosis. That’s why their structure of gene and genome is different from prokaryotic genetic machinery.
The differences between eukaryotic and prokaryotic genes
Unlike Prokaryotes, Eukaryotes:- have chromosomes
- contain a nucleus
- have amounts of DNA that differ between species
- have variations in the number of chromosomes between species
- genes contain introns
- (parallet structure…..”have genes containing introns”)
- may have multiple copies of a gene
There is great divergence of sequence between a given intron in different eukaryotic organisms. The exon sequences are much more conserved. This suggests that the actual sequence of the intron is not very important. If it were important, then any changes that occurred during evolution would be damaging, and the organisms with the changes would not be likely to survive.
Rna splicing
The DNA in eukaryotes is organized into exons and introns. The introns do not carry any genetic information. The process of RNA splicing is responsible for removing introns from precursor RNAs to produce the final RNA product. In the process from pre-mRNA to mRNA, splicing must be extremely accurate. If splicing is off by one nucleotide, the entire coding will be messed up because all of the codons downstream of the mistake will be out of the correct reading frame--they will be out of phase.RNA splicing is carried out by snRNPs which stands for small nuclear RNA containing ribonucleoprotein particles. The snRNPs contain both RNA and proteins. (Each snRNP contains a molecule of snRNA.) In this respect they are very similar to ribosomes, another RNP particle in the cell. In snRNPs, the RNA carries out enzymatic duties, and the proteins hold the snRNPs in the correct configuration to stabilize them.
The role of snrnps
The snRNAs in the snRNPs base pair with the pre-mRNA at splice junctions (and some other sites too). The snRNPs base paired at different splice junctions interact with each other to facilitate the removal of the intron between the snRNPs and to join the adjacent exons.There is an evolutionary benefit to having introns; otherwise, the energy cost to splice would not be compensated.
Sometimes splicing skips over an exon. For example say the pre-mRNA contains A-B-C-D exons. Splicing in some tissues might lead to an A-B-D mRNA (exon C is skipped). Or the splicing could produce an A-C-D mRNA (exon B is skipped). These mRNAs would have the same end exons but different middles. They will code for different proteins. This alternative splicing uses genetic expression to facilitate the synthesis of a greater variety of proteins.