Concept 17.3 Eukaryotic cells modify RNA after transcription

· Enzymes in the eukaryotic nucleus modify the pre-mRNA before the genetic messages are dispatched to the cytoplasm.

· During RNA processing, both ends of the primary transcript are altered. In most cases, certain interior parts of the molecule

are cut out and the remaining parts are spliced together. These modifications help form an mRNA molecule that is ready for

translation.

· At the 5' end of the pre-mRNA molecule, a modified form of guanine is added, the 5' cap.

· At the 3' end, an enzyme adds 50 to 250 adenine nucleotides, the poly-A tail.

*These modifications have several important functions.

They facilitate the export of mRNA from the nucleus.
They help protect mRNA from hydrolytic enzymes.
They help the ribosomes attach to the 5¢ end of the mRNA.

· The parts of the mRNA that will not be translated into protein are referred to as UTRs (untranslated regions).

· The most remarkable stage of RNA processing occurs during the removal of a large portion of the RNA molecule in a cut-and-

paste job of RNA splicing.

○ The average length of a transcription unit along a human DNA molecule is about 27,000 nucleotide pairs.However, it takes

only 1,200 nucleotides to code for an average-sized protein of 400 amino acids.

· Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides. Noncoding segments of

nucleotides called intervening regions, or introns, lie between coding regions.

○ The regions called exons are eventually expressed, usually by being translated into amino acid sequences.

· RNA splicing removes introns and joins exons to create an mRNA molecule with a continuous coding sequence.

· The signal for RNA splicing is a short nucleotide sequence at each end of an intron. Particles called small nuclear

ribonucleoproteins (snRNPs) recognize the splice sites.

· snRNPs are located in the cell nucleus and are composed of RNA and protein molecules. The RNA in an snRNP particle is called

a small nuclear RNA molecule (snRNA). Each RNA molecule is about 150 nucleotides long.

· Several different snRNPs join with a variety of proteins to form a larger assembly called a spliceosome, which is about the size

of a ribosome. The spliceosome interacts with certain sites along an intron, releasing the introns and joining together the two

exons that flanked the introns.

- snRNAs catalyze these processes as well as participating in spliceosome assembly and splice site recognition.

Ribozymes are RNA molecules that function as enzymes.

· The idea of a catalytic role for snRNA arose from the discovery of ribozymes, RNA molecules that function as enzymes.

· In some organisms, RNA splicing occurs without proteins or additional RNA molecules: The intron RNA functions as a ribozyme and catalyzes its own excision.

- For example, in the protozoan Tetrahymena, self-splicing occurs in the production of ribosomal RNA (rRNA), a component of the organism’s ribosomes.

○ The pre-rRNA removes its own introns.

· The discovery of ribozymes rendered obsolete the idea that all biological catalysts are proteins.

· Three properties of RNA allow some RNA molecules to function as ribozymes.

1. Because RNA is single-stranded, a region of the RNA molecule may base-pair with a complementary region elsewhere in the

same molecule, giving the RNA a specific three-dimensional structure that is key to its ability to catalyze reactions.

2. Some of the bases in RNA contain functional groups that participate in catalysis.

3. The ability of RNA to hydrogen-bond with RNA or DNA adds specificity to its catalytic activity.

Introns may play a regulatory role in the cell.

· Specific functions have not been identified for most introns, but some contain sequences that regulate gene expression, and

many affect gene products.

· Due to the presence of introns in genes, a single gene can encode more than one kind of polypeptide.

· Many genes give rise to two or more different polypeptides, depending on which segments are treated as exons during RNA

processing; this is called alternative RNA splicing.

Sex differences in fruit flies are due to differences in how males and females splice the RNA transcribed from certain genes.

· Results from the Human Genome Project suggest that alternative splicing explains why humans can get along with a relatively

small number of genes. Because of alternative splicing, the number of different protein products an organism can produce is

much greater than its number of genes.

· Proteins often have a modular architecture with discrete structural and functional regions called domains.

○ One domain of an enzyme may include the active site, while another might allow the protein to bind to a cellular membrane.

· The presence of introns in a gene may facilitate the evolution of new and potentially useful proteins in a process known as

exon shuffling.

· Introns increase the probability of potentially beneficial crossing over between the exons of alleles of a gene by providing more

terrain for crossovers without interrupting coding sequences. This might lead to a protein with a new combination of exons, a

novel structure, and a novel function.

· Occasionally, exons may be exchanged between completely different (nonallelic) genes.

· Either way, exon shuffling can lead to new proteins through novel combinations of functions.

End

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Describe the process of mRNA processing