Concepts 17.3 lecture outline

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Concepts 17.3 lecture outline

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