Chapter 17 (3)

Enzymes in the eukaryotic nucleus modify ___in specific ways before the genetic messages are dispatched to the cytoplasm.
During this ___, both ends of the __are altered. Also, most of the time, certain interior sections of the RNA molecule are cut out and the remaining parts spliced together. These modifications produce an mRNA molecule ready for __.

Alteration of mRNA ends
Each end of a __ molecule is modified in a particular way.
The __is synthesized first; it receives a __, a modified form of a G nucleotide added onto the __’ end after transcription of the first 20 to 40 nucleotides.

The __’ end of the pre-mRNA molecule is also modified before the mRNA exits the nucleus.
· An enzyme adds 50-250 more A nucleotides forming a __.

The 5’ cap and poly-A-tail share several important functions. What are they?

· 1. They seem to facilitate the export of the mature mRNA from the nucleus.

__ are parts of the mRNA that will not be translated into protein, but they have other functions, such as __.

__is the removal of large portions of the RNA molecule that is initially synthesized- a cut-and-paste job.
The avg. length of a __along a human DNA molecule is about 27,000 base pairs, so the __is also that long.
However, it takes only 1200 nucleotides in RNA to code for the avg-sized protein of 400 __. (Remember, each amino acid is encoded by a triplet of nucleotides.)

This means that most eukaryotic genes and their __have long __stretches of nucleotides, regions that are not translated. Most of these __sequences are interspersed between coding segments of the gene and thus between coding segments of the pre-mRNA.
· In other words, the sequence of DNA nucleotides that codes for a eukaryotic polypeptide is usually not continuous; it is split into segments.

The noncoding segments of nucleic acid that lie between coding regions are called intervening sequence, or __.
The other regions are called _, because they are eventually expressed, usually by being translated into amino acid sequences. (Exceptions are __of the __at the ends of the RNA, which aren’t translated into protein.)

__ and __are used for both RNA and DNA sequences that encode them.
In making a primary transcript from a gene, __transcribes both __ and __ from the DNA, but the __molecule that enters the cytoplasm is an abridged version.

__are cut out from the molecule and the __joined together, forming an mRNA molecule with a continuous coding sequence. This is the process of __.

How is pre-mRNA splicing carried out?
Researchers have learned that the signal for __is a short nucleotide sequence at each end of an __.
Particles called __ recognize these splice sets.

__ are located in the cell nucleus and are composed of RNA and protein molecules.
· The RNA in a __particle is a __; each molecule is about 150 nucleotides long.
· Several different _join with additional proteins to form an even larger assembly called a __, which is almost as big as a ribosome.
o The __ interacts with certain sites along an __, releasing the __and joining together two __that flanked the __.

The idea of a catalytic role for __arose from the discovery of __, RNA molecules that function as enzymes.
In some organisms, __can occur without proteins or even additional RNA molecules: The intron RNA functions as a __and catalyzes its own excision!

Three properties of RNA enable some RNA molecules to function as enzymes.
List the first.
· cont.
A specific structure is essential to the catalytic function of ribozymes, just as it is for enzymatic proteins.

List the second property of RNA tht enablesome o function as enzymes.

· Second, like certain amino acids in an enzymatic protein, some of the bases in RNA contain functional groups that may participate in catalysis.

List th third property.

o Example: complementary base pairing between the RNA of the spliceosome and the RNA of a primary RNA transcript precisely locates the region where the ribozyme catalyzes splicing.

· Third, the ability of RNA to hydrogen-bond with other nucleic acid molecules (RNA or DNA) adds specificity to its catalytic activity.

What could be the biological functions of introns and RNA splicing?
· While specific functions may not have been identified for most introns, at least some contain sequences that regulate gene activity. And the __process itself is necessary for the passage of __from the nucleus to the cytoplasm.

One consequence of the presence of __in genes is that a single gene can encode more than one kind of __.
·

Many genes give rise to two or more different polypeptides, depending on which segments are treated as __during __, called __.

Alternative _is one reason humans can along with a relatively small number of genes; because of __, the number of different protein products an organism produces can be much greater than its number of genes.

Proteins often have a modular architecture consisting of discrete structural and functional regions called ___.
· One __of an enzymatic protein might include the active site, while another might attach the protein to a cellular membrane. In quite a few cases, different exons code for the different __of a protein.

The presence of __in a gene may facilitate the evolution of new and potentially useful proteins as a result of a process known as __.

__increase the probability of potentially beneficial crossing over between the exons of alleles- simply by providing more terrain for crossovers without interrupting coding sequences.

__of either sort could lead to new proteins with novel combinations of functions. While most of the shuffling would result in nonbeneficial changes, occasionally a beneficial variant might arise.