Nucleus

Transcription

^^ DNA is transcribed into pre-mRNA.

Intron

Exon pre-mRNA

Introns are removed.

-j® The mRNA strand leaves the nucleus and enters the cytoplasm for translation into a protein.

^^ The remaining exons are spliced together in mRNA

mRNA

^^ The remaining exons are spliced together in mRNA

mRNA

-j® The mRNA strand leaves the nucleus and enters the cytoplasm for translation into a protein.

Control After Transcription

In prokaryotes, transcription and translation occur within the cytoplasm. In eukaryotes, however, transcription occurs in the nucleus, and then mRNA passes through the nuclear envelope and into the cytoplasm, where translation occurs. The physical separation of transcription and translation by the nuclear envelope gives eukaryotes more opportunities to regulate gene expression.

Unlike prokaryotes, eukaryotes can control gene expression by modifying RNA after transcription. When transcription occurs, both introns and exons are transcribed, as shown in step Q of Figure 11-3. The result is a large molecule known as pre-mRNA. Pre-mRNA is a form of messenger RNA (mRNA) that contains both introns and exons. (Note that the terms intron and exon can be used to describe both DNA segments and the RNA segments that are transcribed.) A molecule of mRNA is formed when introns are removed (step ©) from pre-mRNA and the remaining exons are spliced (joined) to one another (step G). Complex assemblies of RNA and protein called spliceosomes split the pre-mRNA at each end of an intron and join the exons. The end result is an mRNA molecule containing only the exons. The mRNA strand leaves the nucleus and enters the cytoplasm to begin the manufacture of a protein on the ribosomes (step ©). The nucleotides in the removed introns can be used again during the transcription of additional pre-mRNA. Similar RNA splicing occurs following the transcription of transfer RNA and ribosomal RNA.

The removal of introns and splicing of an mRNA molecule have also been found to occur in another way. Scientists have discovered that RNA molecules can act as biological catalysts. RNA itself can act as a catalyst to remove introns from mRNA molecules as they form in the nucleus. Until this discovery, it was thought that all enzymes were proteins. RNA molecules that act as enzymes are called ribozymes.

figure 11-3

Both introns and exons are transcribed to form pre-mRNA. Spliceosomes cut out the introns and join the remaining exons together, forming mRNA.

figure 11-3

Both introns and exons are transcribed to form pre-mRNA. Spliceosomes cut out the introns and join the remaining exons together, forming mRNA.

Modeling PostTranscription Control

Materials felt-tip markers, paper, scissors, tape

Procedure

1. Write a sentence that contains only three-letter words and makes sense.

2. Hide the words in random places in a long sequence of letters. This sequence should contain random letters and other three-letter words that make no sense in the sentence you are hiding. Print the sequence of letters all the same size, equally spaced, and with no breaks between them.

3. Trade papers with another team. Use scissors to cut out the "introns." Find the message, and reassemble it with tape. Analysis What represents pre-mRNA in this activity? What represents mRNA?

Enhancer

Transcription factor (activator)

Promoter Structural gene

Transcription factor mÉ'

Enhancer

Promoter

Structural gene

Transcription begins RNA polymerase figure 11-4

Many enhancers are located far (thousands of nucleotide bases) away from the genes they activate. Transcription factors facilitate transcription by binding to the enhancer and to the promoter.

Control at the Onset of Transcription

Most gene regulation in eukaryotes occurs when RNA polymerase binds to a gene—the onset of transcription. Eukaryotic cells, like prokaryotic cells, have regulatory genes. But eukaryotic gene regulation involves more proteins, and the interactions are more complicated. Regulatory proteins in eukaryotes are known as transcription factors.

Transcription factors help in the placement of RNA polymerase at the correct area on the promoter, as shown in Figure 11-4. Many different transcription factors may influence one gene.

Transcription factors may also bind sequences of DNA called enhancers. In general, enhancers are located at a position far— thousands of nucleotide bases away—from the promoter. A loop in the DNA may bring the enhancer and its activator (the attached transcription factor) into contact with the RNA polymerase and transcription factors at the promoter. Transcription factors bound to enhancers can activate transcription factors bound to promoters, as shown in Figure 11-4.

Sirens Sleep Solution

Sirens Sleep Solution

Discover How To Sleep In Peace And Harmony In A World Full Of Uncertainty And Dramatically Improve Your Quality Of Life Today! Finally You Can Fully Equip Yourself With These “Must Have” Tools For Achieving Peace And Calmness And Live A Life Of Comfort That You Deserve!

Get My Free Ebook


Post a comment