Chapter 3: transcription Part 2

When a region of DNA that contains the genetic information for a protein is isolated from a bacterial cell and inserted into a eukaryotic cell in a proper position between a promoter and a terminator, the resulting cell usually produces the correct protein. But when the experiment is done in the reverse direction (inserting eukaryotic DNA into a bacterial cell), the correct protein is often not produced. Why is this the case?

The eukaryotic DNA contains introns, which the bacterial cell cannot splice out properly. Thus, the correct protein is not produced from the information in the bacterial RNA transcript.

The type of RNA that physically interacts with a ribosome, providing the sequence information for a specific protein, is abbreviated:

mRNA

What is Transfer RNA (tRNA)?

carries individual amino acids for use in translation.

alternative splicing allows for:

different polypeptides to be made from a single gene

Alternative splicing means that:

different spliced forms contain different combinations of exons

What is ribosomal RNA (rRNA)?

essential in translation; In eukrayotic cells, the genes and transcripts for rRNa are concentrated in the nucleolus, a distinct, dense, non-membrane-bound spherical structure observed within the nucleus.

What is alternative splicing?

regulated process during gene expression that results in a single gene coding for multiple proteins

What is primary transcript?

the RNA transcript that comes off the template DNA strand

Describe the steps in RNA splicing

1. The spliceosome brings a specific sequence within the intron into proximity with the 5' end of the intron, at a site known as the 5' splice site. 2. The proximity enables a reaction that cuts the RNA at the 5' splice site, and the lariat is formed. 3. The spliceosome brings the 5' splice site close to the 3' splice site, cleaving the bond of the lariat and exons. The exons are connected and the lariat is released.

Arrange the descriptions of the steps in RNA splicing in order from earliest to latest. Consider the steps in RNA splicing. 1. Exon ends are covalently joined together 2. Components of the spliceosome bind conserved regions in the intron and bring them together in close proximity. 3. Acceptor splice site is cut. 4. Donor splice site is cut, and the end of the intron is joined covalently to the branch site.

2, 4, 3, 1.

What is RNA processing?

Chemical modification that converts the primary transcpit into finished mRNA, nabling the RNA molecule to be transported to the cytoplasm and recognized by the translational machinery.

What is messenger RNA?

RNA molecules that combine with the ribosome to direct protein synthesis is known as mRNA bc it carries the genetic "message" from the DNA to the ribosome.

What are the three mechanisms of RNA processing in eukaryotes?

Three mechanisms of RNA processing (chemical modification of the primary transcript to generate the finished mrRNA) in eukaryotic cells are as follows: a. Addition of the 5' cap, allowing the mRNA to be recognized by the ribosome complex and helps stabilize the mRNA. b. addition of the poly(A) tail. Important in mRNA transcription termination and in the export of the mRNA into the cytoplasm. also helps stabilize the mRNA. c. RNA splicing. introns get removed from primary RNA transcript. A single transcript w multiple introns may be spliced in different ways to generate different mRNAs and different protein products with different function. Thus, this alternative splicing is one more layer contributing to the diversity of the genetic info stored in DNA.

True or false? In prokaryotes, the primary transcript is the mRNA.

True; Even as the 3' end of the primary transcript is still being synthesized, ribosomes bind with special sequences near its 5' end and begin the process of protein synthesis. it's because they have no nuclear envelope.

an intron is:

an RNA sequence that is removed during the processing of an RNA molecule in the nucleus

What is Small nuclear RNA (snRNA)

an essential component of the spliceosome required for RNA processing

What are the two major types of small regulatory RNA?

microRNA (miRNA) and small interfering RNA (siRNA).

RNa processing occurs in the

nucleus

What other feature do primary transcripts for protein-coding genes in prokaryotes not share with eukaryotes?

they often contain the genetic information for the synthesis of two or more different proteins. molecules of mRNA that code for multiple proteins are known as polycistronic mRNA

Explain the addition of the 5' cap

5' cap is a modified nucleotide called 7-methylguanosine attached to the 5'end of the primary transcript via enzyme. It is attached backwards, instead of creating phosphodiester bonds between the 3'-OH group of one nucleotide and the 5' carbon of the next nucleotide, the cap is linked to the RNA transcript by a triphosphate bridge between the 5' carbons of both ribose sugars. The 5' cap is important because in translation, the ribosome recognzies and mRNA by its 5' cap. without the cap, the ribosome would not attach to the mRNA and translation would not occur.

What are the five types of noncoding RNA and their functions?

1) ribosomal RNAs (rRNA), which are found in all ribosomes and aid in transaltion 2) transfer RNAs (tRNA), which carry individual amino acids for use in translation 3) small nuclear RNAs (snRNA), which are involved in eukaryotic gene splicing, polyadenylation, and other processes in the nucleus. 4) microRNAs (miRNA), which inhibit translation; 5) small interfering RNAs (siRNA), which destroy RNA transcripts

RNA processing consists of which three principal types of chemical modifications?

1) the addition of the 5' cap 2) polyadenylation, the addition of about 250 consecutive A-bearing ribonucleotides to the 3' end, forming a poly(A) tail. 3) RNA splicing

What is RNA splicing?

RNA splicing is where noncoding regions (introns) are removed from RNA transcripts, leaving the protein-coding regions (exons).

Why is polyadenylation important?

The addition of a string of about 250 consecutive A-bearing ribonucleotides to the 3' end, forming a polly(A) tail is important for the export of mRNA into the cytoplasm. Both the 5' cap and poly(A) tail stabilize the RNA transcript at both ends until it is translated in the cytoplasm.