Deck 13: RNA Molecules and RNA Processing

Question 1

When a continuous sequence of nucleotides in DNA encodes a continuous sequence of amino acids in a protein, the two are said to be colinear. In eukaryotes, not all genes are colinear with the proteins that they encode.
-What evidence indicated that eukaryotic genes are not colinear with their proteins?

Answer 1

When DNA was hybridized to the mRNA transcribed from it, regions of DNA that did not correspond to RNA looped out.

Question 2

Many eukaryotic genes contain exons and introns, both of which are transcribed into RNA, but introns are later removed by RNA processing. The number and size of introns vary from gene to gene; they are common in many eukaryotic genes but uncommon in bacterial genes.
-What are the four major types of introns?

Answer 2

Group I introns, group II introns, nuclear pre-mRNA introns, and transfer RNA introns

Question 3

Eukaryotic pre-mRNAs are processed at their 5' and 3' ends. A cap, consisting of a modified nucleotide and several methyl groups, is added to the 5' end. The cap facilitates the binding of a ribosome, increases the stability of the mRNA, and may affect the removal of introns. Processing at the 3' end includes cleavage downstream of an AAUAAA consensus sequence and the addition of a poly(A) tail.

-Why are pre-mRNAs capped, but tRNAs and rRNAs aren't?

Answer 3

A protein that adds the 5' cap is associated with RNA polymerase II, which transcribes pre-mRNAs but is absent from RNA polymerase I and III, which transcribe rRNA and tRNAs.

Question 4

Introns in nuclear genes contain three consensus sequences critical to splicing: a 5' splice site, a 3' splice site, and a branch point. The splicing of pre-mRNA takes place within a large complex called the spliceosome, which consists of snRNAs and proteins.

-If a splice site were mutated so that splicing did not take place, what would be the effect on the mRNA?

A) It would be shorter than normal.
B) It would be longer than normal.
C) It would be the same length but would encode a different protein.

Question 5

Alternative splicing enables exons to be spliced together in different combinations to yield mRNAs that encode different proteins. Alternative 3' cleavage sites allow pre-mRNA to be cleaved at different sites.

-Alternative 3' cleavage sites result in

A) multiple genes of different lengths.
B) multiple pre-mRNAs of different lengths.
C) multiple mRNAs of different lengths.
D) all of the above.

Question 6

Individual nucleotides in the interior of pre-mRNA may be changed, added, or deleted by RNA editing. The amino acid sequence produced by the edited mRNA is not the same as that encoded by DNA.
-What specifies the modified sequence of nucleotides found in an edited RNA molecule?

Question 7

All tRNAs are similar in size and have a common secondary structure known as the cloverleaf. Transfer RNAs contain modified bases and are extensively processed after transcription in both bacterial and eukaryotic cells.

-How are rare bases incorporated into tRNAs?

A) Encoded by guide RNAs
B) By chemical changes to one of the standard bases
C) Encoded by rare bases in DNA
D) Encoded by sequences in introns

Question 8

A ribosome is a complex organelle consisting of several rRNA molecules and many proteins. Each functional ribosome consists of a large and a small subunit. Ribosomal RNAs in both bacterial and eukaryotic cells are modified after transcription. In eukaryotes, rRNA processing is carried out by small nucleolar RNAs.

-What types of changes take place in rRNA processing?

A) Methylation of bases
B) Cleavage of a larger precursor
C) Nucleotides are trimmed from the ends of rRNAs.
D) All of the above

Question 9

Small interfering RNAs and microRNAs are tiny RNAs produced when larger double-stranded RNA molecules are cleaved by the enzyme Dicer. Small interfering RNAs and microRNAs participate in a variety of processes, including mRNA degradation, the inhibition of translation, the methylation of DNA, and chromatin remodeling. Piwi-interacting RNAs are found in the germ cells of animals and inhibit transposons.
-How do siRNAs and miRNAs target specific mRNAs for degradation or for the repression of translation?