LECTURE CONNECTIONS

14 | RNA Molecules and RNA Processing

© 2009 W. H. Freeman and Company

14.3 Transfer RNAs, Which Attach to Amino Acids, Are Modified after Transcription in

Bacteria and Eukaryotic Cells

• Each tRNA attaches to a particular aa and carries it to the ribosome.

• Each tRNA is capable of attacjing to only one type of aa.

tRNAAla - attaches to aa alanine

• 20 aa are found in proteins, so there must be a minimum of 20 different types of tRNA

The Structure of tRNA

• Rare modified RNA nucleotide bases

• Ribothymine• Pseudouridine

• Modified bases arise from chemical changes made to the four standard bases after transcription.

(tRNA-modifying enzymes)

• Common secondary structure – the cloverleaf structure

The Structure and processing of tRNA

• All tRNA molecules in both bacterial and eukaryotic cells undergo processing after transcription.

• Eukaryotic cells usually have many copies of each tRNA gene.

• Different tRNAs are processed in different ways, so a generic processing pathway is not possible.

• Some eukaryotic and archeal tRNA genes possess introns of variable length that must be removed in processing.

Concept Check 3

How are rare bases incorporated into tRNAs?

a. Encoded by guide RNAs

b. By chemical changes in one of the standard bases

c. Encoded by rare bases in DNA

d. Encoded by sequences in introns

14.4 Ribosomal RNA (rRNA)

• Ribosomal RNA is a component of the ribosome and is also processed after transcription

• Within ribosomes, the genetic instructions contained in mRNA are translated into the aa sequences of polypeptides.

The Structure of the Ribosome

• Ribosome is one of the most abundant organelles in the cell;

• Ribosomes contain typically about 80% of total cellular RNA;

• Ribosomes are complex organelles (more than 50 proteins and RNA molecules);

• A functional ribosome consists of 2 subunits:

Large ribosome subunit

Small ribosome subunit

A measure of how rapidly an object

sediments in a centrifugal field.

Each ribosome unit consists of one or more pieces of RNA and a number of proteins.

Ribosomal RNA Gene Structure and Processing

• The genes for rRNA can be present in multiple copies and the number vary among species;

• In bacteria, rRNA genes are dispersed, while in eukaryotic cells, they are clustered;

• Ribosonal RNA is processed in both bacterial and eukaryotic cells.

Concept Check 5

What types of changes take place in rRNA processing?

a. Methylation of bases

b. Cleavage of bases

c. Nucleotides are trimmed from the ends of rRNAs.

d. All of the above

14.5 Small RNA Molecules Are Present Extensively in Eukaryotes and Participate in a

Variety of Functions

• RNA interference: limits the invasion of foreign genes and censors the expression of their own genes

• Antisense RNA: single-stranded RNA molecules that bind to mRNA and inhibit translation

• siRNA and microRNAs: doubled-stranded RNA that regulate gene expression by a process called RNA interference.

14.5 Small RNA Molecules Are Present Extensively in Eukaryotes and Participate in a

Variety of Functions

• Both siRNA and miRNA molecules combine with proteins to form an RNA-induced silencing complex (RISC).

• The RISC pairs with a mRNA molecule that possesses a sequence complementary to its siRNA or miRNA component and either: - cleaves the mRNA (leading to degradation or mRNA) or

- represses translation of the mRNA