Objective V and VI

rRNA Processing bacteria vs eukaryotes

Bacteria - contain 3 pieces of rRNA contained on single gene

Eukaryotes - contain 4 pieces of rRNA. 5S piece is a single gene. The other three pieces on one gene

What are the steps to convert hnRNA to rRNA? (5)

• Ribosomal proteins bind to the rRNA as it is being made.

• Endonucleases cut the large rRNA transcript into pieces.

• Exonucleases trim the ends of each rRNA piece.

• Some bases are modified, using SAM as the methyl donor.

• The rRNA folds into complex 3D shapes with ribosomal proteins.

What are the steps to convert hnRNA to tRNA? (5)

• Endonuclease cuts at the mature 5′ end

• Mature 3′ end produced by exonucleases

• A CCA sequence is added to the 3′ end, yielding a mature tRNA.

• Any introns present are removed.

• Several nucleotides are modified

Base modification in tRNA processing (6)

• 1-Methylguanosine

• Ribothymidine

• Inosine

• 4-Thiouridine

• Dihydrouridine

• Pseudouridine

Do we have to know what these are or just names?

What are the steps to convert hnRNA to mRNA in eukaryotes? (3)

• 5′ Capping

• Splicing

• 3′ Polyadenylation

What is 5′ Capping

Addition of a 7-methylguanosine cap for stability and ribosome binding.

What is splicing?

Removal of introns and joining of exons by spliceosomes.

What is 3′ Polyadenylation

Addition of a poly-A tail (~200 adenine nucleotides) for stability and export.

What is alternative splicing and what advantages does it give eukaryotic cells?

Alternative splicing allows different exons of a pre-mRNA to be joined in various combinations.

• It increases protein diversity without increasing gene number.

• Enables tissue-specific and developmental regulation of protein production.

What are constitutive or housekeeping genes?

Genes that are constantly expressed at a steady level because their products are essential for basic cellular functions (e.g., metabolism, structural proteins).

What are regulated or inducible genes?

Genes expressed only under specific conditions such as developmental stages or environmental stimuli. Their expression is controlled and variable.

What is a regulator sequence?

A region of DNA (e.g., enhancers, operators) that binds regulatory proteins like activators or repressors to control transcription.

What is a repressor?

A regulatory protein that binds to an operator or silencer element and inhibits transcription by blocking RNA polymerase activity.

What is an inducer?

A small molecule that binds to a repressor protein, changing its shape and preventing it from binding DNA, thus allowing transcription to occur.

What are the four general mechanisms for controlling gene expression?

• Transcriptional control

• RNA processing control

• mRNA stability control

• Translational control

Transcriptional control

whether and how often a gene is transcribed.

RNA processing control

Alternative splicing or editing.

mRNA stability control

regulation of mRNA degradation rate.

Translational control

efficiency and frequency of translation into protein.