Cell and Molecular Biology Module 2

RNA differs from DNA in that

It is single stranded and can fold into a variety of complex 3D structures

Why can RNA fold into a variety of 3D structures?

It is single stranded

The sigma subunit of bacterial RNA polymerase

Recognizes promoter sites in the DNA

What occurs with termination of transcription in prokaryotes?

RNA polymerase transcribes through the terminator sequence, causing the polymerase to separate from the DNA and release the transcript

What would occur to a transcript in a bacterial strain with a mutation that removes the transcription termination signal?

The transcript RNA from the mutant strain would be longer than normal

What could decrease transcription of only one specific gene in a bacterial cell?

A mutation that introduced extensive sequence changes into the DNA that precedes the genes transcription start site

Transcription in eukaryotes requires which of the following addition to RNA polymerase

General transcription factors (TFs)

What type of polymerase is involved in transcription of mRNA for a globin protein in eukaryotes?

RNA polymerase II

What occurs before a eukaryotic mRNA is exported from the nucleus?

• the strand is polyadenylated at its 3’ end

• RNA polymerase dissociates

• 7-methylguanosine is added in a 5’ to 5’ linkage to the strand

What occurs in eukaryotic gene expression that does NOT occur in prokaryotic gene expression?

A poly A tail is added to the 3’ end of mRNA and a cap is added to the 5’ end

Why are intronic sequences not translated into proteins (eukaryotic)?

They are removed from RNA molecules by the spliceosome which works in the nucleus

A eukaryotic transcription unit that is 8000 nucleotides long may only use 1200 nucleotides to make a protein consisting of 400 amino acids. This is best explained by

many noncoding stretches of nucleotides are present in mRNA

what molecular component of the spliceosome catalyzes the excision reaction?

RNA

snRNA are the RNA components of snRNPs and they

can bind to specific sequences at intron-exon boundaries through complementary base pairing

Alternative RNA splicing

can allow the production of proteins of different sizes from a single mRNA

RNA splicing characteristics

• Conventional introns are not found in bacterial genes

• small RNA molecules in the nucleus perform the splicing reactions necessary for the removal of introns

• splicing occurs after the 5’ cap has been added to the end of the primary transcript

What may occur if an mRNA molecule was inserted after removing the 5’ cap and poly A tail?

• mRNA would have problems exiting the nucleus to be translated

• issues during intron excision

• issues being translated at the ribosome

• may be digested by exonucleases in the cytosol

What is true about the genetic code?

the genetic code is redundant

Why has the TATA box been highly conserved throughout evolution?

any mutation in the sequence is selected against

61 mRNA codons specify an amino acid, but certain cells only use 45 tRNAs. Why is this?

the rules for base pairing between the third base of a codon and tRNA are flexible

The accuracy in the translation of mRNA into the primary structure of a polypeptide depends on the specificity of

the attachment of amino acids to tRNAs (charging) and the bonding of the anticodon to the codon

Ribosome and peptide bonds

• important for catalyzing the formation of peptide bonds

• the catalytic site for peptide bond formation is formed primarily from an rRNA

• rRNA has the catalytic activity that joins amino acids together

If the only product to be translated is a Met-Lys dipeptide that remains attached to the ribosome, how does this inhibit protein synthesis?

it inhibits the movement of the small subunit relative to the large subunit

What is the first event that takes place in translation in eukaryotes?

the small subunit of the ribosome recognizes and attaches to the 5’ cap of mRNA

What is the function of the release factors in the translation process?

they bind to the stop codon in the A site in place of a tRNA

Characteristics of prokaryotic mRNA molecules

• a single prokaryotic mRNA can be translated into several proteins

• mRNAs are not polyadenylated

• Ribosomes can start translating the molecule before transcription is complete

What is used by cells to regulate the amount of protein in the cell?

• genes can be transcribved into mRNA with different efficiencies

• many ribosomes can bind to a single mRNA molecule

• protiens can be tagged with ubiquitin, marking them for degredation

What molecule was thought to have arisen first during evolution?

RNA

The minimum requirement for life to have originated on Earth was the formation of

a molecule that could catalyze its own replication

Ribosomes catalyze what reaction?

RNA splicing

What molecule is likely to contain the genetic information of a virus in a given situation?

RNA

cells in the body have different functions, why is this?

they express mRNAs that others do not

The distinct characteristics of different cell types in a multicellular eukaryotic organism result mainly from the differential regulation of the

genes transcribed by RNA polymerase II

How many genes does the typical differentiated human cell express at any one time?

between 5000 and 15000

Differentiated Cells Characteristic

some of the proteins found in differentiated cells are also found in all cells of a multicellular organism

Housekeeping Proteins

• RNA polymerase

• Histones

• ATP Synthase

a nuclear transplant experiment occured to determine whether DNA is altered irreversibly during development. What is true about the experiment?

the embryo that develops from the nuclear transplant experiment is genetically identical to the donor of the nucleus

DNA microarrays have a impact on genomic studies because

they allow the expression of many or even all of the genes in the genome to be compared at once

Methods of controlling eukaryotic and prokaryotic gene expression

• controlling how often a gene is transcribed

• controlling which mRNAs are translated into protein by the ribosomes

• controlling how rapidly proteins are destroyed once they are made

What method for controlling gene expression occurs in eukaryotes but not prokaryotes?

controlling which mRNAs are exported from the nucleus to the cytosol

Transcriptional regulator characteristics

• form hydrogen bonds, ionic bonds, and hydrophobic interactions with DNA

• DNA binding motifs of TRs usually bind in the major groove of the DNA helix

• the binding of TRs generally does not disrupt the hydrogen bonds that hold the double helix together

Operons

contain a cluster of genes transcribed as a single mRNA

The tryptophan operator

binds to the tryptophan repressor when the repressor is bound to tryptophan

What is true about the Lac operon?

• when the CAP activator is bound to DNA, if lactose is not present, the Lac operon will not be transcribed

• the CAP activator can only bind DNA when it is bound to cAMP

• the Lac operon only produces RNA When lactose is present and glucose is absent

How are eukaryotic transcription regulators able to affect transcription when their binding sites are so far from the promoter?

by looping out the intervening DNA between their binding site and the promoter