Gene Expression - Transcription

What process copies information from DNA into RNA?

Transcription.

What process uses mRNA to make a polypeptide?

Translation.

What is the usual flow of genetic information?

DNA → RNA → protein.

What discovery showed that information can flow from RNA back to DNA?

The discovery of retroviruses and reverse transcriptase.

How can RNA retroviruses infect host cells?

Viral RNA is used as a template to make viral DNA, which becomes incorporated into the host cell’s nuclear genome.

How many amino acids are used to make polypeptides?

Twenty.

How many nucleotide triplets are possible in the genetic code?

Sixty-four.

How many nucleotide triplets code for amino acids?

Sixty-one.

How many stop codons are there?

Three.

What do the three stop codons code for?

Release factors.

What is the main difference between prokaryotic and eukaryotic transcription locations?

Transcription occurs in the cytoplasm of prokaryotes and in the nucleus of eukaryotes.

Where does translation occur?

In the cytoplasm.

What are differences between prokaryotic and eukaryotic gene expression?

They have different promoters and stop sequences, different gene structures, different RNA-processing requirements, different ribosomes, and different starting amino acids.

What is usually found in eukaryotic genes but not prokaryotic genes?

Introns.

What occurs to many eukaryotic RNA transcripts?

hey require RNA processing.

What does AUG code for in prokaryotes?

N-formylmethionine, or fMet.

What does AUG code for in eukaryotes?

Methionine, or Met.

Where does transcription occur in eukaryotes?

In the nucleus.

Where are eukaryotic ribosomal subunits assembled?

In the nucleolus.

How are prokaryotic and eukaryotic ribosomes different?

They have different-sized ribosomes and ribosomal subunits.

What similarities do prokaryotic and eukaryotic gene expression share?

They use the central dogma, have initiation, elongation, and termination, use the same genetic code, use the same start and stop codons, follow the same base-pairing rules, and are regulated at multiple levels.

What are the three fundamental steps of transcription and translation?

Initiation, elongation, and termination.

What is the start codon?

AUG.

What are the stop codons?

UAA, UAG, and UGA.

In what direction is RNA synthesized?

5′ to 3′.

In what direction are polypeptides synthesized?

N-terminus to C-terminus.

What enzyme carries out transcription?

RNA polymerase.

Where does RNA polymerase function in prokaryotes?

In the cytoplasm.

Where does RNA polymerase function in eukaryotes?

In the nucleus.

How many RNA polymerases do prokaryotes have?

One.

What types of RNA does prokaryotic RNA polymerase synthesize?

mRNA, rRNA, and tRNA.

Where does translation take place

in the cytoplasm

How many major RNA polymerases do eukaryotes have?

Three.

What does eukaryotic RNA polymerase I synthesize?

rRNA.

What does eukaryotic RNA polymerase II synthesize?

mRNA.

What does eukaryotic RNA polymerase III synthesize?

tRNA.

What do transcription factors recognize?

Promoter sequences.

What is found approximately 35 bases upstream in a prokaryotic promoter?

The −35 upstream region.

What is the −10 region of a prokaryotic promoter called?

The Pribnow box.

What is the sequence shown for the −35 region?

TTGACA.

What is the sequence shown for the −10 Pribnow box?

TATAAT.

What is the 5′ UTR?

The 5′ untranslated region of the RNA transcript.

Where does transcription initiation occur relative to the translation initiation site?

Before the translation initiation site.

What is required for transcription elongation?

DNA, Mg++, and RNA polymerase.

What is the general chemical reaction for RNA elongation?

NTP + (NMP)n → (NMP)n+1 + PPi.

How does RNA polymerase pair ribonucleotides with DNA nucleotides?

A pairs with U, G pairs with C, C pairs with G, and T pairs with A.

What are the two types of prokaryotic transcription termination?

GC-rich intrinsic termination and Rho-mediated termination.

How does GC-rich intrinsic termination work?

A GC-rich sequence forms an intrastrand loop in the RNA, which terminates transcription.

What is Rho-mediated termination?

Termination caused by a Rho hexamer at C-rich sequences.

What sequences are recognized during eukaryotic transcription initiation?

The CAAT box and TATA box.

What signals transcription termination in eukaryotes?

The mRNA sequence AAUAAA.

How far does transcription continue beyond the AAUAAA sequence?

Approximately 50 base pairs.

Where does the endonuclease cut the transcript?

Approximately 20 base pairs downstream from AAUAAA.

What enzyme adds the poly-A tail?

Poly-A polymerase.

What is added to the 3′ end of eukaryotic pre-mRNA?

A poly-A tail.

How long is the poly-A tail?

Approximately 150–250 adenines.

What is the function of the poly-A tail?

It protects the 3′ end of the transcript.

What is pre-mRNA?

The initial RNA transcript made by eukaryotes that requires processing before becoming mature mRNA.

How does eukaryotic mRNA synthesis differ from prokaryotic mRNA synthesis?

Eukaryotes produce pre-mRNA that must be processed, while prokaryotes synthesize mRNA directly.

What is added to the 5′ end of pre-mRNA?

A 7-methylguanosine triphosphate cap, or 7m-GTP cap.

Why is the 7m-GTP cap added?

It protects the 5′ end from endonuclease digestion.

What are the three main steps of eukaryotic pre-mRNA processing?

Addition of a 5′ 7m-GTP cap, addition of a 3′ poly-A tail, and removal of introns.

What removes introns from pre-mRNA?

Spliceosomes.

What are introns?

Noncoding DNA sequences located within genes that are removed before the final mRNA is produced.

What sequence identifies an intron?

A 5′ GU sequence, an internal A, and a 3′ AG sequence.

In what types of organisms are introns commonly found?

Animals, plants, fungi, and protists.

Are introns found in prokaryotes?

The slides state that introns are not found in prokaryotes.

What can alternative splicing produce?

Different mRNA transcripts and different polypeptides from the same gene.

How does alternative splicing allow different cell types to make different proteins?

Different introns are removed in different cells, producing different transcripts and cell-specific polypeptides.

How many human genes were expected before the genome was sequenced?

Approximately 80,000–100,000 genes.

How many human genes were discovered after genome sequencing?

Approximately 22,000 genes.

Why can humans produce more polypeptides than the number of genes suggests?

Some genes code for more than one polypeptide through alternative splicing.

What is an operon?

A gene cluster that is transcriptionally regulated as a unit.

Where are operons found mostly?

In prokaryotes.

What organism was used to discover the lac operon?

Escherichia coli.

Who discovered the lac operon regulatory mechanism?

Jacob and Monod in 1961.

What does β-galactosidase do?

It cleaves lactose into glucose and galactose.

What does lactose permease do?

It allows lactose to cross the cell membrane into the cell.

What does thiogalactoside transacetylase do?

It helps with cellular detoxification.

What are the three components of the lac operon?

Functionally related protein-coding genes, an operator site, and a regulatory site.

What does the operator site do?

It regulates transcription.

What does the regulatory site do?

It controls the operator site.

How does the lac operon repressor work?

The repressor blocks RNA polymerase from initiating transcription.

What happens to the lac operon when lactose is present?

Lactose binds to the repressor, causing it to dissociate from the transcription initiation site.

What happens after the repressor leaves the lac operon?

RNA polymerase transcribes the three genes involved in lactose metabolism.

Why is the lac operon considered an on-demand system?

It conserves energy when lactose is absent and transcribes the genes when lactose is present.

What is a repressible operon?

An operon that is normally on but can be inhibited.

What type of pathway is usually controlled by a repressible operon?

An anabolic pathway that builds an essential organic molecule.

What is the state of the repressor in a repressible operon before activation?

The repressor is inactive.

What is a corepressor?

The organic molecule being produced that activates the repressor when present in high concentrations.

What happens when the activated repressor binds to the operator?

The operon is shut down.

What is an example of a repressible operon?

The tryptophan operon.

What does the tryptophan operon produce?

Enzymes used in tryptophan synthesis.

When does the tryptophan operon shut down?

When there is already too much tryptophan in the cell.

What types of rRNA do prokaryotes have?

5S, 16S, and 23S rRNAs.

Where are 5S and 23S rRNAs located in prokaryotic ribosomes?

In the large ribosomal subunit.

Where is 16S rRNA located?

In the small ribosomal subunit.

What types of rRNA do eukaryotes have?

5S, 5.8S, 18S, and 28S rRNAs.

How many copies of polycistronic ribosomal RNA genes do humans have?

Approximately 200–1,200 copies.

On which chromosomes are human polycistronic rRNA genes located?

Chromosomes 13, 14, 15, 21, and 22.