bio ch 21

What is the proper order of the following events in the expression of a eukaryotic gene? 1. translation 2. RNA processing 3. transcription 4. modification of protein

3, 2, 1, 4 Transcription RNA Processing Translation Modification of a Protein

Who formulated the one gene-one enzyme hypothesis?

Beadle and Tatum

Which of the following is a post-translational modification of a polypeptide?

Cleavage of a polypeptide into two or more chains Additional steps—post-translational modifications—may be required before the protein can begin doing its particular job in the cell. Certain amino acids may be chemically modified by the attachment of sugars, lipids, phosphate groups, or other additions. Enzymes may remove one or more amino acids from the leading (amino) end of the polypeptide chain. In some cases, a polypeptide chain may be enzymatically cleaved into two or more pieces. For example, the protein insulin is first synthesized as a single polypeptide chain but becomes active only after an enzyme cuts out a central part of the chain, leaving a protein made up of two polypeptide chains connected by disulfide bridges. In other cases, two or more polypeptides that are synthesized separately may come together, becoming the subunits of a protein that has quaternary structure. A familiar example is hemoglobin.

What is a key difference in gene expression between eukaryotic and prokaryotic cells?

In prokaryotic cells, the mRNA transcript is immediately available as mRNA without processing.

Genetic information of eukaryotic cells is transferred from the nucleus to the cytoplasm in the form of __________.

RNA

Which of the following catalyzes the linkage between ribonucleotides to form RNA during gene expression?

RNA polymerase

Which of the following best describes the arrangement of genetic information in a DNA molecule?

The three-nucleotide words of a gene are arranged in a nonoverlapping series on the DNA template strand.

__________ is the synthesis of RNA using information in the DNA.

Transcription

__________ is the synthesis of a polypeptide using information in the mRNA.

Translation

When RNA is being made, the RNA base _________ always pairs with the base __________ in DNA.

U; A

Because the bacterial cell's DNA is not surrounded by a nuclear envelope, __________ occur(s).

coupled transcription and translation

The function of tRNA during protein synthesis is to __________.

deliver amino acids to their proper site during protein synthesis

Stop codons are unique because they __________.

do not code for amino acids that allow a releasing factor to bind to the A site of the ribosome

The TATA box is a __________ that allows for the binding of __________ and __________.

eukaryotic promoter; transcription factors; RNA polymerase II In eukaryotes, a collection of proteins called transcription factors mediate the binding of RNA polymerase and the initiation of transcription. Only after transcription factors are attached to the promoter does RNA polymerase II bind to it. The whole complex of transcription factors and RNA polymerase II bound to the promoter is called a transcription initiation complex.

Insertions and deletions are called __________ mutations.

frameshift

Polysomes may be defined as __________.

groups of ribosomes A single ribosome can make an average-sized polypeptide in less than a minute. Typically, however, multiple ribosomes translate an mRNA at the same time; that is, a single mRNA is used to make many copies of a polypeptide simultaneously. Once a ribosome is far enough past the start codon, a second ribosome can attach to the mRNA, eventually resulting in a number of ribosomes trailing along the mRNA. Such strings of ribosomes, called polyribosomes (or polysomes), can be seen with an electron microscope. Polyribosomes are found in both bacterial and eukaryotic cells. They enable a cell to make many copies of a polypeptide very quickly.

The bonds that hold tRNA molecules in the correct three-dimensional shape are __________.

hydrogen bonds

The sickle-cell β-globin mutation is an example of a __________.

missense mutation

The type of point mutation that results in a premature stop codon is called a _________ mutation.

nonsense

By bombarding the fungus Neurospora crassa with X-rays, Beadle and Tatum were able to study __________ and characterize enzymes in a __________.

nutritional mutants; biochemical pathway

The structures called snRNPs are __________.

part of a spliceosome

One strand of a DNA molecule has the following sequence: 3-AGTACAAACTATCCACCGTC-5. In order for transcription to occur in that strand, there would have to be a specific recognition sequence, called a(n) __________, to the left of the DNA sequence indicated.

promoter

RNA molecules that function as enzymes are called __________.

ribozymes The idea of a catalytic role for the RNAs in the spliceosome arose from the discovery of ribozymes, RNA molecules that function as enzymes. In some organisms, RNA splicing can occur without proteins or even additional RNA molecules: The intron RNA functions as a ribozyme and catalyzes its own excision! For example, in the ciliate protist Tetrahymena, self-splicing occurs in the production of ribosomal RNA (rRNA), a component of the organism's ribosomes. The pre-rRNA actually removes its own introns. The discovery of ribozymes rendered obsolete the idea that all biological catalysts are proteins.

In eukaryotic cells, a __________ by a __________ targets a growing peptide to the endoplasmic reticulum.

signal peptide; signal-recognition particle What determines whether a ribosome is free in the cytosol or bound to rough ER? Polypeptide synthesis always begins in the cytosol as a free ribosome starts to translate an mRNA molecule. There the process continues to completion, unless the growing polypeptide itself cues the ribosome to attach to the ER. The polypeptides of proteins destined for the endomembrane system or for secretion are marked by a signal peptide, which targets the protein to the ER. The signal peptide, a sequence of about 20 amino acids at or near the leading end (N-terminus) of the polypeptide, is recognized as it emerges from the ribosome by a protein-RNA complex called a signal-recognition particle (SRP). This particle functions as an escort that brings the ribosome to a receptor protein built into the ER membrane. The receptor is part of a multiprotein translocation complex. Polypeptide synthesis continues there, and the growing polypeptide snakes across the membrane into the ER lumen via a protein pore. The signal peptide is usually removed by an enzyme. The rest of the completed polypeptide, if it is to be secreted from the cell, is released into solution within the ER lumen.

An exception to the one gene-one enzyme hypothesis is __________.

that not all genes code for enzymes; some genes code for structural proteins such as keratin

Eukaryotic processing of the primary transcript includes __________.

the addition of a 5ꞌ cap, a 3ꞌ poly-A tail, and the splicing out of introns

Bacteria can transcribe and translate human genes to produce functional human proteins because __________.

the genetic code is nearly universal

Gene expression is __________.

the process by which DNA directs the synthesis of proteins

The "triplet code" refers to the fact that _________.

three nucleotides code for a single amino acid Triplets of nucleotide bases are the smallest units of uniform length that can code for all the amino acids. If each arrangement of three consecutive nucleotide bases specifies an amino acid, there can be 64 (that is, 43) possible code words—more than enough to specify all the amino acids. Experiments have verified that the flow of information from gene to protein is based on a triplet code: The genetic instructions for a polypeptide chain are written in the DNA as a series of nonoverlapping three-nucleotide words. The series of words in a gene is transcribed into a complementary series of nonoverlapping three-nucleotide words in mRNA, which is then translated into a chain of amino acids.