Molecular Biology and Gene Expression

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Questions and Answers

What was the primary goal of Beadle and Tatum's experiment involving Neurospora?

To investigate the relationship between genes and enzymes (correct)

To expose the effects of X-rays on bread mold

To develop a new method for synthesizing arginine

To study the ornithine cycle in Neurospora

What was the outcome of Beadle and Tatum's experiment where they exposed Neurospora to X-rays?

Mutants that were unable to survive on minimal media without arginine (correct)

The X-rays had no effect on the Neurospora

Mutants that were able to survive on minimal media without arginine

No mutants were produced

What did Adrian Srb and Norman Horowitz identify in their research on arginine-deficient mutants?

Three classes of mutants that lacked different enzymes (correct)

One class of mutants that lacked a single enzyme

Two classes of mutants that lacked different enzymes

Four classes of mutants that lacked different enzymes

What is the main function of a gene according to the one gene-one enzyme hypothesis?

To synthesize a specific enzyme Signup and view all the answers

What is the primary role of RNA in the process of gene expression?

To act as a bridge between genes and protein synthesis Signup and view all the answers

What is the process of synthesizing a polypeptide using information in mRNA?

Translation Signup and view all the answers

What is the current form of the hypothesis that was originally proposed by Beadle and Tatum?

One gene-one polypeptide Signup and view all the answers

What is the site where translation occurs in a cell?

Ribosomes Signup and view all the answers

What is the direction in which the mRNA base triplets are read during translation?

5′ → 3′ Signup and view all the answers

What is the total number of codons that specify amino acids?

61 Signup and view all the answers

What is the function of the three 'stop' signal codons?

To specify the termination of translation Signup and view all the answers

What is the consequence of not reading codons in the correct reading frame?

The specified polypeptide is not produced at all Signup and view all the answers

What is the significance of the genetic code being nearly universal?

It suggests that a language shared by all living things must have been operating very early in the history of life Signup and view all the answers

What is the main difference between transcription termination in bacteria and eukaryotes?

The location where the RNA transcript is released Signup and view all the answers

What is the primary function of the 5' cap and poly-A tail in eukaryotic mRNA?

To facilitate the export of mRNA to the cytoplasm Signup and view all the answers

What is the role of RNA polymerase in transcription?

To catalyze the synthesis of RNA nucleotides Signup and view all the answers

What is the difference between the base-pairing rules of RNA and DNA synthesis?

RNA synthesis uses uracil instead of thymine Signup and view all the answers

What is the term for the noncoding stretches of nucleotides that lie between coding regions in eukaryotic genes?

Introns Signup and view all the answers

What is the purpose of RNA splicing in eukaryotic cells?

To remove introns from the RNA transcript Signup and view all the answers

What is the first stage of gene expression?

Transcription Signup and view all the answers

What is the term for the process of modifying pre-mRNA in eukaryotic cells?

RNA processing Signup and view all the answers

What is the function of the poly-A tail in eukaryotic mRNA?

To protect mRNA from hydrolytic enzymes Signup and view all the answers

What is the result of RNA splicing in eukaryotic cells?

A mature mRNA molecule without introns Signup and view all the answers

What is the term for the region of the mRNA molecule that is not translated into amino acid sequences?

UTR Signup and view all the answers

What is the primary function of transfer RNA (tRNA) in the process of translation?

To facilitate the binding of amino acids to the growing polypeptide Signup and view all the answers

What is the shape of a tRNA molecule when it is flattened into one plane?

Cloverleaf Signup and view all the answers

What is the purpose of the enzyme aminoacyl-tRNA synthetase in translation?

To match a tRNA with its correct amino acid Signup and view all the answers

What is the term for the flexible pairing at the third base of a codon that allows some tRNAs to bind to more than one codon?

Wobble Signup and view all the answers

How many nucleotides are typically found in a tRNA molecule?

80 Signup and view all the answers

What is the function of the protruding 3′ end of a tRNA molecule?

To act as an attachment site for an amino acid Signup and view all the answers

What is the process by which a cell translates an mRNA message into a protein?

Translation Signup and view all the answers

What is the term for the correct match between a tRNA anticodon and an mRNA codon?

Molecular recognition Signup and view all the answers

What is the main function of spliceosomes in RNA splicing?

To recognize and remove introns from RNA Signup and view all the answers

What is a key property of RNA that enables it to function as an enzyme?

Its ability to form a three-dimensional structure Signup and view all the answers

What is the term for the process by which genes can encode more than one kind of polypeptide?

Alternative RNA splicing Signup and view all the answers

What is the result of exon shuffling in the evolution of new proteins?

The mixing and matching of exons between different genes Signup and view all the answers

What is the term for the discrete regions in a protein that are coded for by different exons?

Protein domains Signup and view all the answers

What is the significance of introns in gene expression?

They regulate gene expression Signup and view all the answers

What is the relationship between the number of genes in an organism and the number of different proteins it can produce?

The number of proteins is much greater than the number of genes Signup and view all the answers

What is the term for RNA molecules that function as enzymes?

Ribozymes Signup and view all the answers

Study Notes

Nutritional Mutants in Neurospora

George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal media.
Some cells grew when minimal media was supplemented with the amino acid arginine.
Adrian Srb and Norman Horowitz identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine.
The results of the experiments provided support for the one gene–one enzyme hypothesis, which states that the function of a gene is to dictate production of a specific enzyme.

The Products of Gene Expression

Not all proteins are enzymes, so researchers later revised the hypothesis to one gene–one protein.
Many proteins are composed of several polypeptides, each of which has its own gene.
Therefore, Beadle and Tatum’s hypothesis is now restated as the one gene–one polypeptide hypothesis.
It is common to refer to gene products as proteins rather than more precisely as polypeptides.

Basic Principles of Transcription and Translation

RNA is the bridge between genes and protein synthesis.
Transcription is the synthesis of RNA using information in DNA.
Transcription produces messenger RNA (mRNA).
Translation is the synthesis of a polypeptide, using information in the mRNA.
Ribosomes are the sites of translation.

Translation

During translation, the mRNA base triplets, called codons, are read in the 5′ → 3′ direction.
Each codon specifies the amino acid (one of 20) to be placed at the corresponding position along a polypeptide.
All 64 codons were deciphered by the mid-1960s.
Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation.
The genetic code is redundant (more than one codon may specify a particular amino acid) but not ambiguous (no codon specifies more than one amino acid).
Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced.

Evolution of the Genetic Code

The genetic code is nearly universal, shared by the simplest bacteria and the most complex animals.
Genes can be transcribed and translated after being transplanted from one species to another.
A language shared by all living things must have been operating very early in the history of life.

Transcription

Transcription is the first stage of gene expression.
RNA synthesis is catalyzed by RNA polymerase, which pryes the DNA strands apart and joins together the RNA nucleotides.
The mRNA is complementary to the DNA template strand.
RNA polymerase does not need any primer.
RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine.

Termination of Transcription

The mechanisms of termination are different in bacteria and eukaryotes.
In bacteria, the polymerase stops transcription at the end of the terminator and the mRNA can be translated without further modification.
In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence; the RNA transcript is released 10–35 nucleotides past this polyadenylation sequence.

Eukaryotic Cells Modify RNA After Transcription

Enzymes in the eukaryotic nucleus modify pre-mRNA (RNA processing) before the genetic messages are dispatched to the cytoplasm.
During RNA processing, both ends of the primary transcript are changed.
Also, in most cases, certain interior sections of the molecule are cut out and the remaining parts spliced together.

Alteration of mRNA Ends

Each end of a pre-mRNA molecule is modified in a particular way.
The 5′ end receives a modified nucleotide 5′ cap.
The 3′ end gets a poly-A tail.
These modifications share several functions:
- They seem to facilitate the export of mRNA to the cytoplasm.
- They protect mRNA from hydrolytic enzymes.
- They help ribosomes attach to the 5′ end.

Split Genes and RNA Splicing

Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions.
The noncoding segments in a gene are called intervening sequences, or introns.
The other regions are called exons because they are eventually expressed, usually translated into amino acid sequences.
Introns are removed through RNA splicing.
The removal of introns is accomplished by spliceosomes.
Spliceosomes consist of a variety of proteins and several small RNAs that recognize the splice sites.
The RNAs of the spliceosome also catalyze the splicing reaction.

Ribozymes

Ribozymes are catalytic RNA molecules that function as enzymes and can splice RNA.
Three properties of RNA enable it to function as an enzyme:
- It can form a three-dimensional structure because of its ability to base-pair with itself.
- Some bases in RNA contain functional groups that may participate in catalysis.
- RNA may hydrogen-bond with other nucleic acid molecules.

The Functional and Evolutionary Importance of Introns

Some introns contain sequences that regulate gene expression and many affect gene products.
Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during splicing.
This is called alternative RNA splicing.
Consequently, the number of different proteins an organism can produce is much greater than its number of genes.
Proteins often have a modular architecture consisting of discrete regions called domains.
In many cases, different exons code for the different domains in a protein.
Exon shuffling may result in the evolution of new proteins by mixing and matching exons between different genes.

Translation: A Closer Look

Genetic information flows from mRNA to protein through the process of translation.
A cell translates an mRNA message into protein with the help of transfer RNA (tRNA).
tRNAs transfer amino acids to the growing polypeptide in a ribosome.
Translation is a complex process in terms of its biochemistry and mechanics.

The Structure and Function of Transfer RNA

Each tRNA molecule enables translation of a given mRNA codon into a certain amino acid.
Each carries a specific amino acid on one end.
Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA.
A tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long.
Flattened into one plane to reveal its base pairing, a tRNA molecule looks like a cloverleaf.
Because of hydrogen bonds, tRNA actually twists and folds into a three-dimensional molecule.
tRNA is roughly L-shaped with the 5′ and 3′ ends both located near one end of the structure.
The protruding 3′ end acts as an attachment site for an amino acid.

Accurate Translation

Accurate translation requires two instances of molecular recognition:
- First: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase.
- Second: a correct match between the tRNA anticodon and an mRNA codon.
Flexible pairing at the third base of a codon is called wobble and allows some tRNAs to bind to more than one codon.

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Description

Quiz about the one gene-one enzyme hypothesis, gene expression, and the role of RNA in synthesizing polypeptides. Learn about the contributions of Beadle, Tatum, Srb, and Horowitz to our understanding of molecular biology.