Biology Chapter 17

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Questions and Answers

What determines the shape of a protein during folding?

The gene that encodes the protein (correct)

The temperature of the environment

The length of the polypeptide chain

The rate of transcription

Where does polypeptide synthesis always begin?

In the nucleus

In the cytosol (correct)

In the ribosomes

In the endoplasmic reticulum

What is the role of the signal recognition particle (SRP) during protein synthesis?

To degrade misfolded proteins

To escort the ribosome to the ER membrane (correct)

To initiate transcription in the nucleus

To bind directly to the polypeptide

What is a polyribosome?

An assembly of ribosomes translating the same mRNA Signup and view all the answers

How do free and bound ribosomes differ in function?

Free ribosomes synthesize proteins that function in the cytosol, while bound ribosomes create proteins for the endomembrane system. Signup and view all the answers

What marks a polypeptide for transport to the endoplasmic reticulum?

A signal peptide Signup and view all the answers

What happens to RNA before it can leave the nucleus in eukaryotes?

It undergoes splicing and processing. Signup and view all the answers

What initiates the binding of a ribosome to the endoplasmic reticulum?

The recognition of a specific signal peptide Signup and view all the answers

What is the primary function of ribozymes?

To act as a catalyst for RNA splicing Signup and view all the answers

How does RNA differ from DNA in terms of structure that allows it to function as an enzyme?

RNA can base-pair with itself Signup and view all the answers

What is alternative RNA splicing?

The modification of mRNA to include or exclude certain exons Signup and view all the answers

What role do tRNA molecules play in translation?

They carry amino acids to the ribosome and match them to mRNA codons Signup and view all the answers

Which statement correctly defines the interaction between tRNA and mRNA during translation?

tRNA uses its anticodon to pair with complementary mRNA codons Signup and view all the answers

What is a potential outcome of exon shuffling in the evolution of proteins?

It may result in the development of new proteins by mixing exons between different genes Signup and view all the answers

How does the structure of RNA contribute to its catalytic properties?

It contains functional groups that can participate in catalysis Signup and view all the answers

What does the complexity of translation refer to in its mechanisms?

The intricate biochemical interactions between mRNA, tRNA, and ribosomes Signup and view all the answers

What occurs at the end of transcription in bacteria?

Transcription stops at the terminator without modification. Signup and view all the answers

Which modification is added to the 5′ end of pre-mRNA in eukaryotes?

A modified nucleotide 5′ cap Signup and view all the answers

What is the purpose of adding a poly-A tail to the 3′ end of mRNA?

To facilitate mRNA export and protect it from degradation. Signup and view all the answers

What are intervening sequences in eukaryotic genes?

Noncoding regions known as introns. Signup and view all the answers

How does RNA splicing impact the resulting mRNA molecule?

It joins together exons and removes introns. Signup and view all the answers

What role do spliceosomes play in RNA processing?

They recognize splice sites and catalyze splicing. Signup and view all the answers

What modification is NOT typically performed on mRNA in eukaryotes?

Addition of disulfide bridges Signup and view all the answers

What is the primary purpose of the mRNA modifications that occur during RNA processing in eukaryotes?

To facilitate ribosome attachment and protect mRNA. Signup and view all the answers

Study Notes

Gene Expression: From Gene to Protein

Gene expression is the process by which DNA directs protein synthesis.
Proteins are the link between genotype and phenotype.
The process includes two key stages: transcription and translation.
In 1902, Archibald Garrod suggested genes dictate phenotypes through enzymes (catalyzing specific chemical reactions).
He also thought that inherited diseases result from an inability to synthesize a specific enzyme due to a metabolic pathway of cells.
Cells synthesize and degrade molecules sequentially.
George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that could not survive on minimal media.
Beadle and Tatum's colleagues, Adrian Srb and Norman Horowitz, identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for arginine synthesis.
Experiments demonstrated that the function of a gene is to dictate the production of a specific enzyme.

Transcription: DNA to RNA

RNA is the bridge between genes and protein synthesis.
Transcription is the synthesis of RNA using information from DNA.
Transcription produces messenger RNA (mRNA).
Translation is the synthesis of a polypeptide (a chain of amino acids) from an mRNA molecule.
mRNA is translated into a chain of amino acids.
Ribosomes are the sites of translation.
In prokaryotes, translation of mRNA can begin before transcription is finished.
In eukaryotes, the nuclear envelope separates transcription from translation.
Eukaryotic RNA transcripts are modified through RNA processing before leaving the nucleus.
A primary transcript is the initial RNA transcript from any gene prior to processing.
The central dogma is the concept that cells are governed by a cellular chain of command. DNA → RNA → protein.
Transcription is the first stage of gene expression.
RNA synthesis is catalyzed by RNA polymerase.
The RNA polymerase pries the DNA strands apart. It then joins together RNA nucleotides.
The RNA is complementary to the DNA template strand).
RNA polymerase does not need any primer.

Codons: Triplets of Nucleotides

The flow of information from gene to protein is based on a triplet code consisting of non-overlapping, three-nucleotide words.
The words of a gene are transcribed into complementary non-overlapping three-nucleotide words of mRNA.
These mRNA words are translated into a chain of amino acids.
One of the two DNA strands acts as a template for the sequence of complementary nucleotides in the RNA transcript.
The template strand is always the same for a given gene.
The nontemplate strand is called the coding strand; it is identical to the mRNA sequence, except that T takes the place of U in the mRNA.
The mRNA molecule is complementary to the template strand.
During translation, the mRNA base triplets (codons) are read in the 5' → 3' direction.
Each codon specifies a particular amino acid.

Cracking the Code

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) for the specified polypeptide to be produced.

Transcription Initiation, Elongation, and Termination

The three main stages of transcription are initiation, elongation, and termination.
Promoters signal the transcription start point.
Transcription factors help the binding of RNA polymerase and initiate transcription.
The completed assembly of transcription factors and RNA polymerase II bound to a promoter is called a transcription initiation complex.
A promoter called a TATA box is crucial in forming the initiation complex in eukaryotes.
As RNA polymerase moves along the DNA, it untwists the DNA double helix, 10–20 nucleotides at a time.
Nucleotides are added to the 3' of the growing RNA molecule.
Transcription progresses at a rate of 40 nucleotides per second in eukaryotes.
A gene can be transcribed simultaneously by several RNA polymerases.
The termination mechanisms vary in bacteria and eukaryotes.
In bacteria, the polymerase stops transcription at the end of the terminator.
In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence; the RNA transcript is released 10–35 nucleotides past this polyadenylation sequence.

RNA Processing

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

Alternative RNA Splicing

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.

Protein Folding

During synthesis, a polypeptide chain begins to coil and fold spontaneously into a specific shape (three-dimensional molecule with secondary and tertiary structure).
A gene determines the primary structure, and the primary structure in turn determines shape.
Post-translational modifications may be required before the protein can begin doing its function in the cell.

Targeting Polypeptides to Specific Locations

Two populations of ribosomes exist in cells: free ribosomes in the cytosol and bound ribosomes attached to the ER.
Free ribosomes mostly synthesize proteins that function in the cytosol.
Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell.
Ribosomes are identical and can switch from free to bound.
Polypeptide synthesis always begins in the cytosol.
Synthesis finishes in the cytosol unless the polypeptide signals the ribosome to attach to the ER.
Polypeptides destined for the ER or for secretion are marked by a signal peptide.
The signal peptide is a sequence of about 20 amino acids at or near the leading end of the polypeptide.
A signal-recognition particle (SRP) binds to the signal peptide.
The SRP escorts the ribosome to a receptor protein built into the ER membrane.
The signal peptide is removed by an enzyme.
Other kinds of signal peptides target polypeptides to other organelles.

Making Multiple Polypeptides

Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome (or polysome).
Polyribosomes enable a cell to make many copies of a polypeptide very quickly.
A bacterial cell ensures a streamlined process by coupling transcription and translation.

Mutations

Mutations are changes in the genetic information of a cell.
Point mutations are changes in just one nucleotide pair of a gene.
The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein.
If a mutation has an adverse effect, the condition is referred to as a genetic disorder or hereditary disease.
Point mutations can be classified as:
- Substitutions (silent, missense, nonsense) and
- Insertions/Deletions (frameshift).

New Mutations and Mutagens

Spontaneous mutations can occur during DNA replication or recombination.
Mutagens are physical or chemical agents that cause mutations.
Chemical mutagens fall into a variety of categories.
Most carcinogens (cancer-causing chemicals) are mutagens.

Gene Editing Using CRISPR

Biologists who study disease-causing mutations have sought techniques for gene editing.
The powerful technique called CRISPR-Cas9 is transforming the field of genetic engineering.
In bacteria, the protein Cas9 acts together with a guide RNA to help defend bacteria from viral infection.
The Cas9 protein cuts any sequence to which it is targeted.
Scientists can introduce a Cas9-guide RNA complex into a cell that they wish to alter (the guide RNA is engineered to target a gene).
Cas9 cuts both strands of the targeted gene.
The broken ends trigger a DNA repair system in which repair enzymes remove or add some random nucleotides while joining the broken ends.

Gene Therapy

To treat genetic disease, researchers have modified this technique.
They can introduce a template with a normal, functional copy of the gene to correct the defective gene.

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Test your knowledge on protein synthesis and the molecular mechanisms involved in it. This quiz covers topics such as ribosomes, RNA processing, and the roles of various molecules like tRNA and SRP. Challenge yourself and deepen your understanding of these critical biological processes!