NAS 2 Cell Protein Syntesis

What essential information does DNA provide to a cell?

DNA contains the instructions needed to build proteins, which help determine the cell’s structure and carry out nearly all cellular functions.

Why can a change in a gene alter both cell structure and cell function?

A gene determines the amino acid sequence of a protein. Changing the gene may change the protein’s shape or activity, affecting structural components, enzymes, or other cellular processes.

Why are proteins central to cellular physiology?

They form many cellular structures and assist with virtually every cellular function, including catalyzing biochemical reactions.

What is the role of an enzyme in a cell?

An enzyme is a protein that speeds up a necessary biochemical reaction without being consumed by the reaction.

DNA replication and microtubule synthesis are examples of what general type of enzyme-assisted reaction?

They are building, or synthesis, reactions in which smaller components are assembled into larger molecules or structures.

Harvesting chemical energy from nutrient molecules commonly requires what general type of enzyme-assisted reaction?

Breakdown reactions in which larger molecules are separated into smaller components.

Distinguish a cell’s genome from its proteome.

The genome is the cell’s complete DNA complement; the proteome is the cell’s complete complement of proteins.

What is a gene?

A gene is a functional segment of DNA that contains the information needed to build a particular protein or other final gene product.

What is gene expression?

Gene expression is the conversion of information encoded in a gene into a final gene product; in this section, the product is a protein.

How does gene expression ultimately influence a cell’s structure and function?

It determines which proteins the cell makes, and those proteins establish cellular structures and perform cellular activities.

What does the nucleotide sequence of a gene specify?

It specifies the amino acid sequence of the corresponding protein.

What is a DNA triplet?

A triplet is a sequence of three consecutive DNA bases that corresponds to a particular amino acid through the genetic code.

In the example used by OpenStax, which amino acid is specified by the DNA triplet CAC?

Valine.

Why can a gene encode an entire protein rather than only one amino acid?

A gene contains many triplets arranged in a specific order, so it specifies many amino acids in the sequence required for the protein.

Place these in order: protein, DNA, mRNA.

DNA → mRNA → protein.

What are the two major stages that convert a DNA code into a protein?

Transcription converts the DNA information into mRNA; translation decodes the mRNA to build a polypeptide.

True or false: DNA is translated directly into protein.

False. DNA is first transcribed into mRNA, and the mRNA is then translated into protein.

In Figure 3.25, what molecule serves as the intermediate between a DNA template and a protein?

RNA, specifically mRNA.

A diagram shows DNA becoming RNA and RNA becoming protein. Name the process shown by each arrow.

DNA → RNA is transcription; RNA → protein is translation.

What relationship does Figure 3.25 emphasize between a gene and its protein?

The nucleotide sequence of the gene ultimately determines the amino acid sequence of the corresponding protein.

What is messenger RNA (mRNA)?

mRNA is a single-stranded nucleic acid that carries a copy of one gene’s genetic code from the nucleus to the cytoplasm for protein production.

Why is an intermediate messenger required for protein synthesis in a eukaryotic cell?

DNA remains in the nucleus, whereas translation occurs in the cytoplasm, so mRNA carries the genetic instructions between the two locations.

How does the strandedness of most RNA differ from DNA?

Most RNA, including mRNA, is single-stranded; DNA is double-stranded.

How does the sugar in RNA differ from the sugar in DNA?

RNA contains ribose, which has one more oxygen atom than the deoxyribose found in DNA.

Which nitrogenous base is found in RNA in place of thymine?

Uracil.

During RNA synthesis, which RNA base pairs with adenine on DNA?

Uracil.

Correct the statement: RNA contains thymine, and DNA contains uracil.

DNA contains thymine; RNA contains uracil.

What is transcription?

Transcription is the synthesis of an mRNA strand complementary to the DNA sequence of a gene.

Why is the process called transcription?

The mRNA is a transcript, or working copy, of the genetic information in the DNA gene.

How is transcription similar to DNA replication?

A DNA region unwinds, the strands separate, and complementary nucleotides are assembled using a DNA strand as a template.

How does transcription differ from DNA replication in the amount of DNA opened?

Transcription separates only the small DNA region containing the gene being expressed, rather than copying the entire DNA molecule.

What is a codon?

A codon is a three-base sequence on mRNA that directly specifies an amino acid or a translation signal.

Distinguish a DNA triplet from an mRNA codon.

A triplet is a three-base sequence in DNA; a codon is the corresponding three-base sequence in mRNA that is read during translation.

Name the three stages of transcription in order.

Initiation → elongation → termination.

What is a promoter?

A promoter is a specific nucleotide sequence at the beginning of a gene that signals the start of transcription.

Which transcription stage begins when the promoter triggers the process?

Initiation.

What does RNA polymerase do?

It unwinds the DNA segment and adds complementary RNA nucleotides to the growing mRNA strand.

How does OpenStax describe the coding strand during transcription?

It is the DNA strand used as the template containing the gene sequence to be transcribed.

During transcription elongation, what happens after RNA polymerase unwinds the DNA segment?

RNA polymerase aligns complementary RNA nucleotides with the exposed DNA bases and joins them into a growing mRNA strand.

A DNA template base is G. Which RNA nucleotide will RNA polymerase add opposite it?

Cytosine (C).

A DNA template sequence is TAC. What complementary mRNA sequence is produced?

AUG.

True or false: RNA polymerase adds thymine to a growing mRNA strand.

False. RNA polymerase uses uracil instead of thymine in RNA.

What is a terminator sequence?

It is a nucleotide sequence at the end of a gene that causes transcription to stop.

How does the terminator sequence end transcription according to the section?

It causes the new RNA to fold on itself, promoting separation of the RNA from the gene and RNA polymerase.

Match each event to its stage: promoter recognition; mRNA chain growth; RNA release.

Promoter recognition—initiation; mRNA chain growth—elongation; RNA release—termination.

What enzyme is shown producing the RNA transcript in Figure 3.26?

RNA polymerase.

In Figure 3.26, what determines the nucleotide sequence of the RNA transcript?

Complementary base pairing with the DNA template strand.

In a transcription diagram, where would you expect DNA to be locally separated?

At the region where RNA polymerase is moving and the RNA transcript is being assembled.

What is pre-mRNA?

Pre-mRNA is the initial RNA transcript before it has completed the modifications required to leave the nucleus and be translated.

Why must pre-mRNA be processed before translation?

It contains regions that must be removed or rearranged so the mature mRNA carries the appropriate coding sequence into the cytoplasm.

What is RNA splicing?

Splicing is the removal of selected regions from pre-mRNA followed by reconnection of the remaining regions.

What is a spliceosome?

A spliceosome is a complex of proteins and other molecules that cuts selected regions from pre-mRNA and joins the remaining segments.

What is an intron?

An intron is a segment removed from a pre-mRNA transcript during splicing.

What is an exon?

An exon is a segment of RNA that remains in the mature transcript after splicing.

What happens to introns and exons during standard pre-mRNA splicing?

Introns are removed; exons are joined together.

Place these events in order: mature mRNA exits the nucleus; transcription produces pre-mRNA; spliceosome removes selected segments.

Transcription produces pre-mRNA → spliceosome removes selected segments and joins the remaining ones → mature mRNA exits the nucleus.

How can different splicing patterns increase protein diversity?

Different coding regions can be retained or removed, producing different mature mRNAs and therefore protein variants with different structures and functions.

Correct the statement: Every intron is always a meaningless, noncoding sequence.

The section notes that some removed introns are not always noncoding; splicing choices can affect which protein variant is produced.

A mutation prevents a spliceosome from functioning. What direct problem would occur?

Pre-mRNA would not be properly spliced, so introns might remain or exons might not be correctly joined, potentially altering the protein.

What structure removes the intron in Figure 3.27?

The spliceosome.

What two types of regions are visible in the pre-mRNA at the start of Figure 3.27?

Introns and exons.

After the process shown in Figure 3.27, what remains connected in the mature RNA?

The exons.

What is translation?

Translation is the process of decoding mRNA to synthesize a chain of amino acids called a polypeptide.

What is a polypeptide?

A polypeptide is a chain of amino acids produced during translation that will form all or part of a protein.

What two major functional aids are required for translation?

A translator molecule that matches codons with amino acids and a substrate or platform on which the mRNA is decoded.

What is the ribosome’s main function in translation?

It provides the platform that aligns mRNA with tRNAs so amino acids can be assembled in the correct order.

What is ribosomal RNA (rRNA)?

rRNA is RNA that combines with proteins to form the structure of a ribosome.

What are the two major structural components of a ribosome before they assemble on mRNA?

A small ribosomal subunit and a large ribosomal subunit.

What happens to the two ribosomal subunits when an mRNA is ready for translation?

The small and large subunits come together and attach to the mRNA.

What proteins are commonly synthesized by ribosomes associated with rough ER?

Proteins destined for processing or transport through the Golgi apparatus.

True or false: Ribosomes function only when attached to rough ER.

False. Ribosomes can also exist free in the cytoplasm; the section notes that many are associated with rough ER.

What is transfer RNA (tRNA)?

tRNA is an RNA molecule that carries a specific amino acid to the ribosome and matches it to the appropriate mRNA codon.

Why is tRNA described as the molecular translator of the genetic code?

Its anticodon recognizes an mRNA codon while its other end carries the amino acid specified by that codon.

What is located at one end of a tRNA molecule?

A binding site for a specific amino acid.

What is an anticodon?

An anticodon is a three-base sequence on tRNA that is complementary to an mRNA codon.

Where are codons and anticodons located?

Codons are on mRNA; anticodons are on tRNA.

If an mRNA codon is GGA, what complementary tRNA anticodon is given in the section?

CCU.

If an mRNA codon is AUG, what is the complementary tRNA anticodon?

UAC.

A tRNA has the correct anticodon but carries the wrong amino acid. What would happen?

The tRNA could pair with the codon, but the wrong amino acid would be inserted into the polypeptide, potentially altering the protein.

Name the three main stages of translation in order.

Initiation → elongation → termination.

What major event defines translation initiation?

A ribosome binds to an mRNA transcript and assembles into a functional translation complex.

What event begins each cycle of translation elongation?

A tRNA anticodon recognizes and pairs with the next complementary mRNA codon.

After codon–anticodon pairing, what happens to the incoming amino acid?

It is attached to the growing polypeptide chain with enzyme assistance and an energy input.

What happens after an amino acid is added to the growing chain?

The tRNA releases the mRNA, the mRNA shifts by one codon through the ribosome, and the next matching tRNA arrives.

Why must the mRNA move exactly one codon at a time through the ribosome?

Moving one three-base unit at a time preserves the correct reading sequence so each codon specifies the next amino acid.

Does attaching each new amino acid to the growing polypeptide require energy?

Yes. The section states that the attachment requires energy and assistance from enzymes.

What signals termination of translation?

A final codon that functions as a stop message.

What is released when a stop message is reached?

The complete newly synthesized protein, and the translation complex ultimately disassembles.

Correct the statement: tRNA supplies the code, while mRNA carries amino acids.

mRNA supplies the codon sequence; tRNA carries the corresponding amino acids.

True or false: The ribosome determines which amino acid corresponds to each codon without help from tRNA.

False. tRNAs match their anticodons to mRNA codons and deliver the corresponding amino acids; the ribosome aligns the molecules.

What would be the immediate effect if a ribosome could bind mRNA but tRNAs could not enter the complex?

Translation could initiate structurally, but elongation would fail because no amino acids could be matched to codons or added to the polypeptide.

Which three major components form the functional complex shown in Figure 3.28?

mRNA, a ribosome, and tRNA molecules.

What emerges from the ribosome in the final panel of Figure 3.28?

A growing polypeptide chain.

In a translation diagram, what should be paired directly with an mRNA codon?

The complementary anticodon of a tRNA.

What does the sequence of tRNAs in Figure 3.28 accomplish?

It delivers amino acids in the order specified by successive mRNA codons.

Where does transcription occur in the cell shown in Figure 3.29?

Inside the nucleus.

Where does translation occur in the cell shown in Figure 3.29?

In the cytoplasm.

According to Figure 3.29, what happens to mRNA between transcription and translation?

It is modified in the nucleus and then transported into the cytoplasm.

Which molecules help decode the mRNA transcript in Figure 3.29?

A ribosome and tRNA molecules.

Trace protein synthesis from a gene to a released protein.

The gene is transcribed into pre-mRNA → pre-mRNA is processed into mature mRNA → mRNA exits the nucleus → ribosomal subunits bind it → tRNAs match codons and add amino acids → a stop signal ends translation and releases the protein.