Molecular Biology Quiz

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Questions and Answers

What is the role of the sigma factor in prokaryotic transcription initiation?

It assists in the termination of transcription
It acts as a chaperone for tRNA molecules
It is involved in proofreading the newly synthesized RNA strand
It helps RNA polymerase recognize the promoter sequence (correct)

Where in the eukaryotic cell do transcription, mRNA processing, and translation occur? What makes this different than prokaryotes?

Transcription in the nucleus, mRNA processing and translation in the cytoplasm; differs from prokaryotes where all processes occur in the nucleus
Transcription and mRNA processing in the nucleus, translation in the cytoplasm; differs from prokaryotes where all processes occur in the cytoplasm (correct)
All processes occur in the cytoplasm; differs from prokaryotes where all processes occur in the nucleus
All processes occur in the mitochondria; differs from prokaryotes where all processes occur in the cytoplasm

What do we mean when we say the genetic code is 'universal' and 'degenerate'?

Universal: The genetic code is shared between DNA and RNA; Degenerate: Each codon codes for multiple amino acids
Universal: The genetic code is the same for all organisms; Degenerate: Each amino acid is coded by only one codon (correct)
Universal: The genetic code is constantly evolving; Degenerate: Each amino acid is coded by only one codon
Universal: The genetic code is unique to each organism; Degenerate: Each amino acid is coded by multiple codons

During mRNA synthesis, where does the energy come from to create bonds between nucleotides?

From the cleavage of ATP to AMP and pyrophosphate (D) Signup and view all the answers

What is the importance of the 2’, 3’, and 5’ carbons of RNA (and/or DNA)?

They are involved in forming the sugar-phosphate backbone of the nucleic acid. (D) Signup and view all the answers

Given that there are 20 amino acids, explain why the genetic code cannot consist of two nucleotides per codon, and why it would be overkill to use four nucleotides per codon.

Two nucleotides per codon would not provide enough combinations to code for 20 amino acids, while four nucleotides per codon would result in redundancy and inefficiency. (A) Signup and view all the answers

What is the role of tRNA? How are tRNAs 'charged'?

tRNA carries specific amino acids to the ribosome during translation; they are 'charged' through the addition of a phosphate group. (B) Signup and view all the answers

What do we mean when we say the genetic code is 'universal'?

The same genetic code is used by all living organisms on Earth. (A) Signup and view all the answers

What is the difference between DNA, gene, and chromosome?

A gene is a specific sequence of DNA that contains the information for making a particular protein, while a chromosome is a structure composed of DNA and proteins that carries many genes. (D) Signup and view all the answers

Why is it important for the initiation of transcription to be tightly controlled in both prokaryotes and eukaryotes?

Tightly controlled initiation ensures that the correct genes are expressed at the right time and in the right amounts. (B) Signup and view all the answers

Where does transcription occur in eukaryotic cells?

In the nucleus (A) Signup and view all the answers

What is the role of the 2’, 3’, and 5’ carbons of RNA (and/or DNA)?

They are involved in forming the backbone structure of the RNA or DNA molecule. (C) Signup and view all the answers

What is the role of the nontemplate strand in transcription?

It acts as the coding strand for protein synthesis. (A) Signup and view all the answers

What is the role of the sigma factor in prokaryotic transcription initiation?

It helps to unwind the DNA double helix at the initiation site. (D) Signup and view all the answers

Why is it overkill to use four nucleotides per codon in the genetic code?

It would make the genetic code too complex and error-prone. (D) Signup and view all the answers

What do we mean when we say the genetic code is 'degenerate'?

It means that the genetic code has redundancy, with some amino acids being specified by more than one codon. (A) Signup and view all the answers

How are tRNAs 'charged'?

They are activated by attaching a specific amino acid to their 3’ end. (D) Signup and view all the answers

What is the difference between a gene and a codon?

A gene is a sequence of nucleotides that encodes a specific protein, while a codon is a three-nucleotide sequence that codes for a specific amino acid. (C) Signup and view all the answers

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Study Notes

Prokaryotic Transcription Initiation

Sigma factor is a protein that binds to RNA polymerase, facilitating the recognition of promoter regions on DNA during transcription initiation.
It aids in the formation of a transcription initiation complex, ensuring the correct start site for RNA synthesis.

Eukaryotic Cellular Processes

Transcription occurs in the nucleus, while mRNA processing takes place in the nucleus as well before it is exported to the cytoplasm.
Translation occurs in the cytoplasm on ribosomes.
Prokaryotes conduct transcription and translation in the cytoplasm simultaneously, lacking a nucleus.

Genetic Code Characteristics

The genetic code is described as 'universal' because it is nearly the same across all living organisms, indicating a common evolutionary ancestor.
It is 'degenerate' due to the redundancy in the genetic code; multiple codons can specify the same amino acid.

Energy Source in mRNA Synthesis

Energy for forming bonds between nucleotides during mRNA synthesis comes from the hydrolysis of high-energy phosphate bonds in nucleoside triphosphates (NTPs).

Importance of RNA/DNA Carbons

The 2’ and 3’ carbons in RNA are crucial for distinguishing RNA from DNA; RNA has a hydroxyl (-OH) group at 2’, while DNA has a hydrogen (-H).
The 5’ carbon is important for attaching the phosphate group, forming the backbone of nucleic acids.

Amino Acids and Codon Length

With 20 amino acids, a two-nucleotide codon system would only allow for 16 combinations, insufficient to code for all amino acids.
Using four nucleotides per codon would lead to 256 possible combinations, far exceeding the number needed, making it excessive.

Role of tRNA

tRNA transfers specific amino acids to growing polypeptide chains during translation, recognizing codons on mRNA through its anticodon.

Charging tRNAs

tRNAs are 'charged' by the enzyme aminoacyl-tRNA synthetase, which attaches the appropriate amino acid to its corresponding tRNA.

Gene vs. Codon

A gene is a segment of DNA that encodes a functional product, typically a protein, while a codon is a sequence of three nucleotides that corresponds to a specific amino acid or stop signal in translation.

Control of Transcription Initiation

Tightly controlling transcription initiation is crucial to regulate gene expression, ensuring proper cellular function and response to environmental stimuli in both prokaryotes and eukaryotes.

Non-template Strand in Transcription

The nontemplate (coding) strand of DNA has the same sequence as the RNA transcript (except for uracil replacing thymine) and serves as a reference for the RNA polymerase during transcription.