40 Questions
What type of sugar is found in messenger RNA (mRNA)?
Ribose
What separates the genes in a polycistronic transcript of bacterial mRNA?
Noncoding RNA
What process in eukaryotes can lead to the expression of several proteins from a single mRNA?
Alternative splicing
Which structure is characteristic of many RNAs?
Typical right-handed stacking pattern
What is a palindrome?
A word or phrase that reads the same backward as forward
What type of symmetry do palindromes exhibit?
Twofold symmetry
Which type of structure requires rotation about both horizontal and vertical axes to superimpose repeats?
Palindromes
How does UV light primarily induce skin cancers?
By inducing dimerization of pyrimidines
Which of the following bases is de-aminated to form inosine in RNA?
Adenosine
How do cells primarily repair UV light-induced modifications in DNA?
Repairing some of the modifications directly
Which base only occurs at the 'wobble position' of the anticodon in tRNA?
Inosine
How does DNA differ from RNA chemically?
DNA has thymine while RNA has uracil
What happens when cytosine deaminates to form uracil in DNA?
It leads to a mutation from a G-C pair to a U-A pair
Why is it selectively advantageous for genetic information to be stored in DNA rather than RNA?
DNA is more resistant to nucleophilic cleavage due to absence of a 2’-OH group
How does the conversion of cytosine to uracil affect DNA repair mechanisms?
It triggers a proofreading process by DNA glycosylases
What is the role of DNA glycosylases in the repair of DNA?
To proofread and correct errors in DNA sequences
How does depurination affect the stability of DNA?
It destabilizes the DNA at the affected position
Which form of DNA is right-handed and is the most stable structure observed in cells?
B-DNA
In the complementarity of DNA strands, how do the two chains run?
Antiparallel
Which enzyme catalyzes the synthesis of a new DNA strand during replication of the genetic code?
DNA polymerase
During DNA replication, which process occurs first?
Strand separation
In the context of DNA and RNA structure, what biological function is associated with mutagenesis?
Creating mutations in nucleic acids
What is a characteristic of palindromic sequences in DNA?
They read the same backward and forward
Which statement accurately describes mirror repeats in nucleic acids?
Each strand has a symmetric sequence within it
What does it mean that a palindrome can form hairpins and cruciforms?
It suggests secondary structures can be formed by palindromes
What is required to superimpose one repeat on another in mirror repeats?
A 180˚ rotation about the vertical axis only
Why is it important that palindromic sequences are capable of forming hairpins and cruciforms?
To allow secondary structures to be created
At what temperature does DNA denaturation occur?
High temperature
What remains intact during DNA denaturation?
Covalent bonds
What happens to UV absorbance during DNA denaturation?
UV absorbance increases
What process is involved in the reversal of DNA denaturation?
Annealing
What is the basis for the polymerase chain reaction (PCR)?
DNA denaturation and annealing
How is DNA denaturation commonly monitored?
By UV spectrophotometry at 260 nm
What influences the melting point (Tm) of DNA during denaturation?
Size and base composition of the DNA only
What is the predominant tautomeric form of uracil at pH 7.0?
Lactam form
Which type of tautomeric forms are structural isomers differing in the location of protons?
Prototropic tautomers
What type of reaction results in the loss of purines by hydrolysis of the N-β-glycosyl bond?
Depurination
Which event occurs at a significant rate in mammalian cells: 100 C → U events per day?
Deamination
In DNA repair mechanisms, what type of bond is hydrolyzed during depurination?
N-glycosidic bond
Which type of tautomerism is common in ketones?
"Keto-enol tautomerism"
Study Notes
DNA and RNA Structure
- DNA contains 2-deoxyribose, whereas RNA contains ribose, making DNA more resistant to alkaline hydrolysis.
- DNA contains thymine (T) instead of uracil (U), which is important for preventing heritable mutations of C-G pairs to U-A pairs.
Palindromic Sequences
- Palindromic sequences can be found in DNA and can form hairpins and cruciforms through intrastrand base pairing.
- Palindromes are sequences of double-stranded nucleic acids with twofold symmetry.
Messenger RNA (mRNA)
- Is synthesized using a DNA template.
- Contains ribose instead of deoxyribose.
- Contains uracil instead of thymine.
- One mRNA can code for more than one protein.
- Bacterial mRNA has monocistronic and polycistronic forms, with noncoding RNA separating the genes.
RNA Structure
- RNA molecules have complex structures, including hairpins and cruciforms.
- Typical right-handed stacking pattern of single-stranded RNA.
- The paired regions have an A-form right-handed helix.
Denaturation and Annealing of DNA
- Denaturation can be induced by high temperature, change in pH, or chemical agents.
- Covalent bonds remain intact, but hydrogen bonds are broken, and base stacking is lost.
- Denaturation may be reversible: annealing.
- Reversible denaturation and annealing (renaturation) of DNA forms the basis for the polymerase chain reaction.
Minor Nucleosides in RNA
- Inosine is sometimes found in the "wobble position" of the anticodon in tRNA.
- Pseudouridine is found widely in tRNA and rRNA and may stabilize the structure of tRNA and help in folding of rRNA.
Radiation-Induced Mutagenesis in DNA
- UV light induces dimerization of pyrimidines, which may be the main mechanism for skin cancers.
- Ionizing radiation causes ring opening and strand breaking.
- Cells can repair some of these modifications, but others cause mutations.
Replication of Genetic Code
- Strand separation occurs first, and each strand serves as a template for the synthesis of a new strand.
- Synthesis is catalyzed by enzymes known as DNA polymerases.
- Newly made DNA molecule has one daughter strand and one parent strand.
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