Biochem lecture 6

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Questions and Answers

What type of sugar is found in messenger RNA (mRNA)?

Glucose

Deoxyribose

Maltose

Ribose (correct)

What separates the genes in a polycistronic transcript of bacterial mRNA?

Uracil

Ribose

Deoxyribose

Noncoding RNA (correct)

What process in eukaryotes can lead to the expression of several proteins from a single mRNA?

Translation

Alternative splicing (correct)

Replication

Transcription

Which structure is characteristic of many RNAs?

Typical right-handed stacking pattern Signup and view all the answers

What is a palindrome?

A word or phrase that reads the same backward as forward Signup and view all the answers

What type of symmetry do palindromes exhibit?

Twofold symmetry Signup and view all the answers

Which type of structure requires rotation about both horizontal and vertical axes to superimpose repeats?

Palindromes Signup and view all the answers

How does UV light primarily induce skin cancers?

By inducing dimerization of pyrimidines Signup and view all the answers

Which of the following bases is de-aminated to form inosine in RNA?

Adenosine Signup and view all the answers

How do cells primarily repair UV light-induced modifications in DNA?

Repairing some of the modifications directly Signup and view all the answers

Which base only occurs at the 'wobble position' of the anticodon in tRNA?

Inosine Signup and view all the answers

How does DNA differ from RNA chemically?

DNA has thymine while RNA has uracil Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

How does the conversion of cytosine to uracil affect DNA repair mechanisms?

It triggers a proofreading process by DNA glycosylases Signup and view all the answers

What is the role of DNA glycosylases in the repair of DNA?

To proofread and correct errors in DNA sequences Signup and view all the answers

How does depurination affect the stability of DNA?

It destabilizes the DNA at the affected position Signup and view all the answers

Which form of DNA is right-handed and is the most stable structure observed in cells?

B-DNA Signup and view all the answers

In the complementarity of DNA strands, how do the two chains run?

Antiparallel Signup and view all the answers

Which enzyme catalyzes the synthesis of a new DNA strand during replication of the genetic code?

DNA polymerase Signup and view all the answers

During DNA replication, which process occurs first?

Strand separation Signup and view all the answers

In the context of DNA and RNA structure, what biological function is associated with mutagenesis?

Creating mutations in nucleic acids Signup and view all the answers

What is a characteristic of palindromic sequences in DNA?

They read the same backward and forward Signup and view all the answers

Which statement accurately describes mirror repeats in nucleic acids?

Each strand has a symmetric sequence within it Signup and view all the answers

What does it mean that a palindrome can form hairpins and cruciforms?

It suggests secondary structures can be formed by palindromes Signup and view all the answers

What is required to superimpose one repeat on another in mirror repeats?

A 180˚ rotation about the vertical axis only Signup and view all the answers

Why is it important that palindromic sequences are capable of forming hairpins and cruciforms?

To allow secondary structures to be created Signup and view all the answers

At what temperature does DNA denaturation occur?

High temperature Signup and view all the answers

What remains intact during DNA denaturation?

Covalent bonds Signup and view all the answers

What happens to UV absorbance during DNA denaturation?

UV absorbance increases Signup and view all the answers

What process is involved in the reversal of DNA denaturation?

Annealing Signup and view all the answers

What is the basis for the polymerase chain reaction (PCR)?

DNA denaturation and annealing Signup and view all the answers

How is DNA denaturation commonly monitored?

By UV spectrophotometry at 260 nm Signup and view all the answers

What influences the melting point (Tm) of DNA during denaturation?

Size and base composition of the DNA only Signup and view all the answers

What is the predominant tautomeric form of uracil at pH 7.0?

Lactam form Signup and view all the answers

Which type of tautomeric forms are structural isomers differing in the location of protons?

Prototropic tautomers Signup and view all the answers

What type of reaction results in the loss of purines by hydrolysis of the N-β-glycosyl bond?

Depurination Signup and view all the answers

Which event occurs at a significant rate in mammalian cells: 100 C → U events per day?

Deamination Signup and view all the answers

In DNA repair mechanisms, what type of bond is hydrolyzed during depurination?

N-glycosidic bond Signup and view all the answers

Which type of tautomerism is common in ketones?

"Keto-enol tautomerism" Signup and view all the answers

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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