DNA Structure, Function, and Applications

Comprehensive practice questions covering the history, structure, replication, protein synthesis, and biotechnological applications of DNA based on the lecture material.

What did Frederick Griffith’s 1928 experiments with Streptococcus pneumoniae demonstrate?

That DNA is the hereditary material through a "transforming principle" where information passed from dead S-strain cells into live R-strain cells.

In the 1944 Avery, MacLeod, and McCarty experiment, which enzyme destroyed the transforming ability of S-strain extracts?

DNase (DNA-digesting enzymes).

How did Hershey and Chase confirm DNA as the genetic material using bacteriophages?

They labeled DNA with radioactive phosphorus ($^{32}P$) and protein with radioactive sulfur ($^{35}S$); only the $^{32}P$ entered the bacteria to produce new viruses.

What are the three components of a DNA nucleotide?

A 5-carbon sugar (deoxyribose), a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, or guanine).

State Chargaff's Rule regarding nitrogenous bases.

The amount of adenine equals thymine ($A = T$), and the amount of cytosine equals guanine ($C = G$), indicating an equal proportion of purines and pyrimidines.

What specific contribution did Rosalind Franklin and Maurice Wilkins make to DNA structure discovery?

They used X-ray diffraction to show DNA has a helical structure with nitrogen bases inside and a sugar-phosphate backbone outside.

Who proposed the double helix structure of DNA in 1953?

James Watson and Francis Crick.

Which enzymes control DNA supercoiling in prokaryotic cells like Escherichia coli?

Topoisomerase I and topoisomerase II.

What are nucleosomes in eukaryotic cells?

Condensed structures formed when double-stranded DNA wraps around an octamer of histone proteins.

What is the function of telomeres at the ends of eukaryotic chromosomes?

They protect chromosome ends from nucleases and maintain the integrity of linear chromosomes.

Which DNA replication model was confirmed by Meselson and Stahl (1958)?

The semi-conservative model, where each new DNA molecule consists of one parental strand and one daughter strand.

During DNA replication initiation, what is the role of helicase?

It breaks the hydrogen bonds connecting the complementary base pairs to unwind the DNA.

Define the "leading strand" in DNA replication.

The strand synthesized continuously in the $5'$ to $3'$ direction from the parent strand.

What are Okazaki fragments?

Short DNA fragments synthesized discontinuously on the lagging strand away from the replication fork.

What is the function of DNA ligase in the elongation step?

It connects Okazaki fragments to form a continuous DNA strand.

What is the estimated error rate for human DNA replication?

Approximately 1 in $1 \times 10^9$ (1 billion) nucleotide pairs.

Which enzymes are responsible for proofreading newly synthesized DNA strands?

DNA polymerase I and II.

Describe the Central Dogma of Genetics.

Genetic information flows unidirectionally and irreversibly from DNA to RNA to protein.

What is the role of RNA polymerase during transcription?

It separates DNA strands and links RNA nucleotides together to produce an RNA molecule, starting at the promoter (TATA box).

Explain the difference between introns and exons in RNA processing.

Introns are non-coding sequences that are spliced out by spliceosomes, while exons are expressed sequences that code for amino acids and are joined together.

Which amino acid is carried by the initiator tRNA during translation initiation?

Methionine.

What triggers the termination of translation?

When one of the three stop codons reaches the A site of the ribosome, causing a release factor to bind and free the polypeptide.

In forensics, what are simple tandem repeats (STRs)?

Variable regions found in satellite DNA used for DNA fingerprinting.

What is "golden rice" in the context of agricultural biotechnology?

A transgenic organism enriched with beta-carotene.

What was the purpose of the Polymerase Chain Reaction (PCR) invented in 1984?

To amplify specific DNA segments, creating billions of copies of a target sequence in a few hours.

Name three diseases that can be diagnosed using PCR.

Huntington's disease, Cystic fibrosis, and Human immunodeficiency virus (HIV).