D1.1

What is the purpose of DNA replication?

To ensure each new cell receives a complete and identical copy of genetic material before cell division.

Where does DNA replication occur?

In the nucleus.

What does semi-conservative replication mean?

Each new DNA molecule contains one original (parent) strand and one newly synthesized strand.

What is the template strand?

The original DNA strand used as a pattern for building the new complementary strand.

What ensures accuracy in base pairing during replication?

Complementary base pairing between adenine-thymine and cytosine-guanine.

Why is DNA replication important in multicellular organisms?

It allows for growth, repair, and replacement of old or damaged cells.

Who proposed the semi-conservative model of DNA replication?

Watson and Crick.

Who experimentally proved the semi-conservative model?

Meselson and Stahl.

Why is it important that one original strand is conserved in replication?

It maintains genetic continuity between generations of cells.

What enzyme unwinds and separates DNA strands?

Helicase.

How does helicase separate DNA strands?

It breaks hydrogen bonds between complementary bases, "unzipping" the DNA double helix.

What enzyme synthesizes new DNA strands?

DNA polymerase.

In which direction does DNA polymerase synthesize new DNA?

In the 5' to 3' direction.

In which direction does DNA polymerase read the template strand?

In the 3' to 5' direction.

What type of reaction joins nucleotides during replication?

A condensation reaction forming phosphodiester bonds between nucleotides.

What bonds link the sugar of one nucleotide to the phosphate of the next?

Phosphodiester bonds.

What bonds form between complementary bases?

Hydrogen bonds.

Why is DNA replication called "semi-conservative"?

Because half of the original DNA molecule (one strand) is conserved in each daughter molecule.

What ensures genetic continuity during replication?

The conservation of one template strand in each new DNA molecule.

What does DNA polymerase require to begin replication?

A free 3' hydroxyl group (-OH) on a primer.

What directionality rule applies to DNA replication?

New nucleotides are always added to the 3' end of the growing strand.

Why can't DNA polymerase start replication by itself?

It requires a short RNA primer to provide a starting point.

What enzyme synthesizes RNA primers?

DNA primase.

What enzyme removes RNA primers and replaces them with DNA?

DNA polymerase I.

What enzyme joins Okazaki fragments on the lagging strand?

DNA ligase.

What is the function of DNA ligase?

It seals nicks between fragments by catalyzing the formation of sugar-phosphate bonds.

What are Okazaki fragments?

Short DNA fragments synthesized on the lagging strand during discontinuous replication.

Why is DNA replication different on the two strands?

Because the two strands are antiparallel, so DNA polymerase moves in opposite directions on each.

What is the leading strand?

The strand synthesized continuously toward the replication fork.

What is the lagging strand?

The strand synthesized discontinuously away from the replication fork.

Why is the lagging strand discontinuous?

Because DNA polymerase can only synthesize in the 5' to 3' direction, opposite to fork movement.

What enzyme proofreads the new DNA strand?

DNA polymerase III.

What happens during proofreading by DNA polymerase III?

It detects and removes incorrectly paired nucleotides, replacing them with the correct ones.

What are mutations?

Errors in DNA replication that alter the nucleotide sequence.

Why are mutations significant?

They can be harmful, beneficial, or neutral, but some cause diseases such as cancer.

What keeps DNA strands apart during replication?

Single-stranded binding proteins.

What is the function of single-stranded binding proteins?

They stabilize separated DNA strands to prevent reannealing.

What direction does replication always proceed in?

From 5' to 3' on the new strand.

What is the role of RNA primers on the lagging strand?

They provide multiple starting points for DNA polymerase III to synthesize Okazaki fragments.

What happens to RNA primers after replication?

They are replaced with DNA by DNA polymerase I.

How does DNA replication prepare for mitosis?

It doubles the DNA content so each daughter cell receives a complete set of chromosomes.

What technique separates DNA fragments by size and charge?

Gel electrophoresis.

Why does DNA move toward the positive electrode during electrophoresis?

DNA is negatively charged due to its phosphate groups.

What gel is commonly used for DNA electrophoresis?

Agarose gel.

What determines how far DNA fragments move in electrophoresis?

Their size—smaller fragments move farther and faster through the gel.

How are DNA fragments visualized after electrophoresis?

By using radioactive probes or fluorescent dyes like ethidium bromide.

What is PCR?

Polymerase Chain Reaction—a method for amplifying DNA in vitro.

What enzyme is used in PCR and why?

Taq polymerase, because it is heat-stable from Thermus aquaticus.

What are the three stages of PCR?

Denaturation (95°C), annealing (50-60°C), and extension (72°C).

What happens during denaturation in PCR?

DNA strands separate as hydrogen bonds break at 95°C.

What happens during annealing in PCR?

Primers bind to complementary sequences at 50-60°C.

What happens during extension in PCR?

Taq polymerase builds new DNA strands at 72°C.

What equipment performs PCR cycles automatically?

A thermal cycler.

How does PCR achieve exponential amplification?

Each cycle doubles the amount of DNA, producing billions of copies after ~30 cycles.

What is DNA profiling?

The process of identifying individuals based on unique DNA patterns.

What are VNTRs?

Variable Number Tandem Repeats—short noncoding DNA sequences unique to individuals.

What steps are used in DNA profiling?

Extraction, PCR amplification, restriction digestion, electrophoresis, and visualization.

How is DNA profiling used in paternity testing?

A child's DNA bands must appear in either the mother's or the father's DNA profile.

How is DNA profiling used in forensics?

To match crime scene DNA with suspects or identify human remains.

What increases the reliability of DNA profiling?

Using more VNTR markers to reduce the chance of false matches.

Why can DNA profiling not be 100% certain?

Because contamination or limited samples can affect accuracy.

What is the overall importance of semi-conservative replication?

It preserves genetic information accurately across generations of cells.