DNA: Genetic Material & Replication – Concise Exam Notes

What did Griffith's experiment in 1928 indicate?

In 1928, Griffith's experiment indicated a heritable 'transforming principle' in Streptococcus.

What did the Avery, MacLeod & McCarty experiment in 1944 identify as the 'transforming principle'?

In 1944, Avery, MacLeod & McCarty biochemically identified DNA as the 'transforming principle', as protein/RNA removal had no effect, while DNase abolished its activity.

What did the Hershey–Chase experiment in 1952 prove regarding viral genetic material?

In 1952, Hershey–Chase showed that only phage DNA, not protein, enters bacteria, proving DNA is the viral genetic material.

What are the fundamental components of DNA?

The components of DNA are 5-carbon deoxyribose sugar, a phosphate group, and nitrogenous bases (purines A/G; pyrimidines T/C).

What do Chargaff's rules state about base pairing in DNA?

Chargaff's rules state that in DNA, the amount of adenine (A) equals thymine (T), and guanine (G) equals cytosine (C), meaning A=T and G=C. The \frac{G{+}C}{A{+}T} ratio differs among species.

Describe the Watson–Crick model of DNA structure.

The Watson-Crick double helix model describes DNA as two antiparallel strands (5′→3′ / 3′→5′) with a phosphodiester backbone on the outside and bases paired inside. Adenine pairs with thymine via 2 hydrogen bonds, and guanine pairs with cytosine via 3 hydrogen bonds.

What did the Meselson–Stahl experiment in 1958 prove about DNA replication?

In 1958, Meselson–Stahl proved that DNA replication is semiconservative, meaning each new DNA molecule consists of one original strand and one newly synthesized strand.

What is the function of Helicase in DNA replication?

Helicase unwinds the DNA double helix during replication.

What is the role of Primase in DNA replication?

Primase synthesizes short RNA primers required for DNA polymerase to start synthesis.

What is the main function of Prokaryotic DNA Pol III?

DNA Polymerase III (Pol III) is the main prokaryotic DNA polymerase involved in DNA synthesis, extending primers in the 5′→3′ direction.

What are the main eukaryotic DNA polymerases and their roles?

DNA Polymerase ε (Pol ε) and DNA Polymerase δ (Pol δ) are the main eukaryotic DNA polymerases, with Pol ε synthesizing the leading strand and Pol δ synthesizing the lagging strand.

What is the function of the Sliding Clamp in DNA replication?

The sliding clamp (β in prokaryotes, PCNA in eukaryotes) increases the processivity of DNA polymerases, loaded by a clamp loader.

What is the role of Topoisomerase (DNA gyrase)?

Topoisomerase (also known as DNA gyrase) relieves torsional strain that builds up ahead of the replication fork as DNA unwinds.

What is the function of SSB proteins?

Single-Strand Binding (SSB) proteins stabilize the separated single DNA strands, preventing them from reannealing (reforming a double helix) or being degraded.

What are the functions of Prokaryotic DNA Pol I?

DNA Polymerase I (Pol I) in prokaryotes replaces RNA primers with DNA and also performs 5′→3′ and 3′→5′ exonuclease activity for proofreading and repair.

What is the role of DNA Ligase?

DNA ligase seals nicks in the DNA backbone, connecting Okazaki fragments on the lagging strand or repairing breaks.

How is the leading strand synthesized during DNA replication?

The leading strand is synthesized continuously in the 5′→3′ direction, moving towards the replication fork.

How is the lagging strand synthesized during DNA replication?

The lagging strand is synthesized discontinuously in short segments called Okazaki fragments, each requiring its own RNA primer, synthesized in the 5′→3′ direction away from the replication fork.

What are Okazaki Fragments?

Okazaki fragments are short, newly synthesized DNA fragments that are formed on the lagging template strand during DNA replication.

What constitutes the Prokaryotic Replisome?

The replisome in prokaryotes is a complex machine consisting of a primosome (helicase + primase + helpers) and two DNA Pol III complexes, responsible for DNA replication.

What are Telomeres and their primary function?

Telomeres are short, tandem repeat sequences at the ends of eukaryotic chromosomes that protect against nuclease degradation and loss of genetic information during replication.

Explain the End-Replication Problem.

The end-replication problem refers to the inability of DNA polymerase to fully replicate the very end of linear chromosomes due to the removal of the final RNA primer on the lagging strand, leading to a 3′ overhang and gradual shortening.

What is Telomerase and what is its function?

Telomerase is a ribonucleoprotein enzyme that extends the 3′ end of telomeres using an internal RNA template, thereby counteracting telomere shortening.

Describe the Photorepair mechanism.

Photorepair is a specific DNA repair mechanism where photolyase, activated by visible light, directly splits UV-induced thymine dimers.

What is Mismatch Repair?

Mismatch repair is a specific DNA repair mechanism that excises mispaired bases that were incorrectly incorporated during DNA replication.

Explain Excision Repair (NER/BER).

Excision repair (Nucleotide Excision Repair/Base Excision Repair) is a nonspecific DNA repair mechanism that recognizes DNA damage, removes a short single-stranded DNA segment containing the damage, and then fills the gap with DNA polymerase and seals it with ligase.