ANSC 221 (Exam 3): G2-DNA Replication

DNA (Deoxyribonucleic Acid)

A molecule composed of nucleotides that carries genetic instructions for life.

Nucleotide

The building block of DNA, consisting of a nitrogenous base, a deoxyribose sugar, and a phosphate group.

Purines

Nitrogenous bases with two rings: adenine (A) and guanine (G).

Pyrimidines

Nitrogenous bases with one ring: cytosine (C) and thymine (T).

Chargaff's Rules

In DNA, the amount of adenine equals thymine, and the amount of guanine equals cytosine.

Double Helix

The twisted-ladder shape of DNA, with two antiparallel strands held together by hydrogen bonds between base pairs.

Antiparallel Strands

DNA strands run in opposite directions: one 5' to 3', the other 3' to 5'.

Base Pairing Rules

Adenine pairs with thymine (A-T), and guanine pairs with cytosine (G-C).

Semiconservative Replication

Each new DNA molecule has one original strand and one newly synthesized strand.

Origin of Replication

Specific DNA sequence where replication begins; multiple origins exist in eukaryotes.

Replication Fork

Y-shaped region where the DNA is unwound and replication occurs.

Helicase

Enzyme that unwinds the DNA double helix at the replication fork.

Single-Strand Binding Proteins (SSBs)

Stabilize and prevent re-pairing of separated DNA strands.

Topoisomerase

Relieves tension ahead of the replication fork by cutting and rejoining DNA strands.

Primase

Synthesizes short RNA primers needed to start DNA synthesis.

DNA Polymerase III

Adds nucleotides to the 3' end of the RNA primer to synthesize new DNA.

Leading Strand

Synthesized continuously toward the replication fork.

Lagging Strand

Synthesized in short fragments (Okazaki fragments) away from the fork.

Okazaki Fragments

Short DNA segments synthesized on the lagging strand.

DNA Polymerase I

Replaces RNA primers with DNA nucleotides.

DNA Ligase

Joins Okazaki fragments into a continuous strand.

Nucleotide Excision Repair

Mechanism where damaged DNA is cut out and replaced.

Chromatin

DNA-protein complex that packages DNA in eukaryotic cells.

Histones

Proteins around which DNA winds to form nucleosomes.

Euchromatin

Loosely packed chromatin; accessible for transcription.

Heterochromatin

Densely packed chromatin; transcriptionally inactive.

What is the basic structure of DNA?

DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a pentose sugar (deoxyribose), and a phosphate group.

What are Chargaff's rules?

In any species, the number of adenine (A) and thymine (T) bases are equal, and the number of guanine (G) and cytosine (C) bases are equal.

Who discovered the double helix structure of DNA?

James Watson and Francis Crick, using Rosalind Franklin's X-ray crystallographic images.

What is the significance of the double helix structure?

It allows for uniform width by pairing a purine (A or G) with a pyrimidine (C or T), explaining Chargaff's rules.

What is the semiconservative model of DNA replication?

Each daughter DNA molecule consists of one old strand from the parental molecule and one newly synthesized strand.

What is the role of helicase in DNA replication?

Helicase unwinds the double helix at the replication forks, separating the two parental strands.

What is the function of DNA polymerase?

DNA polymerase catalyzes the synthesis of new DNA by adding nucleotides to the 3' end of a preexisting chain.

What are Okazaki fragments?

Segments of DNA synthesized on the lagging strand during DNA replication, which are later joined together by DNA ligase.

What is the difference between leading and lagging strands?

The leading strand is synthesized continuously towards the replication fork, while the lagging strand is synthesized in segments away from the fork.

What ensures the fidelity of DNA replication?

DNA polymerases proofread and replace incorrect nucleotides during synthesis, and nucleotide excision repair removes damaged nucleotides.

What is the role of topoisomerase in DNA replication?

Topoisomerase reduces molecular tension caused by unwinding ahead of the replication fork.

What is the structure of chromatin in eukaryotic cells?

Chromatin consists of linear DNA molecules combined with proteins (histones), which help in DNA packing.

What are the two types of chromatin?

Euchromatin (loosely packed, accessible for transcription) and heterochromatin (highly condensed, inaccessible for transcription).

What is the importance of DNA replication before cell division?

It ensures genetic information is accurately transmitted from parent cells to daughter cells during mitosis and meiosis.

What happens during base pairing to a template strand?

Complementary nucleotides are connected to form new strands, with A pairing with T and G pairing with C.

What is the function of single-strand binding proteins?

They bind to unpaired DNA strands to prevent them from re-annealing and stabilize single-stranded DNA.

What is the direction of DNA strand synthesis?

DNA strands are synthesized in the 5' to 3' direction.

What is the role of DNA ligase?

DNA ligase joins Okazaki fragments on the lagging strand into a continuous DNA strand.

What is the replication fork?

The Y-shaped region where DNA strands are being unwound during replication.

What is the significance of the sugar-phosphate backbone in DNA?

It provides structural stability and directionality to the DNA molecule.

What is the role of primase in DNA replication?

Primase synthesizes a short RNA primer that provides a starting point for DNA polymerase.

What is the function of the replication bubble?

Replication bubbles form at origins of replication, allowing DNA to be copied in both directions.

What is the significance of the antiparallel structure of DNA?

The antiparallel orientation affects how the leading and lagging strands are synthesized during replication.