Lecture 8 - DNA Replication and Compaction Mechanics
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Questions and Answers

What was the purpose of spinning the solution in a CsCl density gradient?

  • To maintain the temperature during centrifugation
  • To enhance UV light absorption in DNA
  • To separate proteins from DNA
  • To separate DNA based on its density (correct)

What type of DNA was confirmed to be produced through semi-conservative replication?

  • Only 14N-DNA
  • A mixture of original and newly synthesized strands (correct)
  • Only hybrid DNA strains
  • Only 15N-DNA

Which factors are required for the in vitro replication of DNA as discovered by Arthur Kornberg?

  • An isolated enzyme and a template DNA
  • A template DNA, dNTPs, Mg2+, ATP, and a primer (correct)
  • Only a primer and energy source
  • Only a template DNA and deoxynucleotide triphosphates

What is the primary direction of new DNA strand synthesis mediated by polymerases?

<p>5' to 3' (D)</p> Signup and view all the answers

Which enzyme is essential for DNA replication in E. coli?

<p>Pol III (B)</p> Signup and view all the answers

What additional activities does Pol I possess apart from 5' to 3' DNA polymerizing?

<p>3' to 5' exonuclease and editing functions (D)</p> Signup and view all the answers

What was the conclusion regarding the forms of DNA visible after one generation in the experiment?

<p>All DNA was hybrid (B)</p> Signup and view all the answers

Which of the following was NOT a condition needed for DNA replication as identified by Kornberg?

<p>An anti-sense strand of DNA (D)</p> Signup and view all the answers

Who discovered Pol III, the enzyme essential for DNA replication in E. coli?

<p>Thomas Kornberg (C)</p> Signup and view all the answers

What mechanism describes how DNA replication results in each daughter DNA molecule containing one parental strand?

<p>Semi-conservative model (A)</p> Signup and view all the answers

Which process helps to compact E. coli DNA into its small cell size?

<p>Supercoiling (D)</p> Signup and view all the answers

What is the primary role of nucleoid-associated proteins in E. coli?

<p>Helping to compact DNA (D)</p> Signup and view all the answers

What type of base pairing occurs between adenine and thymine during DNA structure formation?

<p>Two hydrogen bonds (B)</p> Signup and view all the answers

What is the outcome of Meselson and Stahl's experiments on DNA replication?

<p>Support for the semi-conservative model (D)</p> Signup and view all the answers

How long is the total DNA length in a human somatic cell?

<p>~2.2 m (A)</p> Signup and view all the answers

Which of the following statements about DNA replication error rates is true?

<p>Error rates are low due to proofreading mechanisms. (C)</p> Signup and view all the answers

What experimental technique did Meselson and Stahl use to differentiate between DNA replication models?

<p>CsCl equilibrium density gradient centrifugation (A)</p> Signup and view all the answers

What is the approximate size of the human genome in base pairs?

<p>6.4 Gbp (B)</p> Signup and view all the answers

Which model of DNA replication suggests that both parental strands remain intact?

<p>Conservative model (A)</p> Signup and view all the answers

Which process helps compact DNA in E. coli?

<p>Supercoiling (B)</p> Signup and view all the answers

What model of DNA replication did Watson and Crick propose?

<p>Semi-conservative model (C)</p> Signup and view all the answers

What technique did Meselson and Stahl use to test their DNA replication model?

<p>Caesium chloride equilibrium density gradient centrifugation (C)</p> Signup and view all the answers

What is the primary function of DNA polymerases?

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What is required for DNA polymerase to synthesize new DNA?

<p>Deoxynucleotide triphosphates and a primer (B)</p> Signup and view all the answers

Which enzyme is essential for DNA replication in E. coli?

<p>Pol III (C)</p> Signup and view all the answers

What characteristic of DNA replication was demonstrated by the Watson-Crick model?

<p>DNA replication is semi-conservative, with each new strand containing one original and one new strand. (C)</p> Signup and view all the answers

Which model of DNA replication was confirmed by Meselson and Stahl’s experiment?

<p>Semi-conservative (C)</p> Signup and view all the answers

Flashcards

E. coli Genome

A circular double-stranded DNA molecule found in bacteria like Escherichia coli, containing all the genetic information necessary for the organism's survival.

DNA Compaction in Bacteria

The process of packing DNA tightly into a small space, crucial for fitting a very long DNA molecule inside a bacterial cell.

Supercoiling in DNA

A process of twisting DNA into a more compact shape, similar to winding a phone cord, to reduce the overall size of the DNA molecule.

Nucleoid-Associated Proteins

Proteins that assist in the process of DNA compaction, specifically in bacterial cells.

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DNA Compaction in Eukaryotes

The process of packaging DNA inside the nucleus of eukaryotic cells, involving the wrapping of DNA around histone proteins to form nucleosomes, which then further compact into chromatin fibers.

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DNA Replication

The process of copying the DNA molecule before cell division, ensuring that each daughter cell receives a complete copy of the genetic information.

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Watson-Crick Model

The discovery by Watson and Crick that the specific base pairing pattern in DNA (A with T, G with C) suggests a mechanism for replicating the genetic information.

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Semi-Conservative Replication

The principle that each daughter DNA molecule resulting from replication contains one original parental strand and one newly synthesized strand.

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CsCl Equilibrium Density Gradient Centrifugation

A technique used to separate molecules based on their density, by spinning them in a solution of cesium chloride.

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Meselson-Stahl Experiment

The experiment conducted by Meselson and Stahl in 1958, which used CsCl gradient centrifugation to confirm the semi-conservative model of DNA replication.

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CsCl Density Gradient Centrifugation

A technique that uses a density gradient of CsCl to separate DNA molecules based on their density. Heavy DNA settles lower in the gradient, while lighter DNA settles higher.

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Hybrid DNA

A type of DNA molecule containing both a heavy isotope (e.g., 15N) and a light isotope (e.g., 14N) of nitrogen. This occurs after one round of replication in Meselson and Stahl's experiment.

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Semi-Conservative DNA Replication

The process by which DNA is duplicated, resulting in two identical DNA molecules. Each new DNA molecule consists of one original strand (parent strand) and one newly synthesized strand (daughter strand).

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DNA Polymerase

The enzyme responsible for synthesizing new DNA strands. It adds nucleotides one by one to the existing strand, following the template sequence.

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Primer

A short piece of DNA with a free 3' hydroxyl (OH) group, needed by DNA polymerase to initiate the DNA synthesis process. It provides a starting point for the polymerase to add nucleotides.

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DNA Polymerase I (Pol I)

A type of DNA polymerase that has three different activities: 5' to 3' polymerase activity, 3' to 5' exonuclease activity, and 5' to 3' exonuclease activity.

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DNA Polymerase III (Pol III)

A type of DNA polymerase responsible for the majority of DNA replication in E. coli. It is a more efficient enzyme than Pol I, and it is essential for rapid and accurate replication.

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Proofreading

The process by which a DNA polymerase removes incorrect nucleotides from a newly synthesized DNA strand. This ensures the accuracy of DNA replication.

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Deoxyribonucleotide Triphosphates (dNTPs)

The chemical building blocks of DNA, consisting of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (adenine, thymine, cytosine, guanine).

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Magnesium (Mg2+)

A co-factor required by many enzymes, including DNA polymerase, for proper function. It helps to stabilize the enzyme and facilitates the transfer of phosphate groups during DNA synthesis.

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Study Notes

DNA Replication

  • DNA is organised in a circular, double-stranded structure
  • Bacterial DNA (e.g., Escherichia coli) has a size of 4639 kb
  • Base pairs are spaced 0.34 nm apart
  • Total DNA length in E. coli is 1.6 mm
  • Bacterial cell size is 1-2 μm
  • DNA compaction mechanisms include supercoiling and nucleoid-associated proteins
  • Supercoiling reduces size by twisting the DNA
  • Nucleoid-associated proteins help compact DNA within the bacterial cell
  • Human DNA has a genome size of 6.4 Gbp and a total length of ~2.2 m per cell.

Compaction Mechanics

  • Nucleus size is approximately 6 μm in diameter.
  • DNA compaction occurs through nucleosomes and chromatin fibres.
  • Eukaryotic DNA is packed more densely than E. coli DNA.
  • DNA length in humans is sufficient to stretch to the Sun and back approximately 730 times.

DNA Replication Mechanisms

  • DNA must decompact and be copied during cell division
  • Error rate during replication is very low
  • Watson-Crick model: DNA strands can act as templates to build complementary strands, resulting in identical daughter molecules
  • Watson-Crick model proposes that base pairing (A-T, G-C) holds the strands together.
  • Each strand is complementary to the other, therefore, each can act as a template for DNA replication.

Testing Models of DNA Replication

  • Meselson and Stahl (1958) used caesium chloride (CsCl) equilibrium density gradient centrifugation to test DNA replication models
  • The technique separates molecules based on density.
  • Using isotopes, they tracked DNA replication to determine whether replication was conservative, semi-conservative or dispersive.
  • The results confirmed semi-conservative replication, meaning each new DNA molecule consisted of one original strand and one newly synthesized strand.

DNA Polymerases

  • DNA polymerase enzymes are responsible for DNA replication
  • DNA polymerase I can synthesize DNA and has editing and error correction functions
  • It possesses 5' to 3' polymerase, 3' to 5' exonuclease, and 5' to 3' exonuclease activities.
  • DNA polymerase III is essential for DNA replication in E. coli.
  • New DNA strands are extended in a 5' to 3' direction by DNA polymerase.
  • DNA polymerase has two active sites: one for polymerization and one for editing.
  • The editing site is the 3' to 5' exonuclease active site.

DNA Replication Polarity

  • The DNA strands run anti-parallel (5' to 3' and 3' to 5')
  • DNA replication is semi-continuous (one leading and one lagging strand.)
  • New DNA is made in the 5’ → 3’ direction
  • Priming is required for replication, with the primers extended in 5’ → 3’ direction.
  • Lagging strand requires new priming and extension because DNA synthesis occurs in the opposite direction.

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Related Documents

Lecture 8 - DNA Replication PDF

Description

This quiz covers the fundamental concepts of DNA replication, including the structure and compaction mechanisms in both bacterial and eukaryotic cells. Explore the differences in DNA organization, size, and replication processes. Challenge your understanding of key terms and models in genetics.

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