Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

8. DNA Replication

Overview of DNA Replication

Genetics

8. DNA Replication

Overview of DNA Replication

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1:25 minutes

Problem 27b

Textbook Question

DNA polymerases in all organisms add only 5' nucleotides to the 3' end of a growing DNA strand, never to the 5' end. One possible reason for this is the fact that most DNA polymerases have a proofreading function that would not be energetically possible if DNA synthesis occurred in the 3' to 5' direction.
Consider the information in your sketch and speculate as to why proofreading would be problematic.

Verified step by step guidance

Understand the directionality of DNA synthesis: DNA polymerases synthesize DNA in the 5' to 3' direction by adding nucleotides to the 3' hydroxyl (-OH) group of the growing strand. This directionality is crucial for the mechanism of DNA replication.

Recognize the role of proofreading: DNA polymerases have a proofreading function, which involves the removal of incorrectly paired nucleotides using their 3' to 5' exonuclease activity. This ensures high fidelity during DNA replication.

Consider the energy requirements for proofreading: In the 5' to 3' synthesis direction, the incoming nucleotide provides the energy for bond formation via its triphosphate group. If synthesis occurred in the 3' to 5' direction, the energy source for proofreading would be unavailable because the triphosphate group would already be part of the growing strand.

Speculate on the consequences of 3' to 5' synthesis: If DNA synthesis occurred in the 3' to 5' direction, removing an incorrect nucleotide during proofreading would leave the strand without a triphosphate group at the 3' end. This would prevent the addition of new nucleotides, halting DNA synthesis entirely.

Conclude why 5' to 3' synthesis is advantageous: The 5' to 3' direction ensures that proofreading can occur without disrupting the energy supply for continued synthesis, maintaining the efficiency and accuracy of DNA replication.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

DNA Polymerase Function

DNA polymerases are enzymes responsible for synthesizing new DNA strands by adding nucleotides to the growing chain. They can only add nucleotides to the 3' end of the strand, which means that DNA synthesis occurs in a 5' to 3' direction. This unidirectional synthesis is crucial for maintaining the integrity and accuracy of DNA replication.

Proofreading Mechanism

Proofreading is a critical function of DNA polymerases that allows them to correct errors during DNA synthesis. When an incorrect nucleotide is added, the enzyme can detect the mismatch and remove the erroneous nucleotide before continuing synthesis. This mechanism relies on the enzyme's ability to reverse its direction temporarily, which would be energetically inefficient if DNA synthesis occurred in the 3' to 5' direction.

Energetics of DNA Synthesis

The energetics of DNA synthesis refers to the energy requirements for adding nucleotides to a growing DNA strand. The process is driven by the hydrolysis of nucleoside triphosphates (NTPs), which release energy when incorporated into the DNA. If DNA polymerases were to synthesize DNA in the 3' to 5' direction, it would complicate the energy dynamics and potentially hinder the proofreading process, making it less efficient.

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Textbook Question

Why do the genomes of eukaryotes, such as Drosophila, need to have multiple origins of replication, whereas bacterial genomes, such as that of E. coli, have only a single origin?

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Textbook Question

Bloom syndrome (OMIM 210900) is an autosomal recessive disorder caused by mutation of a DNA helicase. Among the principal symptoms of the disease are chromosome instability and a propensity to develop cancer. Explain these symptoms on the basis of the helicase mutation.

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Textbook Question

How does rolling circle replication differ from bidirectional replication?

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Textbook Question

The genome of D. melanogaster consists of approximately 1.7x10⁸ base pairs. DNA synthesis occurs at a rate of 30 base pairs per second. In the early embryo, the entire genome is replicated in five minutes. How many bidirectional origins of synthesis are required to accomplish this feat?

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Textbook Question

DNA polymerases in all organisms add only 5' nucleotides to the 3' end of a growing DNA strand, never to the 5' end. One possible reason for this is the fact that most DNA polymerases have a proofreading function that would not be energetically possible if DNA synthesis occurred in the 3' to 5' direction.

Sketch the reaction that DNA polymerase would have to catalyze if DNA synthesis occurred in the 3' to 5' direction.

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Textbook Question

Assume that the sequence of bases shown below is present on one nucleotide chain of a DNA duplex and that the chain has opened up at a replication fork. Synthesis of an RNA primer occurs on this template starting at the base that is underlined.

In the intact RNA primer, which nucleotide has a free 3'-OH terminus?

3'.......GGCTACCTGGATTCA....5'

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Textbook Question

If the RNA primer consists of eight nucleotides, what is its base sequence?

3'.......GGCTACCTGGATTCA....5'

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Textbook Question

Reiji and Tuneko Okazaki conducted a now classic experiment in 1968 in which they discovered a population of short fragments synthesized during DNA replication. They introduced a short pulse of ³H-thymidine into a culture of E. coli and extracted DNA from the cells at various intervals. In analyzing the DNA after centrifugation in denaturing gradients, they noticed that as the interval between the time of ³H-thymidine introduction and the time of centrifugation increased, the proportion of short strands decreased and more labeled DNA was found in larger strands. What would account for this observation?

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