DNA Replication Study Guide

Key Concepts to Know

• What is DNA?

  • DNA stands for deoxyribonucleic acid.

  • It is the hereditary material found in the nucleus of cells.

  • Structure: Double helix composed of nucleotides (phosphate, sugar, nitrogen base).

• Why is DNA Replication Important?

  • Ensures that each new cell receives an exact copy of the DNA.

  • Occurs during the S phase of interphase before cell division (mitosis or meiosis).

• Where Does Replication Happen?

  • In the nucleus of eukaryotic cells.

• Steps of DNA Replication:

  1. Initiation – The enzyme helicase unwinds the DNA double helix.

  2. Elongation – The enzyme DNA polymerase adds complementary nucleotides to each original strand.

  3. Termination – Two identical DNA molecules are formed.

• Enzymes to Know:

  • Helicase: Unzips the DNA strands.

  • DNA Polymerase: Builds the new DNA strand.

  • Ligase: Joins Okazaki fragments on the lagging strand.

• Semi-Conservative Replication

  • Each new DNA molecule contains one old strand and one new strand.

Vocabulary to Review

• DNA: Deoxyribonucleic acid, the hereditary material found in the nucleus of cells, consisting of a sequence of nucleotides that encodes genetic information.

• Nucleotide: The basic building block of DNA, composed of a deoxyribose sugar, a phosphate group, and a nitrogenous base.

• Base pairing rule (A-T, C-G): The principle that in DNA, adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G) through hydrogen bonds.

• Helicase: An enzyme that unwinds the DNA double helix, separating the two strands of DNA during replication.

• DNA Polymerase: An enzyme that adds complementary nucleotides to the growing DNA strand during replication and also proofreads the newly synthesized DNA for errors.

• Ligase: An enzyme that joins Okazaki fragments on the lagging strand in DNA replication, sealing nicks in the sugar-phosphate backbone.

• Semi-conservative replication: A process of DNA replication in which each new DNA molecule consists of one original strand and one newly synthesized strand.

• Leading Strand: The strand of DNA that is synthesized continuously in the same direction as the replication fork during DNA replication.

• Lagging Strand: The strand of DNA that is synthesized discontinuously in short segments called Okazaki fragments in the opposite direction to the replication fork during DNA replication.

• Okazaki Fragments: Short segments of DNA synthesized on the lagging strand during DNA replication, which are later joined together by the enzyme ligase.

Review Questions for Practice

1. What are the three parts of a nucleotide?

2. What is the first step in DNA replication?

3. What enzyme is responsible for adding nucleotides?

4. Why is DNA replication called “semi-conservative”?

5. What is the difference between the leading and lagging strands?

Important Scientists and Contributions

• Griffith: Discovered transformation in bacteria using mice.

• Avery: Identified DNA as the molecule responsible for transformation.

• Hershey and Chase: Demonstrated that DNA (not protein) carries genetic material using radioactive markers.

Structure of DNA

• Composed of nucleotides (deoxyribose sugar, phosphate group, nitrogenous base).

• DNA structure is a double helix (two strands twisted around each other).

• Strands are antiparallel and held together by hydrogen bonds between base pairs.

Important Scientists in DNA Structure:

• Chargaff: Discovered base-pairing rules (A=T, C=G).

• Franklin: Used X-ray diffraction to suggest DNA’s helical shape.

• Watson and Crick: Created the first accurate model of DNA as a double helix.

Key Enzymes in DNA Replication

• Helicase: Unwinds DNA strands.

• DNA Polymerase: Adds nucleotides to new DNA strand and proofreads.

• Telomerase: Helps replicate DNA at the chromosome ends (telomeres).

Types of Replication

• Prokaryotes: Single origin of replication.

• Eukaryotes: Multiple replication origins.