DNA Structure and Replication Study Guide


Every cell in our body contains DNA or deoxyribonucleic acid. It is the carrier of genetic instructions that makes every species unique and drives the development and functioning of an organism.

DNA Structure

  • The structure of DNA is a double helix, and it looks like a twisted ladder.

  • A pair of nitrogenous bases make up the steps of the DNA ladder, while the sides of the ladder are made of deoxyribose and phosphate molecules.

  • Each strand of DNA is referred to as a polynucleotide chain.

  • Four different types of nitrogenous bases adenine (A), thymine (T), cytosine (C), and guanine (G) are found in DNA, and these have a specific pairing pattern.

  • Hydrogen bonds and the stacking of the base pairs stabilize the helical structure.

Double Helix

  • Two straight strands of double-stranded DNA that run counter to one another, or anti-parallel, and twist together are known as the double helix.

  • Each DNA strand in the double helix is a long, linear molecule consisting of nucleotides, which are smaller building blocks that link together to form a chain.

  • The sugar-phosphate backbones, also known as the chemical backbones of the double helix, are composed of phosphate and sugar molecules joined together by chemical bonds.

  • The two helical strands are joined through interactions between pairs of nucleotides, also termed base pairs. Nucleotide A pairs with T and nucleotide C pairs with G, respectively, in two different types of base pairing.

Chargaff’s Rules

The base pairing was based on the observation of Erwin Chargaff:

  1. In any double-stranded DNA, the purine and pyrimidine base pairs are in equal amounts. A + G = T + C

  2. The proportion of A is equal to T, and the proportion of G is equal to C, but the amount of A + T is not necessarily equal to G + C.

DNA Replication

During the S phase of the eukaryotic cell cycle, DNA undergoes a step wherein it makes a copy of the cell’s entire DNA. This copying is also called replication.

The replication of DNA takes place in three stages: Initiation, elongation, and termination.

1. Initiation

  • DNA replication starts with unwinding the DNA double helix into two single-stranded DNA molecules.
  • An important enzyme called DNA helicase breaks the hydrogen bonds that join the complementary nucleotide bases to each other.
  • This exposes the strands so that these are used as a template to help in replication.

2. Elongation

  • In this step, nucleotides are added to the new DNA strand. This is carried out by an enzyme called DNA polymerase.
  • This enzyme will addd nucleotides one by one, in the exact order set down by the template strand.

3. Termination

  • After the new DNA strands have been synthesized, the process of replication is terminated.
  • DNA replication is semi-conservative as the outcome of replication, is a double helix composed of both an old strand (half of the parent DNA molecule) and a new strand (synthesized during replication).


  • DNA is a nucleic acid that contains the genetic instructions or the hereditary materials from the parent.
  • Chargaff’s rules state that the amount of A is similar to the amount of T, and the amount of G is similar to the amount of C
  • DNA replication is the process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules.


1. How does the structure of DNA relate to DNA replication?

The double helix structure of the DNA opens when the hydrogen bonds break, and every DNA strand acts as a template in the synthesis of a new and complementary strand.

2. What are DNA structure and replication purposes?

DNA is a nucleic acid that contains the cells genetic instructions and replication produces two double helical DNA copies.

3. What are the 4 steps of DNA replication?

  1. Initiation
  2. Primer binding
  3. Elongation
  4. Termination

4. What DNA structure regulates replication?

The replisome is recruited to regulate DNA replication.

5. What is DNA replication called?

It is called a semi-conservative process.

6. What is the main goal of DNA replication?

The main goal of DNA replication is to produce a second and identical DNA double strand.

7. When and Where does DNA replication take place?

DNA replication occurs in the nucleus of eukaryotes and the cytoplasm of prokaryotes. It occurs in the synthesis phase.

8. What are the 7 steps of DNA replication?

  • Initiation
  • Primer Synthesis
  • Leading strand synthesis
  • Lagging strand synthesis
  • Primer removal
  • Ligation
  • Termination

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