DNA Structure, Replication, and Gene Function in Biology

DNA

Deoxyribonucleic acid; the genetic material that stores information in living organisms.

Nucleotide

The building block of DNA; made of a phosphate, deoxyribose sugar, and nitrogenous base.

Nitrogenous base

The A, T, G, or C part of a nucleotide that distinguishes one nucleotide from another.

DNA Function

DNA provides instructions for making proteins, allows cellular replication, and dictates development and function.

Central Dogma

DNA → RNA → protein.

Transcription

The process of copying DNA information into mRNA.

Translation

The process in which mRNA is used to make a polypeptide chain that folds into a protein.

Gene

A discrete unit of hereditary information consisting of a specific nucleotide sequence.

Protein-coding gene

A gene that contains instructions for making a protein.

Regulatory gene

A gene that produces proteins that control the expression of other genes.

Insulin

A protein that helps control blood sugar; INS is an example of a protein-coding gene.

DNA Replication

The process of making an identical copy of DNA before cell division.

Parent/Parental Strand

The original DNA strand that serves as a template during replication.

Daughter Strand

The newly synthesized DNA strand made during replication.

Semiconservative Replication

Replication in which each new DNA molecule contains one original parental strand and one new daughter strand.

Origin of Replication (ORI)

A location where DNA replication begins.

Replication Bubble

The area where DNA has unzipped at an origin of replication.

Replication Fork

The Y-shaped area where DNA is being unzipped and copied.

Hydrogen Bonds

Bonds between nitrogenous bases that hold the two DNA strands together.

Covalent Bonds

Strong bonds that connect the sugars and phosphates in the DNA backbone.

Topoisomerase

Relieves twisting tension and prevents DNA from becoming supercoiled.

Helicase

Unzips the DNA double helix by breaking hydrogen bonds between nitrogenous base pairs.

Single-Strand Binding Proteins (SSBs)

Bind to separated DNA strands to keep them open and stable and prevent them from re-pairing.

Primase

Adds a short RNA primer to give DNA polymerase a starting point.

RNA Primer

A short RNA sequence that provides DNA polymerase with a starting point for DNA synthesis.

DNA Polymerase

Adds new nucleotides to build the new DNA strand.

Phosphodiester Bond

A covalent bond that connects nucleotides in the sugar-phosphate backbone.

Dehydration Reaction

A reaction that forms a bond while producing H₂O as a product.

3′ OH

The hydroxyl group attached to the third carbon of the DNA sugar; DNA polymerase adds new nucleotides to this end.

5′ Phosphate

The phosphate group attached to the fifth carbon of the DNA sugar.

DNA Replication Direction

The parent/template strand is read 3′ → 5′, while the new strand is written 5′ → 3′.

Leading Strand

The DNA strand synthesized continuously in the 5′ → 3′ direction toward the replication fork.

Lagging Strand

The DNA strand synthesized discontinuously in the 5′ → 3′ direction away from the replication fork.

Okazaki Fragments

Short, newly synthesized DNA segments formed discontinuously on the lagging strand during DNA replication.

Ligase

Seals the gaps between Okazaki fragments on the lagging strand, forming a continuous strand.

Why are multiple ORIs used?

Multiple origins allow DNA to be copied at the same time, decreasing the time needed for replication.

Replication Fork Direction

At each ORI, two replication forks move in opposite directions.

Replication Result

Two identical copies of the original DNA are formed.

Enzyme

A macromolecule, usually a protein, that acts as a biological catalyst and speeds up a chemical reaction without being consumed.

Replication Steps

Topoisomerase → Helicase → SSBs → Primase → DNA Polymerase → Ligase.

Order of DNA Replication

1. Topoisomerase → 2. Helicase → 3. SSBs → 4. Primase → 5. DNA Polymerase → 6. Ligase.

Step 1: Topoisomerase

Relieves twisting tension and prevents supercoiling.

Step 2: Helicase

Unzips the DNA double helix by breaking hydrogen bonds.

Step 3: SSBs

Keep the separated DNA strands open and stable.

Step 4: Primase

Adds a short RNA primer to provide a starting point.

Step 5: DNA Polymerase

Adds complementary nucleotides to build the new DNA strand 5′ → 3′.

Step 6: Ligase

Seals the gaps between Okazaki fragments on the lagging strand.