DNA Structure & Complementary Base Pairing

Base Pairing Rules

:

DNA is made of two strands of complementary base pairs.

Adenine (A)

always pairs with

Thymine (T)

, and

Cytosine (C)

always pairs with

Guanine (G)

.

Template Function

:

If the DNA strands are separated, each strand can serve as a

template

to make a

new complementary strand

.

This allows

two identical copies

of DNA to be made from the

original DNA molecule

.

Significance in Cell Division

:

Copying DNA allows each new cell to receive DNA identical to the original cell (

mitosis

).

Overview of DNA Replication

What is DNA replication?

The biological process of duplicating a double-stranded DNA molecule to produce two identical DNA molecules.

What happens in DNA replication?

The double helix unwinds and separates, and each original strand serves as a template for synthesizing a matching complementary strand.

When and why does DNA replication occur?

When

: During the

Synthesis (S) phase

of Interphase, before cell division (mitosis) begins.

Why

: To ensure that every newly formed daughter cell receives an exact and complete copy of genetic material.

Steps of DNA Replication

Step One: Unwinding and Separating the Strands

Before DNA replication can begin, the double helix

unwinds and unzips (hydrogen bonds between base pairs are broken)

.

The complementary strands form a Y shape called a

replication fork

.

Step Two: Complementary Base Pairing

New nucleotides are added to each original strand following

complementary base-pairing rules (A pairs with T, and C pairs with G)

.

The original strand serves as a

template

for the new strand.

Step Three: Termination and Two Identical Molecules Formed

This process produces

two identical DNA molecules

.

Each double-stranded DNA molecule is made up of one

original (parental) strand

and one

new (daughter) strand

(semi-conservative model).

Replication Proteins

DNA Helicase

:

Unwinds and separates the DNA double helix during replication by breaking the hydrogen bonds holding the nitrogenous base pairs together.

DNA Polymerase

:

Adds new nucleotides to each original template strand according to complementary base-pairing rules.

Proofreading Function

: Proofreads and error-checks the new DNA strand, excising and correcting mismatched base pairs to prevent mutations.

Additionally, this proofreading activity enhances the fidelity of DNA replication, ensuring that the genetic information is accurately transmitted during cell division.