Molecular Biology of the Gene

DNA and RNA are nucleic acids composed of long chains (polymers) of chemical units called

nucleotides.

Each nucleotide consists of three components:

• Phosphate group

• Five-carbon sugar (deoxyribose in DNA, ribose in RNA)

• Nitrogenous base (Adenine, Thymine, Cytosine, Guanine in DNA; Adenine, Uracil, Cytosine, Guanine in RNA)

Each strand of DNA is a

polynucleotide (a chain of nucleotides).

DNA is composed of two polynucleotide strands that form a

double helix

The strands are held together by

hydrogen bonds between complementary nitrogenous bases

These base are

• Adenine (A) pairs with Thymine (T)

• Guanine (G) pairs with Cytosine (C)

The backbone of each strand is formed by

alternating sugar and phosphate groups, while the bases project inward, forming the rungs of the helical ladder.

Nucleotides are joined by

covalent bonds between the sugar of one nucleotide and the phosphate of the next, forming a sugar-phosphate backbone.

The sequence of bases along a DNA strand encodes

genetic information.

Complementary base pairing allows

for the precise copying of genetic material during replication.

the process by which a cell copies its DNA before cell division

DNA replication

semiconservative model

• The two strands of the parental DNA molecule separate.

• Each original strand serves as a template for the synthesis of a new complementary strand.

• Each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.

Replication begins at specific locations called origins of replication, forming replication "bubbles".

Initiation

Enzymes involved

DNA polymerase: Synthesizes new DNA strands by adding nucleotides to a pre-existing chain.

DNA ligase: Joins short DNA fragments (Okazaki fragments) on the lagging strand.

DNA synthesis occurs in the 5' to 3' direction.

Directionality

synthesized continuously toward the replication fork.

leading strand

synthesized discontinuously, in short fragments, away from the replication fork.

lagging strand

The building block of nucleic acids, consisting of a phosphate group, a five-carbon sugar, and a nitrogenous base

Nucleotide

A long chain of nucleotides.

Polynucleotide

The structure of DNA, consisting of two intertwined polynucleotide strands.

Double helix

Each new DNA molecule consists of one old strand and one new strand.

Semiconservative replication

The enzyme that synthesizes new DNA strands.

DNA polymerase

The enzyme that joins DNA fragments on the lagging strand.

DNA ligase