Microbiology Ch 5

Heredity and DNA Replication: Key Concepts and Enzymes

Genetics

Study of heredity and variation.

Gene

Segment of DNA (or RNA in viruses) coding for a trait or function.

Genome

Complete set of genetic material in an organism.

Bacteria (Prokaryotes)

DNA, circular chromosome.

Eukaryotes

DNA, linear chromosomes in nucleus.

Viruses

DNA or RNA (single- or double-stranded).

Central Dogma of Genetics

Flow of information: DNA → RNA → Protein.

Prokaryotes (Chromosome Differences)

Single circular chromosome, no nucleus, plasmids common.

Eukaryotes (Chromosome Differences)

Multiple linear chromosomes, in nucleus, wrapped around histones.

Nucleotide Structure

3 parts: phosphate group, sugar, nitrogenous base.

DNA

deoxyribose sugar, bases = A, T, G, C.

RNA

ribose sugar, bases = A, U, G, C.

Base Pairing

Purines (A, G) pair with pyrimidines (T, C, U).

DNA Structure

Double helix shape.

RNA Structure & Differences from DNA

Single-stranded, ribose sugar (extra -OH group), uracil replaces thymine.

Purpose of DNA Replication

Occurs before binary fission in prokaryotes and before cell division in eukaryotes.

Helicase

Unwinds the double helix.

Primase

Lays down RNA primers.

DNA Polymerase III

Synthesizes new DNA strand (5′→3′ direction).

DNA Polymerase I

Removes RNA primers, replaces with DNA.

Ligase

Seals nicks, joins Okazaki fragments on lagging strand.

Topoisomerase/Gyrase

Relieves supercoiling ahead of replication fork.

Single-stranded binding proteins (SSBPs)

Stabilize unwound DNA.

Initiation of Replication

Starts at the origin of replication (ori site).

Directionality & Strand Synthesis

DNA strands are antiparallel → replication is not identical on both sides.

Leading strand

Synthesized continuously (toward replication fork).

Lagging strand

Synthesized discontinuously as Okazaki fragments (away from fork).

Semiconservative Replication

Each new DNA molecule has 1 original (parental) strand and 1 newly synthesized strand.