Replication origins: compared with bacteria that have a single origin, how many origins of replication are typically found across the much larger genomes of mammals and other eukaryotes?

Introduction / Context:
Genome size and chromatin organization influence how replication is initiated and completed within the S phase of the cell cycle. Eukaryotic genomes are large and linear, necessitating many initiation sites to finish replication efficiently and reliably.

Given Data / Assumptions:

• Bacteria such as E. coli have one circular chromosome with a single well-defined origin (oriC).
• Eukaryotes have multiple linear chromosomes and far larger genomes.
• S phase timing constraints require many replication origins firing in regulated patterns.

Concept / Approach:
Eukaryotic chromosomes contain numerous potential origins, only a subset of which fire in any given S phase according to replication timing programs. Mammalian genomes typically use thousands to tens of thousands of origins, ensuring complete duplication within hours. Origin licensing involves ORC, Cdc6, Cdt1, and loading of MCM helicases, which later activate to form replication forks.

Step-by-Step Solution:
Contrast a single bacterial origin with the complexity of eukaryotic chromatin.Consider the scale: billions of base pairs vs. millions in bacteria.Infer the need for numerous origins to complete replication on time.Select the option indicating thousands to tens of thousands of origins.

Verification / Alternative check:
DNA combing, nascent strand mapping, and replication timing analyses demonstrate dense origin usage across mammalian chromosomes.

Why Other Options Are Wrong:
One: true for many bacteria, not for mammals.About 100: too few for mammalian genome sizes and S phase duration.None of these: incorrect since a correct quantitative range is provided.

Common Pitfalls:
Assuming each gene needs its own origin; origins are independent of gene boundaries and are regulated by chromatin state and replication programs.

Final Answer:
Thousands and tens of thousands