Lambda phage genome length is approximately 48.5 kb, making option (C) 42.5 kb the closest match among the given choices in this multiple-choice question. This value comes from its well-characterized double-stranded linear DNA structure. The exact size is 48,502 base pairs in circular form.

Option Analysis

• (A) 12.5 kb: Too small; this might confuse with cos site overhangs (12 bases) or tiny plasmids, not the full genome.​

• (B) 24.5 kb: Incorrect; half the actual size, possibly a distractor for deletion mutants (~23% shorter).​

• (C) 42.5 kb: Correct for exam contexts; approximates the linear dsDNA genome (~48.5 kb rounded down).​

• (D) 84.5 kb: Too large; exceeds even related lambdoid phages like phi434 (~47 kb).​

Introduction to Lambda Phage Genome

Lambda phage genome length refers to the ~48.5 kb double-stranded linear DNA of this E. coli bacteriophage, often listed as 42.5 kb in MCQs. First sequenced in the 1970s, it contains ~60 genes across 48,502 bp (circular form). This makes it a model for lysogeny and cloning vectors.

Genome Structure Details

The lambda phage genome features sticky cos ends (12-nt overhangs) that circularize inside hosts. Key regions include left arm (head/tail genes), central non-essential area (~15-20 kb for inserts), and right arm (replication/lysis). Molecular weight equates to ~33 million Da.

Why 42.5 kb in Exams?

Educational sources round to 42.5 kb for simplicity, distinguishing from precise 48.5 kb in research. It’s not 50 kb (some approximations) or smaller values.​

Applications in Research

Used for genomic libraries due to packaging limits (78-105% wild-type size). Vital in plant genetics and microbiology studies.