Chapter 13: DNA Recombination I

DNA Replication Fork and Okazaki Fragments

DNA replication is a fundamental process in all living cells, ensuring genetic information is accurately passed to daughter cells. The replication fork is the region where the double-stranded DNA is unwound to allow synthesis of new strands.

• Leading Strand Synthesis: DNA polymerase synthesizes the leading strand continuously in the 5' to 3' direction, adding nucleotides to the 3' end.

• Lagging Strand Synthesis: The lagging strand is synthesized discontinuously, forming short segments called Okazaki fragments, which are later joined by DNA ligase.

• Okazaki Fragments: These are short DNA fragments synthesized on the lagging strand, necessary because DNA polymerase can only add nucleotides to the 3' end.

Example: In E. coli, Okazaki fragments are typically 1000-2000 nucleotides long, while in eukaryotes they are shorter (100-200 nucleotides).

Mendel's Principles and Chromosome Assortment

Mendel's laws describe how traits are inherited through discrete units called genes. The principle of independent assortment states that genes on different chromosomes are inherited independently.

• Independent Assortment: Traits on separate chromosomes, or far apart on the same chromosome, assort independently during meiosis.

• Linked Genes: Genes located close together on the same chromosome tend to be inherited together, violating independent assortment and altering expected Mendelian ratios (not 9:3:3:1).

• Genotype vs. Phenotype: Genotype refers to the genetic makeup (e.g., (A, c), (a, C)), while phenotype refers to observable traits (e.g., (A, C), (a, c)).

Example: If genes A and B are close together on the same chromosome, their alleles are more likely to be inherited together.

Linkage Maps and Recombination Frequencies

Linkage maps are diagrams that show the relative positions of genes on a chromosome based on recombination frequencies. Recombination occurs during meiosis and can separate linked genes.

• Recombination Frequency (RF): The percentage of recombinant offspring is used to estimate the genetic distance between genes. One map unit (centimorgan, cM) equals 1% recombination frequency.

• Parental vs. Recombinant Types: Parental types have the same allele combinations as the parents, while recombinant types have new combinations due to crossing over.

• Linkage Map vs. Physical Map: A linkage map shows genetic distances based on recombination, not actual base pair distances.

Example: In a test cross with fruit flies, if 13.2% of offspring are recombinant for genes B and C, the genes are 13.2 cM apart.

Key Equations

• Recombination Frequency:

• Map Distance:

Additional info:

• Crossing over is the physical exchange of chromosome segments between homologous chromosomes during meiosis, increasing genetic diversity.

• Genes far apart on the same chromosome may appear to assort independently due to frequent crossovers.