Introduction
- Recombination:
- During meiosis, DNA is cut and fragments are joined differently.
- This leads to entirely new sets of genetic information.
- Recombination can be directed by the cell or occur as a mutation.
- Two types: general and site-specific recombination.
- Prokaryotes:
- Use transformation, conjugation, and transduction.
- Eukaryotes:
- Undergo sexual recombination.
- Three Ways for Recombination in Bacteria:
- Conjugation: DNA transferred between cells, often plasmid-mediated, requires cell contact.
- Transformation: Cell takes up DNA from the environment.
- Transduction: New DNA inserted by phages.
- General Recombination Principles:
- Affects large genome regions.
- Guided by base-pairing interactions between long sequences; some parts must be homologous.
- Participating sequences are usually on two copies of the same chromosome.
- The xchange site is not strictly defined.
- At the exchange site, one DNA strand base pairs with the opposite DNA strand, forming a heteroduplex joint linking two double helices.
- Heteroduplex can be thousands of base pairs long.
- Crossing over is homologous recombination.
Crossing Over vs. Gene Conversion
- Crossing Over
- Originates from the repair of double-strand breaks in meiotic chromosomes.
- Only a subset of these breaks result in crossing over, with most being repaired by alternative pathways that produce non-crossover DNA.
- Gene Conversion
- Occurs when a missing DNA segment is replaced by a segment copied from a homologous chromosome.
- Essentially, one gene is substituted with the same gene from the other homolog, a process known as gene conversion
- Proteins Involved in General Recombination:
- Endonucleases, which also function as helicases, act at specific sites.
- Proteins supporting single-strand DNA.
- Ligases.
- Proteins provide an energy supply.
- Proteins facilitate synapsis between chromosomes.
Mismatch Repair
- Process
- Crossover DNA strands originate from non-sister chromatids, forming heteroduplex DNA with non-complementary sequences.
- Mismatches are corrected through specific repair mechanisms or during DNA replication.
- Impact
- this leads to gene conversion, resulting in chromosomes that carry identical alleles.
- If allele selection is biased, the frequency of one allele may decrease.
- Functions of General Recombination:
- Generates chromosomes with new allele combinations in gametes.
- Facilitates the repair of damaged DNA after replication, known as recombinative repair.
- Side Effects:
- Uneven crossing over can result in the multiplication of DNA sequences along chromosomes.
- Bloom Syndrome:
- Caused by frequent crossing over due to a recessive mutation in the helicase gene, affecting replication.
- Intensive activation of repair systems in response to DNA abnormalities.
- Crossing-Over in Meiosis (Prophase I):
- Occurs during Prophase I, where homologous chromosomes pair up.
- Crossing-over happens between non-sister chromatids facilitated by the Synaptonemal complex.