Genetic Recombination in Bacteria

Genetic Recombination

• Genetic recombination is an important process that shuffles and mixes genetic material.
• It requires two similar DNA molecules that are not identical.
• Homology allows DNA molecules to line up and recombine precisely.
• Recombination is also known as crossing over.
• It involves enzymatic cutting and pasting of DNA backbones.

Genetic Recombination in Bacteria

• Genetic recombination can occur in bacteria like E. coli.
• Recombination can occur through bacterial conjugation, transformation, and transduction.
• Bacterial conjugation requires a cytoplasmic bridge connecting two bacteria.
• The donor cell sends DNA to the recipient cell, which undergoes recombination.
• Plasmids are circular, non-chromosomal transferable DNA that can carry antibiotic resistance genes.
• Transformation occurs when bacteria take up DNA from disintegrated bacteria.
• Linear fragments recombine through double crossovers.
• Transduction occurs when bacterial phages transfer DNA from one bacterium to another.
• Viral DNA incorporates DNA fragments from the host cell.
• Recombination in bacterial cells can result in partial diploidy.
• Generalized Transduction involves a phage carrying host cell DNA to another bacterium.
• Specialized Transduction occurs when a prophage is excised from the bacterial genome, carrying some of the host cell's DNA.
• Temperate bacteriophages can use both lysogenic and lytic cycles of infection.
• Virulent bacteriophages only use the lytic cycle of infection which always kills the host bacterium.

Genetic Recombination in Eukaryotes: Meiosis

• Meiosis occurs in different locations in different organismal life cycles.
• Meiosis changes both the number of chromosomes and the DNA sequence.
• Meiosis produces four genetically different daughter cells.
• Meiosis contributes to genetic diversity through recombination, random segregation, alternative combinations at anaphase II, and random fertilization.

Sexual Reproduction

• Sexual reproduction creates offspring through the union of male and female gametes (sperm and egg).
• Meiosis produces gametes with half the number of chromosomes.
• Sexual reproduction allows for increased genetic diversity, which is an evolutionary advantage.

Fertilization

• Fertilization is when the nuclei of the egg and sperm fuse, forming a zygote.
• This process restores the parental chromosome number.

Animal Life Cycles

• The diploid phase dominates animal life cycles.
• Meiosis is followed directly by gamete formation.
• The haploid phase is reduced and short, and there is no mitosis.
• Only one nucleus from meiosis becomes the egg in females.

Homologous Chromosome Pairs

• Homologous chromosome pairs consist of one paternal and one maternal chromosome.
• Each pair may have different alleles.
• Meiosis separates homologous pairs.

Meiosis I

• Meiosis I is the first meiotic division.
• Recombination exchanges segments between homologous chromosomes.
• Meiosis I produces two haploid cells with chromatids attached.

Meiosis II

• Meiosis II is the second meiotic division.
• Sister chromatids separate, forming two separate cells.
• Meiosis II produces 4 recombined haploid cells.

Meiotic Cell Cycle

• Prophase I: Sister chromatids condense, chromosomes undergo recombination. This includes synapsis (pairing of homologs) and the formation of tetrads (fully paired homologs).
• Prometaphase I: The nuclear envelope breaks down, and kinetochores attach to polar spindles.
• Metaphase I: Tetrads align on the metaphase plate, and homologs segregate (sister chromatids remain attached) to opposite poles.
• Anaphase I: Homologs segregate, moving to opposite poles.
• Telophase I: No change in chromosomes, spindle disassembles.
• Interkinesis: A short interphase occurring after Telophase I.
• Prophase II: Chromosomes condense, and spindles form.
• Prometaphase II: Nuclear envelope breaks down, and kinetochores attach to microtubules.
• Metaphase II: Chromosomes align on the metaphase plate.
• Anaphase II: Spindles separate chromatids.
• Telophase II: Spindles disassemble, new nuclear envelopes form.
• Result: 4 genetically different haploid cells.

Nondisjunction

• Happens when both members of a homologous chromosome pair connect to spindles from the same pole.
• One pole receives both copies of the pair, while the other receives none.
• Result: Gametes with 2 copies of a chromosome.
• Following fertilization, the zygote has 3 copies of the chromosome.
• This is known as Trisomy.

Sex Chromosomes in Meiosis

• Gametes produced by females receive either X chromosome.
• Gametes produced by males receive either X or Y chromosome.

Meiosis and Mitosis Compared

• Both involve similar cell divisions, but meiosis divides twice.
• Mitosis: Produces 2 identical diploid daughter cells.
• Meiosis: Produces 4 genetically different haploid daughter cells.
• Premeiotic interphases are similar to mitotic interphase.

Genetic Variability

• Four main ways genetic variability arises:
  • Genetic Recombination
  • Random segregation at anaphase I
  • Alternative combinations at anaphase II
  • Random fertilization

Genetic Recombination

• Recombination is the shuffling of alleles that occurs during prophase I.
• Two of the four chromatids in a tetrad exchange alleles.
• This process is also known as crossing over and occurs between non-sister chromatids.

Random Segregation

• Random segregation takes place during metaphase I.
• Each chromosome of a homologous pair can randomly end up at either spindle pole.
• This results in a random combination of maternal and paternal chromosomes being segregated to gametes.

Alternative Combinations at Anaphase II

• During Metaphase II, spindle attachment to sister chromatid kinetochores is random.
• This results in random alignment and even more combinations.

Random Fertilization

• Random chance of male and female gametes forming a zygote.
• This is the last step, where a sperm and egg randomly meet.

The advantages of genetic diversity

• Increased chances of survival in a changing environment.
• Reduces the risk of harmful recessive alleles becoming expressed.
• Contributes to the evolution of species.

Explore the fascinating process of genetic recombination in bacteria, particularly focusing on organisms like E. coli. This quiz covers mechanisms such as bacterial conjugation, transformation, transduction, and the role of plasmids in gene transfer. Test your understanding of how these processes contribute to genetic diversity and antibiotic resistance.