General Biology/Cells/Sexual Reproduction

Sexual

 * Exclusively eukaryotes
 * Fusion of two haploid genomes
 * Fertilization (= syngamy)
 * Forms new individuals in multicellular organisms as result of fusion of egg and sperm
 * Plants
 * Animals
 * Meiosis yields haploid genomes at some point in life cycle

Sexual life cycle
Typical animal life cycle
 * Meiosis occurs in germ line cells in gonads producing haploid gametes
 * All other cells are somatic cells
 * Alternation of generations
 * Sexual intercourse

Meiosis

 * Gives rise to genetic variation
 * Reduction division: 2n to n
 * Preceded by one round of DNA (chromosome) replication
 * Two rounds of nuclear (& usually cell) division
 * Meiosis I
 * Synapsis of homologs
 * Segregation of homologs
 * Reduction division, 2n to n
 * Meiosis II
 * No chromosome replication
 * Segregation of sister chromatids
 * Formation of 4 haploid (n) cells
 * Two nuclear divisions, usually 2 cell divisions, only one round of replication
 * Meiosis I
 * Prophase: synapsis and crossing over
 * Metaphase
 * Anaphase: chromosome segregation
 * Telophase
 * Meiosis II (mitosis-like)
 * Prophase
 * Metaphase
 * Anaphase: sister chromosome segregation
 * Telophase

Prophase I: synapsis

 * Complete alignment of replicated homologs
 * Synapsis occurs throughout the entire length of a pair of homologs
 * Key to chromosome segregation
 * Synapsis, crossing over
 * Subdivided into 5 continuous stages

Crossing over

 * Reciprocal, physical exchange between nonsister chromatids
 * Type of recombination; mixes maternal and paternal genes
 * Visual evidence: chiasmata

Microtubules and anaphase I

 * During prophase microtubules attach to kinetochores on one side of centromere
 * The metaphase checkpoint insures proper attachment
 * A phosphorylation event initiates motor activity and anaphase

Meiosis II

 * Cytologically similar to mitosis
 * No preceding DNA replication
 * Chromatids segregate and move to opposite poles as chromosomes
 * 4 haploid cells produced
 * In animals, these cells differentiate into gametes
 * In plants and many other organisms, these cells divide by mitosis, followed some time later by gamete formation

Evolution of sex

 * Asexual reproduction: all offspring genetically identical to parent
 * Sex: recombination destroys advantageous combinations
 * So why sex?
 * Many hypotheses
 * Effect repair of genetic damage?
 * Much pachytene repair as well as gene conversion
 * Some protists form diploid cells in response to stress
 * Recombination breaks up combinations of genes favoring parasites, thus reducing parasitism?

Consequences of sex

 * Recombination: generates genetic diversity
 * Crossing over
 * Independent assortment
 * Random fertilization
 * Qualities of gamete usually do NOT reflect qualities of genes enclosed in gamete