Sexual Reproduction Chapter 12

Sexual Reproduction In asexual reproduction, a single individual gives rise to offspring that are identical to itself and others [MITOSIS] Bacteria reproduce asexually Most plants and some simple animals can reproduce both asexually and sexually In sexual reproduction, two individuals mix their genetic material Creates a diversity of traits which offers a better chance of surviving environmental change

Sexual Reproduction Somatic (body) cells of sexually reproducing eukaryotes have pairs of homologous chromosomes Carry genes of the same characteristics (one from dad, one from mom) Humans have 23 pairs of homologous chromosomes 22 autosomal pairs, 1 sex pair

Sexual Reproduction Gametes are haploid, autosomal cells are diploid Sexual reproduction mixes up traits from two parents through the fusion of gametes Gamete: mature reproductive [sex] cells – sperm and egg Gametes are haploid, autosomal cells are diploid n = number of chromosomes Haploid: n (cell has 1 copy of every chromosome) Diploid: 2n (cell has 2 of every chromosome, female n + male n)

Meiosis Meiosis: a nuclear division mechanism that occurs in reproductive cells of eukaryotes DNA replication occurs prior to meiosis: The nucleus is diploid (2n) with two sets of chromosomes, one from each parent During meiosis, chromosomes of a diploid nucleus become distributed into four haploid nuclei

Meiosis Meiosis halves the diploid (2n) chromosome number to the haploid (n) chromosome number for forthcoming gametes, ensuring the offspring have the same number of chromosomes as the parents Diploid chromosome number is restored at fertilization

Fertilization Fertilization: when one haploid gamete (egg) combines with another (sperm) In humans, n = 23 and 2n = 46 This number is different for every species! When two haploid gametes fuse, a zygote is formed The first cell of a new individual

Fertilization If meiosis didn’t happen before fertilization, the chromosome number would double with every generation Chromosome number changes can have drastic consequences in animals An individual’s set of chromosomes is like a fine-tuned blueprint that must be followed exactly to have normal functions

Meiosis Two consecutive nuclear divisions Reduction Division: 2nn Interphase: DNA replication Meiosis I Prophase I, Metaphase I, Anaphase I, Telophase I Meiosis II Prophase II, Metaphase II, Anaphase II, Telophase II

Meiosis I Prophase I Metaphase I Anaphase I Telophase I Homologous chromosomes condense, pair up, and swap segments, spindle microtubules attach to chromosomes as the nuclear envelope breaks up Metaphase I The homologous chromosome pairs are aligned midway between spindle poles Anaphase I The homologous chromosomes separate and begin heading toward the spindle poles Telophase I Two clusters of chromosomes reach the spindle poles, a new nuclear envelope forms around each cluster, so two haploid (n) nuclei form

Meiosis II Prophase II Metaphase II Anaphase II Telophase II The chromosomes condense, nuclear envelope breaks up, spindle microtubules attach to each sister chromatid Metaphase II The (still duplicated) chromosomes are aligned midway between poles of the spindle Anaphase II All sister chromatids separate, the (now unduplicated) chromosomes head to the spindle poles Telophase II A complete set of chromosomes clusters at each spindle pole, new nuclear envelope encloses each cluster, four haploid (n) nuclei form

Variation Two events in meiosis introduce new combinations of alleles Crossing Over: chromosome segments are exchanged between homologous chromosomes during prophase I, increases genetic diversity

Variation The second event is the segregation of chromosomes into gametes When homologous chromosomes separate in meiosis I, one of each chromosome pair goes to each of the two new nuclei For each pair, the maternal or paternal version is equally likely to end up in either nucleus Each time a human germ cell undergoes meiosis the four gametes that form end up with one of 8,388,608 (or 223) possible combinations of homologous chromosomes

Mitosis and Meiosis There are striking parallels between mitosis and meiosis II Meiosis may have evolved by remodeling the existing mechanisms of mitosis, or for repairing damaged DNA to maintain the integrity of the chromosome https://www.youtube.com/watch?v=VzDMG7ke69g