-Based on the mitotic division of the nucleus
-An organisms that reproduces asexually may be single-celled like yeast, reproducing itself with each cell cycle, or it may be multicellular like the cholla cactus, that breaks off a piece to produce a new multicellular organism
-Rapid and effective way of making new individuals
-The offspring are clones of the parent organism–The offspring are genetically identical to the parent
-Any genetic variation among the offspring is due to small environmentally caused changes in the DNA=Mutations
-Requires gametes created by meiosis-2 parents eacg contribute one gamete to each of their offspring
-Can produce gametes (and thus offspring) that differ genetically from each other and from the parents
-Because of this genetic variation, some offspring may be better adapted than others to survive and reproduce in a particular environment
-Meiosis thus generates the genetic diversity
-One chromosome of each pair comes from each of the organism’s 2 parents
Homologous Pairs: The members of a pair are similar in size and appearance, except for the sex chromosomes found in some species
-Bear corresponding, although not identical, genetic information
Haploid: The number of chromosomes in a gamete is denoted by n
-2 haploid gametes fuse to form a zygote during fertilization
-The zygote has 2 sets of chromosomes just as somatic cells do
Diploid: The zygote’s chromosomes number is denoted by 2n
-Depending on the organism the zygote may divide either by meiosis or mitosis
-Eventually, the haploid cells or gametes fuse to produce a zygote, beginning the diploid stage of the life cycle
Since the origin of sexual reproduction, evolution has generate may different versions of the sexual life cycle
1)In haplontic organisms, including most protisists, fungi, and some green algae, the tiny zygote is the only diploid cell in the life cycle . After it is formed it immediately undergoes meiosis to produce more haploid cells. These are usually spores, which are the dispersal units for the organism, like the seed of a plant. A spore germinates to form a new haploid organism, which may be single-celled or multicellular. Cells of the mature haploid organism fuse to form the diploid zygote
2)Most plants and some fungi display alternation of generations. Meiosis gives rise to haploid spores, which divide by mitosis to form a haploid life stage called the gametophyte. The gametophyte forms gametes by mitosis, which fuse to form a diploid zygote. The zygote divides by mitosis to become the diploid sporophyte, which in turn produces the gametes by meiosis.
3) In diplontic organisms, which include animals brown algae and some fungi, the gametes are the only haploid cells in the life cycle, and the mature organism ids diploid
-Both of these steps contribute to shuffling of genetic information in the population, so that no 2 individuals have exactly the same genetic constitution
-Diversity provided by sexual reproduction opens up enormous opportunities for evolution
Karyotype: Such a rearranged photomicrograph reveals the number, shapes, and sizes of the chromosomes in a cell
-To ensure that each of the haploid products has a complete set of chromosomes
-To generate genetic diversity among the products
-Homologous chromosomes come together to pair along their entire lengths (No such pairing occurs in mitosis)
-The homologous chromosome pairs separate, but the individual chromosomes, each consisting of 2 sister chromatids, remain intact
Like mitosis, meiosis 1 is precede by an interphase with an S phase, during which each chromosome is replicated=As a result, each chromosome consists of 2 sister chromatids, held together by cohesion proteins
At the end of meiosis 1, 2 nuclei form, each with half o the original chromosomes (one member of each homologous pair)
Since the centromeres did not separate these chromosomes are still double-composed of 2 sister chromatids–The sister chromatids are separated during meiosis 2, which is not preceded by DNA replication=As a result, the products of meiosis 1 and 2 are four cells, each containing the haploid number of chromosomes (the four cells are not genetically identical)
-Begins with a long prophase 1 during which the chromosomes change markedly
-The homologous chromosomes pair by adhering along their lengths in a process called synapsis (does not happen in mitosis)
-This pairing process lasts from prophase 1 to the end of metaphase 1
-The 4 chromatids of each pair of homologous chromosomes form a tetrad or bivalent
-Throughout prophase 1 and metaphase 1 the chromatin continues to coil and compact, so that the chromosomes appear ever thicker
-The chromosomes usually begin exchanging material shortly after synapsis begins, but chiasmata do not become visible until later, when the homologs are repelling each other
-Crossing over results in recombinant chromatids, and it increases genetic variation among the products of meiosis by reshuffling genetic information among the homologous pairs
-A diploid organism has 2 sets of chromosomes-one set derived from its male parent, and the other from its female parent
-As the organism grows and develop, its cells undergo mitotic divisions
-In mitosis, each chromosomes behaves independently of its homolog, and its 2 chromatids are sent to opposite poles during anaphase
-Each daughter nucleus ends up with 2n chromosomes
-Independent assortment is another source of diversity
-It is a matter of chance which member of a homologous pair goes to which daughter cell at anaphase 1
-2 homologous pairs of chromosomes in the diploid parent nucleus-A particular daughter nucleus could receive the paternal chromosomes 1 and the maternal chromosome 2 + it could get paternal 2 and maternal 1, or both paternal, or both maternal chromosomes
-Depends on the way in which the homologous pairs line up at metaphase 1
-Breakdown in the cohesins that keep sister chromatids and tetrads joined together during prophase 1
-These and other proteins ensure that when the chromosomes line up at the equatorial plate, one homolog will face one pole and the other homolog will face the other pole
– If the cohesins break down at the wrong time both homologs may go to one pole
-During the formation of a human egg, both members of the chromosome 21 pair go to the same pole during anaphase 1, the result eggs will contain either 2 copies of chromosome 21 or none at all
-If na egg with two of these chromosomes is fertilized by a normal sperm, the resulting zygote will have 3 copies of the chromosome–It will be trisomic for chromosome 21=Down Syndrome (extra chromosome 21)
