A. Types of Reproduction ( 생식 ) 1. Asexual( 무성 ) Reproduction involves only 1 parent offspring are genetically identical( 동일 ) to the parent (clones) Advantageous( 유리한 ) when organism is well-adapted to a stable( 안정된 ) environment Ex. binary fission, parthenogenesis( 처녀생식, 단위생식 )
2. Sexual( 유성 ) Reproduction involves 2 parents parents produce haploid (1n) gametes which fuse to form a diploid (2n) zygote( 접합체 ) (first cell of offspring). Offspring( 자손 ) are genetically diverse( 다양한 ) advantageous in a changing environment
Variations ( 세대교번 ): Alternation of generations( 세대교번 ) in plants Conjugation in bacteria Haploid gametes are produced by a type of cell division called meiosis.
B. Meiosis ( 감수분열 ) Type of cell division that halves the number of chromosomes. involves 2 nuclear divisions (meiosis I & meiosis II) 1 diploid (2n) cell 4 haploid (1n) cells occurs in germ ( 생식 ) cells In humans, where would germ cells be located?
Germ cell replicates its chromosomes in S phase of the cell cycle. 1. Meiosis I (reduction division) Prophase I replicated chromosomes condense( 응축 ) Spindle( 방추 ) apparatus( 장치 ) forms & fibers attach to chromosomes Nucleolus( 인 ) disappears( 사라진다 ) nuclear membrane breaks down Synapsis ( 상동염색체의 병렬접착 ) & crossing over( 교차 ) occurs
Synapsis - homologous chromosomes (homologs) pair up. Homologs - chromosomes that carry genes for the same traits in the same sequence. A diploid human cell has 23 homologous pairs.
Crossing over - non sister chromatids of homologs exchange chromosomal material which leads to new genetic combinations. Chiasma - visible point of cross over between homologs.
Metaphase I paired homologous chromosomes line up along equator of spindle
Anaphase I homologous chromosomes separate & move to opposite poles
Telophase I spindle apparatus breaks down chromosomes decondense nuclear membranes partially reform around two nuclei Cytokinesis usually occurs between meiosis I & II
2. Meiosis II (equational( 적도판의 ) division) Prophase II chromosomes condense spindle apparatus forms & fibers attach to chromosomes nuclear membrane breaks down Metaphase II chromosomes line up single-file along equator of spindle
Anaphase II centromeres part; sister chromatids (now called chromosomes) separate & move toward opposite poles Telophase II spindle apparatus breaks down chromosomes de-condense nuclear membranes reform nucleoli reappear Cytokinesis divides two cells into four nonidentical cells.
Meiosis II Prophase II Metaphase II Anaphase II Telophase II Four non- identical haploid cells