Sexual Reproduction ( Cell Division ) - Chromosome # s

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1 Sexual Reproduction ( Cell Division ) - Chromosome # s somatic cells: all the cells in the body except for specialized sex cells each somatic cell has a specific # of chromosomes - ( humans have 46, 23 pairs) each pair of chromosomes is called a homologous pair homologous pair: one chromosome from male parent, one from female parent similar is size, shape, and genetic content to each other an organism with all its homologous pairs is said to be diploid (2n) ex. diploid # for humans is 46, haploid # is 23 gametes: specialized haploid sex cells ( n= 23 in humans) sperm in males, eggs in females no homologous pairs of chromosomes - each gamete only contains half of the genetic material

2 Asexual Reproduction involves only one parent cell ( ex. budding, fission, regeneration etc.) uses mitosis for cell division offspring has same genetic information as parent and each other - inherit favorable and unfavorable traits many offspring in short amount of time - rapid population growth Sexual Reproduction involves 2 different parent cells - each contributing half of the genetic information offspring are genetically different from parents - allows for genetic variation ( survival of the fittest) exchange of genetic material by way of gametes (special sex cells; usually a male and female ) fertilization occurs when male and female gametes join together and their nuclei fuse first single cell formed from this fusion is called the zygote

3 Sexual Reproduction - ( Cell Division ) Meiosis very similar to mitosis except stages happen twice takes place in special cells - sex cells - to reduce chromosome number cells are diploid at start -chromosomes replicate only once in first interphase cells then divide twice - producing four haploid daughter cells Prophase I each chromosome has already replicated producing 2 chromatids each pair of chromatids lines up with its homologous pair and they become fastened at centromere ( synapsis ) each group of four chromatids is now called a tetrad Chromatids may twist around each other in the tetrad and segments may become exchanged - this is called crossing over and leads to genetic variation nuclear membrane is disappearing, spindle is forming

4 Metaphase I centromeres of the tetrads line up at the equator of the cell Anaphase I the homologous chromosomes of each tetrad separate from each other and move to opposite ends - disjunction there is now half as many chromosomes as in the original cell -however - each chromosome is double stranded Telophase I this is the end of the first mitotic division cytoplasm divides - 2 daughter cells (haploid) cell immediately starts dividing again No further replication of chromosomes takes place; 2 nd division is exactly like mitosis Prophase II nuclear membrane disappears, spindle fibres form in each of the daughter cells double stranded chromosomes move toward the equator of the cell

5 Metaphase II centromeres of the chromosomes line at equator; each chromosome comsists of 2 strands Anaphase II the centromeres divide and the double stranded chromosomes separate the single chromosomes move to opposite poles Telophase II both daughter cells divide forming four haploid daughter cells nuclear membrane forms again Summary: 1 st Meiotic division produces: 2 haploid cells with double stranded chromosomes 2 nd meiotic division produces: 4 haploid cells with single stranded chromosomes

6 Sexual Reproduction in Simple Organisms Disadvantages of Asexual Reproduction: each individual is exactly like its parent and each other no inherited differences or variations in genetics if environment stays the same, variations are not needed if environment changes, organisms may not be able to adapt Advantages of Sexual Reproduction: offspring are not identical to either parent offspring have new combinations of characteristics (different structures, different functions) increasing genetic variation also increases the chance of survival for species undergoing environmental change successful variations are often passed to the next generation