Chapter 13 Meiosis and Sexual Life Cycles Reproduction Characteristic of all living things Reproduction also involves the transmission of traits from one generation to the next; inheritance Heredity Latin = heres, heir With inherited similarity there is also some variation Offspring differ from parents and siblings 1
Variation Essential to life on earth Produces biodiversity; formation of new species by natural selection Human artificial selection Breeds of dogs; varieties of crops The study of heredity and hereditary information - Genetics Genetics Study may occur at three levels: Molecular level DNA Cell within a single nucleus Organism hereditary information passed 2
It s in your genes!!!! Inheritance of genes Unit of heredity Gene Genes are segments of DNA Sequences of bases along the sugar-phosphate backbone for a genetic language Most genes code for the synthesis of proteins The DNA of eukaryotic organisms is subdivided into chromosomes Each species has a characteristic number of chromosomes; humans have 46 A gene s location on a chromosome is termed the locus. Where did you get those genes??? Inheritance from parents reproduction Two basic forms of reproduction Asexual - no fusion of gametes Sexual involves fusion of gametes 3
Asexual reproduction Single celled organisms reproduce by mitotic division Multicellular organisms can also reproduce asexually Budding by Hydra Organisms produced asexually are clones genetically identical to parent How does change occur? Mutations changes ion DNA sequences Sexual reproduction Sex - fusion of gametes from the Latin sexus (to divide; as in different sexes male and female) Male gamete sperm Female gamete ova or egg Parents give rise to offspring that have unique combinations of genes; not clones Two essential questions have to be asked - How does this genetic variation arise? How are offspring produced with the correct number of chromosomes? Recall that each species has a predetermined number of chromosomes 4
In each human somatic (normal body) cell there are 46 chromosomes The 46 chromosomes occur as homologous chromosome pairs One maternal chromosome One paternal chromosome Each homologous chromosome carry genes controlling the same inherited trait Cells that have pairs of homologous chromosomes are said to be diploid (2n) In humans, 23 pair is the diploid number Cells that have half of a homologous chromosome pair are haploid (n) In humans gametes (sperm and egg) Karyotypes used to observe cells ploidy level Karyotype Ordered display of condensed chromosomes Cells arrested in metaphase Karyotypes are often used to screen for genetic anomalies; Down syndrome Karyotypes can also be used to determine sex male or female Sex chromosomes in humans, 23 rd pair (Other 22 pair are known as autosomes) XY - XX - 5
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recall chromosome structure Replicated homologous chromosomes Sister chromatids centromere What does this have to do with sex???? Sex involves the fusion of gametes during fertilization forming a zygote (2n) Zygote single cell Zygote grows and develops by mitosis In order to reproduce sexually the gametes must be haploid 7
Need for cell division process to produce haploid cells for reproduction Meiosis Cell division with single replication of the genetic material followed by two consecutive cytoplasmic divisions Meiosis I Meiosis II End result : Four genetically unique haploid cells Process of meiosis Very similar to mitosis Be sure to note the differences!!!! These subtle differences allow for genetic variation in cells produced Begin with cell which has completed Interphase DNA replication has occurred 8
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Genetic variation Due to two processes which occur during meiosis Crossing over during prophase I Independent assortment during metaphase I & II Crossing over Synapsis Replicated homologous chromosome pairs line up and are physically connected Form tetrads Chiasma Point where non-sister chromatids overlap and genetic rearrangement occurs This produces recombinant chromosomes carry genes from two different parents 10
Independent assortment metaphase I Tetrads arrange at metaphase plate There is a 50-50 chance that each tetrad will move to each pole Independent assortment Metaphase II Sister chromatids line up at metaphase plate 50-50 chance for movement to each pole 11
The possible number of combinations when chromosomes sort independently is 2 n Where n = haploid number of the organism In humans 2 23 = 8,388,608 Couple this with probabilities which factor in crossing over and then random fertilization (one from male and one from female) 2 23 x 2 23 = 70,368,744,177,664 (>70 trillion) WOW unique we each truly are!!!! Differences between mitosis and meiosis Crossing over Tetrads Separation of homologous chromosomes Meiosis I - reductional division Meiosis II equational division 12