Heredity & Genetic Variation PDF

© Getting Down With Science  Heredity Genetics: the study of heredity and hereditary variation Heredity: the transmission of traits from one generation to the next Traits are passed from parent to offspring through genes Segments of DNA that code for basic units...

© Getting Down With Science  Heredity Genetics: the study of heredity and hereditary variation Heredity: the transmission of traits from one generation to the next Traits are passed from parent to offspring through genes Segments of DNA that code for basic units of heredity ○ Offspring acquire genes from parents by inheriting chromosomes © Getting Down With Science © Getting Down With Science  Asexual vs Sexual Reproduction Asexual Sexual Single individual Two parents No fusion of gametes (male/female) Clones: offspring are Offspring are unique exact copies of parent combinations of Mutations are the only genes from parents source of variation Genetically varied Can produce from parents and asexually through siblings mitosis © Getting Down With Science  Homologous Chromosomes Homologous Homologous pair of chromosomes chromosomes: a pair of chromosomes (same size, length, Paternal Maternal centromere position) that carry the same Sister Replication genetic information chromatids One homologous chromosome is inherited from mom and one is inherited from dad Homologous pair of chromosomes © Getting Down With Science Karyotypes © Getting Down With Science  Karyotypes Karyotypes: a display of chromosome pairs ordered by size and length Pair of homologous duplicated chromosomes Note: in actual karyotypes it is difficult to see the sister chromatids in each pair Sister Chromatids © Getting Down With Science  Cells and Chromosomes Somatic (body) cells ○ Diploid, or 2n: two complete sets of each chromosome Humans: 2n = 46 Gametic (sex) cells: ○ Haploid, or n: one set of each chromosome ○ Humans (sperm and eggs): n = 23 © Getting Down With Science  Cells and Chromosomes Eukaryotes have DNA that is packaged in chromosomes There are two types of chromosomes: ○ Autosomes: chromosomes that do not determine sex (humans have 22 pairs) ○ Sex chromosomes: X and Y Eggs: X (humans: 22 + X) Sperm: X or Y (humans: 22 + X OR 22 + Y) Note: all sexually reproducing organisms have both a diploid and a haploid number © Getting Down With Science  Life Cycles Life cycle: sequence of stages in the reproductive history of an organism from conception to its own reproduction Fertilization and meiosis alternate in sexual life cycles ○ Fertilization is when a sperm cell (haploid) fuses with an egg (haploid) to form a zygote (diploid) © Getting Down With Science Meiosis © Getting Down With Science  Meiosis Meiosis: a process that creates haploid gamete cells in sexually reproducing diploid organisms ○ Results in daughter cells with half the number of chromosomes as the parent cell Example: humans Diploid: 2n= 46 Meiosis produces sperm and eggs that are haploid: n=23 ○ Involves two rounds of division Meiosis I and Meiosis II © Getting Down With Science  Mitosis vs Meiosis While meiosis is similar to mitosis, there are some key differences: Mitosis Meiosis Occurs in somatic Forms gametes cells (sperm/egg) 1 division 2 divisions Results in 2 diploid Results in 4 haploid daughter cells daughter cells Daughter cells are Each daughter cell is genetically identical genetically unique © Getting Down With Science  Homologous Chromosomes žPair of chromosomes (maternal and paternal) that are similar in shape and size. žHomologous pairs (tetrads) carry GENES controlling the SAME inherited traits. LOCI žEach locus (position of a gene) is in the same position on homologues. žHumans have 23 pairs of homologous chromosomes: a. First 22 pairs of autosomes b. Last pair of sex chromosomes 14 © Getting Down With Science  Homologous Chromosomes eye color eye color locus locus hair color hair color locus locus Paternal Maternal 15 © Getting Down With Science  Crossing Over žCrossing over may occur between non- sister chromatids at sites called chiasmata. žCrossing over: segments of nonsister chromatids break and reattach to the other chromatid. žChiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.) žCauses Genetic Recombination 16 © Getting Down With Science  Genetic Recombination nonsister chromatids Tetrad chiasmata: site variation 17 of crossing over © Getting Down With Science  18 © Getting Down With Science  Key Events in Meiosis There are three key events in meiosis that are unique: 1. Prophase I: synapsis and crossing over 2. Metaphase I: tetrads (homologous pairs) line up at the metaphase plate 3. Anaphase I: homologous pairs separate © Getting Down With Science  Meiosis I Interphase: cell goes through G1, S (DNA is copied), and G2 © Getting Down With Science  Meiosis I Prophase I: Synapsis: homologous chromosomes pair up and physically connect to each other forming a tetrad Crossing over (recombination) occurs at the chiasmata and DNA is exchanged between the homologous pairs Every chromatid that is produced has a unique combination of DNA © Getting Down With Science  Meiosis I Metaphase I Independent orientation: tetrads line up at the metaphase plate Anaphase I: pairs of homologous chromosomes separate Sister chromatids are still attached © Getting Down With Science  Meiosis I Telophase I and Cytokinesis: Nuclei and cytoplasm divide There is now a haploid set of chromosomes in each daughter cell © Getting Down With Science  Meiosis II Prophase II: No crossing over Spindle forms © Getting Down With Science  Meiosis II Metaphase II: Chromosomes line up at the metaphase plate Because of crossing over in meiosis I, the chromatids are unique © Getting Down With Science  Meiosis II Anaphase II: Sister chromatids separate and move towards opposite poles © Getting Down With Science  Meiosis II Telophase II and Cytokinesis: 4 haploid cells Nuclei reappear Each daughter cell is genetically unique © Getting Down With Science  Meiosis Review Early Meiosis I Parent cell: 2n = 4 End of Telophase II and cytokinesis Each daughter cell: n = 2 © Getting Down With Science  How Does Meiosis Lead to Genetic Variation? 1. Crossing over a. Produces recombinant chromosomes: they exchange genetic material 2. Independent assortment of chromosomes a. Chromosomes are randomly oriented along the metaphase plate during Metaphase I i. Each can orient with either the maternal or paternal chromosomes closer to a given pole 3. Random fertilization a. Any sperm can fertilize any egg © Getting Down With Science  Putting It All Together Meiosis followed by fertilization ensures genetic diversity in sexually reproducing organisms and provides genetic variation that plays a role in natural selection. This cellular process is driven by the interaction of subcellular components and uses free energy that is required for the growth and reproduction of living systems © Getting Down With Science