Chapter 13: Meiosis and Sexual Life Cycles Genetics: study of heredity. Transmission of traits from gene to gene. Important in medicine, used for agricultural breeding Units of information -?genes, all = genome Genes are sections of chromosomes; each gene has a specific locus Chromosomes, so genes, are made of DNA This chapter-?how sexual reproduction passes chromosomes from parent to child Asexual reproduction: clones, from one parent-?mitotic division only Most single-celled organisms Some multicultural organisms (hydra, strawberry) Variation only if mutation occurs Sexual reproduction: two parents contribute chromosomes
Genetic variation in offspring due to various combinations possible Mechanism = meiosis-special division of chromosomes to make gametes Life Cycle: sequence of steps from conception to production of own offspring Somatic cells (every cell except germ cells and gametes) – diploid (an) Humans: 46 chromosomes, 23 pairs of homologous chromosomes Display = eukaryote 22 pairs of outcomes 1 pair sex chromosomes; XX= female, XX= male Gametes= haploid (n) Egg (ovum in female, sperm in male Fertilization produces diploid zygote, develops by mitosis to mature organism (an), makes own gametes (meiosis, haploid n)
Propagation of germ line Production of gametes by special cell division = meiosis Have chromosomal number All sexually reproducing organisms; alternation of meiosis (haploid) and fertilization (diploid) 3 major types of sexual life cycles (see fig. 13. 6) Humans and animals-gametes are the only haploid cells, short lived phase Fungi and some protests-usually haploid, zygote is diploid and one cell only, immediately becomes haploid via meiosis Plants and algae -alternation of generations, multicultural haploid and diploid stages Diploid= saprophyte, undergoes meiosis —haploid spores
Spores divide mimetically —-commemorate = haploid, multicultural Commemorate eventually makes gametes by mitosis Two gametes Join to start diploid saprophyte, which is multicultural So gametes made by mitosis, spores made by meiosis. Meiosis-makes 4 haploid daughter cells DNA replicates as for mitosis – enterprise I Phases: Meiosis I and Meiosis II (fig. 13. 8 p. 244-245) Proposes I-longer, more complex than mitotic proposes- 90% meiosis Partial condensation synapses (homologous pair up) —–crossing over Injunctions -?schismatic) ——finish condensing Metastases I-both constrictors of a sister chromatic pair face same pole
Anapest l- homologous separate, sister chromatics stay Joined at controvert Telephone I- each pole has haploid set, but 2 sister chromatics for each chromosome Cytokines simultaneous Sometimes interlines-?nuclear membranes and nucleoli reform No further DNA replication before meiosis II Meiosis II: Just like regular mitosis, but only half the number of chromosomes to start out. Anapest II: chromatics separate Meiosis vs.. Mitosis (fig 13. 19) Chromosome number divided in half by meiosis, not mitosis Meiosis makes haploid gametes, mitosis makes clones Proposes I-?crossing over occurs between homologous
Metastases I-bivalents align on metastases plate, Joined by schismatic Anapest l- separates homology, not sister chromatics Errors occasionally occur during meiosis, chromosomal abnormalities in Chapter 1 5 Variation important in population, encouraged by sexual reproduction Chromosomes assort independently In AN = 4, then 4 combinations possible. If AN =6, then 8 combinations (NAN) Humans 2 “’23 = > 8 million Crossing over = exchange to genes between chromosomes Synapses very precise, involves proteins to form sensationally complex Mechanism not understood yet
New genetic combinations created (fig 12. 9-genetic recombination) Likelihood of recombination correlates with distance apart on chromosome Random fertilization of egg by one of millions of sperm Evolution: depends on genetic variation Evolution occurs due to differential reproductive success Adaptation: the accumulation of genetic variations favored by the environment Environmental crisis or change-?population survives if some members can cope Variation from sex and from mutation (Chi. 17) Darwin knew the heritable variation is necessary for evolution, but couldn’t explain how it worked.