Meiosis and Sexual Life Cycles Chapter 13

Heredity and Inheritance Extreme variation among living things Due to heredity – transmission of traits from one generation to the next Transferred in the form of genes – specific sequences of DNA nucleotides

Heredity and Inheritance Genes are passed via gametes (sex cells) Uniting of gametes causes fusing of the nuclei DNA is packaged into chromosomes A specific genes location on a chromosome is its locus Somatic cells have a full set of chromosomes Somatic cells are body cells Gametes sperm and egg Chromosomes can contain a few hundred to a few thousand genes Locus = location

Sexual versus Asexual Reproduction Two types of reproduction exist Sexual Typically two parents Genetic variability Asexual One parent Creates clone Asexual reproduction is when parent passes all their traits on to the next generation

Chromosomes Human cells have 23 pairs of chromosomes Distinguished from one another by banding patterns – homologous chromosomes Same gene, same position BUT possible different versions One difference – X and Y – sex crhomosome Arranged on a karyotype Karyotype is an ordered arrangement of paired homologs Homologous chromosomes will have the same length, centromere position and staining pattern Many of the genes on X do not have counterparts on the Y

Chromosomes One chromosome inherited from each parent Number of chromosomes in a single set is represented by n Diploid cell has two sets (normal for humans) of chromosomes (2n) Haploid cell has a single set (n) Sperm and egg are haploid Somatic cells are diploid

Chromosomes and Fertilization Fertilization results in a zygote Zygote then begins mitosis to generate somatic cells Only cells not produced by mitosis are eggs and sperm – why? Zygote is a fertilized egg and becomes diploid

Meiosis I Prophase I A tetrad is made of each homologous set due to replication Homologs connected along length – synaptonemal complex (synapsis) Crossing over occurs resulting in chiasmata Rest remains the same as mitosis – metaphase, anaphase, telophase No duplication of chromosomes a the end of meisosis I Followed by meiosis II

Variety of Sexual Life Cycles 3 types of life cycles 1st - Normal Meiosis 2nd - Alternation of generations Both diploid and haploid multicellular organisms 3rd – Gametes fuse to make diploid zygote Only haploid cells produced Alternation of generations Multicellular diploid stage is called a sporophyte Meiosis of sporophyte produces haploid cells called spores Spore divides mitotically generating a haploid gametophyte (multicellular) Gametophyte generates gametes by mitosis Fusion of two gametes at fertilization results in a diploid sporophyte Occurs in plants and some algae 3rd Meiosis does not produce games but rather mitosis produces haploid unicellular organisms or haploid multicellular organisms Haploid organisms produce gametes by mitosis Only diploid stage is the zygote Occurs in most fungi and some protists

Meiosis vs. Mitosis Meiosis Mitosis Sexual reproduction Reduction from diploid to haploid Homologous chromosomes Synapsis and crossing over 4 cells produced Mitosis Asexual reproduction Conserves the diploid number Sister chromatids No synapsis/crossing over 2 cells produced

Genetic Variation by Sexual Reproduction 50% chance of a gamete of getting either the maternal or paternal chromosome Independent Assortment The chromosomes will assort independently of all other chromosomes Crossing over Produces recombinant chromosomes Random Fertilization 8.4 million possible chromosome combination Fertilization produces 70 trillion possible combinations Recombinant chromosomes are chromosomes from 2 different parents

Evolution and Genetic Variation Individuals best suited for an environment leave more offspring Changing environment-sexual reproduction Stable environment-asexual reproduction? Bdelloid rotifer Asexual ensures passing on of favorable traits, less energy expensive Bdelloid does parthonegenesis for reproduction Live in a wide variety of environments-does this mean parthonegenesis is better? In certain environments, the area may dry and the rotifers go into a suspended state-during this time their cell membranes can crack allowing other rotifer or even other organisms DNA to enter