University:
Rutgers University Newark
Course:
BIO 203 | General Biology
Academic year:

2021
Views:

398

Pages:

3
Author:

emplanasf3fe

Chapter 13: Meiosis and Sexual Life Cycles 13.1 “Offspring acquire genes from parents by inheriting chromosomes”  Parents endow their offspring with coded information in the form of hereditary units called genes  Tens of thousands of genes we inherit from our mothers and fathers constitute our genome  Genes are segments of DNA  In asexual reproduction, a single individual is the sole parent  The genomes of the offspring are virtually exact copies of the parent’s genome  An individual that reproduces asexually gives rise to a clone  Genetic differences occasionally arise due to mutations  In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents  In contrast to a clone, their offspring vary genetically from their siblings and both parents 13.2 “Fertilization and meiosis alternate in sexual life cycles”  Each somatic cell, any cell other than a gamete, has 46 chromosomes (Diploid 2n=46)  During mitosis, the chromosomes become condensed enough to be visible under a light microscope  Sex chromosomes determine an individual’s sex (X and Y)  Females= XX Males= XY  Gametes contain 23 chromosomes (Haploid n=23)  The human life cycle begins when a haploid sperm cell from the father fuses with a haploid ovum from the mother  Meiosis reduces the number of sets of chromosomes from two to one in the gametes, compensating for the doubling that occurs at fertilization  Plants and some type of algae exhibit a second type of life cycle called alternation of generations  This type of life cycle includes both diploid and haploid multicellular stages  The multicellular diploid stage is called sporophyte  Meiosis in the sporophyte produces haploid cells called spores  A spore divides mitotically to generate a multicellular haploid stage called the gametophyte  A third type of life cycles occurs in most fungi and some protists, including some algae 13.3 “Meiosis reduces the number of chromosome sets from diploid to haploid”  Meiosis has two stages: Meiosis I (separates homologous chromosomes) and Meiosis II (separates sister chromatids)  Theses division result in 4 daughter cells instead of two, each with only half as many chromosomes as the parent cell  Stages of Meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase I and Cytokinesis  Stages of Meiosis II: Prophase II, Metaphase II, Anaphase II, Telophase II and Cytokinesis  Meiosis I is called the reductional division because it halves the number of chromosome sets per cell—a reduction from two sets to one set  The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis 13.4 “Genetic variation produced in sexual life cycles contributes to evolution”  The events of sexual reproduction that contribute to genetic variation in a population are independent assortment of chromosomes during meiosis, crossing over during meiosis I, and random fertilization of egg cells by sperm  Genetic variation is the raw material for evolution by natural selection  Mutations are the original source of this variation; the production of new combinations of variant genes in sexual reproduction generate additional genetic diversity