Chapter 9: Sexual Reproduction

Sexual Reproduction: Meiosis Due to meiosis, two individuals can create off-spring that are genetically different not only from themselves but also from their siblings Meiosis is a special type of nuclear division that results in 4 gamete cells Spermatogenesis in males Oogenesis in females Meiosis serves 2 functions: 1. Reducing the chromosome number 2. Shuffling the chromosomes and gens to produce genetically different gametes Fertilization by the fusion of the sperm and egg allows even more shuffling of genes

Homologous Chromosomes Chromosomes occur in pairs One pair contains sex chromosomes (X and Y) The larger chromosome is the X and the smaller is the Y chromosome Autosomes are all the pairs of chromosomes not including X and Y The 23 pairs of chromosomes (46 altogether) are called the diploid (2n) number Haploid (n) number is half of the diploid number Homologous chromosomes (homologues) are members of a chromosome pair Alternate versions of a gene for a particular trait are called alleles.

Human Life Cycle The human life cycle involves both mitosis and meiosis Mitosis occurs during development and after birth as part of growth and tissue repair Somatic (body) cells have the diploid number due to mitosis Meiosis occurs during sexual reproduction and the number of chromosomes are reduced from diploid to haploid Spermatogenesis is the production of sperm in males by the process of meiosis Occurs in the testes Oogenesis is the production of eggs in females via meiosis This occurs in the ovaries The fertilization of an egg by a sperm gives rise to a zygote (a diploid) The zygote has homologous pairs of chromosomes

Overview of Meiosis Meiosis consist of 2 divisions: Meiosis I and Meiosis II In meiosis I, the homologous chromosomes of each pair come together and line up side-by-side in an event called synapsis A tetrad (2 homologous chromosomes consisting of 2 chromatids each) arise as a result One chromosome from each homologous pair goes to each daughter nucleus 2 daughter cells are produced with half the number of chromosomes (n) but the chromosomes are still duplicated Chromosomes composed of 2 sister chromatids are called dyads There is no duplication of chromosomes between meiosis I and meiosis II During meiosis II, the sister chromatids of each dyad separate and are daughter chromosomes 4 daughter cells arise at the end of meiosis II Because no rules restrict which chromosome goes to which daughter nucleus…all possible combinations of chromosomes may occur with the gametes

The Importance of Meiosis Meiosis keeps the chromosome number constant by producing haploid daughter cells that later become gametes Meiosis introduces genetic variations through 1. Crossing-over where the nonsister chromatids from the tetrad exchange genetic material Crossing-over allows shuffling of alleles and brings about genetic recombination Crossing-over increases the diversity of the gametes and of the offspring 2. Many possible combination of chromosomes can occur in the daughter cells

Phases of Meiosis Meiosis I: diploid to haploid Prophase I : nuclear envelope fragments and nucleolus disappears Spindle fibers appear Chromosomes condensed and homologues undergo synapsis to produce tetrads Crossing-over of nonsister chromatids and shuffling of alleles between chromosomes Metaphase I : tetrads align at the spindle equator with each homologue facing opposite spindle poles Anaphase I : Homologous separate and dyads move to the poles Telophase I : daughter nuclei are haploid because each daughter received one duplicated chromosome from each homologous pair Cytokinesis follows Interkinesis: The period between meiosis I and meiosis II where no replication of DNA occurs Meiosis II: haploid to haploid Prophase II : nuclear envelope fragments, nucleolus disappears, spindle appears Each dyad attaches to the spindle Metaphase II : Dyads lined up at the spindle equator and sister chromatids are facing opposing poles Anaphase II : Sister chromatids separate from the dyad and move to the poles Telophase II : Spindle fibers disappear and nuclear envelopes are formed 4 genetically different haploid daughter cells arise after cytokinesis

Meiosis I

Meiosis II

Meiosis Compared to Mitosis Meiosis requires 2 nuclear divisions mitosis requires only 1 nuclear division Meiosis produces 4 haploid daughter cells (half the chromosome number of the parent cell) Mitosis produces 2 diploid daughter cells (same as the parent cell) Meiosis produces 4 daughter cells that are genetically different to each other and to the parent cell Mitosis produces 2 daughter cells that are genetically similar to each other and to the parent cell

Meiosis v Mitosis

Abnormal Chromosome Inheritance Nondisjunction is the failure of homologous chromosomes or daughter chromosomes to separate during meiosis I or meiosis II Meiosis I: both members of a homologous pair go into the same daughter cell Meiosis II: sister chromatids fail to separate and both daughter chromosomes go into the same gamete Trisomy: one type of chromosome is present in 3 copies Monosomy: one type of chromosome is present in a single copy Down syndrome (trisomy 21): an individual has 3 copies of chromosome 21 Characteristics: short stature, an eyelid fold, stubby fingers, a wide gap between the first and second toes; large, fissured tongue, round head; mental disabilities

Abnormal Sex Chromosome Number Klinefelter syndrome (47, XXY): individual is a male The extra X is a Barr body Testes and prostate gland are underdeveloped, no facial hair, large hands and feet, long arms and legs Slow to learn but not mentally handicapped unless there are more than two X chromosomes Nondisjunction during oogenesis or spermatogenesis can lead to abnormal sex traits due to too many or too few X and Y Normal females have only one functioning X chromosome (the other X chromosome becomes an inactive mass known as a Barr body) The presence of a Y chromosome almost always determine maleness Turner syndrome (45, XO): the individual is a female The O indicates the lack of a second sex chromosome Characteristics: short female with broad chest and webbed neck, small and underdeveloped ovaries, oviducts, and uterus Turner females do not undergo puberty or menstruate They have normal intelligence