Sexual Reproduction
•What is sexual reproduction?•Why is it beneficial?•What kinds of cells are involved?•How are these cells formed?
Diploid vs Haploid
•Diploid nuclei have pairs of homologous chromosomes.
•Haploid nuclei have one chromosome of each pair.
Meiosis•Meiosis is the process by which sex cells,
or gametes, are formed.▫In humans this takes place in the testes and
ovaries•Meiosis involves 2 stages of cell division.•In the end, daughter cells contain half the
chromosome number of the parent cell = haploid.
•Halving of the chromosome number allows a sexual life cycle with fusion of the gametes.
Stages of Meiosis: (Make a table as shown on the
board)
• Meiosis I:▫ Purpose: Reduce chromosome number▫ Process: Prophase I, Metaphase I, Anaphase I,
Telophase I▫ Results: Two Daughter cells each containing half the
number of chromosomes (still in the form of sister chromatids).
• Meiosis II:▫ Purpose: Separate sister chromatids▫ Process: Prophase II, Metaphase II, Anaphase II,
Telophase II▫ Results: Total of Four haploid Daughter cells (which
may become gametes).
Crash Course Biology Video:https://www.youtube.com/watch?v=qCLmR9-YY7o
Meiosis I•Prophase I:
▫Nuclear membrane dissolves▫Centrioles split and move to opposite poles▫Spindle fibers form▫Chromosomes come together in
homologous pairs Each chromosome is composed of a pair of
sister chromatids The whole structure is referred to as a tetrad
because it contains 4 chromatids▫The intertwined chromatids may break and
exchange segments of genetic material = crossing over
Meiosis I
•Metaphase I:▫Homologous chromosomes attach to
spindle fibers▫In pairs, they line up along the equatorial
plate
Meiosis I
•Anaphase I:▫“Reduction division” occurs▫One member of each Homologous pair
moves towards either pole. Each chromosome still contains sister
chromatids
Meiosis I
•Telophase I:▫Membranes form to create 2 new nuclei▫Each of the daughter nuclei contains one
member of each chromosome pair.
▫Telophase is followed by cytokinesis, the division of the cytoplasm.
Meiosis II
•Metaphase II:▫Chromosomes, each with two chromatids,
line up along the equatorial plate.
Meiosis II
•Anaphase II:▫Attachment between the sister chromatids
is broken▫Individual chromatids move to opposite
poles
Meiosis II
•Telophase II:▫Nuclear membranes form▫Form haploid nuclei have been produced
▫Telophase is followed by cytokinesis.
Bivalent formation and crossing over• The early stages of
meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
• Bivalent = a pair of homologous chromosomes
• Synapsis = pairing process
• Junction = the area where chromosomes break and rejoin
• Chiasmata = the point of connection
Random Orientation of the bivalents
• Orientation of pairs of homologous chromosomes prior to separation is random.
• Orientation = the pole to which each chromosome is attached depends on the way it is facing.
• For every additional bivalent, the number of possible chromosome combinations in a cell produced by meiosis doubles (2n).▫ Calculate the number of possible
combinations in humans
Non-disjunction• Non-disjunction = when
homologous chromosomes fail to separate in anaphase.
• Both of the chromosomes move to one of the poles.
• The result is a gamete with either an extra chromosome or deficient in a chromosome.
• If this gamete goes on to be fertilized, the result will be an individual with 45 or 47 chromosomes.
• Example: Down Syndrome