Biology
Class Notes
Lesson 23 Meiosis and Genetic Variation
Objective: 3.2.1
*With the exception of identical twins, no two individuals of the same species are identical.
*Genetic diversity increases the probability of survival for a species.
*The process that creates genetic diversity among Eukaryotes that reproduce sexually is called meiosis.
Sexual Reproduction and Variation
Sexual Reproduction: Occurs when sex cells (i.e. gametes with half the required amount of DNA) from two
parents join to form a new individual.
Ex: Sperm and Egg, Pollen and Ovule
Key Points:
 Somatic Cells (i.e. cells other than sex cells/gametes) replicate through mitosis to form identical copies.
 If two somatic cells (one from each parent) joined together to form a new offspring, the new individual
would have twice the necessary amount of DNA. Therefore, gametes/sex cells are produced by a
process called meiosis that result in each one cell having half the necessary DNA.
Mitosis Review from Lesson 5 (Do Not Copy)
--------------------------------------------------------------------------------------------How do multicellular eukaryotes replace damaged cells and how do single cellular prokaryotes and eukaryotes
reproduce?
Cell Division: The process by which a “parent cell” divides to form two or more new “daughter cells”.
Cell Cycle: The continuous process in which individual cells grow, make copies of their chromosomes (aka
c’somes), and then divide to form daughter cells.
The Cell Cycle
Interphase
• G1 Phase- Cells grow to mature size
•
•
• Longest phase of the cell cycle (18-20 hours).
S Phase- Cell’s DNA is copied
G2 Phase- Cell prepares for cell division
• Shortest phase of the cell cycle
Mitosis
• M Phase of Cell Cycle
• Division of the nucleus during cell division.
• Number of c’somes is same in mother and daughter cells
• Used for homeostasis/cell maintenance and asexual reproduction by plants, and unicellular eukaryotes.
Prophase
• First phase of mitosis.
• Nuclear envelope dissolves.
• DNA shortens and tightens into c’somes.
• Spindle fibers form from centrioles and drag c’somes to ends of cell.
• Two copies of c’somes form, each one is called a chromatid.
– Chromatids are connected by a centromere, which creates the ‘x’ shape.
Metaphase
•
•
•
•
Second phase of mitosis.
C’somes visible w/ a light microscope for the first time.
Spindles move c’somes to center of the cell, line them up, and hold them in place.
Anaphase
• Third phase.
• Chromatids split and are pulled by spindles to opposite poles of the dividing cell.
• Once they separate, they are now individual chromosomes.
Telophase
• Fourth phase.
• C’somes reach opposite ends of the cell.
• Nucleolus and nuclear envelope reform around c’somes.
• The spindle fibers dissolve.
Cytokinesis (New Cell is Complete)
• Cytoplasm divides and splits at the Cleavage Furrow: Area of the membrane that
pinches to divide one cell to become two. Note in plants, a Cell Plate develops and
divides the two new cells.
Pros and Cons of Asexual Reproduction
Pros
o Produces many offspring in a short period of time
o Partner is not needed
Cons
o Genetically identical offspring can all be harmed by the same disease.
o No variety, so ‘bad’ genes are passed on.
End of Lesson 5 Mitosis Review
Sexual Reproduction
• Two parent cells join together to form a new individual.
• Offspring contain DNA from both parents.
• Sex cells are haploid (1n): Contain ½ the # of c’somes for the species.
*When the cells recombine, the haploid cells add together to make cells that are diploid (2n), which
contain a complete set of c’somes.
Meiosis
• Process that produces sex cells/gametes/sperm and eggs.
• A form of cell division that cuts the # of c’somes in half.
• C’somes are copied once, but nucleus divides twice.
• Produces 1 diploid mother cell → 4 haploid grand-daughter cells
• Only occurs in reproductive organs. (i.e. testes and ovaries)
• Involves Homologous chromosomes: Two copies of each chromosome contained within the diploid
cells of a reproducing individual that are the same size and shape and that carry genes for the same
traits.
• One from each of the individual’s parents.
• Same genes are in the same position on both c’somes
< Interphase of the cell
cycle/DNA is replicated.
Stages of Meiosis
Meiosis I
1. Prophase I-Crossing Over
2. Metaphase I-Independent Assortment
3. Anaphase I
4. Telophase/ Cytokinesis I
Meiosis II
1. Prophase II
2. Metaphase II
3. Anaphase II
4. Telophase/Cytokinesis II
Stage Details for Meiosis I
Prophase I
• DNA coils into c’somes
• Spindle fibers appear
• C’somes pair w/ their homologue. (This does not occur in mitosis)
• Crossing over occurs. (This does not occur in mitosis)
Crossing Over
• Homologous c’somes pair up and align their genes.
• Portions of the chromatids switch between the c’somes.
• Results in genetic recombination, which creates a new mixture of genetic material.
Metaphase I
• Chromatid pairs line up randomly with their homologue and are pulled apart as intact
chromatids.
• Spindle fibers attach and hold them there.
Anaphase I
• Each homologue moves to opposite ends of the cell.
• The separation of chromatids is called independent assortment, which results in the random
separation of mixed maternal and paternal c’somes and leads to genetic variation.
Telophase I
• Chromatids reach opposite ends of cell.
• Cytokinesis occurs.
Products of Meiosis I
• Two diploid daughter cells, from 1 mother cell.
• Cells are diploid- contain total # of c’somes for the species.
• Due to crossing over and independent assortment, daughter cells are not identical to mother
cells.
Stage Details for Meiosis II (much like mitosis)
• Prophase II
• Spindle fibers form, and c’somes move toward the center.
• Metaphase II
• Chromatids line up at center.
• Anaphase II
• Chromatids separate and move to opposite poles.
• Telophase/ Cytokinesis II
• Nuclear membrane forms, and the cell divides.
Products of Meiosis II
• Four granddaughter (GD) cells/from two daughter cells/from one parent cell.
• GD cells are haploid which means they contain ½ the DNA from original mother cell.
• Haploid cells are called gametes: The egg cells from females and the sperm cells from males.
Diagram of Meiosis I
Diagram of Meiosis II
Fertilization
• Sex cells combine to create a cell with a complete set of c’somes (1/2 from each parent sex cell).
• Offspring have a mix of DNA from both parents.
• Offspring are not identical to parents.
• Offspring cells are diploid.
Pros and Cons of Sexual Reproduction
Pros
– Genetic variety means they are more resistant and adaptable.
– Unfavorable genes can be eliminated.
Cons
– Requires partners.
– Complicated process, more room for error.
CellsAlive.com View “Meiosis Animation”
Problem Sets: Meiosis internet lesson
http://biologycorner.com/worksheets/meiosis_internet.html
Lab: Meiosis w/ pipe-cleaners and straws
Key Words:
Mitosis
Sexual Reproduction
Asexual Reproduction
Gamete
Somatic Cell
Variations
Haploid number
Fertilization
Diploid number
Homologous chromosomes
Meiosis
Crossing over
Independent assortment