Meiosis and Genetic Diversity

Heredity

• Genetics: The study of heredity and hereditary variation

• Traits are passed from parent to offspring through genes

  • Segments of DNA that code for basic units of heredity

• Offspring acquire genes from parents by inheriting chromosomes

Asexual VS Sexual Reproduction

Asexual

• Single individual

• No fusion of gametes

• Clones: offspring are exact copies of parent

• Mutations are the only source of variation

• Can produce asexually through mitosis

Sexual

• Two parents (male/female)

• Offspring are unique combinations of genes from parents

• Genetically varied from parents and siblings

Homologous Chromosomes

Homologous chromosomes: a pair of chromosomes (same size, length, centromere position) that carry the same genetic information

• One homologous chromosome is inherited from mom and one is inherited from dad

Karyotypes

Karyotypes: A display of chromosome pairs ordered by size and length

Note: in actual karyotypes it is difficult to see the sister chromatids in each pair

Cells and Chromosomes

• Somatic (body) cells

  • Diploid, or 2n: Two complete sets of each chromosome

    • Humans: 2n=46

• Gametic (sex) cells:

  • Haploid, or n: one set of each chromosomes

  • Humans (sperm and eggs): n=23

Cells and Chromosomes

Eukaryotes have DNA that is packaged in chromosomes

• There are two types of chromosomes

  • Autosomes: chromosomes that do not determine sex (in humans 22 pairs)

  • Sex chromosomes: X and Y

    • Eggs: X (humans 22+x)

    • Sperm: X or Y (Humans 22+x or 22+y)

Life Cycles

• Life cycle: Sequence of stages in the reproductive history of an organism from conception to its own reproduction

• Fertilization and meiosis alternate in sexual life cycles

  • Fertilization is when a sperm cell (haploid) fuses with an egg (haploid) to form a zygote (diploid)

Meiosis

• Meiosis: a process that creates haploid gamete cells in sexually reproducing diploid organisms

  • Results in daughter cells with half the number of chromosomes as the parent cell

    • Example: humans

      • Diploid: 2n= 46

      • Meiosis produces sperm and eggs that are haploid: n=23

  • Involves two rounds of division

    • Meiosis I and Meiosis II

Mitosis vs Meiosis

Although Mitosis and Meiosis are similar, they have key differences

Mitosis

• Occurs in somatic cells

• 1 division

• Results in two diploid daughter cells

Meiosis

• Forms gametes

• 2 divisions

• Results in 4 haploid  daughter cells

• Each daughter cell is genetically unique

Key Events in Meiosis

Meiosis I

Prophase I: Synapsis and crossing over

Metaphase I: Tetrads (homologous pairs) line up at the metaphase plate (middle)

Anaphase I: Homologous pairs separate

Telophase I and Cytokinesis: Nuclei and cytoplasm divide; There is now a haploid set of chromosomes in each daughter cell.

How does crossing over lead to genetic diversity?

Ans: Crossing over allows homologous chromosomes to randomly exchange information between each other, allowing certain traits to be swapped randomly, creating a new combination of alleles.

Meiosis II
 

Prophase II: (Unlike P1, there's no DNA replication) The nuclear membrane disintegrates, spindle fibers go to opposite poles of the cell

Metaphase II: The chromosomes line up at the equator and attach to the spindle fibers

Anaphase II: Sister chromatids separate and move to opposite poles of the cell

Telophase II and Cytokinesis: Spindle fibers disappear and the nuclear membrane reforms. Finally, the cells separate into four haploids.