Meiosis and Sexual Reproduction

Why Meiosis?• Advantages:• Dramatic increase in genetic

variation• Keeps genetic material from

multiplying (ploidy)• Replication done with somatic cells

(body cells) would double the amount of DNA each time

• Disadvantages:• Reproduction is more complex• Meiosis requires more energy• Must find other half of genetic

material to reproduce• Sexual Reproduction fusion of two

haploid male and female cells (gametes)• Fertilization union of male and female

nuclei; produces a zygote

The Cells of Meiosis• What type of cells do meiosis?

– Eukaryotes• Types of meiotic cells:1) Spermatozoa (sperm)

produced by male gonads (testes); contain paternal chromosomes

2) Ova (eggs) produces by female gonads (ovaries); contain maternal chromosomes

• Source of variation though chromosomes are the same (homologous) they have different DNA for the same genes (alleles)

Phases of Meiosis• Two Phase Groups:1) Meiosis I – Prophase I Telophase I– Homologous chromosomes

separate 2) Meiosis II – Prophase II Telophase II– Sister chromatids separate

(like mitosis) • Both phases have

cytokinesis

Meiosis I1) Early Prophase I

– Chromosomes condense– Homologous pairs form tetrads through

pairing (synapsis)– Crossing-over occurs

2) Late Prophase I– Nucleus breaks down– Spindle fibers attach to kinetochores (one for

each sister chromatids)3) Metaphase I

– Homologous pairs line up at metaphase plate

4) Anaphase I– Homologous pairs separate to opposite poles

5) Telophase I– Nucleus reforms, spindles break down, and

short interkinesis

Crossing-Over• Protein complex carefully

exchanges alleles on homologous chromosomes

• Event is random and happens in 2-3 places on every chromosome

• 2 (of 4) chromatids exchange alleles results in two recombinant chromosomes and two parental chromosomes

• Double crosses possible?– Yes, depending on other alleles

and distance

Meiosis II• Follows mitosis:1) Early Prophase II

– Chromosomes condense– Spindles form

2) Late Prophase II– Nucleus breaks down– Spindles attach to kinetochores

3) Metaphase II– Chromosomes move to metaphase plate

4) Anaphase II– Sister chromatids separate to opposite

poles5) Telophase II

– Nucleus reforms, chromosomes decondense, spindles disappear

• End result is 4 haploid cells

Mendel’s Dihybrid Crosses• The Law of Segregation – Gametes will get only 1 copy of 1

allele– All alleles separate when sister

chromatids separate• The Law of Independent

Assortment– Genes for different traits are

inherited independently of each other

– Shown in meiosis when homologous chromosomes separate

– Genes on chromosome 1 move different than genes on chromosome 2, 3, 4, etc…

Producing Variation• Where in meiosis does variation

come from?- Law of Independent Assortment

• Random separation of paternal and maternal DNA

• 8,388,608 possible combinations with our 23 pairs

– Random recombination of haploid material • 1:7.0x1013 possible you

– Switching of genes during cross-over• Too random to measure

• Creates enough variation that no two humans ever be the same…ever (expect for twins)