Chapter 9: Meiosis

Sexual Reproduction Eukaryotic division -Meiosis -- Meiosis 1 and 2 Genetic Variability

Autosomes

numbered These chromosomes are the same for both sexes

Sex chromosomes

xx is for female
xy is for male

Homologous pair

look alike and carry the same sequence of genes for the same traits
received one from ma and one from pa

Alleles

alternative versions of the same gene

Meiosis 1

separates the pairs for homologues, with each daughter nucleus receiving one

Meiosis 2

Separates the chromatids and parcels one chromatid into each of 2 more daughter nuclei

Prophase I

Chromosomes become visible; nuclear envelope breaks down; crossing-over occurs.

Metaphase I

paired homologous chromosomes line up at the equator of the cell
spindle fibers attach to the kinetochore of each chromosome

law of independent assortment

the law that states that genes separate independently of one another in meiosis

Anaphase I

Homologous chromosomes separate
- one duplicated chromosome of each homologous pair moves to the opposite sides of the cell

Telophase I

Two haploid clusters of duplicated chromosomes form

• Spindle microtubules disappear

• cytokinesis usually occurs during telophase I

• Nuclear envelopes may re-form

• Telophase I is followed my Miosis II with no interphase

• Chromosomes do NOT replicate between Meiosis I and Meiosis II

• 2 Haploid cells result from Meiosis I

Meiosis II

Separates sister chromatids into 4 daughter nuclei

Prophase II

• Chromosomes recondense

• spindle fibers re-form and capture duplicated chromosomes

• Kinetochores of chromatids in each chromosome are attached by microtubules to opposite poles

Metaphase II

Duplicated chromosomes line up singly, perpendicular to the spindle, as they do in mitosis

Anaphase II

Chromatids separate and move to opposite poles

Telophase II

Cytokinesis occurs, nuclear membranes re-form, and the chromosomes decondense
-2 nuclear divisions of meiosis produce 4 haploid cells from a single diploid cell

Genetic variability

Makes sure new generations result in individuals with unique genotypes

Crossing over

2 homologous chromosomes exchange genetic material during prophase I

Recombination

the genetic process by which one chromosome breaks off and attaches to another chromosome during reproductive cell division

Independent assortment

chromosome pairs align randomly during metaphase I

monozygotic twins

identical twins formed when one zygote splits into two separate masses of cells, each of which develops into a separate embryo

dizygotic twins

twins who are produced when two separate ova are fertilized by two separate sperm at roughly the same time

heteropaternal superfecundation

A multiple pregnancy involving multiple ova as well as more than one father

Nondisjunction

Error in meiosis in which homologous chromosomes fail to separate.

Down Syndrome

a condition of intellectual disability and associated physical disorders caused by an extra copy of chromosome 21.

Meiosis produces

Sperm in males and eggs in females