Unit 6: Meiosis

Sexual reproduction

Reproduction requiring two parents where each contributes half of their DNA to offspring

Gametes

Sex cells (sperm and egg) that are haploid and carry one set of chromosomes

Fertilization

The union of a sperm and egg cell

Zygote

A fertilized egg that is diploid

Advantage of sexual reproduction

Produces genetic diversity which increases adaptability and survival

Disadvantages of sexual reproduction

Requires a mate

Diploid (2N)

A cell containing two sets of homologous chromosomes

Haploid (1N)

A cell containing one set of chromosomes

Homologous chromosomes

Chromosome pairs with the same genes but possibly different alleles

Chromosome number in humans

Humans have 46 chromosomes or 23 homologous pairs

Purpose of meiosis

To reduce chromosome number by half to form gametes

Meiosis

A two-division process that produces four haploid cells from one diploid cell

Meiosis I

Division where homologous chromosomes separate

Meiosis II

Division where sister chromatids separate

Crossing over

Exchange of genetic material between homologous chromosomes in prophase I

Tetrad

A structure of four chromatids formed during prophase I

Interphase (meiosis)

Phase where DNA is replicated before meiosis begins

Prophase I

Homologous chromosomes pair and crossing over occurs

Metaphase I

Tetrads line up at the center of the cell

Anaphase I

Homologous chromosomes separate

Telophase I and cytokinesis

Two haploid cells form

Prophase II

Chromosomes condense and spindle fibers reform

Metaphase II

Chromosomes line up in the middle of the cell

Anaphase II

Sister chromatids separate

Telophase II and cytokinesis

Four haploid daughter cells are produced

Independent assortment

Random orientation of homologous chromosomes during metaphase I

Random fertilization

Any sperm can fertilize any egg increasing variation

Nondisjunction

Failure of chromosomes to separate properly

Aneuploidy

Abnormal number of chromosomes

Monosomy

Missing one chromosome

Trisomy

Extra chromosome present