Unit 5 - Heredity (5.1 - Meiosis)

Asexual reproduction

Single cell passes genes to its offspring without the fusion of gametes (ex: binary fission)

Sexual reproduction

Two parent cells give rise to offspring that have unique combinations of genes inherited from both

Chromosomes in human cells

Human somatic cells have 23 pairs of chromosomes

• each pair consists of 2 homologous chromosomes, or homologs (1 from each parent)

Karyotype

An ordered display of the pairs of chromosomes

Meiosis

Like mitosis, chromosomes are first duplicated… then meiosis happens in two sets of cell divisions, meiosis I and meiosis II

• instead of 2 diploid daughter cells, the 2 sets of cell divisions result in 4 haploid daughter cells

Stages of Meiosis I - Prophase I

• chromosomes condense

• Homologous chromosomes pair up and form tetrads

• Crossing over occurs = non sister chromatids exchange DNA

  • average of 1-3 crossing over events per pair in humans

Synapsis and crossing over

• zipper-like structure called synaptonemal complex forms during this synapsis (attachment)

• DNA breaks at matching points and a piece of paternal chromatid replaces piece of maternal chromatid, and vice versa, producing recombinant (recombined) chromosomes

• Each homologous pair has 1 or more X-shaped regions called chiasmata

Metaphase I

• homologous pairs line up randomly at the equator via independent assortment (randomly)

• Microtubules from one pole attach to kinetochore of one chromosome of each tetrad

Independent assortment of chromosomes

• Each pair of chromosomes sorts maternal and paternal homologs pair into daughter cells independently of the other cells

• Number of combinations possible is 2n (humans: 2²3)

Anaphase I

• pairs of homologous chromosomes separate

• One chromosome move toward each pole

• Sister chromatids remain attached at the centromere and move as one unit

Telophase I and Cytokinesis

At the beginning of telophase I, each half of the cell has haploid set of chromosomes!

• each chromosome still consists of two sister chromatids

• Cytokinesis usually occurs simultaneously, forming two haploid daughter cells

• No new chromosome duplication before meiosis II

Prophase II

• Spindle apparatus forms

• Chromosomes still composed of two chromatids

Metaphase II

• Chromosomes are arranged at the equator

• Because of crossing over in meiosis I, sister chromatids are no longer genetically identical

• Kinetochores attach to microtubules extending from opposite poles

Anaphase II

• Sister chromatids separate and move as two newly individual chromosomes

Telophase II and Cytokinesis

• Nuclear membranes form, and chromosomes start to loosen

• Final result is four daughter cells, each with haploid set of unduplicated chromosomes

• Each daughter cell is genetically distinct from others, and from the parent cell

Origins of Genetic Variation Among Offspring

*Mechanisms contribute to genetic variation:

1) crossing over

2) independent assortment

3) random fertilization

• any sperm can fuse with any egg

Crossing over

Mutations are original source of genetic diversity

Independent assortment

Reshuffling of genes during meiosis produces more genetic variation

Random fertilization

Natural selection results in the accumulation of genetic variations favored by the environment