Some human traits and diseases are completely heritable, some partiallyheritable, and some are not influenced by genes at all

Results of class survey

Results of class survey

II. Mitosis and Meiosis

II. Mitosis and MeiosisHuman karyotype (a photograph of the matched chromosomes)

22 pairs of homologous chromosomes (one paternal, one maternal)1 pair of sex chromosomes

II. Mitosis and MeiosisA cell cycle

II. Mitosis

Replication of the DNA of each chromosome. In the formof chromatin, DNA is NOT condensed. It is long and stringy.

II. Mitosis

Note that the chromosomes havebeen duplicated. The two membersof the duplicated pair are knownas sister chromatids. Centriolesmigrate to opposite poles of the cell.Centrioles may be responsible fororganizing microtubules intospindle fibers.

sister chromatids

Figure depicts two chromosomes (long and short), and male (blue)and female (orange) homologous copies of those chromosomes

II. Mitosis

Figure depicts two chromosomes (long and short), and male (blue)and female (orange) homologous copies of those chromosomes

II. Mitosis

Period of movement of sisterchromatids to the equatorialplane of the cell. Movementoccurs by binding ofmicrotubules to thekinetochore, a centromericstructure composed of layeredproteins.

Figure depicts two chromosomes (long and short), and male (blue)and female (orange) homologous copies of those chromosomes

Note that homologous pairs of chromosomes do not pair upThis is important because they do pair up in Meiosis I

II. Mitosis

II. MitosisFigure depicts two chromosomes (long and short), and male (blue)and female (orange) homologous copies of those chromosomes

The sister chromatids of eachchromosome disjoin. For completedisjunction to occur, each centromericregion must be split in two, signalingthe initiation of anaphase.

Once anaphase occurs, each sister chromatid is referred to as a daughterchromosome.

II. Mitosis and MeiosisBasic chromosome anatomy

II. Mitosis

cytokinesis - division or partitioning of the cytoplasm

II. Mitosis

II. Mitosis and Meiosis

II. Mitosis and Meiosis

Unlike Mitotic Prophase, members of each homologouschromosome synapse and crossing over (i.e. recombination) occurs

synapsis,crossing over

homologysearchbegins

chiasmatavisible

II. Mitosis and Meiosisalignment of tetrads is random,resulting in random separationof dyads. This is the basis forthe Mendelian postulate ofindependent assortment.

a single centromere holds thesister chromatids together. Itdoes not divide duringMeiosis I.

The separation of homologouschromosomes duringAnaphase I is referred to asdisjunction

Meiosis I

II. Mitosis and MeiosisThe products of Meiosis I are twocells, each with a pair ofrecombined homologous chromosomes. Each pair is referredto as a dyad. The two chromosomesthat make up the pair are alsoreferred to as sister chromatids.

The products of Meiosis II are fourcells (two cells from each of thecells produced by Meiosis I), eachwith a single recombinedchromosome. These chromosomesare referred to as monads.

II. Mitosis and Meiosis

The development of gametes varies during spermatogenesis and oogenesis

In humans, the firstdivision of all oocytesbegins in the embryonicovary, but arrests inProphase I. Meiosis resumes just prior toits ovulation.