Mitosis & Meiosis

Chromatin – Thin, uncoiled strands of DNA & proteins (histones)

Chromosomes – Rod-shaped structures composed of coiled strands of DNA & proteins.

Chromosomes are made up of two sister chromatids held together at a central point called a centromere.

Chromosomes appear in pairs in all sexually reproducing organisms.◦ Called homologous chromosomes ◦ Same shape & same size

Diploid number (2N) – A cell containing both chromosomes of a homologous pair.

Haploid number (1N) – A cell containing one chromosome of a homologous pair.

SOMATIC CELL REPRODUCTION◦Somatic cells are body cells◦Germ cells are reproductive/sex cells

(gametes)◦Mitosis creates two daughter cells that

are exact replicas of the original cell◦Size causes a cell to divide

G1 Period – Cell growth S Period – DNA

Replication G2 Period – Rapid cell

growth Mitosis – Cell division Cytokinesis –

Cytoplasmic division

Comprised of the G1, S, & G2 periods. A period of preparation for mitosis.

Period of nuclear division◦Prophase◦Metaphase◦Anaphase◦Telophase

Chromatin coils to form chromosomes. Nucleolus & nuclear membrane disappear. Spindle fibers begin to develop.

◦ Kinetochore fibers (pole to chromosomes); ◦ Polar fibers (pole to pole)

Formation of asters. Asters: Protein fibers that radiate from each pole.

Chromosomes move to the equator/center of the cell.

Metaphase plate formation

Centromeres of each pair of chromatids divide.

Chromatids separate & move to opposite poles.

Centrioles & spindle fibers disappear

Chromatids unwind to form chromatin

Nuclear membrane & nucleolus reappear

The division of the cytoplasm ANIMAL CELL – cleavage furrow PLANT CELL – cell plate formation

Cell Plate

Process of nuclear division that reduces chromosome number in half.

Used to create germ cells (gametes) Two divisions

◦ Meiosis I◦ Meiosis II

Prophase I◦ Nuclear membrane/nucleolus disappear◦ Chromosomes form from chromatin◦ Homologous chromosomes line up together (tetrads)◦ Crossing over occurs (Genetic Recombination)◦ Spindle fibers form.

Metaphase I◦ Homologous chromosomes move to equator.

Anaphase I◦ Homologous chromosomes separate & move to poles.

Telophase I◦ Cytoplasm divides (forms 2 daughter cells)◦ Nuclear membrane & nucleolus reappear

Crossing Over

Most like Mitosis Prophase II

◦ Nuclear Membrane & nucleolus disappear Metaphase II

◦ Chromosomes line up on equator Anaphase II

◦ Chromatids separate Telophase II

◦ Nuclear membrane & nucleolus reappear◦ Chromosomes uncoil

Spermatogenesis◦ Occurs in males.◦ Four sperm cells are

formed. Oogenesis

◦ Occurs in females.◦ I egg (ovum) formed.◦ 3 polar bodies

formed, but eventually disintegrate.

Not all cells move through the cell cycle at the same rate.◦ Nerve cells don’t divide at all once developed &

cells of skin grow & divide rapidly. Controls on cell growth can be turned on &

off. Cyclins: Proteins that regulate the cell

cycle in eukaryotic cells. Cancer: Cells fail to respond to signals that

regulate the cell growth.