Mitosis and Cell Division

Mitosis Cells divide to make more cells. -this allows organisms to grow, develop, maintain and repair themselves Mitosis is Asexual (1 parent) While all of the organelles can be randomly separated into the daughter cells, the chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA.

The Challenge of Mitosis Normal human cells have 23 pairs of chromosomes - each pair consists of one copy of the chromosome from each parent – 46 total - after replication, the total number of chromosomes = 92 Each cell produced by cell division needs exactly the correct set of 46 chromosomes

Still More Chromosomes Each of the chromatids is a copy of the original chromosome In mitosis, the two chromatids of each chromosome separate, with each chromatid going into a daughter cell.

Machinery of Mitosis Cells build a structure called the spindle, which is made of microtubules to facilitate the movement of chromosomes during mitosis The spindle fibers are attached to each chromosome at structures called the kinetochore found at the centromere

Machinery of Mitosis: The Spindle In animal cells, the spindle fibers are anchored at either end to a centrosome - each centrosome is composed of a pair of centrioles Plants lack centrosomes and their spindle formation is poorly understood spindle is thought to originate from the centrosomes. In plant cells, it is thought to originate from the region of the nucleus In all cells, the opposite ends of the cell are referred to as the poles

Mitosis Occurs in Phases mitosis occurs in a logical sequence of steps - the occurrences in one phase prepare the cell for the next stage in the process -The phases of mitosis: Prophase Metaphase Anaphase Telophase

Interphase Preparatory phase Chromosomes replicate here Where cell spends most of its time G1-cell growth to mature size S-cell DNA copied (synthesized) G2- preparation for cell division

Prophase – the first step in cell division 1. DNA Condenses. -Chromatin coils into chromosomes 2.The nucleus & nucleolus disappear 3. Centrioles migrate to poles 4. The spindle starts to form, chromosomes attach to spindle fibers 5. Longest Phase The double membrane that surround the nucleus dissolves into a collection of small vesicles, freeing the chromosomes to use the whole cell for division In plant cells, the microtubules seems to originate from the nucleus

Metaphase The chromosomes line up along the middle of the cell – known as the equator Everything is now aligned for the rest of the division process to occur. Shortest Phase.

Anaphase the chromosomes separate at the centromere. - the sister chromatids move toward opposite poles Then the spindle fibers shorten, and the chromosomes are pulled to opposite poles, towards the centrosomes.

Telophase The chromosomes uncoil back to chromatin The spindle disintegrates The nuclear envelope re-forms around the two sets of chromosomes, forming nucleus and nucleolus The cytoplasm is divided into 2 separate cells, the process of cytokinesis begins.

Cytokinesis The organelles get divided up into the 2 daughter cells passively: they go with whichever cell they find themselves in. Plant and animal cells divide the cytoplasm in different ways.

Plant Cell Cytokinesis No pinching because of cell wall. Cell plate forms from the middle to split the cytoplasm. Cell plate is formed by the Golgi Bodies.

Animal Cell Cytokinesis In animal cells, a ring of actin fibers (microfilaments are composed of actin) forms around the cell equator and contacts, pinching the cell in half. Cleavage furrow-cell membrane pinches inward.

Summary of Mitosis Prophase: Chromosomes condense Nuclear envelope disappears Centrioles move to opposite sides of the cell Spindle forms and attaches to centromeres on the chromosomes Metaphase Chromosomes lined up on equator of spindle centrosomes at opposite ends of cell Anaphase Centromeres divide: each 2-chromatid chromosome becomes two 1-chromatid chromosomes Chromosomes pulled to opposite poles by the spindle Telophase Chromosomes uncoil Spindle disappears Nuclear envelope reappears Cytokinesis: the cytoplasm is divided into 2 cells

Vocabulary Histones nucleosomes chromosomes chromatin centromere kinetochore spindle centrosomes poles Prophase metaphase anaphase telophase