Cell Division (Mitosis) Section 10-2

Chromosomes In our cells the genetic information that is carried from one generation to the next is carried by chromosomes Chromosomes are made up of DNA Every organism has a specific number of chromosomes Fruit Fly =8 (4 pair) Human = 46 (23 pair) Carrot = 18 (9 pair)

Chromosomes Chromosomes are not visible in most cells except during cell division Before cell division, each chromosome is replicated, or copied Because of this, at the beginning of cell division each chromosome consists of two identical sister chromatids Each pair of chromatids are attached at an area called a centromere

Replicated Chromosomes Centromere Sister Chromatids

The Cell Cycle During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells, each of which then begins the cycle again The cell cycle consists of two distinct phases Interphase M (Mitotic)-phase

The Cell Cycle (Mitosis)

Interphase Interphase is broken down into three different steps (This is not necessarily part of mitosis) G1(Gap1)Phase This is a period in which the cell does most of its growing S (Synthesis)Phase This is when the chromosomes are replicated G2(Gap2) Phase The organelles and other molecules required for cell division are produced

Cell Cycle When the events of G2 are complete the cell is ready to enter the M (mitotic)-Phase Mitosis can be divided into four different phases: Depending on the type of cell: the four phases of mitosis can last a few minutes or several days

1. Prophase The longest phase of mitosis The chromosomes become visible Centrioles, which are two tiny structures located in the cytoplasm near the nuclear envelope, separate and move to opposite sides of the nucleus

Prophase The spindle fibers form These are fanlike microtubule structure that helps to separate the chromosomes Chromosomes become attached to the spindle fibers at the centromere of each chromatid Nucleolus (ribosomes are made here) disappears as does the nuclear envelope

Prophase

2. Metaphase Second phase of mitosis Lasts only a few minutes The chromosomes line up across the center of the cell

Metaphase

3. Anaphase Third phase of mitosis The centromeres that join the sister chromatids separate Allows the chromatids to separate and become individual chromosomes These chromosomes continue to move until they have separated into two groups on opposite sides of the cell

Anaphase

4. Telophase Fourth phase of mitosis Everything that happened during prophase is reversed The spindle fibers disappear Two new nuclear membrane are formed Chromosomes are wrapped up Chromosomes turns back into chromatins

Telophase

Cytokinesis At the end of mitosis, two nuclei--each with a duplicate set of chromosomes--are formed All that remains to complete the cell cycle is cytokinesis, the division of the cytoplasm Cytokinesis usually occurs at the same time as telophase

Cytokinesis Cytokinesis can take place in a number of ways In animal cells the cell membrane is drawn inward until the cytoplasm is pinched into two nearly equal parts Each part contains its own nucleus and organelles

Cytokinesis in Animal Cells

Cytokinesis In plants a cell plate forms midway between the divided nuclei The cell plate gradually develops into a separating membrane A cell wall then begins to appear in the cell plate

Cytokinesis in Plant Cells

Mitosis in Onion Root Tip