Mitosis

What is Mitosis?Mitosis is the division of the cell’s nucleus

Why Do Cells Need to Divide?Cells want to divide for several reasons:

1) Large cells can’t control their functions as well as small cells

-- as a cell gets bigger, it still has the same amount of DNA to do the same jobs

2) Large cells can’t exchange materials as well

-- the ratio of surface area (cell membrane) to cell volume decreases as a cell gets

bigger, making it harder for food to get in and wastes to go out

3) As cells age, they aren’t as efficient

The Cell CycleCells aren’t constantly dividing; instead, cells grow, live their lives and then divide when necessary.

-- this process of growth and division is known as the cell cycle

The cell cycle consists of 3 parts:

Interphase

Mitosis

Cytokinesis

Image of the Cell Cycle

InterphaseThe cell cycle begins when a new daughter cell is formed and immediately begins interphase. During interphase, the cell undergoes the following life “phases”

G1 Phase

In G1, cells do most of their growing

Cells also synthesize new proteins and organelles in G1

S Phase

In S, cells replicate their DNA so that they have a copy for each new daughter cell

Once S phase begins, it is ready for mitosis

Interphase (continued)G2 Phase

In G2, the cell grows a little bit more

The cell also makes the organelles and molecules required for mitosis while in G2

Once G2 ends, mitosis begins!

About ChromosomesOnly when the cell is about to begin mitosis (and during mitosis) do chromosomes become visible.

Chromosomes consist of a chromatid (a strand of DNA) with a condensed area called the centromere.

-- usually, centromeres are near the middle of a chromatid

At the end of each chromosome is an area of repetitive DNA called a telomere

DNA (and therefore chromosomes) were replicated during the S phase of interphase, so each chromosome actually consists of two “sister” chromatids, and looks like the image on the next page

Humans have 23 pairs of chromosomes, for a grand total of 46 chromosomes

More About TelomeresWhen DNA is replicated (during the S phase of the cell cycle), the enzyme DNA polymerase cannot go all the way to the end of the strand.

The telomere is a non-coding region of repetitive DNA that allows the DNA to replicate all the genetic info without replicating the entire chromosome.

-- this also prevents chromosome fraying and chromosomes from falling apart and attaching to each other

When a new strand of DNA is constructed, enzymes called telomerases rebuild the telomeres to their proper length

Image of a Chromosome

MitosisAs mentioned earlier, mitosis is divided into 4 parts. The parts of mitosis are:

-- prophase

-- metaphase

-- anaphase

-- telophase

ProphaseProphase is the longest phase of mitosis

During prophase:

-- the centrioles line up on opposite sides of the nucleus-- spindle fibers made of microtubules form, and are organized by the centriole

-- the chromatin condenses into visible chromosomes

-- nucleolus disappears and nuclear envelope breaks down

MetaphaseMetaphase only last for a few minutes

During metaphase:

-- the chromosomes line up across the center of the cell

-- microtubules connect to the centromeres

AnaphaseDuring anaphase:

-- the centromeres that join sister chromatids separate and the chromatids become individual chromosomes

-- the chromosomes move apart until they reach the centrioles near the end of the spindle

-- anaphase ends when the centrioles stop moving

TelophaseDuring telophase:

-- the chromosomes begin to tangle back into chromatin and lose their visibility

-- a nuclear envelope begins to re-form around each set of chromosomes

-- the spindle breaks apart

-- nuceloli become visible in each new nucleus

-- this is the end of mitosis!

CytokinesisAlthough cells have split up their genetic material and formed new nuclei, the cells are not split.

Cytokinesis is the process by which cells split their cytoplasm into two new daughter cells

In animals, cytokinesis happens when the cell membrane is drawn in and the cell cleaves into two new cells

In plant, a structure made of wall-building vesicles, called a cell plate forms between the two new nuclei, and eventually becomes a dividing membrane

Full Diagram of Mitosis