Mitosis&

The Cell Cycle

Unit – Cell Reproduction

Biology

How many cells do you think your body has?

Each of us began as a single cell,

so one important question is:

How did that single cell develop into a body with more than a

trillion cells?

Cell Division

The production of such a large number of body cells is accomplished by many, many repeats of a cycle of cell division in which one cell gives rise to two cells, each of which in turn gives rise to two cells, etc.

Thus, cell division is needed for growth.

Even in a fully grown adult, cells still undergo cell division.

Why is this useful?

Think about your skin, for example.

Cell Division

The two cells that come from the division of one cell are called daughter cells.

Each of the daughter cells needs to have a complete set of chromosomes.

What are chromosomes?

Chromosomes are

compact

spools

of

DNA

Chromosomes

• The chromosome (left in the picture) was isolated from a dividing cell.

• Notice that the chromosome consists of two identical halves

• Each half is called a chromatid

ChromosomesThe point

where the two chromatids touch and are held together is called a centromere

Review chromosome structure

Let’s make some chromosomes

Obtain 2 white pipe cleaners and 2 of another color.

Cut white pipe cleaners into 2:

4 inch long pieces

3 inch long

2 inch long

1½ inch long

Let’s make some chromosomes

• Pick two different colored beads. You will need 8 of each color

• Put one of the colored beads on each of the white pipe cleaners and move them to the center of the pip cleaners.

• Repeat by putting one of the other colored beads on each of the colored pipe cleaners

Chromosome Numbers

Each species has a certain number of chromosomes in each cell.

46

18

7852

Sex chromosomes

• Chromosomes that determine the sex of the organism

• In humans, sex chromosomes are either X or Y

XX XY

Autosomes

• All the other chromosomes in an organism

Unsorted Human Chromosomes

KaryotypePhotomicrograph of the chromosomes in a dividing cell found in a human

What is the sex of the person whose chromosomes are shown here?

Question

• What percent of your genes come from your mom?

• What percent of your genes come from your dad?

Where did my chromosomes come from?

• We inherit one member of each chromosome pair from each parent. 

Where did my chromosomes come from?

• The 46 chromosomes in our somatic cells are actually two sets of 23 chromosomes—a maternal set (from our mother) and a paternal set (from our father.)

Homologous chromosomes• The two copies of each chromosome

• They are the same size and shape and carry genes for the same traits

Diploid Cells

• Cells having two sets of chromosomes

• All normal human cells are diploid cells (except reproductive cells – egg and sperm cells)

• Commonly abbreviated as 2n

• In humans the diploid number is 46

Haploid Cells

• Human sperm cells and egg cells

Haploid Cells

• Contain only 1 set of chromosomes• Haploid cells have only 1 chromosome of

each homologous pair and only 1 sex chromosome

• Abbreviated as 1n• When a sperm (1n) and egg cell (1n)

combine, the new cell will be diploid (2n)

A comparison

Haploid Diploid

Cell Division

How do you think each daughter cell gets a complete set of chromosomes?

In each cycle of cell division, the cell first makes a copy of all of the DNA in each of the chromosomes.

Cell Division

After the DNA in each chromosome has been copied, the cell undergoes a type of cell division called mitosis, which carefully separates the two copies of each chromosome to opposite ends of the dividing cell, so each daughter cell ends up with a complete set of chromosomes.

What is the cell cycle?

• The repeating set of events that make up the life of a cell

• Cell division is one phase of the cell cycle

What is the cell cycle?• Interphase

– G1, S, G2 phase

• Mitosis (M phase)

– Prophase – Metaphase– Anaphase – Telophase

• Cytokinesis

Interphase• The time between cell divisions

• Divided into 3 phases (G1, S, G2 phases)

G1 phase• Offspring cells grow to mature size

S phaseCell’s DNA is copied

S phase

Cell’s DNA is copied

G2 phaseThe time to prepare for cell division

G2 phase• Centrosomes – two pairs of dark spots; it

is the microtubule organizing center

• Centrioles - structures in the cytoplasm from which the spindle fibers form

G0 phase• Cells can exit the cell cycle and enter a

state called the G0 phase

• Cells do not copy DNA and do not prepare for cell division

MitosisDivision of the nucleus

Prophase• DNA tightens

and coils into chromosomes

• Nuclear membrane breaks down and disappears

Metaphase• Chromosomes

line up in the center of the cell

Anaphase• The chromatids

of each chromosome separate at the centromere

• chromatids move to opposite poles of cell

Telophase

• Chromosomes return to chromatin state

• A nuclear membrane forms around each set of chromosomes

Cytokinesis• The cytoplasm of the cell divides

• The area of the cell membrane that pinches in and eventually separates the dividing cell into two cells is called the cleavage furrow.

Cytokinesis

Cytokinesis• Vesicles formed by the

Golgi apparatus fuse at the midline of the dividing cell, forming a membrane-bound cell wall called the cell plate.

Plant cells in various stages of mitosis

Cell Cycle

Cancer cells

Cancer cells

Cancer cells

Cancer cells