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

Centromere

Sister Chromatids

Replicated Chromosomes

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)PhaseThis is when the chromosomes are replicated

– G2(Gap2) PhaseThe 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