CHAPTER 10.2 Cell Division

A cell has to go through many preparations to divide. This is important to make sure that each new cell has all the necessary parts to survive.

In eukaryotic cells (cells with a nucleus) the cell undergoes 2 stages in cell division – mitosis – which divides the nucleus, and cytokinesis – division of the cytoplasm.

Most unicellular organisms reproduce through mitosis and cytokinesis.

CHROMOSOMES

Chromosomes are made up of DNA – genetic coded information for the cell, makes proteins.

Every organism has a specific number of chromosomes – humans have 46 chromosomes, fruit flies have 8, and a carrot has 18.

The only time chromosomes are visible is during cell division because normally chromosomes are in the form of chromatin – loosely bound DNA. When getting ready to go through mitosis the chromatin coils into chromosomes.

Before cell division each chromosome makes a copy of itself and so each chromosome is made of 2 identical chromatids – these chromatids will separate and one will go into each new cell. Sister chromatids are held together by a centromere.

THE CELL CYCLE

Cell cycle is like the cells life cycle – it is the events that happen during the life of a cell.

The cell cycle – the cell grows, prepares for division, and divides to form 2 daughter cells. These new daughter cells will then begin their cell cycle.

There are 4 stages of the cell cycle – mitosis and cytokinesis takes place during the M phase. Chromosomes copy during the S phase, there are then 2 growth and activity stages called G1 and G2.

EVENTS OF THE CELL CYCLE

Interphase – the growth stage takes the longest in the cell cycle and G1, S, and G2 are all parts of interphase.

G1 is when the cell does most of their growing – they grow and they make new proteins and organelles.

S is when chromosomes are replicated.

G2 happens after the S phase. This is the shortest of the 3 interphase stages. During this time organelles and molecules necessary for cell division are produced.

After G2 the M phase takes place.

MITOSIS

There are 4 phases of mitosis: prophase, metaphase, anaphase, and telophase.

PROPHASE

This is the 1st and longest stage. The chromosomes become visible during this time and the centrioles take place at opposite ends of the cell. Centrioles are very important for separating sister chromatids. They organize the spindle – a structure that holds and separates the chromosomes.

METAPHASE

During this phase the chromosomes line up at the center (middle) of the cell.

ANAPHASE

During this phase the sister chromatids will separate and start to move to opposite poles.

TELOPHASE

The chromosomes will loosen into chromatin and a nuclear envelope will reform about each new nucleus.

CYTOKINESIS

At the end of mitosis we have a cell with 2 nuclei. Cytokinesis is division of the cytoplasm.

In animal cells the cell membrane is drawn inward until it pinches the cell into 2 new cells.

In plants a cell plate form between the 2 new nuclei and then the plate will become a new membrane and a cell wall will form between the 2 new cells.