UNIT 1: REPRODUCTION

UNIT A IN YOUR GRADE 9 SCIENCE 9 PROBE TEXTBOOK

2.1 The importance of Cell Division (pg 36)

You have probably encountered the idea that cells divide to create new cells. This process is called cellular division and it provides three main functions:

• Growth – If cell division did not happen all of us would only be one cell big. From the moment of conception onward our cells are dividing to help us grow bigger. So why don’t our cells just get bigger? Why do we need more? As a cell gets bigger it becomes less efficient. That means being made of many, smaller, cells is better.

• Repair – Your cells get damaged all of the time. In order to deal with these damages your cells divide and produce new healthy cells to replace the old damaged ones. Old and dead cells are replaced every second. The cells that you started off life with are long gone.

• Reproduction – Unicellular organisms reproduce by growing larger and then splitting in half. When a bacteria splits in half it creates two identical copies of the original cell.

Some interesting questions exist around cell division. Like why do some cells divide more often than others?

3.2 Cell Structures Involved in Cell Division (Sec 2.2 pg 39) When a cell undergoes cell division all parts of the

cell get copied so the two new cells will have all the parts that they need. Some of those parts are more important to cell division and we will focus on them now:

• The nucleus – This organelle directs cell division. It also contains all of the DNA. During cell division the nuclear membrane dissolves so that the DNA can duplicate and get into each new cell.

• Chromosomes – This is the name for the segments of DNA. Humans typically have 46 (23 pairs) of chromosomes. These get copied during cell division.

• Nucleolus – This part of the nucleus creates ribosomes

• Ribosomes and Endoplasmic Reticulum (ER) – The ribosomes make proteins which help the cell function properly. The rough ER folds and transports the proteins to where they are needed in the cell. The smooth ER manufacturers and transports fats to where they are needed in the cell.

• Cytoplasm – This is the fluid that contains all of the organelles in the cell. The cytoplasm also contains a network of microtubules that act like a rail system and skeleton for the cell. During cell division a segment of the microtubules, called the centrioles, help the cell divide evenly in half.