2. Cell division

Cell Division and Mitosis

As seen in the image above cells divide to make new cells. This is a necessary function for both multicellular and unicellular organisms for different reasons. Unicellular organisms, such as Amoeba, divide in order to reproduce i.e. make more unicellular organisms. If they didn’t do this then the species would die out! In multicellular organisms, it is a little bit different. As seen in the picture above, and from your work on fertilisation in S1 you know that you started out life as a single fertilised cell. Just one cell. Now look at you! A lot has changed since then. Now you are made up of millions of cells. Without cells being able to divide and make more cells you would never have been able to grow. So one of the reasons for cell division in multicellular organisms is for growth.

There are two other reasons for cells to divide in multicellular organisms.

1. Replacing dead or worn-out cells
2. Repairing tissue (e.g. cut skin or broken bones

Note: your cells do NOT get repaired. If they are damaged then they will die. However tissues such as skin, muscle, bone etc can be repaired if they are damaged by making more cells – please do not confuse the two.

In mature animals i.e. your parents compared to you, cells will only divide for the latter two reasons. They will not divide for growth. Mature animals will have stopped growing so cells will only continue to divide to replace dead cells or repair tissue. In mature plants, it is a slightly different story and cells at the root and shoot tips always continue to grow.

We looked at the basics of cell division, looking the whole cell in the cases of plant and animal cells.

So in both cases, we start with a single parent cell, the nucleus divides into 2, the remainder of the cell divides and 2 new daughter cells are formed. As the nucleus contains all the genetic information for a cell to function, and being the nucleus, is in control of cell division we must take a closer look at what is actually happening in the nucleus.

Genetic information is stored in the nucleus in long stretches of DNA called chromosomes. Chromosomes are thread-like structures and without them, the cell can not develop properly.

A chromosome is made of tightly coiled DNA (we’ll come to DNA later in the year) and looks like an X-shape under the microscope.

A X-shaped chromosome is made of 2 sister chromatids, joined together by the centromere. The sister chromatids are essentially copies of each other and are genetically identical. This is very important as we’ll see later on. In humans, every normal body cell (i.e. not the sex cells) contains 46 chromosomes. Each and every one of these chromosomes contains important information, without with the cell wouldn’t survive. Having just one chromosome missing or extra will mostly likely result in cell death.

The image above shows all 46 chromosomes from a single cell. They have been stained to see clearly and seen under a very powerful microscope. This type of image, with all the chromosomes paired in this way is called a Karyotype. While all human body cells contain 46 chromosomes, this is not the same for all organisms.

Each species of organism has a specific and characteristic number of chromosomes within each cell. This is known as the chromosome complement.  Since chromosomes provide the genetic information for each species, it is essential that each cell receives the full complement of chromosomes. This ensures that all animal and plant cells are provided with all the characteristics of their species.

So if a single human body cell has 46 chromosomes in its nucleus, and that cell divides to produce 2 new cells each also having 46 chromosomes in their nucleus – how does that work? That is where we have to look closer at how mitosis works.

Mitosis (cell division) is a cycle of steps that cells undergo when dividing to produce 2 new daughter cells.

Stages of Mitosis handout

This clip from BBC shows a time lapse of mitosis happening in a cell.

You must be able to describe what is happening in each of the stages in terms of the chromosomes/chromatids as practised in class.

The mitosis song is added below. Remember you will NOT be asked the names of the stages – you just need to be able to describe what is happening.

This video from youtube provides a very full summary of the work we have done so far. It goes into more detail that you need to know so ignore all the extra protein names and concentrate on the chromosomes/chromatids and how he describes the steps. I’ve added it because it is a good summary and some of you like to push yourselves but don’t get bogged down in all the extra terms he uses. In case you are wondering, no we will not be going on to look at meiosis (cell division in sex cells).

Scientists take advantage of cell division by growing cells in the lab. The process of growing cells in the lab in either solid or liquid media is called cell culture. Cells are grown in culture to make food and drink such as mycoprotein (Quorn), beer and wine, to test antibiotics, vaccine and medicines and to learn about cell division. It is possible to grow a layer of cells in dishes/bottles which can then be used for a variety of purposes. Current research is being done to use cells grown in the lab as skin grafts for burn victims and even to grown whole entire organs – this is very much still experimental!

This clip from the BBC shows cell culture to grow new skin.

We did our own little version of cell culture by taking a yeast culture in medium and preparing a streak plate to grow the yeast on solid agar (jelly). When carrying out cell culture it is extremely important that any chance of contamination is limited. So there are certain rules we follow to make sure only the cell of interest is grown and microbes from the surrounding environment don’t interfere with your cells. These rules and way of working is called ‘aseptic technique’.

The video below demonstrates a very similar procedure to that which you carried out (up to about 6 mins – after that is a different procedure).