3.1 Binary fission

3.1

Binary fission

KEY IDEAS

The cell cycle

All cells come from pre-existing cells (biogenesis). Single-celled organisms reproduce through the process of cell division. Multicellular organisms grow from the division of a single cell (a fertilised egg), which continues to divide and grow to become a whole organism. This process needs to continue throughout a multicellular organism’s life to maintain normal day-to-day function, growth and repair. An example of this is the constant production of red blood cells by the stem cells in your bone marrow.

The process of cell division is an important part of the cell cycle, from the first time a cell is formed from the division of its parent cell, until its own division to produce two new daughter cells.

For division to be successful, each daughter cell must be genetically identical to its parent. This means the DNA of each parent cell must first make a copy of itself before the cell can divide into two.

biogenesis

the production of new living organisms from Binary fission other living organisms Prokaryotes (bacteria) have a single circular cell cycle DNA molecule that carries their genetic the process of a cell material. Copying this DNA is the first step growing, dividing and dying in the cell division that is called binary fission (Figure 1). binary fission a form of asexual During binary fission, a cell grows to reproduction in which almost twice its size before splitting into two the parent cell splits equal-sized daughter cells. Prokaryotes do into two equal-sized daughter cells not have a nucleus, so their genetic material is located in the cytoplasm of the cell. The DNA that makes up a prokaryote’s genetic material is more than 500 times longer than the cell. To fit inside the cell, the DNA molecule is tightly wound around proteins to form a chromosome. When the prokaryote replicates, the DNA molecule unwinds to make a single copy of itself. Each copy of DNA then attaches to a different part of the cellular membrane. As the cell grows longer (elongates), the two chromosomes are pulled apart. Once the elongated cell is large enough, the plasma membrane pinches in the centre, dividing the bacterial cell in half. A new cell wall forms around the two genetically identical daughter cells (Figure 2).

In this topic, you will learn that: ✚ the cell cycle produces new cells from existing cells ✚ reproduction of prokaryotic cells occurs through binary fission. Study tip It is important to describe daughter cells as ‘genetically identical to the parent cell’ rather than ‘identical to the parent cell’. This is because the daughter cells may have different numbers of individual organelles from the parent cell or other daughter cells and so are not completely identical. FIGURE 1 Binary fission occurs when a cell divides into two equal-sized daughter cells. Here, Paramecium aurelia is going through binary fission. DRAFT ONLY - NOT FOR SALE

Binary fission has many benefits. Only one parent cell is needed to produce two cloned daughter cells. Binary fission can also occur rapidly, producing many daughter cells in a short time. For example, some bacteria can double their numbers in as little as 20 minutes. Worked example 3.1 demonstrates the effect of this rapid bacterial growth. The disadvantage of binary fission is that genetically identical daughter cells are all affected in the same way by changing environmental conditions. This is an advantage to humans when antibiotics are used to treat a bacterial infection. If one of the bacterial cells is vulnerable to the antibiotics, then they all will be. 4 Two identical daughter cells are produced. FIGURE 2 Prokaryotic cells divide by binary fission to produce two genetically identical daughter cells.

WORKED EXAMPLE 3.1 CALCULATING NUMBER OF CELLS

A person sneezed while preparing dinner and one bacterial cell from the sneeze fell onto the warm food. The food containing the bacterial cell was then left on the bench for 2 hours before being put into the fridge, where it took another hour to cool to below 4°C and for the cells to stop reproducing. If the single bacterial cell was able to reproduce (and double in number) every 20 minutes before it cooled to below 4°C, how many bacterial cells would be present in the food?

SOLUTION

One cell doubles every 20 minutes. After 1 hour, the cell has doubled three times and after 3 hours, the cell has doubled nine times. To calculate the number of cells at the end of 3 hours, use the formula: Number of cells = 2n where n represents the number of replications. Number of cells after 3 hours = 29 = 512 bacterial cells

Cell wallPlasma membrane DNA (nucleoid) 2 The cell elongates. 3 The two chromosomes are pulled apart. The plasma membrane pinches in the centre. The cell divides. 1 DNA unwinds and makes a copy of itself. Video Worked example 3.1: Calculating number of cells DRAFT ONLY - NOT FOR SALE

CASE STUDY 3.1

Antibiotic resistance

Describe and explain

1 Explain the term ‘genetically identical daughter cells’. 2 Define ‘binary fission’. 3 Describe the process of binary fission. Use a series of diagrams to illustrate your answer.

Apply, analyse and compare

4 Compare the advantages and disadvantages of binary fission. The replication of DNA molecules is not always accurate and errors can be made. In most cells, the errors are detected and repaired. CHECK 5 Read Case study 3.1. Explain how bacteria can pass on antibiotic resistance to their daughter cells or to other bacterial cells. Design and discuss 6 Discuss why it is important that food is not left out of the fridge for too long. YOUR LEARNING 3.1 Some errors can be useful for the organism. For example, a change in the DNA can make a bacterium resistant to an antibiotic. When all the other bacterial cells are dying, the newly resistant bacteria will survive and reproduce. This can mean a patient starts to get well, but then, after a short period of time, the sickness returns again. Bacteria can pass on their antibiotic resistance through the exchange of a type of DNA called a plasmid. This DNA found in plasmids is not part of the cell’s chromosome, but it can carry sections of DNA that allow the cell to be resistant to a specific antibiotic. Bacterial cells can exchange plasmids with each other through direct cell contact or by a hair-like bridge called a pilus (Figure 3). When a bacterial cell replicates its chromosome, it also makes copies of the plasmid to pass on to both daughter cells. FIGURE 3 Bacteria can pass extra genetic material (plasmids) from other cells through the pilus. Pilus DRAFT ONLY - NOT FOR SALE 7 Design a poster that explains how to avoid food contamination.