New cells are important for both growth and repair of damage tissues, so our cells need to be able to replicate themselves. They do this during the cell cycle, this includes mitosis.

The cell cycle

In multicellular organisms, not all cells keep their ability to divide. The ones that do follow a process called the cell cycle. The cycle starts when a cell has been produced by cell division and ends with the cell dividing to produce two identical cells. The cell cycle (see below) consists of a period of cells growth and DNA replication, called the interphase, and a period of cell division called mitosis. Interphase (cell growth) is subdivided into three separate growth stages. These are called G₁, S and G₂.

The cell cycle.png

Mitosis

There are two type of cell division - mitosis and meiosis. Mitosis occurs as part of the cell cycle and is used for growth and repair (meiosis is specifically for the replication of gametes (sex cells)). In mitosis, a parent cell divides to produce two genetically identical daughter cells (they contain an exact copy of the DNA of the parent cell).

While mitosis is really one continuous process, it is described as a series of division stages - prophase, metaphase, anaphase and telophase.

More information on the cell cycle is in this handy Amoeba Sisters video:

The structure of chromosomes in mitosis

As mitosis begins, the chromosomes are made of two strands joined in the middle by a centromere. The separate strands are called chromatids. Two stands on the same chromosome are called sister sister chromatids. There are two to start because each chromosome has already made an identical copy of itself during the interphase. When mitosis is over, the new chromatids end up as one-strand chromosomes in the new daughter cells.

1. Prophase

The chromosomes condense, getting shorter and fatter. Tiny bundles of protein called centrioles start moving to opposite ends of the cell creating a network of fibres across it called the spindle. The nuclear envelope (the membrane around the nucleus) breaks down and the chromosomes lie free in the cytoplasm

2. Metaphase

The chromosomes (each with two chromatids) line up along the middle of the cell and become attached to the spindle by their centromere.

3. Anaphase

The centromeres divide, separating each pair of sister chromatids to opposite poles (ends) of the spindle, centromere first. The chromatids appear in a V-shape at this time.

4. Telophase

The chromatids reach the opposite poles on the spindle. They uncoil and become long and thin again. They are now called chromosomes again. A nuclear envelope forms around each group of chromosomes, so there are now two nuclei. The cytoplasm divides (cytokinesis) and there are now two daughter cells that are genetically identical to the original cell and to each other. Mitosis is finished and each daughter cell is ready to start the interphase part of the cell cycle to get ready for the next round of mitosis.

This link also provides a useful animation of the process

Here is a great Amoeba Sisters video to help you learn this:

Cell replication in bacteria

Prokaryotic cells replicate by a process called binary fission. In binary fission, the cell replicates copies of its genetic material before physically splitting into two daughter cells.

The process of binary fission

Step 1:

The circular DNA and plasmids replicate. The main DNA loop is only replicated once, but plasmids can be replicated many times.

Step 2:

The cell gets bigger and the DNA loops move to opposite 'poll's' of the cell.

Step 3:

The cytoplasm begins to divide (and new cell walls form)

Step 4:

The cytoplasm divides and two daughter cells are produced. Each daughter cell has one copy of the DNA, but can have variable number of copies of plasmids.

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Viral replication

Owing to the fact they are not alive, and are not cells, viruses do not undergo cell division. Instead, they inject their DNA or RNA into the host cell - this hijacked cell then uses its 'own' machinery (such as enzymes, ribosomes) to do the job on behalf of the virus.

In order to inject their DNA or RNA, viruses first attach to the host cell using the host cell surface. They use their attachment proteins to bind to complementary receptor proteins on the cell surface membrane of the host cell. Different viruses have different attachment proteins and therefore require different receptor proteins on the host cells. As a result, some viruses can only infect one type of cell (e.g. some viruses can only infect one species of bacteria), while others can infect lots of different cells (e.g. influenza).

Meiosis

Meiosis is a special type of cell division in sexually-reproducing organisms used to produce the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome. Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and female will fuse to create a cell with two copies of each chromosome again, the zygote.

For more info, watch this Amoeba Sisters video on You Tube