0610 · 17.2Mitosis

Mitosis and genetic information

0:00Think about a small cut in your skin. Over time, new cells help replace the damaged tissue, but where do those new cells come from? Existing cells divide to produce them, and the new cells need the same genetic instructions as the cells they replace. In this video, we will explain how mitosis preserves those instructions, why it matters, and how stem cells fit into the picture. First, we need a little background about the nucleus.

0:24In the cells we are considering, the nucleus contains chromosomes, which carry genetic information. Genes are sections of this genetic material, providing instructions that influence how cells function. When a cell produces new cells, passing on a complete set of these instructions is essential. Mitosis is nuclear division giving rise to genetically identical cells. Nuclear division means that the nucleus divides, producing two nuclei with the same genetic information. The cell usually then divides too, producing two daughter cells.

0:56Daughter cells simply means the cells produced by division, not cells that are female. There is an important distinction here. Mitosis describes the division of the nucleus, rather than every part of cell division. The division of the rest of the cell produces the two separate daughter cells. For this topic, you do not need the names or details of the stages of mitosis. You do need to understand how copying and separating chromosomes preserves genetic information.

Chromosome replication and separation

1:21Imagine trying to give two people a complete instruction manual when you have only one copy. Tearing the manual in half would leave each person missing important instructions. Instead, you would first make an exact copy, then give each person a complete manual. A dividing cell faces a similar problem, except its instructions are carried on chromosomes. Before mitosis, exact replication of chromosomes occurs. Replication means copying, so each chromosome now consists of two joined, identical copies.

1:50Notice the timing. The copying happens before mitosis, preparing the genetic material for division. The amount of genetic material has doubled, but each joined pair is still counted as one chromosome at this point. During mitosis, the copies of chromosomes separate. One copy of every chromosome goes into each new nucleus, so both nuclei receive a complete matching set. The chromosome number is therefore maintained in each daughter cell. This is not a process of sharing out half the instructions.

2:18It is a process of distributing two complete sets. Let us work through an example using a hypothetical cell with four chromosomes. We choose four simply because it makes the diagram easy to follow. Before mitosis, each of those four chromosomes is replicated, producing four joined pairs of identical copies. The cell now has enough genetic material to supply two complete nuclei, rather than having to divide its original instructions between them.

2:43During mitosis, the two copies in each pair separate, with one going into each new nucleus. Each nucleus therefore receives four chromosomes, matching the original chromosome number. When the cell divides, each daughter cell has four chromosomes, not two and not eight. Two would mean missing chromosomes, while eight would mean receiving both complete sets instead of one. Now suppose both daughter cells grow, replicate their chromosomes, and divide again.

3:09The two cells become four cells, but each still has four chromosomes. Another round would produce eight cells, each still with four chromosomes. The number of cells increases through repeated division, while the chromosome number within each daughter cell stays the same.

Roles of mitosis and stem cells

3:24This explains why mitosis is important for growth. A growing organism needs more cells, and mitosis allows those new cells to receive the same genetic instructions. For example, cell division near a plant root tip helps produce cells that contribute to root growth. Cells can also enlarge during growth, but mitosis specifically contributes by increasing cell number. Two other roles are repair of damaged tissues and replacement of cells. Repair involves producing new cells where tissue has been damaged, as in our skin example.

3:56Replacement happens during normal life too, because cells can die or be lost even when there is no obvious injury. Mitosis supplies new cells with the genetic information needed to maintain the tissue. Mitosis also has a role in asexual reproduction. Here, one parent produces offspring without the fusion of gametes, and mitosis passes on the parent's genetic information. For example, mitosis contributes to the growth of a new plant from a cutting.

4:21The important connection is that copying and separating chromosomes allows new cells to receive the same genetic instructions as the parent cells. Now consider stem cells, which connect cell division with the different jobs cells perform. Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions. Unspecialised means they have not yet developed the features needed for a particular specialised role.

4:47Becoming specialised means developing structures and abilities suited to a specific job. For example, stem cells in the skin can produce daughter cells that become specialised skin cells. Mitosis supplies the daughter cells with matching genetic information, while becoming specialised prepares them to perform particular functions. These are two different ideas, producing cells and developing their specialised features. Genetically identical does not mean that cells must always end up with identical shapes or jobs.

Common mistakes and exam practice

5:15Watch out for three common mistakes. First, chromosome replication happens before mitosis, not as a way of replacing missing chromosomes afterwards. Second, mitosis maintains chromosome number. It does not halve it. Third, a stem cell is not already a specialised cell, although its daughter cells can become specialised. In an examination, use the command word to guide your answer. If asked to state a role of mitosis, name a role such as growth or repair of damaged tissues.

5:45If asked to describe what happens to chromosomes, give the relevant sequence. Understanding that sequence helps you explain why the daughter cells receive complete matching sets, rather than simply repeating the word identical. Try this exam-style question. A cell containing six chromosomes divides by mitosis. State the chromosome number in each daughter cell and describe how this number is maintained. Begin by recognising that mitosis preserves the original chromosome number.

6:12Then connect the preparation before mitosis with the separation of chromosome copies during mitosis. Each daughter cell contains six chromosomes. Exact replication of chromosomes occurs before mitosis. During mitosis, the copies of chromosomes separate. Each daughter cell receives one copy of every chromosome, maintaining the chromosome number. Let us bring everything together. Mitosis is nuclear division giving rise to genetically identical cells, with chromosome replication occurring beforehand. During mitosis, chromosome copies separate so that each daughter cell receives a complete set and retains the original chromosome number.

6:48This supports growth, repair of damaged tissues, replacement of cells and asexual reproduction, while stem cells produce daughter cells that can become specialised for specific functions.