Describe the cell cycle and mitosis as the process that produces two genetically identical daughter cells, and explain its role in growth, repair, and asexual reproduction (MA STE HS-LS1-4, HS-LS3-2 supporting).

What this topic is asking

The Massachusetts STE framework expects you to understand how cells divide to build and maintain a body, and how that division keeps the genetic information stable. On the High School Biology MCAS, mitosis is usually tested by comparing it with meiosis (the next topic), or by asking about chromosome number, the genetic make-up of daughter cells, and why the body needs this kind of division. The crosscutting concepts are stability and change and structure and function.

The cell cycle

The order matters. A cell does not divide its DNA randomly; it first copies every chromosome (this is DNA replication, covered in DNA structure and replication), so that there is a full set to give to each new cell. Only then does it divide. Copying first is what makes the two daughter cells identical.

What mitosis produces

In mitosis, the copied chromosomes are separated so that each daughter cell receives one complete set:

• The two daughter cells are genetically identical to each other and to the parent cell.
• Each daughter cell has the same number of chromosomes as the parent. A human body cell has 46 chromosomes, so mitosis produces two cells of 46 chromosomes each.

This is the defining feature of mitosis and the main thing the MCAS checks: same number of chromosomes, identical genetic make-up. It contrasts sharply with meiosis, which halves the chromosome number and produces variation (see meiosis and sources of variation).

Why the body needs mitosis

Mitosis serves several roles, all of which depend on making more identical cells:

• Growth. A multicellular organism grows by making more cells, all carrying the same genetic instructions.
• Repair. Damaged tissue is healed by producing new cells to replace those that were lost.
• Replacement. Worn-out cells (such as skin and gut-lining cells) are constantly replaced.
• Asexual reproduction. Some organisms reproduce by mitosis, producing offspring that are genetically identical to the parent (a clone).

In each case, the new cells must carry the correct instructions, which is why genetic identity is so important: a new skin cell must be able to function as a skin cell.

Try this

Q1. State how the daughter cells produced by mitosis compare with the parent cell. [2]

• Cue. They are genetically identical to the parent and to each other, with the same number of chromosomes.

Q2. Explain why a cell copies its DNA before dividing by mitosis. [2]

• Cue. So there is a complete set of chromosomes for each daughter cell, and both cells receive identical genetic information.