Replication and division of nuclei

1. Overview

Before a cell divides it must make an exact copy of its DNA and then share that DNA equally between two new nuclei. This is the job of the mitotic cell cycle: a repeating sequence of interphase, mitosis (nuclear division) and cytokinesis (division of the cytoplasm). Mitosis produces two genetically identical daughter nuclei, which is essential for growth, replacement of dead or damaged cells, tissue repair and asexual reproduction. This topic also covers the structure of a chromosome, the protective role of telomeres, the importance of stem cells in repair, and how a failure of the controls on cell division can lead to a tumour.

Key Definitions

• Chromosome: a structure made of one very long DNA molecule wound around histone proteins, carrying genetic information.
• Histone proteins: positively charged proteins around which DNA is coiled to package and organise it within the nucleus.
• Sister chromatids: the two identical copies of a chromosome, joined at the centromere, that result from DNA replication.
• Centromere: the region holding sister chromatids together and where spindle fibres attach during nuclear division.
• Telomere: a repeated, non-coding DNA sequence at each end of a chromatid that protects the chromosome ends.
• Mitosis: nuclear division that produces two daughter nuclei genetically identical to each other and to the parent nucleus.
• Cell cycle: the sequence of events from one cell division to the next, made up of interphase, mitosis and cytokinesis.
• Mitotic index: the proportion of cells in a sample that are in mitosis, calculated as the number of cells in mitosis divided by the total number of cells.
• Stem cell: an unspecialised cell that can divide repeatedly and differentiate into one or more specialised cell types.
• Tumour: a mass of cells formed by repeated, uncontrolled mitosis.

Content

Chromosome structure

A chromosome is a single, very long molecule of DNA associated with histone proteins. Histones are positively charged (because they are rich in basic amino acids), so they attract the negatively charged phosphate groups of DNA. The DNA coils tightly around the histones, packaging an enormous length of DNA into the small space of the nucleus and helping to control which genes are active.

After DNA has been copied, each chromosome consists of two identical sister chromatids joined at the centromere. Each chromatid is one complete DNA copy. The centromere is the point where spindle fibres attach during nuclear division, allowing the chromatids to be pulled apart. At each end of every chromatid is a telomere — a short, repeated DNA sequence that does not code for proteins.

Counting telomeres — the rule:

• One chromatid has 2 telomeres (one at each end).
• A chromosome made of two sister chromatids (i.e. after the S phase) has 4 telomeres.
• A chromosome made of a single chromatid (before the S phase, or after the chromatids have separated) has 2 telomeres.

So a chromosome is not always made of two chromatids. Always decide which stage the cell is in before you count.

The role of telomeres

DNA replication cannot copy the very end of a DNA strand completely, so a small length is lost each time a chromosome is copied. Telomeres are repeated, non-coding "buffer" sequences at the chromosome ends. Because they carry no genes, the shortening that happens at each replication removes telomere sequence rather than important genes. Telomeres therefore prevent the loss of genes from the ends of chromosomes during repeated rounds of DNA replication, and stop the ends from sticking to one another.

The mitotic cell cycle

The cell cycle has two main parts: a long interphase and a short period of division.

• Interphase is divided into three stages. In G1 the cell grows and makes new organelles and proteins. In the S (synthesis) phase the DNA is replicated, so each chromosome becomes two sister chromatids. In G2 the cell continues to grow and prepares for division, including checking the copied DNA. Interphase is the most active, longest stage — the DNA is not "resting".
• Mitosis is the division of the nucleus into two genetically identical nuclei. The replicated chromosomes condense, line up, and the sister chromatids are separated and pulled to opposite ends of the cell so that each new nucleus receives an identical set.
• Cytokinesis is the division of the cytoplasm to form two separate daughter cells, each containing one of the new nuclei.

The result is two daughter cells that are genetically identical to each other and to the parent cell, because every chromosome was first copied exactly in the S phase and then shared equally.

Measuring division: the mitotic index

You will often be asked to work with the cycle quantitatively, by counting cells in a stained sample of dividing tissue (such as a root tip). The mitotic index is the fraction of cells that are currently in mitosis:

$$\text{mitotic index} = \frac{\text{number of cells in mitosis}}{\text{total number of cells}}$$

A higher mitotic index means a greater proportion of cells are actively dividing. The same counting idea lets you estimate how long a phase lasts: because each stage takes up roughly the same fraction of the cell cycle as the fraction of cells seen in that stage, a stage that contains more cells is a stage that takes more time.

Why genetically identical daughter cells matter

Producing identical cells is essential in several situations:

• Growth of a multicellular organism: many rounds of mitosis increase cell number while keeping all body cells genetically the same.
• Replacement of cells that are constantly lost, such as skin and gut-lining cells.
• Repair of tissues after injury by replacing damaged or dead cells with new, identical ones.
• Asexual reproduction, where a single organism produces offspring that are genetically identical to it (for example, a strawberry plant forming runners, or yeast budding).

The role of stem cells

Stem cells are unspecialised cells that can keep dividing by mitosis and can differentiate into specialised cell types. They are the source of new cells for growth, replacement and repair. For example, stem cells in the bone marrow divide to form new red and white blood cells, and stem cells in the base of the skin replace cells lost from the surface. Because they divide by mitosis, the new specialised cells they give rise to are genetically identical to the original tissue.

Uncontrolled cell division and tumours

The cell cycle is normally tightly controlled by genes that act as checkpoints, ensuring a cell only divides when it should and that the DNA has been copied correctly. Mutations in these controlling genes can cause a cell to divide repeatedly and uncontrollably, ignoring the normal signals to stop. This produces a growing mass of cells called a tumour. A tumour is therefore the result of mitosis happening when and where it should not, rather than mitosis itself being faulty.

Worked example

Exam-style question: A scientist examines a cell taken from a plant root tip in the G2 stage of interphase. The cell contains 8 chromosomes. State, with a reason, the total number of chromatids and the total number of telomeres present. [3]

Model answer:

• The cell is in G2, which is after the S phase, so DNA replication has already taken place and each chromosome consists of two sister chromatids. (1)
• Number of chromatids = 8 chromosomes × 2 = 16 chromatids. (1)
• Each chromatid has two telomeres (one at each end), so number of telomeres = 16 × 2 = 32 telomeres. (1)

Worked example

Exam-style question: A student stains a slice of onion root tip and counts the cells in one field of view. They record: interphase 160, prophase 18, metaphase 9, anaphase 7, telophase 6. Calculate the mitotic index for this field of view, and explain what your value suggests about the proportion of cells that are dividing. [3]

Model answer:

• Cells in mitosis = prophase + metaphase + anaphase + telophase = $18 + 9 + 7 + 6 = 40$ cells. (1)
• Total number of cells = $160 + 40 = 200$ cells, so mitotic index $= \dfrac{40}{200} = 0.20$. (1)
• A mitotic index of 0.20 means about 20% of the cells are in mitosis, so only a minority are actively dividing at this moment — the rest are in interphase. (1)
• Note: mitotic index has no units (it is a ratio). Leave it as a decimal or fraction, or convert to a percentage ($0.20 = 20\%$) only if the question asks for one.

Worked example

Exam-style question: Using the counts from the previous field of view (40 cells in mitosis out of 200 cells in total), and given that one complete cell cycle in this tissue lasts 24 hours, estimate the time the cells spend in mitosis. [2]

Model answer:

• Assume the fraction of cells seen in a stage equals the fraction of the cycle spent in that stage, so time in a phase $=$ (cells in phase $\div$ total cells) $\times$ total cycle time. (1)
• Time in mitosis $= \dfrac{40}{200} \times 24\ \text{h} = 0.20 \times 24 = 4.8\ \text{h}$ (about 4 hours 48 minutes). (1)

Key Equations

This topic is mostly qualitative, but one calculation is expected:

$$\text{mitotic index} = \frac{\text{number of cells in mitosis}}{\text{total number of cells}}$$

When counting chromosome structures, use these rules instead of an equation:

• One chromatid has 2 telomeres.
• A replicated chromosome (two sister chromatids) has 4 telomeres.

Common Mistakes to Avoid

• Mis-counting telomeres. A single chromatid has two telomeres, so a chromosome made of two sister chromatids has four. Do not assume every chromosome has only one telomere, or that it always has four regardless of whether it has been replicated.
• Calling the nuclear double membrane the "nuclear membrane". When you describe the double membrane around the nucleus, use the precise term nuclear envelope.
• Describing interphase as a "resting phase". Interphase is the longest and most active part of the cycle — the cell grows in G1 and G2 and copies its DNA in the S phase.
• Saying mitosis makes the cell "divide into two". Be precise: mitosis is division of the nucleus; the cytoplasm divides separately in cytokinesis.
• Confusing chromosomes and chromatids. Before the S phase a chromosome is one chromatid; after the S phase it is two sister chromatids joined at the centromere. State which stage you mean before counting.
• Slipping up on unit conversions in microscopy calculations. When a cell-division question involves a measured size or a stage micrometer, convert carefully between mm, µm and nm ($1\ \text{mm} = 1000\ \mu\text{m}$ and $1\ \mu\text{m} = 1000\ \text{nm}$). Mixing these up — especially confusing µm with nm — gives an answer that is wrong by a factor of a thousand, so always check the result is a sensible order of magnitude for the structure.
• Saying a tumour forms because mitosis is "broken". A tumour forms because the control of the cell cycle fails (often due to a mutation), causing uncontrolled, repeated mitosis — the process of mitosis itself still copies and separates DNA normally.

Exam Tips

• Learn the order of the cycle precisely: G1 → S → G2 (all interphase) → mitosis → cytokinesis, and remember that DNA replication happens only in the S phase.
• For "genetically identical" questions, link it back to the S phase: identical daughter nuclei arise because each chromosome was copied exactly and then shared equally.
• Use the term differentiate when describing what stem cells do, and give a named example (e.g. bone marrow stem cells forming blood cells) to secure the mark.
• In calculation questions on chromosome number, decide first whether the cell is before or after the S phase, then apply the 2-chromatids / 4-telomeres rule.
• For mitotic-index questions, remember that only mitosis cells (prophase, metaphase, anaphase, telophase) go on the top of the fraction, and that all counted cells go on the bottom; the answer is a proportion between 0 and 1.
• When magnification or measurement appears in a cell-division question, convert units carefully between mm, µm and nm and check the answer is a sensible order of magnitude for the structure.
• For tumour questions, name the cause as uncontrolled cell division / loss of control of the cell cycle, not simply "too much mitosis".