Chromosome structure

· Chromosome = long molecule of DNA wound around histone proteins.
· Histones help package DNA into a compact structure so it can fit inside the nucleus.
· After DNA replication, each chromosome consists of two genetically identical sister chromatids.
· Sister chromatids are joined at the centromere.
· Centromere = region where spindle fibres attach during nuclear division.
· Telomeres = protective DNA sequences at the ends of chromosomes.
· Key exam point: after S phase, a chromosome has two sister chromatids, but it is still counted as one chromosome until the chromatids separate.

This diagram shows how DNA is packaged with histone proteins to form chromosomes. It helps link the molecular structure of DNA to the visible chromosome seen during cell division. Source

Importance of mitosis

· Mitosis produces genetically identical daughter cells.
· Daughter cells have the same number and type of chromosomes as the parent cell.
· Important for growth of multicellular organisms by increasing cell number.
· Important for replacement of damaged or dead cells.
· Important for repair of tissues by cell replacement.
· Important in asexual reproduction, where offspring are genetically identical to the parent.
· Exam phrase: mitosis maintains genetic uniformity because DNA is replicated and then equally distributed into daughter nuclei.

The mitotic cell cycle

· Mitotic cell cycle = sequence of events from one cell division to the next.
· Main stages: interphase, mitosis, cytokinesis.
· Interphase = longest part of the cycle; cell is active and prepares for division.
· G₁ phase: cell grows and produces organelles/proteins.
· S phase: DNA replication occurs, forming sister chromatids.
· G₂ phase: further growth and preparation for mitosis.
· Mitosis = division of the nucleus to produce two genetically identical nuclei.
· Cytokinesis = division of the cytoplasm to form two separate daughter cells.
· Do not confuse mitosis with cytokinesis: mitosis divides the nucleus; cytokinesis divides the cell.

This diagram summarises the mitotic cell cycle, showing growth phases, DNA replication and division. It is useful for remembering that S phase is when DNA is copied before mitosis. Source

Telomeres

· Telomeres are found at the ends of chromosomes.
· Their role is to prevent loss of genes from chromosome ends during DNA replication.
· DNA replication can shorten chromosome ends, so telomeres act as protective buffers.
· Key exam phrase: telomeres reduce the risk that important coding DNA / genes are lost during repeated cell divisions.
· Telomeres are not genes for proteins; they are protective repetitive DNA sequences.

This image helps show that telomeres are located at the ends of chromosomes. Their protective position explains why they help prevent gene loss during DNA replication. Source

Stem cells, cell replacement and tissue repair

· Stem cells are unspecialised cells that can divide by mitosis.
· Stem cells can produce more stem cells and can also produce cells that differentiate into specialised cell types.
· In tissue repair, stem cells replace damaged or dead cells by mitotic division.
· This is important in tissues that need frequent replacement, such as skin or blood-forming tissues.
· Exam phrase: stem cells support cell replacement and tissue repair because they can divide and produce new specialised cells.

This diagram shows how stem cells can divide and produce cells that become specialised. It links stem cell division by mitosis to tissue repair and cell replacement. Source

Uncontrolled cell division and tumours

· Normal cells have controls that regulate when they divide.
· Uncontrolled cell division occurs when these controls fail.
· Repeated uncontrolled mitosis can produce a mass of abnormal cells called a tumour.
· A tumour may disrupt normal tissue function by occupying space and competing with normal cells.
· Key exam phrase: a tumour forms when cells continue to divide by mitosis without normal regulation.
· Avoid saying all tumours are cancerous unless the question specifies malignant tumour or cancer.

This diagram shows how repeated uncontrolled cell division can produce a tumour. It is useful for linking loss of cell-cycle control to abnormal growth. Source

High-yield exam wording

· Use genetically identical daughter cells, not just “same cells”.
· Use DNA replication in S phase, not “DNA replication in mitosis”.
· Use mitosis = nuclear division and cytokinesis = cytoplasmic division.
· Use sister chromatids joined at a centromere when describing replicated chromosomes.
· Use telomeres prevent loss of genes from chromosome ends for telomere questions.
· Use uncontrolled cell division forms a tumour for tumour questions.

Common mistakes to avoid

· Do not say chromosomes are made only of DNA; they also include histone proteins.
· Do not say sister chromatids are different; they are genetically identical copies.
· Do not say mitosis creates variation; mitosis maintains genetic uniformity.
· Do not say DNA replicates during G₁ or G₂; DNA replication occurs in S phase.
· Do not confuse growth of a cell with growth of an organism; multicellular growth occurs mainly by increasing cell number through mitosis.
· Do not describe detailed prophase/metaphase/anaphase/telophase events unless the question is specifically asking about chromosome behaviour in mitosis.