Cell division, cell diversity and cellular organisation (2.1.6)

Cell division, cell diversity and cellular organisation (2.1.6) An overview of cell division, cell diversity and cellular organisation (2.1.6) from OCR A level Biology including: mitosis, meiosis and stem cells

12 min

DNA replication occurs during the S Phase (synthesis), resulting in each chromosome consisting of two identical sister chromatids. This means that the DNA content of the cell doubles, though the number of chromosomes remains the same. The two daughter cells will each receive an equal amount of DNA, identical to the original cell.

Centrioles (centrosomes), which will organise the mitotic spindle, also duplicate during this phase.

During the G₂ Phase (second growth phase), the cell continues to grow and builds energy stores to ensure it is ready to enter mitosis. The cell’s DNA is also checked for errors.

Synthesis of microtubules and other proteins necessary for spindle formation occurs.

Mitosis is the nuclear division process during which a eukaryotic cell separates its duplicated chromosomes into two identical nuclei. It consists of prophase, metaphase, anaphase and telophase and is followed by cytokinesis.

Cytokinesis is the division of the cytoplasm following nuclear division, resulting in the formation of two daughter cells.

Diagram showing stages of cell division: Interphase with tangled chromatin, Prophase with condensed chromosomes, Metaphase with chromosomes aligned at the center, Anaphase with separated chromosomes moving to poles, Telophase with two distinct nuclei forming, and Cytokinesis with two separate cells. Each stage is labeled accordingly.

Checkpoints control the cell cycle:

G₁ checkpoint: determines whether conditions such as cell size and nutrient levels are suitable for entry into the S phase or whether the cell exits the cycle into G₀ (resting).
G₂ checkpoint: ensures that all DNA is replicated correctly and that the cell is ready to divide.
Metaphase checkpoint: makes sure that all chromosomes have been attached to spindle fibres at the equator of the cell.

Prophase is the first stage of mitosis. Chromatin condenses into chromosomes, each consisting of two sister chromatids joined at the centromere.

The nuclear envelope breaks down and the nucleolus disappears.

Centrosomes (centrioles) move to opposite poles of the cell, forming spindle fibres that guide chromosome movement.

The image shows two diagrams labeled 'Early prophase' and 'Late prophase'. In the 'Early prophase' diagram, there are two centrioles with spindle fibers extending from them, positioned outside a nucleus. Inside the nucleus, labeled structures include the nucleolus, and several chromosomes depicted as pairs of connected blue and red lines. The nuclear envelope surrounds the nucleus. The 'Late prophase' diagram shows spindle fibers extending from centrioles towards the center where chromosomes, depicted as red and blue paired lines, are more condensed and aligned. Labels indicate the chromosomes and spindle fibers, with the centrioles positioned at opposite poles.

During metaphase, chromosomes align at the cell equator, also called the metaphase plate.

Spindle fibres from opposite poles attach to each centromere.

Diagram labeled 'Metaphase' showing a cell with a light blue circular background. In the center, blue and red structures labeled 'Chromosomes' are aligned vertically. Each chromosome has a central black dot labeled 'Centromere.' Gold lines extend from the centromeres to structures labeled 'Centriole' at each end of the cell. These lines are labeled 'Spindle fibres.' The entire cell is labeled 'Cytoplasm.'

During the anaphase, the centromeres divide, allowing sister chromatids to separate.

The spindle fibres shorten, moving the chromatids towards opposite poles of the cell.

The image illustrates the anaphase stage of cell division. Inside a blue circle representing the cytoplasm, two groups of chromatids, depicted as red and blue curved lines, are being pulled apart toward opposite sides. Each group is connected to a centriole, represented as a yellow structure with radiating lines, located on either side of the circle. Labels indicate 'Chromatids' pointing to the red and blue structures, 'Centriole' pointing to the yellow structure, and 'Cytoplasm' pointing to the blue circle. The word 'Anaphase' is written at the top of the image.

During telophase, the chromatids are at the cell’s poles and are now chromosomes. The nuclear envelope reforms around each set of chromosomes, creating two nuclei.

The chromosomes uncoil back into chromatin, and the nucleolus is formed.

The mitotic spindle breaks down as the cell prepares for cytokinesis.

The image illustrates the telophase stage of cell division, showing two circular cells connected in the center. Each cell contains a labeled centriole with radiating spindle fibers. Inside each cell, a nuclear envelope encloses chromatids, depicted as red and blue curved lines, some connected by black dots. Labels include 'Centriole', 'Chromatids', 'Nuclear envelope', 'Nucleolus', and 'Cytoplasm'. The title 'Telophase' is centered at the top of the image. The copyright notice '© Medify' is at the bottom center.

Cytokinesis is the final stage of the cell cycle in which the cytoplasm, cell membrane, and organelles divide, resulting in two daughter cells that are genetically identical.

In animal cells, the plasma membrane folds, pinching the cell membrane inwards, forming a cleavage furrow, until the membrane meets in the middle and the cell is split into two.
In plant cells, vesicles migrate to the equator of the cell, and fuse with each other and the cell membrane, dividing the cell in two. New sections of cell walls then form.

Diagram comparing cell division in animal and plant cells. On the left, labeled 'Animal cell,' three stages are shown vertically. Top stage: two blue circles representing cells with visible nuclei, red and blue structures inside, and a 'Cleavage furrow' label at the center where cells are pinching inward. Middle stage: two separate cells with completed division. Bottom stage: two fully separated cells. On the right, labeled 'Plant cell,' three stages are shown vertically. Top stage: two green rectangles representing cells with visible nuclei, red and blue structures, and a 'Cell plate' label at the center where division occurs. Middle stage: two separate but closely aligned cells. Bottom stage: two fully separated cells. Both sequences include arrows indicating progression from top to bottom.

Mitosis is the process by which a cell divides to produce two genetically identical daughter cells.

This process is vital for asexual reproduction, growth and repair of tissue in plants, fungi and animals.

Meiosis is a type of cell division that produces four genetically distinct gametes (sperm and egg cells) and generates genetic variation through independent assortment of chromosomes and crossing over of alleles between chromatids.

It reduces the chromosome number from diploid to haploid.

Meiosis involves two sequential divisions:

Meiosis I – separates homologous chromosomes.
Meiosis II – separates sister chromatids.

The diagram illustrates the stages of meiosis with labeled phases. At the top, a circle labeled 'Interphase' contains two pairs of loosely aligned red and blue squiggly lines. Below, an arrow points to a circle labeled 'Meiosis I' with more condensed and crossed red and blue lines. This circle splits into two circles; the left one has a pair of crossed red and blue lines, and the right one has two sets of red and blue lines. Two arrows point downward from each circle. The left-side arrow leads to a circle with separated red and blue lines. The right-side arrow, labeled 'Meiosis II,' leads to two circles, each containing separate red and blue lines. The entire diagram is labeled with the copyright '© Medify.'

During interphase before meiosis, DNA is replicated during the S phase. Each chromosome is duplicated and consists of two genetically identical sister chromatids joined at the centromere.

During prophase I of meiosis:

Chromosomes condense and become visible.
Homologous chromosomes pair to form bivalents.
Crossing over occurs between non-sister chromatids of homologous chromosomes at chiasmata, exchanging sections of DNA and generating genetic variation.
The nuclear envelope breaks down and spindle fibres form.

The image illustrates the prophase stage of cell division. At the center is a circular cell with a labeled nuclear envelope. Inside the envelope are several chromatids depicted in red and blue. Outside the nuclear envelope, there are two structures labeled spindle fibers, each radiating from a central point. The labels include 'Prophase' at the top, 'Spindle fibre' pointing to the spindle structures, 'Chromatids' pointing to the chromatids, and 'Nuclear envelope' pointing to the nuclear boundary.

During metaphase I of meiosis:

Bivalents line up along the cell equator.
Each homologous pair aligns independently of other pairs, leading to independent assortment of maternal and paternal chromosomes, generating genetic variation.

The diagram illustrates the metaphase stage of cell division. It shows a blue circular cell with chromosome pairs aligned at the center. Two sets of chromatids, one red and one blue, are connected at the centromeres, which are labeled. Spindle fibers extend from the centromeres to the cell poles, where they attach to structures resembling star-shaped centers, also labeled. Annotations include 'Metaphase' at the top, 'Chromatids' pointing to the chromosome pairs, 'Spindle fibre' pointing to the fibers, and 'Centromere' pointing to the central region of the chromatids.

During anaphase I of meiosis:

Homologous chromosomes are pulled to opposite poles by spindle fibres.
Sister chromatids remain joined at their centromeres.

Diagram labeled 'Anaphase' showing a cell during anaphase of mitosis. The cell is oval-shaped with a blue background. Inside, there are two sets of chromosomes, each with two chromatids, one set shown in red and blue on the left and the other on the right. Spindle fibers, shown as yellow lines, extend from centrosomes at each pole of the cell towards the chromosomes. Labels include 'Chromatids' pointing to the chromosomes, 'Spindle fibre' pointing to the spindle fibers, and 'Centromere' pointing to the central region of the chromosomes where the spindle fibers attach. The diagram is © Medify.

During telophase I of meiosis:

Nuclear envelopes may reform around each set of chromosomes (depending on the species).
The cytoplasm divides to form two haploid cells.
Each chromosome still consists of two sister chromatids joined at the centromere.

Illustration labeled 'Telophase' showing two identical cells side by side. Each cell contains a circular nuclear envelope enclosing two pairs of chromatids, one red and one blue. Spindle fibers are visible outside the nuclear envelopes, extending from spindle apparatuses, which are depicted as star-like structures. Labels point to 'Chromatids', 'Spindle fibre', and 'Nuclear envelope'.

During prophase II of meiosis:

Chromosomes condense (if they had decondensed after meiosis I).
Spindle fibres form.
The nuclear envelope breaks down.

The image shows two circular diagrams labeled 'Prophase II'. Each diagram depicts a cell in prophase II of meiosis. The top diagram shows spindle fibres extending from two centrosomes at opposite poles, with two pairs of chromatids, one red and one blue, in the center. Annotations include 'Spindle fibre' pointing to the spindle fibres and 'Chromatids' pointing to the chromatids. The bottom diagram is similar, with spindle fibres and chromatids, and includes an annotation 'Nuclear envelope' pointing to the cell's boundary. © Medify is noted at the bottom.

During metaphase II of meiosis:

Chromosomes line up individually along the equator of the cell.
Spindle fibres from opposite poles attach to the centromeres.

The image shows two cells undergoing Metaphase II, labeled at the top. Each cell is depicted as a blue circle with spindle fibers extending from centrosomes at opposite poles. The spindle fibers are attaching to chromatids, which are depicted as X-shaped structures with red and blue arms. The top cell has a label pointing to the chromatids on the left and to the spindle fiber on the right. The bottom cell is similar in structure with no additional labels. © Medify is noted at the bottom.

During anaphase II of meiosis:

The centromeres divide.
Sister chromatids separate and are pulled to opposite poles by the spindle fibres.

The image illustrates Anaphase II of meiosis in two cells. Each cell shows chromatids being pulled apart by spindle fibers. The top cell has red and blue chromatids with spindle fibers labeled and extending from centrioles at each pole, pulling the chromatids toward opposite ends. Text labels indicate 'Anaphase II', 'Spindle fibre', and 'Chromatids'. The bottom cell shows a similar arrangement with red and blue chromatids and spindle fibers connecting to centrioles, pulling chromatids apart. Both cells are depicted against a blue background, representing the cell environment. © Medify is noted at the bottom.

During telophase II of meiosis:

Nuclear envelopes reform around each set of chromosomes.
The cytoplasm divides (cytokinesis) resulting in four haploid daughter cells, each genetically unique.

The image depicts Telophase II stage of cell division, showing two cells in the process of cytokinesis. Each cell contains a pair of chromatids, one red and one blue, enclosed within a nuclear envelope. There are spindle fibers extending from centrosomes located outside the nuclear envelopes. Labels identify 'Chromatids', 'Spindle fibre', and 'Nuclear envelope'.

When cells differentiate, they become specialised and adapted for specific functions.

Examples of specialised animal cells include:

Spermatozoa / sperm cells: contain many mitochondria to provide ATP for movement. They are long and streamlined with a flagellum for swimming. The acrosome contains enzymes that digest the outer layers of the ovum. The head contains a haploid nucleus and very little cytoplasm.
Squamous epithelial cells: are thin and flattened cells providing a short diffusion pathway and a smooth surface for fluids to move more easily across.
Ciliated epithelial cells: possess cilia that move substances across a cell surface. They contain many mitochondria to supply ATP for ciliary movement.

Examples of specialised plant cells include:

Palisade mesophyll cells: contain chloroplasts to maximise the rate of photosynthesis. Their elongated shape allows them to pack closely near the leaf surface, maximising light absorption.
Root hair cells: have long, hair-like extensions that increase the surface area available for water and mineral ion absorption. They contain many mitochondria to provide ATP for the active transport of ions from the soil.
Guard cells: are specialised cells that have unevenly thickened cell walls, with a thickened, less-elastic wall next to the stomata. Guard cells contain chloroplasts and use ATP to actively transport potassium ions into and out of the cells. This changes their water potential, causing water to enter or leave by osmosis. The resultant turgidity or flaccidity controls the opening and closing of the stomata.

Multicellular organisms are organised hierarchically, with increasing levels of complexity at each level.

Cells are the basic structural and functional units of life. Cells are specialised for particular functions. Examples include muscle cells and nerve cells in animals, and palisade mesophyll cells in plants.

A tissue is a group of similar cells working together to perform a specific function.

Tissue examples include:

Squamous epithelium: thin, flat cells for diffusion (e.g. lining of lungs).
Ciliated epithelium: cells with cilia to move substances (e.g. trachea).
Muscle tissue: contracts for movement.
Cartilage: provides support and flexibility.
Xylem: transports water and minerals in plants.
Phloem: transports sugars in plants.

An organ is made of different tissues working together to perform a specific function.

Organ examples include:

Heart: composed of muscle, nerve, and connective tissue.
Leaf: composed of palisade, spongy mesophyll, xylem and phloem tissue.

An organ system is a group of organs working together to perform major body functions.

Organ system examples include:

digestive system
circulatory system
respiratory system.

All organ systems together form a complete organism, such as a human.

Stem cells are undifferentiated cells that can divide by mitosis to produce more stem cells.

Alternatively, they can differentiate into specialised cell types that make up tissues and organs.

There are several sources of stem cells:

Embryonic stem cells, obtained from an early embryo.
Umbilical cord blood.
Adult stem cells- found in certain tissues such as bone marrow.
Induced pluripotent stem cells (iPS cells) — cells developed in a lab to behave like embryonic stem cells.

Stem cells in bone marrow can differentiate into specialised cells. Examples include:

Erythrocytes (red blood cells): specialised to transport oxygen. They have a large surface area to volume ratio for rapid gas diffusion, are flexible, so can travel easily through capillaries and contain fewer organelles (including lacking a nucleus), so there is more space for haemoglobin.
Neutrophils: specialised to engulf and destroy pathogens. Larger than erythrocytes, they move towards sites of infection via chemotaxis.

Although both cells come from the same stem cell, different genes are expressed during differentiation, causing the cells to develop different structures and functions.

Plant growth occurs in specialised regions of active cell division called meristems. Cells produced by meristems differentiate to form specialised tissues such as:

Xylem vessels: form when lignin is deposited in cell walls (killing the cell), and the end of the cells break down to form continuous tubes.
Phloem sieve tube elements: lose their organelles and form sieve plates; the companion cells retain their organelles to be able to provide ATP to the sieve tube elements.

Stem cells can be used to repair or replace damaged tissues. For example, stem cells from bone marrow can be transplanted to treat blood disorders such as leukaemia.

Stem cells also offer potential treatments for neurological conditions and have uses in developmental biology research.

Cell division, cell diversity and cellular organisation (2.1.6)

On this page

The cell cycle (2.1.6.a)

Interphase (2.1.6.a)

Mitosis (2.1.6.b, 2.1.6.c, 2.1.6.d, 2.1.6.e)

Meiosis (2.1.6.f, 2.1.6.g)

Specialised cells (2.1.6.h)

Levels of organisation (2.1.6.i)

Stem cells (2.1.6.j, 2.1.6.k, 2.1.6.l, 2.1.6.m)