Summary
Biology OCR A level Cell division Summary Notes
Biology OCR A level Cell division Summary Notes
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1 of 11B6- Cell division
2.1.6 Cell division, cell diversity and cellular organisation
(a) The cell cycle.
Key words:
Cell cycle- the highly ordered sequence of events that takes place in a cell, resulting in division of
the nucleus and the formation of two genetically identical daughter cells.
Interphase- growth period of the cell cycle, between cell divisions (mitotic phase). Consists of
stages G1, S and G2.
Mitosis- nuclear division stage in the mitotic phase of the cell cycle.
Cytokinesis- cell division stage in the mitotic phase of the cell cycle that results in the production
of two identical daughter cells.
Parent cell- the initial cell in cell division.
Daughter cell- one of two cells, identical to the parent, produced by cell division.
• Eukaryotic cells, the cell cycle has two main phases-
interphase and mitotic phase.
Interphase
• Sometimes referred as the resting phase as cells are not
actively dividing.
• Not a stage in cell division but stage between cell
divisions.
During interphase:
• DNA is replicated and checked for errors in the nucleus.
• Protein synthesis occurs in the cytoplasm.
• Mitochondria grow and divide, increasing in number in the
cytoplasm.
• Chloroplasts grow and divide in plant and algal cell
cytoplasm, increasing in number.
• The normal metabolic processes of cells occur.
The three stages of interphase:
• G1- the first growth phase: proteins from
which organelles are synthesised are
produced and organelles replicate. The cell
increases in size.
• S- synthesis phase: DNA is replicated in the
nucleus.
• G2- the second growth phase: the cell
continues to increase in size, energy stores
are increased and the duplicated DNA is
checked for errors.
Mitosis
• The nucleus divides.
Cytokinesis
• The cytoplasm divides and two cells are
produced.
(b) How the cell cycle is regulated.
Key words:
Checkpoints- control mechanisms of the cell cycle.
G0
• Phase when the cell leaves the cycle, temporarily or permanently.
• Differentiation- cell that becomes specialised to carry out a particular function are no longer able
to divide. It will carry out this function indefinitely and not enter the cell cycle again.
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• DNA of a cell may be damaged, in which case it is no longer viable. A damaged cell can no
longer divide and enters a period of permanent cell arrest. The majority of normal cells only
divide a limited number of times and eventually become senescent.
• The number of these cells in your body increases.
Cell cycle control
• Order and timing of processes in the cell cycle are under tight control.
• Cycle has checkpoints, at the end of G1 and G2, that verify whether each phase of the cycle
has been completed correctly.
• These checkpoints are controlled by proteins called cyclins and cyclin-dependent kinases.
• Cell cycle can be halted when errors are detected at a checkpoint.
(c) The main stages of mitosis.
Key words:
Asexual reproduction- the production of genetically identical offspring from a single parent.
Chromatids- two identical copies of DNA (a chromosome) held together at a centromere.
Centromere- region at which two chromatids are held together.
Chromatin- uncondensed DNA in a complex with histones.
Chromosomes- structures of condensed and coiled DNA in the form of chromatin.
Chromosomes become visible under the light microscope when cells are preparing to divide.
Stages of mitosis
Stage Features Appearance
Prophase - Chromatin fibres (comprising DNA and histone
proteins) coil and condenses and chromosomes
become visible
- Nucleolus disappears
- Protein microtubules form spindle shaped
structures linking the poles
- Centrioles move to the poles of the cell
- Spindle fibres attach to specific areas on the
centromeres and start to move the chromosomes
to the centre of the cell (end of prophase/
beginning of metaphase)
- Nuclear envelope breaks down
Metaphase - Spindle fibres (organised by the centrioles) attach
to centromeres (towards the end of prophase/
beginning of metaphase)
- Chromosomes line up along the centre of the cell
forming the metaphase pate
Anaphase - Spindle fibres shorten
- Centromeres divide
- Chromatids are separated and pulled to opposite
poles of the cell
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