1.6 Cell Division
Cell cycle
Cell cycle: describes the behavior of cells as they grow and divide.
Involves 3 main stages:
1. Interphase: a very acive phase of the cell cycle with many processes occurring in the nucleus
and cytoplasm.
- the longest phase of the cell cycle
- processes include:
➢ DNA replication in the nucleus
➢ protein synthesis in the cytoplasm (DNA transcription/translation)
➢ increase in the number of organelles e.g mitochondria and/or chloroplast
➢ cell growth
- It is divided into 3 phases; G1, S and G2.
➢ G1 – the cell grows larger/ protein synthesis/ mitochondria replication/ chloroplast
replication/ number of mitochondria and chloroplasts increase.
➢ S (synthesis) – DNA is replicated.
➢ G2 – cell grows and prepare for mitosis
1. Mitosis: The division of the nucleus to form two genetically identical daughter nuclei.
It is divided into: prophase, metaphase, anaphase and telophase.
2. Cytokinesis is the last stage during which the cytoplasm divides to create two daughter cells. In
animal cells the cell is pinched in two while plant cells form a plate between the dividing cells.
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By Marwa Bkerat – Updated October 22, 2020
Supercoiling of chromosomes
DNA wraps around histones → nucleosomes →further wrapped on top of each other → chromatin →
more coiling → chromosomes
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During the S-phase, DNA is copied
Chromosomes condense by a process called supercoiling
The two copies of each chromosome are held together by a protein
structure called a centromere.
Each chromosome in this state is represented by a pair of sister
chromatids.
Sister chromatids are two identical DNA molecules held together by
centromere.
Mitosis
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The division of the nucleus to form two genetically identical daughter nuclei.
It is divided into: prophase, metaphase, anaphase and telophase.
Mitosis involves the following processes:
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Growth
embryonic development
tissue repair
asexual reproduction
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By Marwa Bkerat – Updated October 22, 2020
Phases of mitosis
Phase
1. Prophase
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2. Metaphase
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3. Anaphase
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4. Telophase
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Event
The spindle microtubules start growing and start to extend from
each pole to the equator.
Sister chromatids condense by supercoiling.
Sister chromatids thicken, shorten and become visible.
The nuclear envelope starts breaking down
The sister chromatids move to the equator and line up separately.
The spindle microtubules from each pole attach to each
centromere on opposite sides.
The spindle microtubules are fully developed
the spindle microtubules contract to pull the sister chromatids
apart splitting the centromeres.
This splits the sister chromatids into chromosomes.
Each identical chromosome is pulled to opposite poles.
the spindle microtubules break down
the chromosomes uncoil/decondense and so are no longer
individually visible.
The nuclear membrane reforms.
The cell then divides by cytokinesis to form two daughter cells with
identical genetic nuclei.
To avoid confusion in terminology, use the following:
- Early stages of mitosis – sister chromatids
- From anaphase onwards – chromosomes.
By Marwa Bkerat – Updated October 22, 2020
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Identification of phases of mitosis in cells viewed with a microscope or in a micrograph.
interphase
prophase
metaphase
anaphase
telophase
The Mitotic index
Mitotic index =
number of cells in mitiosis
(counted in the micrograph)
Total number of cells
The mitotic index is an important prognostic tool for predicting the response of cancer cells to
chemotherapy.
Cytokinesis:
Occurs after mitosis and is different in plant and animal cells.
Plant cell
Mitosis
Cytokinesis
No centrioles
The cell wall forms a plate between the
dividing cells.
Animal cell
Centrioles are present
The cell is pinched in two forming a
cleavage furrow.
Cyclins and the control of the cell cycle
Cyclins:
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a group of proteins
ensure that the tasks of every phase are performed at the correct time so that the cell can move
on to the next stage.
control of the cell cycle
bind to enzymes called cyclin-dependent kinases.
These kinases become active and attach phosphate groups to other proteins in the cell.
The attachment of phosphate triggers the other proteins to become active and carry out tasks
specific to one of the phases of the cell cycle.
CDK (inactive) + specific cyclin → active → phosphorylate other proteins (adds P) → proteins become active → carry out
specific tasks
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By Marwa Bkerat – Updated October 22, 2020
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There are 4 main types of cyclins in human cells:
➢ Cyclin D – triggers cells to move from G0 to G1, and from G1 to S phase
➢ Cyclin E – prepares the cell for DNA replication in S phase
➢ Cyclin A – activates DNA replication inside the nucleus in S phase
➢ Cyclin B – promotes the assembly of the mitotic spindle and other tasks in the cytoplasm
to prepare for mitosis.
Tumor formation and cancer
Mutagens, oncogenes and metastasis are involved in the development of primary and secondary
tumors.
Tumors:
- are the results of uncontrolled cell division
- Occur in any organ or tissue.
- Could be of two types:
1. Primary tumors: when cells adhere to each other and don’t invade nearby tissue. These
tumors are unlikely to cause any harm and are classified as benign.
2. Secondary tumors: when tumor cells become detached, move elsewhere in the body
(metastasis) and develop into secondary tumours. These tumors are very likely lifethreatening and are classified as malignant, known as cancer.
- Agents that cause cancer are known as carcinogens.
- Carcinogens may include:
➢ Mutagens: agents that cause gene mutations, which can cause cancer.
➢ Some viruses
➢ High energy radiation such as X-rays and short-wave ultraviolet light.
Mutations: random changes to the base sequence of genes.
Most genes don’t cause cancer if they mutate. The few genes that cause cancer after mutating are
known as oncogenes. In a normal cell, oncogenes are involved in the control of the cell cycle and cell
division. This is why mutations in them can cause tumor formation.
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By Marwa Bkerat – Updated October 22, 2020
Metastasis: the movement of cells from a primary tumor to set up secondary tumors in other parts of
the body.
Smoking and cancer
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There is a strong positive correlation between cigarette smoking the death rate due to cancer
(incidence of cancers).
Surveys have shown that the more cigarettes that one smokes per day, the higher the death
rate due to cancer.
The main cancers involved are cancer of the mouth, pharynx, larynx, esophagus and lungs
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By Marwa Bkerat – Updated October 22, 2020