Mitotic Cell Cycle
1) Structure of a Chromosome
Chromosomes are thread-like structures present in the nucleus of a plant and animal cell.
They are vehicles that carry DNA wrapped around by Histone Proteins.
Histones organise and condense the DNA tightly.
Chromatids
Each chromatid is made up of one very long, condensed DNA molecule.
The centromere holds 2 chromatids together
The chromatids, centromere and telomeres make up the chromosome
The ends of the chromatids are sealed with protective structures “telomeres”
These telomeres play a crucial role in maintaining chromosome stability and preventing degradation during cell division.
Chromatins
Tightly coiled combinations of DNA and proteins.
2) Role of Telomeres
Made up of DNA with short base sequences that are repeated many times.
this allows for the DNA polymerase enzyme — responsible for DNA replication to bind and to be able to copy the whole DNA.
Telomeres prevents the loss of genes from the ends of chromosomes during DNA replication.
Telomeres makes sure mutation does not occus in the synthesis of DNA.
The Nucleotides in Telomeres do not contain genetic information, but they elongate the DNA so that all the important information can be copied by DNA polymerase.
3) Cell Cycle pt. 1
The sequence of events that cells got through between one cell division and the next.
Interphase → Mitosis → Cytokinesis
4) Cell Cycle pt. 2
Interphase: period of growth and DNA replication between cell divisions.
1) G (Gap) 1 Phase: the cells simply get bigger in size
Organelles are replicated
RNA produced
Enzymes and othe proteins are synthesised
This leads to cell growth
2) S (Synthesis) Phase: controlled by enzymes
DNA Replication Occurs
DNA in the cell replicates - resulting in each chromosome consisting of two identical sister chromatids.
Protein production is very low during this step.
Loose DNA known as chromatin gathers and condenses into chromatids with the help of histone proteins.
3) G2 Phase
Where The Cells Prepare For Mitosis
Also known as the “Gap 2” phase
The cell continues to grow and the duplicated Chromosomes are “double checked” - any errors are usually repaired.
Preparations for division are made (e.g. in production of Tubulin; a protein essential for producing microtubules for mitotic spindle)
Mitotic Spindle, Microtubules, Centrioles, Centrosomes?
Mitotic Spindle:
Cytoskeletal structures of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells.
Long microtubules will extend from the centrioles in all possible directions forming a ‘spindle’.
Microtubules:
Long, rigid, hollow tubes mde of a protein called tubulin: Make up the centrioles and later on during the spindle, help chromosomes move during cell division, & help cell organelles to move inside the cell.
Centrioles:
2 cylindrical structures made of microtubules, and are found outside of the nucleus in animal cells, in the centrosome.
Centrosome:
The region where the centrioles lie, located at the poles of the cell during mitosis.
Each centrosome consists of a pair of centrioles surrounded by a large no. of proteins.
Kinetochore:
A protein structure found at the centromere of a Chromatid
Centromere:
Middle part of a Chromosome, which holds Chromatids during mitosis.
During Mitosis:
Spindle microtubules attach the centromere.
Each Metaphase chromosome has 2 kinetochore as its centromere, one on each chromatid.
Kinetochores will connect the centromere to the spindle microtubules.
Mitosis: The process in which a eukaryotic cell’s nucleus splits into 2, followed by the division of the parent cell into 2 daughter cells.
The Concept of Mitosis: To make more Diploid cells
Process:
Chromosome Duplication (each chromosome is copied)
Separation (Copies are separated to opposite sides of the cell)
Cell Division: Cell divides down the middle
Outcome: Each new cell recieves its own copy of each chromosome
Mitosis
Also called the ‘M-phase’.
Cell growth stops when its reaches here.
Cell divides its copied DNA into two sets and divides its cytoplasm, to make 2 new cells.
2 distinct phases: Karyokinesis & Cytokinesis
Prophase
(Early Prophase)
Chromatins condense becoming chromosomes.
Centrioles separate and start moving to the opposite ends of the cells.
Mitotic Spindle begin to emerge from the Centrosomes.
Nuclear envelope starts to ‘disappear’.
Nucleolus - part of the cell where ribosomes are produced, disappears.
(Late Prophase)
Nuclear envelope already dissolved.
Nucleolus is gone.
Mitotic Spindle starts to form.
Chromosomes condenses (connected by centromere).
Centrioles (surrounded by microtubules) moves to the cell.
Prometaphase (or Late prophase)
The nuclear envelope fragments enabling microtubules to invade the nuclear area.
They gradually attach with the condensed chromosomes.
Nucleolus diappears & Centiroles have moved to opposite poles.
Kinetochores apear at the centromeres.
Mictrotubules bind to chromosomes with the help of a specialised protein present in the centrosomes with the help a specialised protein present in the centromere - Kinetochore.
At the end of prophase, a spindle is completely formed & chromosomes are fully condensed.
Animal cells: more mictrotubules that extends from the microtubule organising centre centrosome towards the edge of the cells forming a structure called Aster.
3. Metaphase
Chromosomes are condensed maximally.
Chromosomes are lined up across the equator of the cell - Metaphase plate.
Centrosomes are at opposite poles of the cell and project Mitotic spindle to connect each chromosome.
4. Anaphase
Chromosomes pulled apart into 2 Chromatids - by spindle fibers.
Sister Chromatids separate at the centromere.
Spindles contract and pull chromatids to opposite poles of the cell.
Non-kinetochore spindle fibers lengthen, elongating the cell.
5. Telophase
Telo means complete or final.
The stage where the daughter cell formation is ‘nearly’ complete.
Chromatids have reached the poles of the spindle.
They begin to de-condense/uncoil to Chromatins.
Their form now: long, slender, and extended.
Nuclear envelope material reappears and surrounds each set of chromosomes —> 2 daughter nuclei.
Spindle fibers break down —> tubulin monomers.
Nucleolus reforms at the end of the telophase.
Control of the Cell Cycle
Regulatory proteins called cyclins control the cell cycle at checkpoints:
G1 Checkpoint — decides whether or not cell will divide.
S Checkpoint — determines if DNA has been properly replicated.
Mitotic Spindle Checkpoint — ensures chromosomes are aligned at mitotic plate.
Cytokinesis
2nd main stage of the Mitotic phase:
→ physical separation of cytoplasmic components into the 2 daughter.
“Cyto” means Cellular.
“Kinesis” means Movement.
In animal cells, 2 proteins: Actin & Myosin separate the daughter cells by a Cleavage Furrow.
In plant cells, daughter cells are spearated by a cell plate —> as constriction is not possible, due to the presence of a rigid cell wall.
At the end of Cytokinesis, there are now 2 cells with the same structure and no. of Chromosomes as the parent cells.
Behaviour of Chromosomes in plant cells is identical to that in animal cells, however:
Plant cells do not have centrosomes.
After nuclear division, a new cell wall must form between the daughter nuclei/daughter cells.
Mitosis in order:
Prophase → Prometaphase → Metaphase → Anaphase → Telophase → Cytokinesis
5) Importance of Mitosis
Growth of multicellular organisms.
Replacement of dead/ damaged cells (e.g. skin cells that only live 2 weeks)
Repair of tissues by cell replacement (e.g. healing wounds – some animals are even able to regenerate whole parts of the body)
Asexual reproduction – offsprings are clones of their parents e.g. Binary fission of Amoeba cells
one cell divides to form two new cells that are identical to each other and the original cell.
Budding: bud on parent organism develops into a new organism through repeated cell division
Mitosis occurs in the initial formation of the bud
Immune response: cloning of B- and T-lymphocytes is dependent on mitosis
Active infection — immune cells are activated and proceed through mitosis and cell differentiation.
This allows for a rapid increase in immune cells.
Stem Cells
Cells that can divide an unlimited number of times by mitosis.
This is done to replenish various types of cells.
They have the ability to develop into several distinct cell types in the body
The power of a cell to dod this is called potency.
They can remain as they are, or differentiate into different types of specialised cells.
e.g. blood, bones, and muscle cells.
They can repair damaged cells.
2 types of stem cells:
Embryonic
Adult
When a stem cell divides, each new cell has the potential to either remain a stem cell or become another type of specialised cell.
6) Cancer
Many different diseases caused by a common mechanism:
uncontrolled cell division → forms a tumour.
7) Factors that increase the risk of cancer:
Carcinogens: Substances/environmental factors that can lead to cancer, such as:
→ Ionising radiation (UV light)
→ Tar
→ Asbestos
→ Virus infections (HPV)
Genetic Mutation
Mutated genes that cause cancer are referred to as oncogene.
→ Results in a malignant tumour.
Hereditary Predisposition: Cancer is inherited.
70% Sporadic
20% Familial
10% Inherited
Development of Cancer
Oncogenes transformed by carcinogen.
Cells do not respond to cell signals from other cells so it continues to divide.
Mitosis.
Cancer cells get nutrients not treated by immune system.
Rapid mitosis.
Tumour gets bigger.
Tumour supplied with blood and lymph vessels (if it’s malignant tumour, tumour cells spread in blood and lymph to other parts of the body.
Metastasis: tumour cells invade other tissues (secondary cancers form throughout the body).
Types of Tumour
Benign tumour
Non-cancerous.
Doesn’t invade surrounding tissues/spread (metastasize) throughout the body.
Grows at a slow rate.
Well-defined boundary: usually have clear, smooth edges (prevents invasion).
Non-invasive.
Malignant tumour
Cancerous.
Invades surrounding tissues and spreads (metastasize) throughout the body.
Uncontrolled growth (grows rapidly).
Irregular boundary: uneven, jagged edges (invade neighbouring tissues + difficult to remove).
Invasive.