1.6 Cell division
Understanding:
Mitosis is division of the nucleus into two genetically identical daughter nuclei.
The Role of Mitosis:
Interphase is a very active phase of the cell cycle with many processes occurring in the nucleus and cytoplasm.
The cell cycle is the sequence of events between one cell division and the next. It consists of two main phases: interphase and cell division (mitosis and cytokinesis).
The Role of Mitosis:
- nucleus of eukaryotic cells can divide to form two genetically identical nuclei by mitosis
- DNA must be replicated, this occurs during Interphase (the period before mitosis)
- each chromosome is converted from a single DNA molecule to two identical DNA molecules, called chromatids
- in mitosis, one of these chromatids passes to each daughter nucleus
- mitosis' four phases: prophase, metaphase, anaphase and telophase
- Growth: multicellular organisms increase in size through mitosis
- Asexual Reproduction: certain eukaryotic organisms reproduce asexually
- Tissue Repair: damaged tissue can recover by replacing dead or damaged cells
- Embryonic Development: a fertilized cell (zygote) will undergo mitosis and cell differentiation in order to develop into an embryo
Interphase is a very active phase of the cell cycle with many processes occurring in the nucleus and cytoplasm.
The cell cycle is the sequence of events between one cell division and the next. It consists of two main phases: interphase and cell division (mitosis and cytokinesis).
Interphase:
Protein Synthesis: the cell synthesizes key proteins and enzymes to enable it to grow, copy its contents and divide
ATP Production: undergoes cellular respiration
Increase in Organelles: mitochondria and chloroplasts are produced for the daughter cells
DNA Replication: genetic material must be duplicated before cell division (S Phase)
Chromosomes condense by supercoiling during mitosis.
Supercoiling of Chromosomes:
Cytokinesis occurs after mitosis and is different in plant and animal cells
Cytokinesis:
- Active Phase:
Protein Synthesis: the cell synthesizes key proteins and enzymes to enable it to grow, copy its contents and divide
ATP Production: undergoes cellular respiration
Increase in Organelles: mitochondria and chloroplasts are produced for the daughter cells
DNA Replication: genetic material must be duplicated before cell division (S Phase)
- numbers of mitochondria increase during interphase because of the grown and division of mitochondria
- in plants/algae, number of chloroplasts increase in the same way, and they also synthesize cellulose and use vesicles to add it to their cell walls
- consists of three phases: G1 Phase, S Phase and G2 Phase
- S Phase: cell replicates all the genetic material in the nucleus
- some cells do not go beyond G1 because they are never going to divide so they don't prepare for mitosis
- so then they enter a phase called G0 which may be temporary or permanent
Chromosomes condense by supercoiling during mitosis.
Supercoiling of Chromosomes:
- the DNA molecules in the chromosomes are immensely long
- human nuclei are on average 5µm in diameter but the DNA molecules in them are more than 50,000µm long
- process of supercoiling is called condensation of chromosomes (occurs during prophase)
- condensation: repeatedly coiling the DNA molecule to make the chromosome shorter and wider
- proteins called histones that are associated with DNA in eukaryote chromosomes help with supercoiling (enzymes are also involved)
Cytokinesis occurs after mitosis and is different in plant and animal cells
Cytokinesis:
- occurs after the cell has two genetically identical nuclei
- cells actually divide during cytokinesis
- Animals Cells: the plasma membrane is pulled inwards around the equator of the cell to form a cleavage furrow (achieved by a ring of contractile protein immediately inside the plasma membrane at the equator) the proteins are actin and myosin (similar to proteins that cause muscle contraction) when the cleavage furrow reaches the centre, the cell is pinched apart into two daughter cells
- Plant Cells:
Cyclins are involved in the control of the cell cycle.
Cyclins and the Control of the Cell Cycle:
Cyclins and the Control of the Cell Cycle:
- cyclins are a group of proteins that ensure the tasks are performed at the correct time and that the cell only moves on to the next stage of the cycle when it is appropriate
- cyclins bind to enzymes called cyclin-dependent kinases, then these kinases become active and attache 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)
- there are four main types of cyclins in human cells
- graph shows how levels of these cyclins rise and fall
- unless these cyclins reach a threshold concentration, the cell does not progress to the next stage of the cell cycle
- therefore, cyclins control the cell cycle and ensures that cells divide when new cells are needed, but not at other times
Mutagens, oncogenes and metastasis are involved in the development of primary and secondary tumours.
Tumour Formation and Cancer:
Tumour Formation and Cancer:
- tumours are abnormal groups of cells that develop at any stage of life in any part of the body
- sometimes, cell stick with each other and do not invade nearby tissues or move to other parts of the body, these tumours are unlikely to cause harm
- other tumours can become detached and move elsewhere and develop into secondary tumours, these tumours can be life-threatening
- these "tumours" (that can move) are know as cancer and have diverse causes
- carcinogens is a type of chemical that causes cancer including chemical mutagens and short-wave UV light (this happens because mutagens are agents cause mutations and mutations can cause cancer)
- mutations are random changes to the base sequence of genes
- not all mutations are cancer causing
- the few genes that become cancer-causing after mutating are known as oncogenes
- in a normal cell, oncogenes are involved in the control of the cell cycle and cell division (which is why mutations in them can result in uncontrolled cell division, therefore tumour formations)
- several mutations must occur in the same cell for it to become a tumour cell
- when a tumour cell has been formed, it divides repeatedly to form two, four, eight, so on (this group of cells is called a primary tumour)
- metastasis is the movement of cells from a primary tumour to set up secondary tumours in other parts of the body
Skills:
Identification of phases of mitosis in cells viewed with an microscope.
Prophase:
Prophase:
- chromosomes become fatter and shorter by supercoiling
- microtubules grow from structures called microtubule organizing centres (MTOC) to form a spindle-shaped array that links the poles of the cell
- at the end of prophase, the nuclear membrane breaks down and the chromosomes are now visible
Metaphase:
- microtubules continue to grow and attack to the centromeres on each chromosome
- the two attachment points on opposite sides of each centromere allow the chromatids of a chromosome to attach to microtubules from different poles
- the microtubules are all put under tension to test whether the attachment is correct (this happens by shortening of the microtubules at the centromere, if the attachment is correct then the chromosomes stay on the equator of the cell)
Anaphase:
- centromeres divide, allowing sister chromatids to separate
- spindle microtubules pull them rapidly towards the poles of the cell
- this works because of the attachment earlier in metaphase
Telophase:
- chromatids have reached the poles and are now called chromosomes
- chromosomes are pulled into a tight group near the MTOC and a nuclear membrane reforms around them
- the chromosomes uncoil and a nucleolus is formed
- by this stage of mitosis, the cell is already dividing and the two daughter cells enter interphase again
Determination of a mitotic index from a micrograph.
The Mitotic Index:
The Mitotic Index:
- the ratio between the number of cells in mitosis in a tissue and the total number of observed cells
- Mitotic Index = (number of cells in mitosis) / (total number of cells)
Nature of science:
Serendipity and scientific discoveries: the discovery of cyclins was accidental.
Discovery of Cyclins:
Discovery of Cyclins:
- Tim Hunt discovered a protein that increased in concentration after fertilization then decreased in concentration, unlike other proteins which continued increase (during research into control of protein synthesis in sea urchin eggs)
- the protein was being synthesized over a period of about 30 minutes and then soon after was being broken down
- further experiments showed that the protein went through repeated increases and decreases in concentration that coincided with the phases of the cell cycle
- Hunt named the protein cyclin
Applications:
The correlation between smoking and incidence of cancers.
Smoking and Cancer:
Smoking and Cancer:
- positive correlation between cigarette smoking and the death rate due to cancer
- results of survey also show huge increases in the death rate due to cancers of the mouth, pharynx, larynx and lung (as smoke makes contact with these parts of the body)