1.6 - Cell division

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The role of mitosis

mitosis: the division of the nucleus into two genetically identical daughter nuclei

before mitosis, all the DNA must replicate (this happens during interphase)

each chromosome is converted from a single DNA molecule into two identical DNA molecules, called chromatids

mitosis is involved whenever cells with genetically identical nuclei are required in eukaryotes: during embryonic development, growth, tissue repair and asexual reproduction

it has four phases: prophase, metaphase, anaphase, and telophase

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Interphase

it's a very active phase in the like of a cell when many metabolic reactions occur

some of these (like the reactions of cell respiration) also occur during cell division

but DNA replication and protein synthesis only happen during interphase

during interphase: the numbers of mitochondria in the cytoplasm increase due to the growth and division of mitochondria

in plant cells and algae the number of chloroplasts increase in the same way, they also synthesize cellulose and use vesicles to add it to their cell walls

interphase consists of three phases: G1 phase, S phase, G2 phase.

Some cells don't progress beyond the G1 phase because they will never divide, they enter a phase called G0

S phase: the cell replicates all the genetic material in the nucleus

G1 phase: cellular contents apart from chromosomes are duplicated

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Supercoiling of chromosomes

because chromosomes are extremely long so they need to be packaged into much shorter structures

this process is known as condensation of chromosomes and it occurs during the first stage of mitosis

condensation occurs by repeatedly coiling the DNA molecule to make the chromosome shorter and wider

this process is called supercoiling

proteins called histones that are associated with DNA in eukaryote chromosomes help with supercoiling and enzymes are also involved

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Prophase

supercoiling happens

the nucleolus breaks down

microtubules grow to form a spindle-shaped array that links the poles of the cell

at the end, the nuclear membrane breaks down

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Metaphase

microtubules continue to grow and attach the centromeres to each chromosome

the two attachment points on opposite sides allow the chromatids to attach microtubules from different poles

the microtubules are shortened at the centromere to test whether the attachment is correct

if the attachment is correct, the chromosomes remain on the equator of the cell

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Anaphase

at the start, each centromere divides, allowing the pairs of sister chromatids to separate

the spindle microtubules pull them rapidly towards the poles of the cell

two genetically identical nuclei are produced because sister chromatids are pulled to opposite poles, this is ensured by the way the spindle microtubules were attached

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Telophase

the chromatids have reached the poles and are now called chromosomes

at each pole, the 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, the cell is usually already dividing and the two daughter cells enter interphase again

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Cytokinesis

the process of cell division is called cytokinesis

it usually begins before mitosis is completed

in animal cells:

the plasma membrane is pulled inwards around the equator of the cell to form a cleavage furrow

this is done using a ring of contractile protein immediately inside the plasma membrane of the equator

the proteins are actin and myosin, they're similar to proteins that cause a contraction in muscle

when the cleavage furrow reaches the center, the cell is pinched apart into two daughter cells

in plant cells:

vesicles are moved to the equator where they fuse to form tubular structures across the equator

with the fusion of more vesicles, these tubular structures merge to form two layers of membrane across the whole of the equator,

which develop into the plasma membranes of the two daughter cells

they are connected to the existing plasma membranes at the sides of the cell, completing the division of the cytoplasm

in the next stage in plants is for pectins and other substances to be brought in vesicles and deposited by exocytosis between the two new membranes

this forms the middle lamella that will link the new cell walls

both of the daughter cells then bring cellulose to the equator and deposit it by exocytosis adjacent to the middle lamella

as a result, each cell builds its own cell wall adjacent to the equator

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Cyclins and the control of the cell cycle

cyclins are groups of proteins that are used to ensure that tasks are performed at the correct time and the cell only moves on to the next stage of the cycle when it's appropriate

cyclins bind to enzymes called cyclin-dependent kinases

the 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

unless the cyclins reach a threshold concentration, the cell does not progress to the next stage of the cell cycle

cyclins control the cell cycle and ensure that cells divide when new cells are needed, but not at other times

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Tumor formation and cancer

tumors are abnormal groups of cells that develop at any stage of life in any part of the body

benign tumors

the cells adhere to each other and do not invade nearby tissues or move to other parts of the body, these are unlikely to cause much harm

malignant tumors

become detached and move elsewhere in the body and develop into secondary tumors, they are very likely to be life-threatening

diseases due to malignant tumors are commonly known as cancer and have diverse causes

chemicals and agents that cause cancer are known as carcinogens because carcinomas are malignant tumors

there are various types of carcinogens including some viruses

all mutagens are carcinogenic, both chemical mutagens and also high energy radiation such as X-rays and short-wave ultraviolet light

this is because mutagens are agents that cause gene mutation and mutations can cause cancer

oncogenes: genes that can become cancer-causing after mutating

in a normal cell, oncogenes are involved in the control of the cell cycle and cell division

so if mutations occur one them, it can result in uncontrolled cell division and therefore tumor formation

several mutations must occur in the same cell for it to become a tumor cell

because there is a vast number of cells in the body, the total chance of tumor formation during a lifetime is significant

when a tumor cell has been formed it divides repeatedly to form two, then four, then eight cells and so on

this group of cells is called a primary tumor

metastasis: the movement of cells from a primary tumor to set up secondary tumors in other parts of the body