CIE AS LEVEL Biology: The Mitotic Cell Cycle

What are chromosomes

Chromosomes are made of one very long, condensed DNA molecule associated with proteins (in eukaryotic cells)

What are histones

The main proteins present are the large positively charged globular proteins called histones, their role is to organise and condense the DNA tightly so that it fits into the nucleus.The other proteins are enzymes used in copying and repairing the DNA

What is chromatin

The tightly coiled combination of DNA and proteins is called chromatin - this is what chromatids, and therefore chromosomes, are made of

How are sister chromatids

During interphase (S phase) the DNA replicates to create two identical strands of DNA called chromatids, joined together by a narrow region called the centromere The two chromatids that make up the double structure of a chromosome are known as 'sister chromatids'

Why is it important that sister chromatids

It is important that the sister chromatids are identical (contain the same genes) because this is key to cell division, as one chromatid goes into one daughter cell and one goes into the other daughter cell during mitosis, ensuring the daughter cells are genetically identical. Each chromatid is made up of one very long, condensed DNA molecule, which is made up of a series of genes.The ends of the chromatids in chromosomes are 'sealed' with protective structures called telomeres

What is mitosis

Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent nucleus

Importance of mitosis

Growth of multicellular organisms
Replacement of cells and repair of tissues
Asexual reproduction

Importance of mitosis in growth of multicellular organisms

The two daughter cells produced are genetically identical to one another (clones) and have the same number of chromosomes as the parent cell This enables unicellular zygotes (as the zygote divides by mitosis) to grow into multicellular organisms Growth may occur across the whole body of the organism or be confined to certain regions, such as in the meristems (growing points) of plants

Importance of mitosis in Replacement of cells and repair of tissues

Damaged tissues can be repaired by mitosis followed by cell division As cells are constantly dying they need to be continually replaced by genetically identical cells In humans, for example, cell replacement occurs particularly rapidly in the skin and the lining of the gut Some animals can regenerate body parts, for example, zebrafish can regenerate fins and axolotls regenerate legs and their tail amongst other parts

Importance of mitosis in Asexual reproduction

Asexual reproduction is the production of new individuals of a species by a single parent organism - the offspring are genetically identical to the parent For unicellular organisms such as Amoeba, cell division results in the reproduction of a genetically identical offspring For multicellular organisms (as seen with many plant species) new individuals grow from the parent organism (by cell division) and then detach ('bud off') from the parent in different ways. Some examples of these are budding in Hydra and yeast and runners from strawberries

What are the phases of the cell cycle

The cell cycle is the regulated sequence of events that occurs between one cell division and the next The cell cycle has three phases:
interphase
nuclear division (mitosis)
cell division (cytokinesis)

What are cyclins

The movement from one phase to another is triggered by chemical signals called cyclins

What happens during interphase

During Interphase the cell increases in mass and size and carries out its normal cellular functions (eg. synthesising proteins and replicating its DNA ready for mitosis) Interphase consists of three phases:G1 phase,S phase,G2 phase
It is at some point during the G1 phase a signal is received telling the cell to divide again. The DNA in the nucleus replicates (resulting in each chromosome consisting of two identical sister chromatids). This phase of the interphase stage of the cell cycle is called the S phase - S stands for synthesis (of DNA). The S phase is relatively short
The gap between the previous cell division and the S phase is called the G1 phase - G stands for gap Cells make the RNA, enzymes and other proteins required for growth during the G1 phase
Between the S phase and next cell division event the G2 phase occursDuring the G2 phase, the cell continues to grow and the new DNA that has been synthesised is checked and any errors are usually repairedOther preparations for cell division are made (eg. production of tubulin protein, which is used to make microtubules for the mitotic spindle)

What happens during cytokinesis

Follows M phase,Once the nucleus has divided into two genetically identical nuclei, the whole cell divides and one nucleus moves into each cell to create two genetically identical daughter cells In animal cells, cytokinesis involves constriction of the cytoplasm between the two nuclei and in plant cells a new cell wall is formed

What are telomeres

The ends of the chromatids in chromosomes are 'sealed' with protective structures called telomeres.They are made of non-coding DNA (DNA that does not contain genes) that is made up of short base sequences that are repeated many times (multiple repeat sequences) In telomeres, one strand is rich in the base guanine (G) and the other strand is rich in the complementary base cytosine (C)

What is the main function of telomeres

The main function of telomeres is to ensure that the very ends of the DNA molecules are included in DNA replication during mitosis (the copying enzyme responsible for DNA replication is unable to run right to the very end of the DNA molecule and stops a little short of the end)
If this end part of the DNA molecule contained an important gene, that piece of genetic information would be lost during DNA replication In each subsequent cell division, a little more genetic information would be lost
Telomeres therefore act as a 'buffer' region of non-essential DNA and ensure that no important coding sections near the ends of the DNA molecules are left out of the replication process. This ensures no genes are lost during cell division (the loss of vital genes can even result in cell death) and allows for continued replication of a cell

What do some cells do to prevent loss of genes

To avoid the risk of losing genes most cells have an enzyme called telomerase that adds additional bases at each end (the telomeres) Some cells (generally specialised cells) do not have telomerase to 'top up' their telomeres and therefore after a certain number of cell divisions the cell dies, this has been connected with the ageing process

What is a stem cell

A stem cell is a cell that can divide (by mitosis) an unlimited number of times. Each new cell (produced when a stem cell divides) has the potential to remain a stem cell or to develop into a specialised cell such as a blood cell or a muscle cell (by a process known as differentiation)

What is potency

This ability of stem cells to differentiate into more specialised cell types is known as potency

What are the types of potency

Totipotency
Pluripotency
Multipotency

What is totipotency

Totipotency - totipotent stem cells are stem cells that can differentiate into any cell type found in an embryo, as well as extra-embryonic cells (the cells that make up the placenta). The zygote formed when a sperm cell fertilises an egg cell is totipotent, as are the embryonic cells up to the 16-cell stage of human embryo development

What is pluripotency

Pluripotency - pluripotent stem cells are embryonic stem cells that can differentiate into any cell type found in an embryo but are not able to differentiate into extra-embryonic cells (the cells that make up the placenta)

What is multipotency

Multipotency - multipotent stem cells are adult stem cells that have lost some of the potency associated with embryonic stem cells and are no longer pluripotent

Why are adult stem cells multipotent

As tissues, organs and organ systems develop, cells become more and more specialised Having differentiated and specialised to fulfil particular roles, most adult cells gradually lose the ability to divide until, eventually, they are no longer able to divide However, small numbers of stem cells (known as adult stem cells) remain to produce new cells for the essential processes of growth, cell replacement and tissue repair Although these adult stem cells can divide (by mitosis) an unlimited number of times, they are only able to produce a limited range of cell types - they are multipotent

Example of multipotent stem cells

the stem cells found in bone marrow are multipotent adult stem cells - they can only differentiate into blood cells (red blood cells, monocytes, neutrophils and lymphocytes). In adults, stem cells can be found throughout the body (eg. in the bone marrow, skin, gut, heart and brain)

How do cancers divide

Cancers demonstrate how important it is that cell division is precisely controlled, as cancers arise due to uncontrolled mitosis Cancerous cells divide repeatedly and uncontrollably, forming a tumour (an irregular mass of cells)

How do cancers start

Cancers start when changes occur in the genes that control cell division. A change in any gene is known as a mutation. If the mutated gene is one that causes cancer it is referred to as an oncogene. Mutations are common events and don't lead to cancer most of the time Most mutations either result in early cell death or result in the cell being destroyed by the body's immune system As most cells can be easily replaced, these events usually have no harmful effect on the body

The mutations that result in the generation of cancerous cells do not result in early cell death or in the cell being destroyed by the body's immune system This means that the harmful mutation occurring in the original cell can be passed on to all that cell's descendants

What is carcinogens

Carcinogens are any agents that may cause cancer (eg. UV light, tar in tobacco smoke, oncoviruses and X-rays). If the agent causes cancer it is described as carcinogenic

What are benign tumours

Some tumours (such as warts) do not spread from their original site - these are known as benign tumours and do not cause cancer

Malignant tumours

Some tumours spread through the body, invading and destroying other tissues - these are known as malignant tumours and cause cancer

Metastasis

Malignant tumours interfere with the normal functioning of the organ / tissue in which they have started to grow (eg. they may block the intestines, lungs or blood vessels
Malignant tumour cells can break off the tumour and travel through the blood and / or lymphatic system to form secondary growths in other parts of the body
The spreading of cancers in this way is known as metastasis
Metastasis is very dangerous as it can be very difficult to detect, locate and remove secondary cancers

Stages of mitosis

Propane
Metaphase
Anaphase
Telophase

Prophase

Chromosomes condense and are now visible when stained
The chromosomes consist of two identical chromatids called sister chromatids (each containing one DNA molecule) that are joined together at the centromere
The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles (opposite ends of the nucleus)
Spindle fibres (protein microtubules) begin to emerge from the centrosomes (consists of two centrioles in animal cells)
The nuclear envelope (nuclear membrane) breaks down into small vesicles

Metaphase

Centrosomes reach opposite poles
Spindle fibres (protein microtubules) continue to extend from centrosomes
Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles
Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres
Each sister chromatid is attached to a spindle fibre originating from opposite poles

Anaphase

The sister chromatids separate at the centromere (the centromere divides in two)
Spindle fibres (protein microtubules) begin to shorten
The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules)

Telophase

Chromosomes arrive at opposite poles and begin to decondense
Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes
The spindle fibres break down

What are meristemz

Growth in plants occurs in specific regions called meristems
The root tip meristem can be used to study mitosis
The root tip meristem can be found just behind the protective root cap
In the root tip meristem, there is a zone of cell division that contains cells undergoing mitosis

How to prepare a slide for an onion

Garlic or onion (Allium cepa) root tips are most commonly used (the bulbs can be encouraged to grow roots by suspending them over water for a week or two)
Remove the tips of the roots (about 1cm) and place in a suitable stain (eg. warm, acidified acetic orcein, which stains chromosomes a deep purple)
The stained root tip is gently squashed on a glass slide using a blunt instrument (eg. the handle of a mounting needle)
Cells undergoing mitosis (similar to those in the images below) can be seen and drawn
Annotations can then be added to these drawings to show the different stages of mitosis