The cell cycle, mitosis and meiosis The cell cycleLiving cells go through a series of stages known as the cell cycle. The cells grow, copy their chromosomes, and then divide to form new cells.

The cell cycle

G1 phase. The cell grows.

S phase. The cell makes copies of its chromosomes. Each chromosome now consists of two sister chromatids.

G2 phase. The cell checks the duplicated chromosomes and gets ready to divide.

M phase. The cell separates the copied chromosomes to form two full sets (mitosis) and the cell divides into two new cells (cytokinesis).

The period between cell divisions is known as 'interphase'.

Cells that are not dividing leave the cell cycle and stay in G0.

Mitosis and meiosisCells divide into two different ways to make new cells.

Mitosis

Mitosis is used to produce daughter cells that are genetically identical to the parent cells. The cell copies - or 'replicates' - its chromosomes, and then splits the copied chromosomes equally to make sure that each daughter cell has a full set.

Meiosis

Meiosis is used to make special cells - sperm cells and egg cells - that have half the normal number of chromosomes. It reduces the number from 23 pairs of chromosomes to 23 single chromosomes. The cell copies its chromosomes, but then separates the 23 pairs to ensure that each daughter cell has only one copy of each chromosome. A second division that divides each daughter cell again to produce four daughter cells.

 

 

Mitosis and meiosis

Mitosis

Your body contains trillions of cells (thousands of millions). But you started life as a single cell - a fertilised egg cell. This cell then divided and divided to make more cells through a process called mitosis.

Mitosis is a way of making more cells that are genetically the same as the parent cell. It plays an important part in the development of embryos, and it is important for the growth and development of our bodies as well. Mitosis produces new cells, and replaces cells that are old, lost or damaged.

In mitosis a cell divides to form two identical daughter cells. It is important that the daughter cells have a copy of every chromosome, so the process involves copying the chromosomes first and then carefully separating the copies to give each new cell a full set.

Before mitosis, the chromosomes are copied. They then coil up, and each chromosome looks like a letter X in the nucleus of the cell. The chromosomes now consist of two sister chromatids. Mitosis separates these chromatids, so that each new cell has a copy of every chromosome.

 

 

A diagram of a cell ready for mitosis. The copied chromosomes consist of two chromatids joined at the centromere

The process of mitosis involves a number of different stages. The following diagram sets out the stages, and the main events that occur in each stage.

 

The phases of mitosis

MIOSIS- the context of mitosis - explaining its position in the sequence of processes that, together, form the "cell cycle" for somatic cells.

Definition: MitosisT is defined as the type of cell division by which a single cell divides in such a way as to produce two genertically identical "daughter cells". This is the method by which the body produces new cells for both growth and repair of aging or damaged tissues throughout the body - as opposed to for sexual reproduction (when meiosisapplies).

Mitosis is the simplest of the two ways (mitosis and meiosis) in which the nucleus of a cell can divide - as part of a

process of whole cell division. The four stages of mitosis (prophase, metaphase, anaphase and telophase) are illustrated and described below.

 

Mitosis (Nuclear Division)

0.

Interphase

Interphase is not part of mitosis but is included here as a reminder that interphase preceeds mitosis. (Hence, it has the number 0.)

Chromatin is material in a cell nucleus consisting of DNA and protein. This is the substance that chromosomes are made from. It can be stained with dyes in order to watch the process of mitosis using a microscope.

1.

Above: Early Prophase

Above: Late Prophase

Prophase

Early in the prophase stage the chromatin fibres shorten into chromosomes that are visible under a light microscope. (Each prophase chromosome consists of a pair of identical double-stranded chromatids.)

Later in prophase, the nucleolus disappears, the nuclear envelope breaks down, and the two centrosomes begin to form the miotic spindle (which is an assembly of microtubules).

As the microtubules extend in length between the centrosomes, the centrosomes are pushed to opposite "poles" (extremes) of the cell.

Eventually, the spindle extends between two opposite poles of the cell.

2.

Metaphase

Metaphase is characterized by the "metaphase plate". This is a mid-point region within the cell that is formed/defined by the centromeres of the chromatid pairs aligning along the microtubules at the centre of the miotic spindle.

3.

Above: Early Anaphase 

Above: Late Anaphase

Anaphase

The centromeres split seperating the two members of each chromatid pair - which then move to the opposite poles of the cell: When they are seperated the chromatids are called chromosomes.

As the chromosomes are pulled by the the microtubules during anaphase, they appear to be "V"-shaped because the centromeres lead the way, dragging the trailing arms of the chromosomes towards the pole/s.

4.Telophase

Telophase begins after the chromosomal movement stops. The identical sets of chromosomes - which are by this stage at opposite poles of the cell, uncoil and revert to the

long, thin, thread-like chromatin form. A new nuclear envelope forms around each chromatin mass. Nucleoli appear. Eventually the miotic spindle breaks-up.

0.

Two new "daughter cells" - the cycle is about to repeat itself.

... then the cytoplasm begins to divide around the two new nuclei - which is called Cytokinesis (Cytoplasmic Division).

 Meiosis

Some simple organisms - such as bacteria - can reproduce by simply dividing into two new individuals. Other organisms, including human beings, reproduce through sexual reproduction. New individuals are formed by the joining together of two special cells: a sperm cell and an egg cell.

The cells in our bodies contain 23 pairs of chromosomes - giving us 46 chromosomes in total. Sperm cells and egg cells contain 23 single chromosomes, half the normal number, and are made by a special form of cell division called meiosis.

Meiosis separates the pairs of matching (or 'homologous') chromosomes, so that sperm cells and egg cells have only one copy of each. That way, when an egg cell fuses with a sperm cell, the fertilised egg has a full set: that is, two copies of every chromosome.

Meiosis involves two cell divisions: Meiosis I and Meiosis II.

Meiosis I separates the matching - or 'homologous' - pairs of chromosomes. Meiosis II divides each chromosome into two copies (much like mitosis). In Meiosis I, each daughter cell receives a mix of chromosomes from the two sets in the parent cell. In addition, the chromosomes in each matching pair swap some genetic material before they are parted in a process called crossing over. These processes produce new combinations of genes in the sperm cells and egg cells.

Interphase

Meiosis interphase is divided into three phases G1 phase or Growth 1 phase is a very active period. In this period the cell synthesizes vast range of

proteins which includes the enzymes and structural proteins necessary for growth of the cell. In the G1 phase the chromosome are made of single molecule of DNA, at this point, in humans, the number of chromosomes per cell is 46 which is 2N and identical to the somatic cells.

S phase or the Synthesis phase - There is replication of genetic material in this phase. Chromosomes duplicate, each of the 46 chromosomes become a complex of two sister, identical chromatids.

G2 phase or Growth phase is not present in meiosis.

The interphase stage is followed by meiosis I and meiosis II.

Meisois is divided into meiosis I and meiosis II stages. It is further divided into Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis II respectively.

Meiosis I

The pairs of homologous chromosomes, made up of two sister chromatids are split into two cells. The resulting daughter cells contains one entire haploid set of chromosomes. The first meiotic division reduces the ploidy of original cell by a factor of two. It produces two haploid cells (N chromosomes, 23 in humans). Hence meiosis I is referred to as a reductional division. A diploid human cell contains 46 chromosomes and is said to be 2N because it contains 23 pairs

of homologous chromosomes. Meiosis II is an equational division similar to mitosis, where the sister chromatids split and

creating 4 haploid cells, two from each daughter cells from meiosis I.

Prophase I

Prophase I is the longest phase of meiosis I. During this phase, there is exhange of DNA between homologous chromosomes, this process is

known as homologous recombination. This process often results in chromosomal crossover. The DNA created are of new combinations, and during crossover they are a significant source of

genetic variation. This may result in beneficial new combinations of alleles. The chromosomes that are paired and replicated are called bivalents or tetrads. They have two chromosomes and four chromatids, each chromosome comes from each parent.

Pairing of homologous chromosomes is called synapsis. At the stage of synapsis formation, the non-sister chromatids may cross-over at points called chiasmata.

Leptotene Leptotene is the first stage of prophase I and is also known as leptonema, which is derived from a

Greek word which means "thin threads". In this stage, individual chromosomes consists of two sister chromatids. The chromosomes condense into visible strands within the nucleus. The two sister chromatids are tightly bound, that they are not distinguishable from one other. During this phase the lateral elements of the synaptonemal complex assemble. This stage is of very short duration and progressive condensation and coiling of chromosome takes

place.

Zygotene Zygotene is also known as zygonema, it is derived from Greek word which means 'paired threads'. The chromosomes in this line up with each other into homologous chromosome pairs. This stage is known as bouquet stage, due to the way the telomeres cluster at on end of the

nucleus. Synapsis of homologous chromosomes takes place in this stage, it is facilitated by the assembly of

central element of the synaptonemal complex. Pairing of chromosomes happens in a zipper like fashion and starts at the centromere (procentric)

or at the chromosome ends (proterminal) or at any other portion (intermediate). Two chromosomes in a pair are equal in length and in position of the centromere, making the

pairing highly specific and exact. These paired chromosomes are called bivalent or tetrad chrmosomes.

Pachytene The pachytene stage is also known as pachynema and is derived from Greek which means "thick

threads".This is the stage where chromosomal crossing over occurs. Nonsister chromatids of homologous chromosomes exchange segments over homologous regions. Sex chromosomes are not identical and they exchange information over a small region of

homology. Chiasmata is formed where exchange happens. There is exchange of information between the non-sister chromatids and this results in a

recombination of information. Every chromosome has a complete set of information and there are no gaps formed as the result of

the process.

Diplotene The diplotene stage is also known as diplonema, which is derived from Greek word meaning "two

threads". During this stage there is degradation of synaptonemal complex and the homologous

chromosomes separate a little from one another. The chromosomes in this stage uncoil a little, this allows transcription of DNA. The bivalent homologous chromosomes remain tightly bound at the region of the chiasmata,where

crossing over occurred. The chiasmata regions remains on the chromosomes until they are separated in the anaphase.

In the oogenesis of humans the developing oocytes in the fetal stage stop at this stage of diplotene before birth. This state is referred to as the dictyotene stage and it remains in this suspended stage until puberty.

Diakinesis

During the stage of diakinesis the chromosomes condense further. The word diakinesis is derived form Greek word which means "moving through". This stage is the first part of meiosis where the four arms of the tetrads are visible. The sites where crossing over has occurred entangle together, overlapping effectively and making

the chiasmata visible clearly. This stage resembles the prometaphase of mitosis, where the nucleoli disappear and the nuclear

membrane disintergrates into vesicle and also there is formation of the meiotic spindle.

Metaphase I

The homologous pairs of chromatids move together along the metapahse plate. The kinetochore microtubules from the centrioles attach to their kinetochores respectively. The homologous chromosomes align along the equatorial plane, this alignment happens due to the

continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the kinetochores of the homologous chromosomes.

Anaphase I

During this phase the kinetochore microtubules shorten, this severs the recombination nodules and pulls the homologous chromosomes apart.

As each chromosome has only one functional unit of a pair of kinetochores, the whole chromosomes are pulled towards the opposite poles which results in the formation of two haploid sets.

Each chromosomes contains a pair of sister chromatids. Disjunction occurs during this time, this is one of the process that leads to genetic diversity as the

chromosomes end up in either of the daughter cells.

The nonkinetochore microtubules lengthen and pushes the centrioles farther apart. The cell is elongated and it prepares for division at the center.

Telophase I

The first phase of meiotic division ends when the chromosomes arrive at the poles. The daughter cells now have half the number of chromosomes, the chromosomes consists of a pair

of chromatids. The microtubules of the spindle network disappear and nuclear membrane surrounds each haploid

set. The chromosomes uncoil and return back to the chromatin stage. The process of cytokinesis occurs where, the cell membrane in the animals cells is pinched off,

and in plant cells there is formation of the cell wall in between the daughter cells. This completes the creation of two daughter cells. The sister chromatids remain attached during

the telophase I stage.

Meiosis II

It is the second part of the meiotic process and is also known as equational division. Meiosis II is similar to mitosis. The genetical results are fundamentally different from that of mitosis. The end result of mitosis II is the production of four haploid cells from two haploid cells produced in meiosis I, each cell consisting of 23 chromosomes in humans and the chromosomes consists of two sister chromatids.In meiosis II there are four steps

Prophase II Metaphase II Anaphase II Telophase II.

Prophase II

During this stage there is disappearance of the nucleoli and the nuclear envelope, also there is shortening and thickening of the chromatids.

The centrioles move to the polar region and the spindle fibers are arranged for the second meiotic division.

Metaphase II

During this stage the centromeres that contain two kinetochores attach to the spindle fibers at each pole from the centrioles.

The equatorial plate formed here is rotated by 90 degrees, compared to meiosis I and is perpendicular to the previous metaphase plate.

Anaphase II

The metaphse II is followed by the anaphase II, in the anaphase II stage the centromeres are cleaved, this allows the microtubules attached to the kinetochores to pull the sister chromatids apart.

The sister chromosomes move towards the opposing poles.

Telophase II

The meiosis II process ends at this stage, this stage is similar to the telophase I. In this phase there is uncoiling and lengthening of the chromosomes and disappearance of the

spindle. There is also reformation of nuclear

The following diagrams set out the main stages of Meiosis I and Meiosis II in males. (A similar process in females produces egg cells rather than sperm cells.)