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Structure

Chromosomes

Chromosomes are created by chromatin, which is made out of DNA tightly wound around a histone protein. One of these is called a nucleosome.
This is shaped as a double helix, which resembled a twisted ladder.
Chromosomes only become visible when chromatin condenses prior to cell division, once the molecules have replicated.
These replicated chromosomes create a X shape, and are known as sister chromatids. They are held together by a centromere, same as a singular chromatid.
Different animals have different chromosome number, which have no correlation to intelligence or size. Humans have 46, while a potato has 48.

Ploidy level

A complete set of chromosomes contains all coding necessary to code for required polypeptides and information for the organism to function.
The amount of chromosomes in a complete set is the haploid number, given the symbol n.
Many organisms receive one complete set from each parent. An example is humans.
- In these organisms, chromosomes occur in matching pairs, known as homologous pairs.
- Humans have 23 pairs of these homologous chromosomes, and therefore described as diploid, with a 2n symbol.
The number of complete chromosome sets is the ploidy level, and animals with more than two complete sets are known as polyploid.

Cell cycle

The cycle cycle is a regular pattern dividing cells go through.
This occurs in tissue repair sites for animal cells, and only in meristems in plant cells.
This is a continuous process. There are three major steps not equal in time:
- Interphase - the biggest step in the cycle, where the cell does its main synthesis and growth.
- Mitosis - the splitting of the cell, resulting in two genetically identical daughter nuclei. This has four stages, prophase, metaphase, anaphase and telophase.
- Cytokinesis - the division of the cytoplasm to form two genetically identical daughter cells.

Interphase

This phase has plenty of metabolic activity throughout.
The cell grows and replicates organelles lost in the last cell division.
DNA replicates to double its amount.
Proteins, such as enzymes and histones, are synthesised, requiring ATP.
Chromosomes are not visible during this stage, as chromatin, the nuclear material, is dispersed throughout the nucleus.

Prophase

This is the longest of the four mitosis stages

Chromosomes condense in the nucleus, becoming shorter and thicker. They become visible as long, thin threads. Chromosomes are visible as pairs of chromatids.
In animal cells, centrioles separate and move to opposite sides of the cell, arranging a partner as they move. They are in pairs again once they reach the poles.
Protein microtubules form from the centrioles, which create spindle fibres. These can extend from pole to pole.
The nuclear envelope disintegrates and the nucleolus disappears.
Pairs of chromatids lie free in the cytoplasm.

Metaphase

A relatively short stage.

A this point, chromosomes are pairs of sister chromatids joined at the centromere.
These centromeres attach to the spindle fibres so that chromosomes are aligned on the equator.
From the side, the chromosomes appear in a line. From the pole, they appear spread out.

Anaphase

A rapid, short phase.

The centromeres divide, separating the chromatids.
The spindle fibres retract, pulling the chromatids to the pole centromere first.

Telophase

The final stage of mitosis.

The chromatids reach the poles of the cell, and are referred to as chromosomes again.
Nuclear envelopes reform around each group of chromosomes, along with nucleolus’.
In animal cells, the spindle fibres break down. In plant cells, these remain through cell wall formation.

Cytokinesis

This is the division of the cytoplasm in order to make two separate cells.

In animal cells, it occurs with the formation of a cleavage furrow, where the mother cell pinches inwards.
- This further contracts, and the cell membranes on each side join up to make two daughter cells.
In plant cells, carbohydrate containing vesicles formed by the golgi body collect on the equator. These fuse together to form a cell plate.
- These stretch across the cell, forming a middle lamella. Cellulose builds up on each side of the lamella to form the cell walls of two daughter cells.

Meiosis

Meiosis has two stages of division, meiosis 1 and meiosis 2. These stages each have a prophase, metaphase, anaphase and telophase.
At the end of stage one, there are two daughter cells, with the normal amount of DNA contained. After the next stage, it has half this DNA and four daughter cells.
Each gamete is genetically unique.

Prophase 1

During this, maternal and paternal chromosomes match together in homologous pairs. This process is known synapsis, and the pairs are known as bivalents.
Prophase then continues as normal, with chromosomes condensing and spindle fibres forming.
However, the homologous chromosomes wrap around each other and partially repel, although they stay joined at points called chiasmata.
At a chiasma, equivalent segments of DNA may be exchanged between the bivalents. This swapping is known as crossing over.
This leads to more genetic variation, especially as it can happen at several places along the chromatid.
As normal, the nuclear envelope and nucleolus disappear.

Metaphase 1

Pairs of homologous chromosomes align at the equator randomly, so the paternal and maternal chromosomes could face either pole.
This means the combination of maternal and paternal chromosomes are random in each cell, known as independent assortment.
This is done to again increase genetic diversity.

Anaphase 1

The bivalent chromosomes are separated, the maternal and paternal going in different directions.
The separation happens as normal, with the retraction of the spindle fibres.

Telophase 1

In some species, the nuclear envelope reforms around the haploid chromosomes and they therefore decondense.
In many, however, they remain in their condensed form.

Cytokinesis

Occurs as normal, forming two haploid cells.

Phase 2

Prophase 2 - New spindles are formed at right angles to the old ones.
Metaphase 2 - Independent assortment occurs when the chromosomes are separated, as the chromatids can face either pole.
Anaphase 2 - Occurs as normal, with spindle fibres shortening.
Telophase 2 - Chromatids lengthen and can no longer be distinguished under the microscope, spindles disintegrate and nuclear envelope and nucleoli reform.
Cytokinesis - occurs as normal, producing 4 haploid daughter cells.

Mitotic index

Mitotic index is a calculation of how many cells are currently going through mitosis, therefore how high mitosis is.
This is calculated by visible cells ÷ visible cells going through mitosis. Then converted to a percentage.
Different tissues have different mitotic indexes, such as meristems, skin cells and gut cells. Meristems need to multiply more due to growth, and gut cells due to damage.

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Science

A-Level Biology

1.6: The cell cycle and division

Structure

Chromosomes

Chromosomes are created by chromatin, which is made out of DNA tightly wound around a histone protein. One of these is called a nucleosome.
This is shaped as a double helix, which resembled a twisted ladder.
Chromosomes only become visible when chromatin condenses prior to cell division, once the molecules have replicated.
These replicated chromosomes create a X shape, and are known as sister chromatids. They are held together by a centromere, same as a singular chromatid.
Different animals have different chromosome number, which have no correlation to intelligence or size. Humans have 46, while a potato has 48.

Ploidy level

A complete set of chromosomes contains all coding necessary to code for required polypeptides and information for the organism to function.
The amount of chromosomes in a complete set is the haploid number, given the symbol n.
Many organisms receive one complete set from each parent. An example is humans.
- In these organisms, chromosomes occur in matching pairs, known as homologous pairs.
- Humans have 23 pairs of these homologous chromosomes, and therefore described as diploid, with a 2n symbol.
The number of complete chromosome sets is the ploidy level, and animals with more than two complete sets are known as polyploid.

Cell cycle

The cycle cycle is a regular pattern dividing cells go through.
This occurs in tissue repair sites for animal cells, and only in meristems in plant cells.
This is a continuous process. There are three major steps not equal in time:
- Interphase - the biggest step in the cycle, where the cell does its main synthesis and growth.
- Mitosis - the splitting of the cell, resulting in two genetically identical daughter nuclei. This has four stages, prophase, metaphase, anaphase and telophase.
- Cytokinesis - the division of the cytoplasm to form two genetically identical daughter cells.

Interphase

This phase has plenty of metabolic activity throughout.
The cell grows and replicates organelles lost in the last cell division.
DNA replicates to double its amount.
Proteins, such as enzymes and histones, are synthesised, requiring ATP.
Chromosomes are not visible during this stage, as chromatin, the nuclear material, is dispersed throughout the nucleus.

Prophase

This is the longest of the four mitosis stages

Chromosomes condense in the nucleus, becoming shorter and thicker. They become visible as long, thin threads. Chromosomes are visible as pairs of chromatids.
In animal cells, centrioles separate and move to opposite sides of the cell, arranging a partner as they move. They are in pairs again once they reach the poles.
Protein microtubules form from the centrioles, which create spindle fibres. These can extend from pole to pole.
The nuclear envelope disintegrates and the nucleolus disappears.
Pairs of chromatids lie free in the cytoplasm.

Metaphase

A relatively short stage.

A this point, chromosomes are pairs of sister chromatids joined at the centromere.
These centromeres attach to the spindle fibres so that chromosomes are aligned on the equator.
From the side, the chromosomes appear in a line. From the pole, they appear spread out.

Anaphase

A rapid, short phase.

The centromeres divide, separating the chromatids.
The spindle fibres retract, pulling the chromatids to the pole centromere first.

Telophase

The final stage of mitosis.

The chromatids reach the poles of the cell, and are referred to as chromosomes again.
Nuclear envelopes reform around each group of chromosomes, along with nucleolus’.
In animal cells, the spindle fibres break down. In plant cells, these remain through cell wall formation.

Cytokinesis

This is the division of the cytoplasm in order to make two separate cells.

In animal cells, it occurs with the formation of a cleavage furrow, where the mother cell pinches inwards.
- This further contracts, and the cell membranes on each side join up to make two daughter cells.
In plant cells, carbohydrate containing vesicles formed by the golgi body collect on the equator. These fuse together to form a cell plate.
- These stretch across the cell, forming a middle lamella. Cellulose builds up on each side of the lamella to form the cell walls of two daughter cells.

Meiosis

Meiosis has two stages of division, meiosis 1 and meiosis 2. These stages each have a prophase, metaphase, anaphase and telophase.
At the end of stage one, there are two daughter cells, with the normal amount of DNA contained. After the next stage, it has half this DNA and four daughter cells.
Each gamete is genetically unique.

Prophase 1

During this, maternal and paternal chromosomes match together in homologous pairs. This process is known synapsis, and the pairs are known as bivalents.
Prophase then continues as normal, with chromosomes condensing and spindle fibres forming.
However, the homologous chromosomes wrap around each other and partially repel, although they stay joined at points called chiasmata.
At a chiasma, equivalent segments of DNA may be exchanged between the bivalents. This swapping is known as crossing over.
This leads to more genetic variation, especially as it can happen at several places along the chromatid.
As normal, the nuclear envelope and nucleolus disappear.

Metaphase 1

Pairs of homologous chromosomes align at the equator randomly, so the paternal and maternal chromosomes could face either pole.
This means the combination of maternal and paternal chromosomes are random in each cell, known as independent assortment.
This is done to again increase genetic diversity.

Anaphase 1

The bivalent chromosomes are separated, the maternal and paternal going in different directions.
The separation happens as normal, with the retraction of the spindle fibres.

Telophase 1

In some species, the nuclear envelope reforms around the haploid chromosomes and they therefore decondense.
In many, however, they remain in their condensed form.

Cytokinesis

Occurs as normal, forming two haploid cells.

Phase 2

Prophase 2 - New spindles are formed at right angles to the old ones.
Metaphase 2 - Independent assortment occurs when the chromosomes are separated, as the chromatids can face either pole.
Anaphase 2 - Occurs as normal, with spindle fibres shortening.
Telophase 2 - Chromatids lengthen and can no longer be distinguished under the microscope, spindles disintegrate and nuclear envelope and nucleoli reform.
Cytokinesis - occurs as normal, producing 4 haploid daughter cells.

Mitotic index

Mitotic index is a calculation of how many cells are currently going through mitosis, therefore how high mitosis is.
This is calculated by visible cells ÷ visible cells going through mitosis. Then converted to a percentage.
Different tissues have different mitotic indexes, such as meristems, skin cells and gut cells. Meristems need to multiply more due to growth, and gut cells due to damage.