Cell & Nuclear Division (DP IB Biology)

Equal cytokinesis is when the cytoplasm divides evenly between two daughter cells, resulting in cells of similar size.

Mitochondria are needed for cellular respiration, and chloroplasts are needed for photosynthesis in plant cells. A cell (or whole organism) could not function without these two processes.

Unequal cytokinesis occurs when the cytoplasm divides unevenly, resulting in daughter cells of different sizes.

Oogenesis in humans and budding in yeast are examples of unequal cytokinesis.

During oogenesis, a primary oocyte divides to form a secondary oocyte and a polar body, followed by another division to form an ovum and a second polar body.

Polar bodies degenerate and become part of the final ovum during oogenesis.

Two processes by which the nucleus of a eukaryotic cell can divide are mitosis and meiosis.

The nucleus must divide before cell division to avoid the production of anucleate cells, which are cells without a nucleus.

Mitosis gives rise to genetically identical cells and is used for growth, repair of damaged tissues, replacement of cells, and asexual reproduction.

Mitosis maintains the chromosome number by producing daughter cells that are diploid (2n) and genetically identical to the parent cell.

Meiosis gives rise to genetically different cells and is used to produce gametes (sex cells).

Meiosis produces haploid (n) daughter cells with half the chromosome number of the parent cell, leading to genetic variation.

DNA replication, during interphase, must occur to create two identical strands of DNA called chromatids. This must happen before mitosis and meiosis.

During anaphase one chromatid from each chromosome is pulled to opposite poles of the cell, eventually becoming individual chromosomes.

DNA condensation is necessary to fit the long DNA molecules into the small nuclei of cells.

Supercoiling, aided by histone proteins and enzymes, repeatedly coils the DNA molecule to condense it into chromosomes.

Microtubules are responsible for the movement of chromosomes by lengthening and shortening to facilitate their movement.

Microtubule motors are motor proteins that carry chromosomes along the microtubules to the equator of the cell in preparation for nuclear division.

Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced, each having the same number of chromosomes as the parent cell.

There are four main stages of mitosis: prophase, metaphase, anaphase, and telophase.

During prophase chromosomes condense and become visible. Each chromosome consists of two identical sister chromatids joined at the centromere.

During prophase the two centrosomes move toward opposite poles of the nucleus, and spindle fibers begin to emerge from them.

During prophase, the nuclear envelope breaks down into small vesicles, and the nucleolus disappears.

During metaphase chromosomes line up at the equator of the spindle, also known as the metaphase plate.

During metaphase, spindle fibres attach to the centromeres of the chromosomes via kinetochores. Each sister chromatid is connected to a spindle fibre from opposite poles.

The key event of anaphase is when the sister chromatids separate at the centromere and are pulled to opposite poles by the shortening spindle fibres.

During telophase, chromosomes arrive at opposite poles and begin to de-condense.

Nuclear envelopes reform around each set of chromosomes, spindle fibres break down, and new nucleoli form within each nucleus.

The stages of the cells X, Y and Z are:

• X = anaphase

• Y = metaphase

• Z = anaphase

Meiosis is a type of cell division that produces genetically different cells and is used to produce gametes (sex cells).

Meiosis consists of two rounds of division: Meiosis I and Meiosis II.

Meiosis produces haploid gametes, reducing the chromosome number by half, which is essential for maintaining the species' chromosome number across generations during sexual reproduction.

During Meiosis I, homologous chromosomes are separated, resulting in two haploid nuclei, each still containing chromosomes with two chromatids.

A bivalent is a pair of homologous chromosomes that come together during prophase I of meiosis.

Meiosis I is called reduction division because it reduces the chromosome number by half, from diploid (2n) to haploid (n).

During Meiosis II, the chromatids of each chromosome are separated, resulting in four haploid nuclei, each containing chromosomes with a single chromatid.

False.

There is no chromosome replication between Meiosis I and Meiosis II.

Crossing over results in the exchange of genetic material between homologous chromosomes, creating recombinant chromosomes and increasing genetic variation.

The random orientation of bivalents during metaphase I leads to the independent assortment of chromosomes, which generates a variety of possible genetic combinations in gametes.

Genetic variation is important for natural selection and increases a species' ability to adapt and survive in changing environments.

Non-disjunction occurs when chromosomes fail to separate properly during meiosis, leading to gametes with an abnormal number of chromosomes.

Down Syndrome, also known as Trisomy 21, occurs when non-disjunction results in an extra copy of chromosome 21, leading to a total of 47 chromosomes in the affected individual, instead of 46.

The risk of non-disjunction increases with the age of the mother, older ova are more likely to undergo incorrect chromosome separation.

Cell proliferation is the process of cells dividing to promote growth, replace cells, and repair tissues.

True.

Cell proliferation occurs in plant meristems.

Interphase is a metabolically active phase in which the cell grows, synthesises DNA, and prepares for mitosis.

The main stages of interphase are G1, S, and G2 phases.

Cyclins regulate the cell cycle by increasing and decreasing in concentration, ensuring checkpoints are met.

False.

Cytokinesis occurs after mitosis, dividing the cytoplasm to complete cell division.

In the S phase, DNA is replicated to ensure each daughter cell has a full set of genetic information.

True.

Cyclins must reach a threshold concentration to ensure cell cycle checkpoints are passed.

Cytokinesis is the process where the cell's cytoplasm divides, resulting in two daughter cells.

Cell proliferation replaces damaged cells in the skin, promoting tissue repair and wound healing.

False.

Cyclins are protein molecules that control the cell cycle. When a threshold concentration is reached, the next stage of the cell cycle can begin.

Cytokines are protein molecules involved in cell signalling within the immune system.

A proto-oncogene normally regulates cell growth and division. A mutation in a proto-oncogene converts it into an oncogene, leading to uncontrolled cell division.

A tumour suppressor gene is a gene that regulates cell division and prevents uncontrolled growth.

True.

Mutations in tumour suppressor genes can prevent synthesis of the proteins required to inhibit cell division, this results in uncontrolled cell division.

Benign tumours do not invade other tissues or spread, while malignant tumours can invade tissues and cause cancer.

Metastasis is the spread of cancer cells from a primary tumour to form secondary tumours in other parts of the body.

True.

A primary tumour is the original location of tumour growth where the initial development is supported by the formation of blood vessels to supply the tumour with nutrients, oxygen and growth factors.

A secondary tumour forms when cancer cells from a primary tumour break off and spread to another location in the body via the blood or lymphatic system.

An oncogene is a mutated proto-oncogene. It is permanently activated and therefore causes uncontrolled cell division and tumour formation.

False.

Benign tumours do not invade tissues and do not cause cancer.

Malignant tumours have the capacity for invasion and metastasis, spreading cancer to other tissues.

False.

Whilst a benign tumour carries less risk due to it being non-cancerous, it can still have significant negative effects on the patient due to damage to surrounding tissues e.g. blockages or pressure.

Proto-oncogenes regulate cell growth and division.

In order to calculate the mitotic index of a sample of cells, we need to know...

• The number of cells with visible chromosomes (undergoing mitosis)

• The total number of cells in the sample

The mitotic index is calculated using the following formula:

The mitotic index can be calculated as follows:

True.

Chromosomes condense during prophase and are visible throughout the cell division process of mitosis or meiosis.

True.

The mitotic index reflects the rate of cell division within a population.

This might indicate that the cells in the sample are dividing more quickly than normal, possibly as a result of uncontrolled cell division.

The mitotic index can be calculated as follows:

To calculate the number of cells in mitosis, we need to carry out the following calculation:

0.54 x 104 = 56.16 = 56 cells undergoing mitosis